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2013 119 Mónica Delgado Gracia Analysis of microencapsulated phase change material slurries and phase change material emulsions as heat transfer fluid and thermal storage material Departamento Director/es Ingeniería Mecánica Zalba Nonay, Belén Lázaro Fernández, Ana Director/es Tesis Doctoral Autor Repositorio de la Universidad de Zaragoza – Zaguan http://zaguan.unizar.es UNIVERSIDAD DE ZARAGOZA Departamento Director/es Mónica Delgado Gracia ANALYSIS OF MICROENCAPSULATED PHASE CHANGE MATERIAL SLURRIES AND PHASE CHANGE MATERIAL EMULSIONS AS HEAT TRANSFER FLUID AND THERMAL STORAGE MATERIAL Director/es Ingeniería Mecánica Zalba Nonay, Belén Lázaro Fernández, Ana Tesis Doctoral Autor Repositorio de la Universidad de Zaragoza – Zaguan http://zaguan.unizar.es UNIVERSIDAD DE ZARAGOZA Departamento Director/es Director/es Tesis Doctoral Autor Repositorio de la Universidad de Zaragoza – Zaguan http://zaguan.unizar.es UNIVERSIDAD DE ZARAGOZA Mónica Delgado Gracia Zaragoza, August 2013 Ph.D. Thesis Analysis of microencapsulated phase change material slurries and phase change material emulsions as heat transfer fluid and thermal storage material    Analysis of microencapsulated phase change material slurries and phase change material emulsions as heat transfer fluid and thermal storage material Dissertation presented by MÓNICA DELGADO GRACIA in fulfillment of the requirements for the degree of Doctor at the University of Zaragoza. Advisors: Prof. Mª Belén Zalba Nonay, Ph.D. Prof. Ana Lázaro Fernández, Ph.D. Escuela de Ingeniería y Arquitectura (EINA) Departamento de Ingeniería Mecánica Área de Máquinas y Motores Térmicos Instituto de Investigación en Ingeniería de Aragón (I3A) Universidad de Zaragoza Zaragoza, August 2013  AGRADECIMIENTOS / ACKNOWLEDGMENTS  i Me gustaría que estas líneas sirvieran para expresar mi agradecimiento a quienes han financiado mi trabajo durante este tiempo, a la empresa CIAT y al Vicerrectorado de Investigación de la Universidad de Zaragoza por la ayuda concedida para la realización de este trabajo. Al Programa Europa de becas de la Obra Social de la CAI y a la COST Action TU 0802 por financiar mi estancia de investigación en Alemania. Así como a las empresas e institutos de investigación que han colaborado con el envío de muestras: BASF, Rubitherm, Fraunhofer UMSICHT, AERO. A mis directoras de tesis Belén y Ana, por su apoyo técnico y personal, por su rigor académico, por su sensatez y complementariedad, por su constante empuje y por haber confiado en mí para la realización de este trabajo. To Stefan Gschwander and all the people of the group “Thermally Active Materials and Solar Cooling” from Fraunhofer ISE Institute, for their warm welcome during those months in Freiburg. To all the people that I met there and made more bearable those days away from my friends and family. A mis amigos, compañeros y familia, a los que están más cerca y a los que están más lejos, por todos esos ratos de ocio que requiere todo esfuerzo continuado. Al grupo GITSE por su buena acogida, especialmente a Luis Serra y a José María Marín. A mis compañeros de trabajo y sala durante estos años, por ser además de compañeros buenos amigos, por compartir innumerables momentos con ellos: especialmente a Pablo, Conchita, Nuria, Sergio, Javi, Mateo, Marisa, Monica C., José Luis y Gemma. La atmósfera de trabajo junto a ellos ha sido simplemente perfecta. A mi hermano, por ser mi referente en este camino, por sus consejos académicos, por sus continuo buen humor, y como no, a mis sobrinos Hugo y Andrea, porque la mejor manera de despejarme y olvidar por un momento la tesis han sido esos ratos junto a ellos. A mis padres, por ser el pilar fundamental en todo lo que soy, para los que no hay suficientes palabras para agradecerles todo su esfuerzo y amor. Y ya para terminar a Sebas, por su paciencia, compresión, cariño y amor. Por el placer cotidiano de estar junto a él en el día a día. Analysis of microencapsulated phase change material slurries and phase change material emulsions as heat transfer fluid and thermal storage material ii INDEX  ix 5.1 Introduction ................................................................................. 127 5.2 Description of the experimental installation ................................. 127 5.2.1 Description of the devices of the experimental installation . 128 5.3 Validation of the experimental installation ................................... 134 5.3.1 Validation of the measurement of pressure drop ................ 136 5.3.2 Validation of the heat flux ................................................... 137 5.3.3 Validation of the wall temperature measurements .............. 138 5.4 Empirical model for the correction of the wall temperature measurements ............................................................................ 142 5.5 Data acquisition programme ....................................................... 145 5.6 Conclusions ................................................................................. 146 Chapter 6. Analysis of microencapsulated PCM slurries as heat transfer fluid 149 6.1 Aim of the tests ........................................................................... 150 6.2 Experimental results .................................................................... 151 6.2.1 Verification of the energy balance ...................................... 151 6.2.2 Measurements of pressure drop and comparison to water 158 6.2.3 Measurements of wall temperature and determination of the internal forced convective coefficient. Comparison to water .. 163 6.3 Conclusions ................................................................................. 172 Chapter 7. Analysis of applications: tanks of PCM slurries. Comparison with other thermal energy storage systems ...................................................................... 175 7.1 Introduction ................................................................................. 176 7.1.1 Review of latent TES systems. Water exchange ................ 178 7.1.2 Considerations about heat transfer in latent TES systems ....................................................................................... 182 7.1.3 Objetives ............................................................................ 182 7.2 TES system with spherical capsules of PCM .............................. 183 7.2.1 PCM used .......................................................................... 183 7.2.2 Development of the numerical model for the melting of a sphere ......................................................................................... 185 7.2.3 Model of the tank with PCM spheres .................................. 190 7.3 TES system with water and with microencapsulated PCM slurry 197 7.3.1 Microencapsulated PCM slurry and description of the tank ............................................................................................. 197 7.3.2 Model of the tank with water and with the microencapsulated PCM slurry .................................................... 198 7.4 Comparison among the results of the studied TES systems ....... 205 7.5 Conclusions ................................................................................. 207 Chapter 8. Conclusions and future work .......................................................... 209 8.1 Contributions ............................................................................... 210 8.1.1 Bibliographic review ........................................................... 210 8.1.2 Thermophysical properties ................................................. 210 Analysis of microencapsulated phase change material slurries and phase change material emulsions as heat transfer fluid and thermal storage material  x 8.1.3 Rheological properties ........................................................ 211 8.1.4 Analysis of stability and compatibility of TES systems with PCMs .......................................................................................... 233 8.1.5 Results of heat transfer and fluids mechanics .................... 212 8.1.6 Thermal and technical behavior of TES systems ............... 214 8.2 Dissemination of results .............................................................. 214 8.3 Future work ................................................................................. 215 Capítulo 8. Conclusiones y trabajo future ........................................................ 219 8.1 Aportaciones ............................................................................... 220 8.1.1 Revisión bibliográfica ......................................................... 220 8.1.2 Propiedades termofísicas ................................................... 220 8.1.3 Propiedades reológicas ...................................................... 221 8.1.4 Análisis de estabilidad y compatibilidad de sistemas de almacenamiento de energía térmica con PCMs .......................... 222 8.1.5 Resultados sobre la transferencia de calor y mecánica de fluidos .......................................................................................... 223 8.1.6 Comportamiento térmico y técnico de sistemas de almacenamiento de energía térmica ........................................... 224 8.2 Difusión de resultados ................................................................. 225 8.3 Líneas futuras ............................................................................. 227 Bibliographic references ............................................................................... 229 Appendix I. Handling and storage of PCM slurries ..................................... 249 Appendix II. Technical specifications of the equipments and calibration certificates ...................................................................................................... 255 Appendix III. Numerical models of simulation in EES of TES systems ..... 271 Appendix IV. Substances analyzed .............................................................. 277 INDEX OF FIGURES   xi INDEX OF FIGURES Figure 1.1 Energy density of different Thermal Energy Storage systems. ....... 12 Figure 1.2 Morphology of microcapsules through a SEM microscope (Su et al. 2007 a) ........................................................................................................ 15 Figure 1.3 Microencapsulation process from (Schmidt 2008) .......................... 16 Figure 1.4 Drying process from BASF (Schmidt 2008) .................................... 16 Figure 1.5 Subcooling and hysteresis phenomena .......................................... 24 Figure 1.6 Effect of two different types of seeds on the nucleation process (Günther et al. 2011) ........................................................................................ 27 Figure 1.7 Instability processes in emulsions (Huang et al. 2009) ................... 29 Figure 1.8 Relationship between pumping power and heat transfer (Chen et al. 2006) ........................................................................................................... 33 Figure 1.9 Friction factors vs. Reynolds number (Wang et al. 2007) ............... 35 Figure 1.10 Viscosity values of different PCM dispersions studied in literature ......................................................................................................................... 36 Figure 1.11 Thermal conductivity values for different mPCM slurries studied in literature ........................................................................................................ 38 Figure 2.1 Aspect of the slurries prepared from PCM microcapsules. PCM mass fractions 10, 20 and 30%. Left: BASF manufacturer; Right: Microtek Laboratories ..................................................................................................... 51 Figure 2.2 Emulsion Fraunhofer UMSICHT with thickener. Deformation of the plastic container ............................................................................................... 52 Figure 2.3 Installation of the T-history method for the determination of the Enthalpy-Temperature curves (Lázaro 2008) ................................................... 54 Figure 2.4 Enthalpy-Temperature curves for the different PCM emulsions and mPCM slurries analyzed ............................................................................ 55 Figure 2.5 Enthalpy-Temperature curves of the candidate mPCM slurries ..... 56 Figure 2.6 Installation of the test bench for the determination of the thermal conductivity of materials in the laboratory for determination of thermophysical properties. Left: DSC; Right: Laser Flash equipment ....................................... 58 Figure 2.7 Image of the sampleholder for liquids from Netzsch ...................... 60 Figure 2.8 Drawing of the sampleholder for liquids .......................................... 62 Figure 2.9 Signal of the infrared sensor with the empty sampleholder and with water ........................................................................................................ 63 Figure 2.10 Thermal diffusivity values of water under vacuum or otherwise ... 64 Analysis of microencapsulated phase change material slurries and phase change material emulsions as heat transfer fluid and thermal storage material  xii Figure 2.11 Values of thermal conductivity of liquids in comparison with their reference values ............................................................................................... 65 Figure 2.12 Thermal conductivity values measured for the candidate slurries DS 5007 and DS 5045 with different PCM microcapsule mass fractions ......... 66 Figure 3.1 Control stress rheometer AR-G2 from TA Instruments ................... 70 Figure 3.2 Oscillatory strain (geomtry seen from above) ................................. 72 Figure 3.3 Left image: Elastic behaviour; Right image: Viscous behaviour (Moreno 2006) .................................................................................................. 73 Figure 3.4 Left image: Strain or stress sweep; Right image: Frequency sweep (Moreno 2006) ...................................................................................... 73 Figure 3.5 Left image: “Solvent trap” placed on the geometry and on the Peltier plate. Right image: Plate geometry for Peltier configuration with “solvent trap” with sample of slurry placed. ..................................................... 75 Figure 3.6 DS 5007 sample. Time sweep. Temperature=28ºC, strain=10%, frequency=1 Hz. Plate geometry of 25 mm without solvent trap. Sample loaded with a narrow pipette and without pre-shear ........................................ 76 Figure 3.7 DS 5007 sample. Time sweep. Temperature=28ºC, strain=10%, frequency=1 Hz. Plate geometry of 25 mm without solvent trap. Sample loaded with a narrow pipette and without pre-shear. ........................................ 76 Figure 3.8 DS 5007 sample. Time sweep. Temperature=28ºC, strain=10%, frequency=1 Hz. Plate geometry of 25 mm without solvent trap. Sample loaded with spatula and without pre-shear. ...................................................... 77 Figure 3.9 DS 5007 sample. Time sweep. Temperature=28ºC, strain=10%, frequency=1 Hz. Plate geometry of 40 mm with solvent trap. Sample loaded with spatula and without pre-shear. .................................................................. 78 Figure 3.10 Viscosity-Shear rate for the DS 5007 slurry at a temperature of 27ºC and for the DS 5045 slurry at a temperature of 29ºC ............................... 79 Figure 3.11 Formation of layers under the shear. Shear thinning or pseudoplastic behavior (Barnes 2000) ............................................................. 79 Figure 3.12 Velocity and shear rate profile for water under laminar flow (uaverage=0.2 m/s) and under turbulent flow (uaverage =1 m/s) .............................. 81 Figure 3.13 Viscosity-Temperature for the DS 5007 slurry with a microcapsules mass fraction of 30%, shear rate=100 1/s ................................ 82 Figure 3.14 Stress sweep at different temperatures (melted phase and transition phase). Frequency=1 Hz. Gap ∼0.5mm. .......................................... 87 Figure 3.15 Comparison between the melting and solidification curves obtained from the oscillatory temperature steps. Frequency=1 Hz. Shear stress=10 Pa. Gap∼0.5mm. ............................................................................. 89 INDEX OF FIGURES   xiii Figure 3.16 Variation of the gap when controling normal force during the phase change of octadecane. .......................................................................... 89 Figure 3.17 Comparison of the flow curve and the frequency sweep to check if the Cox-Merz rule is fulfilled. Gap∼0.4mm. Temperature=29ºC. Conditions of the flow curve: see in text. Conditions of the frequency sweep: shear stress=1 Pa ...................................................................................................... 90 Figure 3.18 Torque applied by the rheometer during the measuremts of the flow curve. The red area points out the non-reliable results due to the minimum torque of the rheometer ................................................................... 91 Figure 3.19 Melting and solidification curves with a gap of 1.4 mm for the different heating and cooling rates. Frequency=1 Hz. Shear stress=1 Pa. ....... 93 Figure3.20 Melting and solidification curves with a gap of 0.4 mm for the different heating and cooling rats. Frequency=1 Hz. Shear stress=1 Pa. ......... 93 Figure 3.21 Influence of the applied stress (within the linear viscoelastic region) on the Complex viscosity-Temperature curves. Gap=1.4 mm. Heating rate=0.5ºC/min. ................................................................................................ 94 Figure3.22 Influence of the applied stress (within the linear viscoelastic region) on the Complex viscosity-Temperature curves. Gap=0.4 mm. Heating rate=0.5ºC/min. ................................................................................................ 95 Figure 3.23 Influence of the frequency on the Complex ViscosityTemperature curves. Shear stress=1 Pa. Gap=0.5 mm. Heating rate=0.5ºC/min ................................................................................................. 95 Figure 4.1 Strain sweeps for the four samples of DS 5007; Temperature=27ºC; f=1 Hz .............................................................................. 101 Figure 4.2 Oscillatory response for real systems (Barnes 2000) ..................... 102 Figure 4.3 Frequency sweeps for the four DS 5007 samples: Temperature=27ºC; Strain=0.1 ........................................................................ 102 Figure 4.4 Creaming observed in the four samples at t=10080 minutes.......... 104 Figure 4.5 Creaming percentage over time of PCM microcapsules in suspension ....................................................................................................... 105 Figure 4.6 Relationship between the G’ modulus and the creaming percentage at t=31703 minutes. ....................................................................... 105 Figure 4.7 Relationship between the cohesive energy and creaming percentage ....................................................................................................... 107 Figure 4.8 Balanced valve of mass flow clogged by the PCM microcapsules from DS 5007 slurry with a mass fraction of 30% ............................................. 108 Figure 4.9 DS 5007 non-thermal-mechanical cycled sample, observed by an enviromental SEM. ........................................................................................... 109 Analysis of microencapsulated phase change material slurries and phase change material emulsions as heat transfer fluid and thermal storage material  xiv Figure 4.10 DS 5007 sample cycled during 3 weeks observed by an environmental SEM (dehydration process) ...................................................... 109 Figure 4.11 DS 5007 sample cycled during two weeks observed with an environemental SEM ........................................................................................ 110 Figure 4.12 DS 5045 non-thermal-mechanical cycled sample, observed with an environmental SEM. Blurry image ............................................................... 110 Figure 4.13 DS 5045 sample observed with an environmental SEM. Top left image: sample with a 40% mass fraction. Top right image: sample with a 35% mass fraction pumped during 5 weeks. Lower left image: sample with a 35% mass fraction pumped during 2 weeks. Lower right image: sample with a 25% mass fraction pumped during 4 weeks. ................................................. 111 Figure 4.14 DS 5007 contaminated after a storage period of 12 months. Culture in non-selective medium ...................................................................... 112 Figure 4.15 Spherical capsules analyzed in the compatibility study with PCMs ......................................................................................................................... 113 Figure 4.16 Mass loss of the plastic spheres over time for different PCMs ..... 115 Figure 4.17 Corrosion tests. Test specimens immersed in beakers that contain the slurry without contamination and the slurry with microbiological contamination ................................................................................................... 119 Figure 4.18 Results of the corrosion tests on the aluminum and copper specimens ........................................................................................................ 120 Figure 4.19 Aluminum specimen slightly oxidized ........................................... 122 Figure 5.1 Diagram of the experimental installation ......................................... 128 Figure 5.2 Temperatures of the copper tube .................................................. 129 Figure 5.3 Temperatures of the PEX tube ....................................................... 129 Figure 5.4 Picture of the Coriolis mass flow meter used for the mass flow measurement ................................................................................................... 131 Figure 5.5 Picture of the balancing valve where the points for the measurement of differential pressure are shown .............................................. 132 Figure 5.6 Left image: Pt100 sensor for the measurement of the fluid temperature. Right image: Thermocouples type T for the measurement of the wall temperature ............................................................................................... 133 Figure 5.7 Picture of the heating resistance .................................................... 134 Figure 5.8 Picture of the ammeter and of the electronic power regulator with phase angle regulator ..................................................................................... 135 Figure 5.9 Picture of the data acquisition system ............................................ 135 Figure 5.10 Picture of the experimental installation ......................................... 136 Figure 5.11 Calculated values of pressure drop in comparison to the measured values .............................................................................................. 137 INDEX OF FIGURES   xv Figure 5.12 Detail of the arrangement of the thermocouple in the heat transfer section ................................................................................................. 139 Figure 5.13 Influence of the interruption of the heat flux for the arrangement of the thermocouple on the measured temperatures ........................................ 140 Figure 5.14 Zone of calculated temperatures in comparison to zone of measured temperatures ................................................................................... 141 Figure 5.15 Difference of measured temperatures (Tmeasured-Tcalculated) in comparison to the temperatures difference estimated for the sensor in position x=1.42 m ............................................................................................ 143 Figure 5.16 Algorithm for the application of the empirical model of correction of wall temperatures ......................................................................................... 144 Figure 5.17 Application of the correction model. Tenv=25ºC; Mass flow=20 kg/h; Q=430W .................................................................................................. 145 Figure 5.18 Image of the results screen .......................................................... 146 Figure 6.1 Enthalpy-Temperature curves obtained by energy balance to the heat transfer section for the DS 5007 slurry. Top: 14% mass fraction; Middle: 20% mass fraction; Below: 30% mass fraction ................................................. 152 Figure 6.2 Enthalpy-Temperature curves obtained by energy balance to the heat transfer section for the DS 5045 slurry with a mass fraction of 20% ........ 155 Figure 6.3 Enthalpy-Temperature curves obtained by energy balance to the heat transfer section for the DS 5045 slurry with a mass fraction of 30% (sample with an initial mass fraction of 35% initially before deteriorating) ........ 156 Figure 6.4 Analysis of the hysteresis of the slurry on the enthalpy .................. 157 Figure 6.5 Measurements of pressure drop for the DS 5007 slurry with PCM microcapsule mass fractions of 14, 20 and 30% and comparison to water ...... 158 Figure 6.6 Improvement ratio vs. Average velocity of the fluid for the DS 5007 slurry with mass fractions of 14, 20 and 30%. Temperature difference=3ºC (21-24ºC) ................................................................................. 159 Figure 6.7 Pumping power vs. Transported Thermal Energy for the DS 5007 slurry with mass fractions of 14, 20 and 30%. Temperature difference=3ºC (21-24ºC) .......................................................................................................... 160 Figure 6.8 Measurements of pressure drop for the DS 5045 slurry with a mass fraction of 20 and 30%. Comparison to water. ........................................ 161 Figure 6.9 Pumping power vs. Transported energy for the DS 5045 slurry with mass fractions of 20 and 30%. Thermal difference=6ºC (22-28ºC) ........... 162 Figure 6.10 Improvement ratio vs. Average fluid velocity for the DS 5045 slurry with mass fractions of 20 and 30%. Thermal difference=6ºC (22-28ºC) . 162 Analysis of microencapsulated phase change material slurries and phase change material emulsions as heat transfer fluid and thermal storage material  xvi Figure 6.11 DS 5007 sample. Wall temperature-Position in the tube for different mass fractions. Top: mass flow=20 kg/h; Below: mass flow=50 kg/h186 ............................................................................................................ 164 Figure 6.12 DS 5007 sample. Heat transfer coefficient by convectionPosition in the tube for the different PCM mass fractions. Top: mass flow=20 kg/h; Below: mass flow=50 kg/h ....................................................................... 165 Figure 6.13 DS 5007 sample. Average decrease of the wall temperature in comparison to water depending on the PCM microcapsules mass fraction .... 167 Figure 6.14 DS 5007 sample. Average improvement of the heat transfer coefficient by convection in comparison to water depending on the PCM microcapsules mass fraction ............................................................................ 167 Figure 6.15 Influence of the flow patterns of thermal development on the heat transfer phenomenon ............................................................................... 169 Figure 6.16 DS 5045 sample. Wall temperature for the DS 5045 slurry with a 20% mass fraction in comparison to water ...................................................... 170 Figure 6.17 DS 5045 sample. Heat transfer coefficient by convection for the DS 5045 slurry with a 20% mass fraction in comparison to water .................... 171 Figure 6.18 DS 5045 sample. Wall temperature and convective heat transfer coefficient for the DS 5045 slurry with a mass fraction of 35% with a mass flow of 25 kg/h. RTO=0.66 ................................................................................ 171 Figure 7.1 Diagram of the solar cooling installation by absorption with the unit of thermal energy storage and dry air cooler, instead of wet cooling (Helm et al. 2009) ............................................................................................. 177 Figure 7.2 h-T curves for parafin RT6 .............................................................. 184 Figure 7.3 cp-T curve measured and cp-T curved fitted for the melting .......... 184 Figure 7.4 Comparison between the solution of the finite difference scheme and the analytical solution ................................................................................ 188 Figure 7.5 Verification of the energy balance. Energy that has entered the sphere until an instant j. Case of a water sphere. ............................................. 189 Figure 7.6 Temperature profiles of the sphere. ................................................ 189 Figure 7.7 Unit cell of the hexagonal compact packaging. .............................. 190 Figure 7.8 Sphere distribution in the tank for different relationships of dp/Dtank seen from above ............................................................................................... 191 Figure 7.9 Graph of the mesh used in a model of approximation of porous medium (Arkar and Medved, 2005). ................................................................. 192 Figure 7.10 Scheme of the model suggested .................................................. 193 Figure 7.11 Temporal evolution of the PCM temperature inside the sphere for two heights in the tank and different sphere diameters .............................. 195 INDEX OF FIGURES   xvii Figure 7.12 Pressure drop of the water when flowing through the tank with spheres of dp=0,044 m according to the different models of pressure drop and according to the interstitial velocity ............................................................ 195 Figure 7.13 Pressure drop of the water when flowing through the sphere tank according to the sphere size and according to the interstitial velocity, calculated by the Ergun correlation (Ergun 1952) ............................................ 196 Figure 7.14 cp-T curve estimated for the RT6 slurry for melting ...................... 197 Figure 7.15 Image of the tank with a helical coil .............................................. 198 Figure 7.16 Temporal evolution for different mass flow of the water temperature at different heights of the tank and temporal evolution of the heat transfer fluid at the outlet .................................................................................. 202 Figure 7.17 Temporal evolution for different mass flow of the microencapsulated RT6 slurry temperature at different heights of the tank and temporal evolution of the heat transfer fluid at the outlet .......................... 203 Figure 7.18 Pressure drop experienced by the heat transfer fluid (water) when flowing through the interior of the helical coil of the tank according to different models ............................................................................................... 204 Figure 7.19 Power according to the different configurations of the TES systems ............................................................................................................ 205 Figure 7.20 Comparison of the TES systems in terms of average power and time during which the system is capable of supplying water with a temperature lower than 10ºC ............................................................................ 206 Analysis of microencapsulated phase change material slurries and phase change material emulsions as heat transfer fluid and thermal storage material  xviii NOMENCLATURE xxv ∆U Voltage [V] Greek symbols α Thermal diffusivity [m2/s] γ Strain [-]  γ Shear rate [-] δ Phase lag [rad, º] ε Tank porosity [-] η Dynamic viscosity [Pa·s] Θi Sphere wall temperature in the tank model in layer i [ºC] λ Thermal conductivity [W/(m·ºC)] λe Effective thermal conductivity [W/(m·ºC)] ρ Density [kg/m3] σ Standard deviation τ Shear stress [Pa] Φ Microcapsules diameter [μm] ω Angular frequency [rad/s] Subscripts c Curved (for coils) calc Calculated cil Cylinder conv Convection cr Critical d Dispersion e Efecctive env Environment ext External f Fluid Analysis of microencapsulated phase change material slurries and phase change material emulsions as heat transfer fluid and thermal storage material xxvi fd,t Fully thermal development i Position in space in Inlet ini Initial int Internal j Position in time m Melting max Maximum mea Measured m1 Beginning of the melting m2 End of the melting p particle, sphere prev In the previous time suspPCM microencapsulated PCM slurry out outlet x local position ½ Half of the maximum value del valor máximo 0 At very low shear rates ∞ At very high shear rates Preamble: motivation, objectives and framework of the thesis 1 Preamble: motivation, objectives and framework of the thesis Analysis of microencapsulated phase change material slurries and phase change material emulsions as heat transfer fluid and thermal storage material  2 The International Energy Agency (IEA) published at the end of 2012 the World Energy Outlook (WEO) 2012, a report which presents the evolution of all energy sectors and their projections until 2035. According to the IEA, the global demand for energy will increase by more than a third compared to today due to the increasing demand of countries such as China, India and the Middle-East. The increase in energy demand in the OECD countries will be barely noticeable and a gradual change in the primary energy sources from coal, oil and nuclear energy to natural gas and renewable energies will be observed. According to the IEA 2012 report, the continuous rise of hydraulic energy and the fast expansion of wind power and solar energy has consolidated the position of renewable energies as an essential part of the energy mix. In 2035, renewable energies will represent almost a third of the total production of electricity. Solar energy is growing more quickly than any other renewable technology. Renewable energies will become the second source of electricity generation in the world by around 2015, producing approximately half that of coal. By 2035, they will come near to matching coal as the primary source of electricity generation. The depletion of non-renewable energy sources has led to a situation where the EU increasingly depends on importing primary energy to satisfy its demand. In Spain, the dependence on energy imports is noticeably higher than the average of the 27 EU countries, reaching 80% (data from September 2011, Eurostat). The development of renewable energy sources is thus a crucial part of national energy policy because besides decreasing our dependence and diversifying our supply sources, it can contribute efficiently to the reduction of greenhouse gas emissions, in particular CO2. For these reasons, the Directive 2009/28/EC of the European Parliament and the Council of Europe of 23 April 2009 on the promotion of the use of energy from renewable sources established the general objective, of achieving a share of 20% of energy coming from renewable energies of the gross final energy consumption of the European Union. For this purpose, it established objectives for each of the member states to be achieved by 2020 with minimum guidelines to be followed until that year. In Spain, the objective is for renewable sources to account for at least 20% of energy consumption in 2020, the same objective as the EU average. Preamble: motivation, objectives and framework of the thesis 3 The EU has identified R&D&i policy as one of the most effective tools for addressing the new challenges of the energy sector and combating climate change. The European R&D Framework Programme has assigned a significant part of its budget to energy and climate change programmes. Motivation In the search for a sustainable energy model, thermal energy storage (TES) represents a significant contribution to the efficient use of energy. Examples of this contribution include energy savings in applications such as bioclimatic architecture or free cooling, and the adaptation of the demand and production curves with renewable energies such as solar energy, where this discrepancy is one of the main problems for its establishment or for the reduction of costs due to the consumption of electrical energy taking advantage of off-peak hours and therefore the night rate. The applications of thermal energy storage using solid-liquid phase change are spreading due to the constant incorporation of new materials with very different properties and phase change temperature ranges. The main R+D lines of these new materials known as PCMs (Phase Change Materials) include the development of materials, the development of encapsulation, the determination of their thermophysical properties, the analysis of economic costs and their integration into systems. Recently, a new technique has been proposed to face some of these challenges. This technique consists of forming a biphasic fluid from the mix of a fluid such as water and a PCM. This gives rise to a fluid that allows storing thermal energy during the phase change of the PCM in suspension. This new fluid could be used as a thermal energy storage material and as a heat transfer fluid. However, there are still some critical points that must be addressed before these materials can be more extensively implemented. These critical issues are: 1) subcooling and hysteresis, 2) lack of physical stability, with regard to stratification problems and rupture of microcapsules in suspension, and 3) analysis of the heat transfer phenomenon to analyze the improvement in comparison to conventional heat transfer fluids. The results of different authors concerning this heat transfer phenomenon are not conclusive. With this work, it is hoped to make further progress towards the solution of these difficulties, and to analyze the thermal behavior and technical viability of systems with these biphasic fluids in comparison with sensible storage systems with water, or with Analysis of microencapsulated phase change material slurries and phase change material emulsions as heat transfer fluid and thermal storage material  4 storage systems in which the PCM has to date been macroencapsulated. The objectives of this thesis are set out in the following section. Thesis structure and specific objectives The working methodology and the thesis structure are summarized in figure I. There are four main blocks: 1. State of the art of microencapsulated PCM slurries and PCM emulsions. 2. Experimental characterization. 3. Study of applications. 4. Conclusions and future work. Figure I Methodology and thesis structure State of the art. mPCM slurries and emulsions Study of applications Experimental characterization of mPCM slurries and emulsions Determination of thermophysical properties Determination of rheological properties Compatibility Stability Enthalpy Thermal conductivity Viscosity of PCM slurries Comparison against macroencapsulated PCM systems PCM-Macroencapsulate PCM slurry-Corrosion Physical Microbiological Creaming Rupture of microcapsules Viscosity of PCM in the melted phase Analysis as HTF and as thermal storage material Heat transfer Pressure drop Conclusions and future work CHAPTER 1 CHAPTER 2 CHAPTER 3 CHAPTER 4 CHAPTER 5 AND 6 CHAPTER 7 CHAPTER 8  The specific objectives of the present thesis are enumerated below: Preamble: motivation, objectives and framework of the thesis 5 1. Exhaustive review and analysis of the literature about microencapsulated PCM slurries and PCM emulsions focusing on: a) Manufacturing processes. b) Synthesis in tables of the microencapsulated PCM slurries and PCM emulsions developed in the commercial market, universities and research institutes, together with their thermal properties. c) Problems of subcooling and hysteresis. d) Rheological behavior. e) Analysis of the heat transfer phenomenon. 2. Determination of some PCM thermophysical and rheological properties: specifically, obtaining the Enthalpy-Temperature curve, the Thermal Conductivity-Temperature curve and the Viscosity-Shear rate curve. In the case of these two last properties, a methodology is proposed for their correct characterization. 3. Measurement methodology of the viscosity of melted PCMs and during the phase transition, for application in studies of natural convection. 4. Rheological study for the analysis of the destabilization processes in microencapsulated PCM slurries and PCM emulsions. 5. Analysis of the rupture of PCM microcapsules in suspension when subjected to thermal-mechanical cycles. 6. Compatibility study of PCMs with the plastic material of encapsulated geometries. 7. Corrosion measurements in typical metallic alloys that make up thermal installations when coming into contact with microencapsulated PCM slurries and PCM emulsions. 8. Design, start up, validation and series of tests of an experimental installation for the study of the heat transfer phenomenon, the pressure drop and the technical viability of microencapsulated PCM slurries and PCM emulsions. 9. Comparison of a thermal energy storage system using PCM slurries with systems with macroencapsulated PCM or sensible storage systems. Comparison in terms of energy density, thermal power and pressure drop. Framework of the thesis The development of this thesis is linked to a grant for Research Personnel in Training (reference PIF-UZ-2009-TEC-02) awarded by the Research ViceDeanship of the University of Zaragoza. Analysis of microencapsulated phase change material slurries and phase change material emulsions as heat transfer fluid and thermal storage material  6 This thesis comes within the research line into Thermal Energy Storage with Phase Change Materials carried out by the Thermal Engineering and Energy Systems Group (GITSE), a research group recognized as a consolidated group by the Government of Aragón and belonging to the Aragón Institute for Engineering Research (I3A). This research line was initiated in 1998 with the doctoral thesis of Dr. Belén Zalba, co-advisor of this thesis. The continuation and relevance of this line of research is reflected in the defense in 2009 and 2011 of two theses by two members of the GITSE group (Ana Lázaro and Pablo Dolado) on the characterization and modeling of storage equipment for heat transfer by air. In addition, two theses are currently being developed within the group: low cost PCM such as by-products from industry carried out by the researcher Mª Concepción Peñalosa; and the study of the inclusion of PCM in active elements of construction, carried out by the researcher Javier Mazo. This academic interest is being corroborated in the national sphere at an institutional level by the consecutive granting of four projects by the National R&D&I Programme in public calls. This thesis has been developed within the framework of the following projects:  Project of the National R&D&I Programme (2009 to 2011): “Contribution of thermal energy storage to energy efficiency in buildings and industrial applications”. Reference ENE2008-06687-C02-02.  Project of the National R&D&I Programme (2012 to 2014): “Improvement of energy efficiency in buildings through thermal energy storage”. Reference ENE2011-28269-C03-01.  Project with a private company: “R&D Project on thermal energy storage through phase change materials and its application in solar heating and cooling systems”. With the objectives of knowing the state of development of the relevant fields of work through knowledge exchange and of contributing as much as possible to this development, while working on this thesis the Ph.D. candidate has collaborated with other international groups within the joint working group of the programmes “Energy Conservation through Energy Storage” and “Solar Heating and Cooling” of the International Energy Agency Task 42-Annex 24: Compact Thermal Energy Storage, Material Development and System Integration. She has also collaborated in the European Project of Cooperation COST Action Preamble: motivation, objectives and framework of the thesis 7 TU0802; and in two projects of cross-border cooperation with the Université de Pau et des Pays de L’Adour. A result of these collaborations was a research stay by the Ph.D. student in the Fraunhofer Institute ISE, in Freiburg, Germany, partially funded by grants awarded in public calls by the COST Action TU0802 and by the Europe Programme of the Social Activities from CAI. The most relevant results of this thesis have been made public in international scientific journals and in diverse scientific forums. To date, as result of this work 5 papers have been published in international scientific journals indexed in JCR and 11 works have been presented at both national and international conferences. Analysis of microencapsulated phase change material slurries and phase change material emulsions as heat transfer fluid and thermal storage material  8 CHAPTER 1. State of technology. Review on PCM emulsions and microencapsulated PCM slurries: Materials, heat transfer studies and applications.  15 the microcapsules, conserving size and spherical form. The texture is smoother and the spherical form is more regular, compared to one-step coacervation, where microcapsules with many protrusions, rougher, coarser and more porous are obtained (Su et al. 2007 a). The morphology can be observed in figure 1.2. In the interfacial polymerization technique, the polymer constituting the microcapsule cover is formed from two monomers in separate phases (an aqueous phase and an organic phase) that react in the interface of both phases, at the moment of microencapsulation. Figure 1.2 Morphology of microcapsules through a SEM microscope (Su et al. 2007 a) In the in situ polymerization, firstly a PCM emulsion is prepared and then the synthesis of the prepolymer solution is carried out through the mixture of two polymers, which will form the cover, and water. This prepolymer is added to the emulsion in the form of droplets, while the emulsion is agitated during a specific time. The emulsion is cooled and filtered, obtaining the microcapsules, which have to be dried. According to the study of Yang et al. (2003), the polymers that present the best characteristics at the time of microencapsulation are the flexible plastics, such as the polymethyl methacrylate (PMMA) and polyethyl methacrylate (PEMA). Besides, the viscosity of the slurry is not significantly affected by the material of the microcapsule shell. The main manufacturer of PCM microcapsules and mPCM slurries is BASF. BASF manufactures its microcapsules from a polymerization process of a PCM emulsion. This microencapsulation process can be observed in figure 1.3 Analysis of microencapasulated phase change material slurries and phase change material emulsions as heat transfer fluid and thermal storage material  16 (Schmidt 2008). Firstly a paraffinic emulsion is elaborated in hot water with agitation and detergents. Then the monomers are added and the growing process of the polymeric capsule begins in the interface of paraffin and water. Finally a highly-reticulated polymer forms a dense capsule over each paraffin droplet. This process results in a liquid product, in which the capsules are dispersed in water. The microcapsules are obtained after the dispersion passes through a drying tower, as shown in figure 1.4. The diameter of the dispersion microcapsules is between the range 2-20 µm. Nevertheless, after the drying process, secondary particles with a larger size are created for safety reasons against the possible inhalation and cutaneous exposure. Figure 1.3 Microencapsulation process from (Schmidt 2008) Figure 1.4 Drying process from BASF (Schmidt 2008) CHAPTER 1. State of technology. Review on PCM emulsions and microencapsulated PCM slurries: Materials, heat transfer studies and applications.  17 1.3 Compilation of PCM emulsions and microencapsulated PCM slurries Table I.1 shows information on the PCM microcapsules and mPCM slurries studied in literature, together with some thermophysical properties, as well as additional interesting information given by the authors. Table I.2 shows a compilation of PCM emulsions studied in literature. At the moment there are few PCM manufacturers approaching these techniques. Table I.3 collects data on commercially available PCM emulsions, mPCM slurries and PCM microcapsules up to date. As is known, inorganic PCM, usually salt hydrates, present certain advantages when compared to organic PCM, such as a higher energy storage density, higher thermal conductivity, non-flammability, and are cheaper. Nevertheless, inorganic PCM also present a series of inconveniences, such as the corrosion problem, phase segregation and subcooling. When the material in the melted phase, it must be cooled below its melting temperature so that it starts to solidify is named subcooling. As a consequence of this phenomenon (which in microscopic geometries would be worse due to probabilistic reasons), there are still few studies on inorganic PCM microencapsulation. After systematic reviews, the only manufacturer of inorganic microencapsulated PCM is Salca BV, selling microcapsules of salt hydrates and the only consulted work is that of Salaün et al. (2008). This last work investigates the effects of the preparation conditions (solvent evaporation-precipitation process) on the morphology of the capsule and on the efficiency of the sodium phosphate dodecahydrate encapsulation. Analysis of microencapasulated phase change material slurries and phase change material emulsions as heat transfer fluid and thermal storage material  Ref. Microencapsulation process Core material Shell material Nucleation agent Size distribution Melting temperature Phase change enthalpy Yamagishi et al. (1999) n.a. octadecane Melamine formaldehyde n.a. 2-10 µm Ø average:6.3 µm 27.85ºC Volumetric concentration (latent heat) 7% 10.2 kJ/kg 12% 17.4 kJ/kg 15% 21.8 kJ/kg 25% 36.3 kJ/kg 30% 43.6 kJ/kg Su et al. (2007 a) Two step coacervation n-octadecane Melamine formaldehyde n.a. 2 µm n.a. n.a. Yang et al. (2003) In situ polymerization Tetradecane 1) PVAc (Polyvinyl acetate) 2) PS (poliestirene) 3) Polymethyl methacrylate (PMMA) 4) Polyethyl methacrylate (PEMA) Tetradecane content of capsule: 40% n.a. 2) 5-40 μm 3 ) 5-30 μm 4) 5-30 μm 1) No manisfested phase change 2) 2.06ºC 3) 5.97ºC 4) 5.68ºC 2) ~0 3) 66.26 kJ/kg 4) 80.62 kJ/kg (melting enthalpy) Su et al. (2005) n.a. lauryl alcohol Melamine formaldehyde n.a. 5-10 µm 24ºC n.a. Fan et al. (2005) n.a. n-octadecane n.a. n.a. 1-2 µm Ø average:1.3 µm n.a. n.a. Su et al. (2007 b) Interfacial polymerization n-octadecane Poliurethane n.a. 5-10 µm n.a. n.a. Zhang et al. (2005) Interfacial polymerization n-octadecane n-nonadecane n-eicosane Urea Melamine formaldehyde l-tetradecanol paraffin l-octadecanol n.a. n.a. 160 kJ/kg Zhang et al. (2004 b) In situ polymerization n-octadecane Urea Melamine formaldehyde n.a. 0.2-5.6 µm n.a. n.a. CHAPTER 1. State of technology. Review on PCM emulsions and microencapsulated PCM slurries: Materials, heat transfer studies and applications.  Ref. Microencapsulation process Core material Shell material Nucleation agent Size distribution Melting temperature Phase change enthalpy Jin et al. (2008) In situ polymerization paraffin Urea-formaldehyde n.a. 5-20 µm (depending on the amount of emulsifier) ~54ºC 157.5 kJ/kg (76.9% core content) (melting enthalpy) Alkan et al. (2009) Polymerization of emulsion docosane PMMA (Polymethyl methacrylate) n.a. 0.14-0.466 µm Ø average: 0.16 µm 41ºC 54.6 kJ/kg Fang et al. 2008 In situ polymerization of a nanoemulsion assisted by ultrasound n-octadecane Poliestirene n.a. 50-200 nm Ø average: 124 nm n.a. 124.4 kJ/kg Zhang et al. (2004 a) In situ polymerization n-octadecane Melamine formaldehyde n.a. n.a. 30.5ºC 170 kJ/kg (melting enthalpy) Sari et al. (2009) Polymerization n-octacosane PMMA (Polymethyl methacrylate) n.a. 0.15-0.33 µm Ø average: 0.25 µm 50.6ºC 86.4 kJ/kg Hawlader et al. (2003) Coacervation Spray-drying Paraffin wax (Merck) n.a. n.a. n.a. n.a. 145-240 kJ/kg (depends on the ratio core/shell and on the method) Li et al. (2007) In situ polymerization n-octadecane Melamine formaldehyde n.a. Ø average: 2.2 µm 40.6ºC 144 kJ/kg Alvarado et al. (2007) Coacervation 1) 99.8% Tetradecane+0.2% silica fume 2) 98% tetradecane+2% tetradecanol 3) 94% tetradecane+6% tetradecanol Gelatin Silicon fume Tetradecanol 1) 90-150 µm Ø average: 100 µm 2) 70-260 µm Ø average: 145 µm 3) 2-10 µm Ø average: 4.4 µm n.a. 3) 202.1 kJ/kg (melting enthalpy) Analysis of microencapasulated phase change material slurries and phase change material emulsions as heat transfer fluid and thermal storage material  Ref. Microencapsulation process Core material Shell material Nucleation agent Size distribution Melting temperature Phase change enthalpy Rao et al. (2007) n.a. n-octadecane n.a. n.a. 1-5 μm Ø average: 4.97 μm Melting range: 24-29ºC 147.1 kJ/kg (melting enthalpy) Chen et al. (2008) n.a. l-bromohexadecane Amino plastic Ratio core-shell:7 Capsule thickness: 0.3 µm n.a. Ø average: 8.2 μm 14.3ºC (melting starting temperature) 5 wt % 6.5 kJ/kg 10 wt % 13 kJ/kg 15.8 wt % 20.5 kJ/kg (melting enthalpy) Diaconu et al. (2010) n.a. RT6 n.a. n.a. n.a. Melting range: 4-6.8ºC For a 45 wt % concentration: 55 kJ/kg (melting enthalpy) Zhang y Zhao (2011) n.a. DPNT06-0182 (Ciba Specialty Chemicals) n.a. n.a. 10-100 µm ~35ºC 96.968 kJ/kg 10% 8.074 kJ/kg 25% 15.194 kJkg 35%37.213 kJ/kg (melting enthalpy) n.a. Micronal DS 5008X (BASF) n.a. n.a. 1-20 µm ~29ºC 102.008 kJ/kg Table I.1 PCM microcapsules and mPCM slurries studied in literature CHAPTER 1. State of technology. Review on PCM emulsions and microencapsulated PCM slurries: Materials, heat transfer studies and applications.  Ref. PCM Nucleation agent Surfactant Emulsifying method Size distribution Melting temperature Phase change enthalpy Royon et al. (1998) Mixture of nalkanes n.a. Non-ionic surfactant Ultrasonic generator Power: 500W Frequency: 20 kHz 2 µm 9.5ºC 50% 78.9 kJ/kg (Melting enthalpy) Monllor (2007) 20% Tetradecane n.a. 6% surfactant (67.7% Tween60, 32.3% Span60) Phase inversion temperature method 200-250 nm n.a. 43 kJ/kg (Melting enthalpy) Yang et al. (2003) Tetradecane n.a. n.a. n.a. Ø average 10% 18.18 μm 20% 18.56 μm 30% 16.05 μm 10% 5.06ºC 20% 5.84ºC 30% 5.84ºC 10% 18.5 kJ/kg 20% 112.3 kJ/kg 30% 150.8 kJ/kg (Melting enthalpy) Choi et al. (1994) Hexadecane n.a. n.a. n.a. <0.1 mm 16.5ºC n.a. Choi y Cho (2001) C22H46 n.a. n.a. n.a. 10-40 µm n.a. n.a. Lorsch et al. (1997 b) Mixture of hexadecane and tetradecane 70/30 n.a. n.a. n.a. n.a. n.a. n.a. Huang et al. (2010 a) 30% RT6, RT10, RT20 2.5 % paraffin with a melting temperature of 50ºC 1.5% alcohol ethoxylate n.a. n.a. n.a. For a temperature range of 6ºC and 30 wt % concentration: RT6=75 kJ/kg RT10=50 kJ/kg RT20=44kJ/kg (Total capacity of storage) Günther et al. (2010) Hexadecane n.a. -SDS -Tween -Disperser -Ultrasonic generator 0.05-30 μm (depending on the method) n.a. n.a. Huang et al. (2010 b) -tetradecane -hexadecane -RT20 n.a. -SDS -Tween40 -Surfactant mixture -Ultrasonic generator - Rotor-stator system 0.05-30 μm (for the hexadecane emulsion depending on the method) 15.4-17.4ºC (for the hexadecane emulsion) n.a. Analysis of microencapasulated phase change material slurries and phase change material emulsions as heat transfer fluid and thermal storage material  Ref. PCM Nucleation agent Surfactant Emulsifying method Size distribution Melting temperature Phase change enthalpy Zou et al. (2010) Paraffin n.a. n.a. Phase Incursion Method Ø average: 0.304 μm n.a. n.a. Huang et al. (2010 c) 30% RT 10 n.a. n.a. n.a. n.a. Melting range: 4-11.5ºC 55 kJ/kg (5-11ºC) (Total capacity of storage) Table I.2 PCM emulsions studied in literature CHAPTER 1. State of technology. Review on PCM emulsions and microencapsulated PCM slurries: Materials, heat transfer studies and applications.  Manufacturer Product Type of product PCM Concentration Particle / droplet size Melting temperature Latent heat BASF DS 5000 mPCM slurry Paraffin 42% n.a. 26ºC 45 kJ/kg DS 5007 mPCM slurry Paraffin 42% n.a. 23ºC 41 kJ/kg DS 5030 mPCM slurry Paraffin 42% n.a. 21ºC 37 kJ/kg DS 5001 Powder Paraffin n.a. n.a. 26ºC 110 kJ/kg DS 5008 Powder Paraffin n.a. n.a. 23ºC 100 kJ/kg DS 5030 Powder Paraffin n.a. n.a. 21ºC 90 kJ/kg Microtek Laboratories MPCM -30D Powder n-decane n.a.. 17-20 μm -30ºC 140-150 kJ/kg MPCM -10D Powder n-dodecane n.a. 17-20 μm -9.5ºC 150-160 kJ/kg MPCM 6D Powder n-tetradecane n.a. 17-20 μm 6ºC 157-167kJ/kg MPCM 18D Powder n-hexadecane n.a. 17-20 μm 18ºC 163-173 kJ/kg MPCM 28D Powder n-octadecane n.a. 17-20 μm 28ºC 180-195 kJ/kg MPCM 37D Powder n-eicosane n.a. 17-20 μm 37ºC 190-200 kJ/kg MPCM 43D Powder Paraffin mixture n.a. 17-20 μm 43ºC 100-110 kJ/kg MPCM 52D Powder Paraffin mixture n.a. 17-20 μm 52ºC 120-130 kJ/kg Salca Thermusol HD35SE Microcapsules Salt hydrate n.a. n.a.. 30-40ºC 200 kJ/kg Thermusol HD60SE Microcapsules Salt hydrate n.a. n.a. 50-60ºC 160 kJ/kg Table I.3 Commercially available PCM microcapsules Analysis of microencapasulated phase change material slurries and phase change material emulsions as heat transfer fluid and thermal storage material  24 1.4 Main characteristics of PCM emulsions and microencapsulated PCM slurries 1.4.1 Hysteresis and subcooling When different results between the measurements of the cooling and heating processes are obtained, this phenomenon is called hysteresis. This hysteresis can encompass the hysteresis phenomenon as a property of the material, and the apparent hysteresis phenomenon, due to the measurement conditions. In the case of inorganic materials the hysteresis phenomenon is much more noticeable than in the case of organic materials. There are several effects that originate hysteresis due to the material, and the most common is subcooling. When a liquid has to be cooled to a temperature below its melting temperature so that the crystallization process to begin, this process is called subcooling. Subcooling can be a serious issue in PCM investigation and application fields, as the operation temperature range of the storage systems will be wider, worsening the energy efficiency of the systems. Despite the fact that many PCM do not present the subcooling phenomenon in macroscopic geometries, the problem occurs in microscopic geometries. The hysteresis and subcooling phenomena are represented in figure 1.5. Figure 1.5 Subcooling and hysteresis phenomena Phase transformation, in this case solidification, can be described as a nucleation process and posterior growth of the stable nuclei formed. Depending on the nature of the material and on the transformation, two types of nucleation can occur: homogeneous and heterogeneous nucleation. Homogeneous CHAPTER 1. State of technology. Review on PCM emulsions and microencapsulated PCM slurries: Materials, heat transfer studies and applications.  31 of cyclohexane in the core has a remarkable effect on morphology, thermal stability and permeability. After thermal treatment of capsules, the volatile character of cyclohexane caused an expansible space to be formed, which improved the thermal stability of microcapsules. These microcapsules presented a greater resistance to temperature and less mass loss. This weight loss was attributed mainly to the loss of n-octadecane in some broken capsules. Alvarado et al. (2007) presented thermophysical properties of a microencapsulated tetradecane slurry. In concrete, with respect to microparticle stability, it was observed that the size of the microcapsule should be smaller than 20 μm to avoid rupture. If a considerable amount of microcapsules were broken during durability tests, smaller capsules or capsules with a higher thickness/diameter ratio were tested. Microcapsules with a 2-10 μm diameter showed less damage during circulation through a progressive pump. Jin et al. (2008) prepared and studied the behavior of paraffin microcapsules with a phase change temperature of 50.2ºC, specifically the influence of the weight percentage with respect to the PCM core on the stability during phase change. They observed that the microcapsules suffered coalescence when the content of the capsules was inferior to 2.1% in weight as a consequence of microcapsule rupture. When the content of the capsule was 16.7% in weight, the microcapsules suffered partial coalescence. When this percentage increased to 28%, the capsules remained intact after an elevated number of thermal cycles and presented a better thermal stability than bulk paraffin. Alkan et al. (2009) characterized the behavior of docosane microencapsulated into PMMA (Polymethyl methacrylate) capsules, which were synthesized by them. Degradation of bulk docosane took place in a single stage at a temperature of approximately 120ºC, while microcapsules degraded in two stages: at 240ºC and at 323ºC. No significant changes were observed in the temperature or phase change enthalpies, after thermal cycles (1000, 3000, and 5000 cycles). Griffiths and Eames (2007) pumped a mPCM slurry through a chilled ceiling. During the experiments, deposition of microcapsules was not observed in the pipe elbows and around valves. Degradation of the slurry was not observed either. It was observed that dry pumps were adequate to pump the slurry. From this review on the stability of PCM emulsions and mPCM slurries it can be concluded that, for the case of PCM emulsions and mPCM slurries, the creaming phenomenon and microcapsule rupture are the most frequent problems. In the case of emulsions, the creaming problem can be solved through the reduction of the PCM droplet size. In the case of the mPCM slurries, microcapsule rupture is solved through the reduction of particle size Analysis of microencapasulated phase change material slurries and phase change material emulsions as heat transfer fluid and thermal storage material  32 and/or increase of the relationship thickness capsule/thickness core for the PCM. 1.4.3 Rheological behavior, viscosity and pressure drop The viscosity measurement and rheological characterization of these slurries systems, PCM emulsions and mPCM slurries, must be taken into account as although it seems homogeneous at a macroscopic level, the presence of other phases at a microscopic level modifies their behavior significantly. The concept of viscosity is based on the supposition that fluids are homogeneous and that, as a consequence, can be treated as continuous. Nevertheless, disperse systems exhibit a behavior that diverges strongly from the behavior observed in homogeneous fluids. This is due to the presence of two or more phases, which generates one or more interfaces of different nature. As a consequence, the behavior of the flow can range from simply Newtonian in diluted particle systems with no interaction, to highly non-Newtonian behaviors in concentrated or diluted systems where there is elevated interaction between particles. The main physical properties that influence rheological behavior are the content of the disperse phase; size, form and distribution of particle size and temperature. For the case of emulsions, two other factors must also be considered: degree of droplet deformation and disperse phase viscosity (Barnes 2000).  Rheological behavior of PCM emulsions Choi et al. (1994) observed that in their 10% hexadecane emulsion, pressure drop decreased significantly starting from the melting point of PCM particles. The focus of the work of Royon et al. (1998) was on the study of the thermorheological behavior of an emulsion with a 50% n-alkane concentration in weight. The results of the rheological experiments showed a pseudoplastic behavior and the apparent viscosity of the emulsion followed the power law (τ=K·φn). Viscosity decreased with the increase of temperature and the n and K indices were functions of temperature. The K index decreased with an increase in temperature, and the relationship between the K index and temperature was exponential. The n index showed a slight rise close to 9.5ºC (phase change temperature), mainly caused by the phase change of the component in dispersion. Chen et al. (2006) built an experimental installation to study the rheological characteristics of a tetradecane emulsion with a weight concentration of 30%, prepared by the phase inversion temperature method. The emulsion could be considered a Newtonian fluid. The friction factor was adjusted to the classical function fD=64/Re for laminar flow. Viscosity of the emulsion was 5.57 times that of water. The pump power decreased CHAPTER 1. State of technology. Review on PCM emulsions and microencapsulated PCM slurries: Materials, heat transfer studies and applications.  33 considerably for the same amount of heat transfer when compared to water, due to phase change, as can be seen in figure 1.8. Figure 1.8 Relationship between pumping power and heat transfer (Chen et al. 2006)  Pollerberg and Dötsch (2006) presented the comparison of three slurries systems in distribution system applications: ice slurries, tetradecane emulsion, and microencapsulated tetradecane slurry. One of the points studied was the rheological behavior of these slurries. The emulsion shows a Newtonian behavior in a weight concentration range of 10-20% and its viscosity was between 2 and 8 times that of water, depending on the tetradecane concentration. Both the mPCM slurry and the PCM emulsion caused a higher pressure drop than water in laminar flow. Nevertheless, the pressure drop curves overlap in turbulent regime. In the work of Huang et al. (2009), all samples prepared presented a pseudoplastic behavior. The K and n coefficient of the power law were practically constant with a weight fraction of 15-50%. A decrease in n and an increase in K were observed when concentration exceeded 50%. Besides, viscosity increased with the increment of solidified fraction. One reason for this behavior is that the solid droplets were not suffering significant deformation, resulting in a higher viscosity than when the droplets were melted. A working limit fraction was established at 50%. Huang et al. (2010 a) observed a pseudoplastic behavior in their PCM emulsions, in which viscosity decreased along with shear rate and then tended to remain constant when shear rate was higher than 30 s-1. Temperature influenced significantly, especially phase change. The reason for this phenomenon could Analysis of microencapasulated phase change material slurries and phase change material emulsions as heat transfer fluid and thermal storage material  34 be that the solid particles were not very deformed by shearing, causing a higher viscosity than that of liquid droplets. Cho et al. (1991) observed that the increase in pumping work in the case of their emulsion with a 25% volumetric concentration of PCM was small in the turbulent region, in comparison to water. In the case of the emulsion of Lorsch et al. (1997 b), a fraction of volume up to 20% increased the pressure drop only by 3% compared to water. When the mixture was cooled to a temperature below PCM solidification, the pressure drop increased from 3 to 5%. Zou et al. (2010) verified that for a given PCM concentration, there is a flow rate that causes the greatest savings in pumping consumptions. Rheological behavior of microencapsulated PCM slurries Yamagishi et al. (1996) studied the feasibility of n-tetradecane and n-dodecane microencapsulated slurries in low temperature applications. One of the studied aspects was the viscosity of these slurries. Specifically, viscosity was measured by a cylindrical Couette viscometer and it was observed that apparent viscosity depended on several parameters: PCM concentration, temperature of slurry, and microcapsule size. When adding addictives such as surfactant agents, the slurry stopped behaving as a non-Newtonian fluid and started behaving as a Newtonian fluid. Yang et al. (2003) prepared slurries with microencapsulated tetradecane from different materials and observed that the capsule material did not influence the viscosity of the sample. Rao et al. (2006) centered their study on the flow characteristics of microencapsulated n-octadecane slurries with concentrations from 5 to 20%, circulating through horizontal minichannels. The friction factor of the slurries under laminar regime increased along with the PCM concentration. Compared to the friction fraction of water, a slight increase was observed in low-concentration (5%) slurries. Nevertheless, when concentration was 10% or higher, the increment in friction factor was more marked. The increment in PCM concentration of the slurries tended to suppress the generation of turbulence in the flow. When concentration was at least 15%, no obvious transition was observed from laminar to turbulent flow as occurred in low-concentration PCM at Re=2000. The pressure drop of the mPCM slurries flowing through the minichannels increased throughout the speed range when concentration increased. As previously mentioned, Pollerberg and Dötsch (2006) compared an ice slurry, a tetradecane emulsion and a microencapsulated tetradecane emulsion, the three systems with a 20% PCM concentration. The slurry presented the behavior of a pseudoplastic fluid, and viscosity was between 120 and 550 times that of water. Both the mPCM slurry and the PCM emulsion studied in this work presented a higher pressure drop CHAPTER 1. State of technology. Review on PCM emulsions and microencapsulated PCM slurries: Materials, heat transfer studies and applications.  35 than water in laminar regime. Nevertheless, the pressure drop curves overlapped in turbulent regime. In the microencapsulated tetradecane slurries developed by Alvarado et al. (2007) with concentrations oscillating between 5 and 17.7%, the relative viscosity seemed to be independent of temperature (in the phase change temperature range) for all concentrations. The slurries behaved as a Newtonian fluid, until mass fractions of 17.7%. The results also indicated that the pressure drop increased slightly when PCM microcapsules were used, but did not affect significantly the pumping work. In some cases the pressure drop was lower than water, maybe due to a possible rupture of microcapsules and liberation of phase change material. The viscosity of the slurries prepared by Wang et al. (2007) with concentrations ranging from 5 to 27.6% of l-bromohexadecane adjusted to the values predicted by the Vand model. All slurries presented a Newtonian behavior. The pressure drop measurements showed a marked transition when the flow changed from laminar to turbulent flow. The friction factors in turbulent flow adjusted well to the classic Hagen Poseiuille model, while the friction factors in laminar flow were lower than those calculated with the Blasius equation, as observed in figure 1.9. Figure 1.9 Friction factors vs. Reynolds number (Wang et al. 2007) In the experimental work of Heinz and Streicher (2006), it was observed that with a concentration up to 30%, the pressure drop was not considerably higher that water, for which this concentration resulted to be a good compromise between storage capacity and pressure drop. Chen et al. (2008) observed that pumping power decreased considerably in comparison to water due to the higher transported heat. The decrease could be of up to 67.5%. Zhang and Analysis of microencapasulated phase change material slurries and phase change material emulsions as heat transfer fluid and thermal storage material  36 Zhao (2011) investigated the rheological properties of two mPCM slurries. Overall these mPCM slurries could be considered as Newtonian fluids when the shear rate is higher than 200 s-1 and PCM microcapsules concentration lower than 35%. The viscosity was higher for slurries with bigger PCM microcapsules. Figure 1.10 compiles the viscosity values of different PCM dispersions studied in literature. It is observed that from mass fractions of 30-40%, viscosity starts to increase significantly, in comparison to water. All consulted studies coincide when remarking that even with PCM concentrations close to 20-30%, the pressure drop of mPCM slurries and emulsions is slightly superior to water, and practically the same in turbulent regime. Figure 1.10 Viscosity values of different PCM dispersions studied in literature 1.4.4 Thermal properties: thermal conductivity One of the main disadvantages of thermal energy storage systems with PCMs is their low thermal conductivity. This circumstance results in slow charging and discharging. In the bibliography, there are numerous studies aimed at the improvement of the thermal conductivity of the PCM, either by embedding structures of materials with high thermal conductivity, or by using finned heat exchangers or encapsulating the PCM in containers with a high surface / volume ratio. This is the reason why PCM microcapsules are interesting. Due to the microscopic size of the PCM microcapsules or droplets, the mPCM slurry can be treated as a homogeneous material. This assumption implies in that the CHAPTER 1. State of technology. Review on PCM emulsions and microencapsulated PCM slurries: Materials, heat transfer studies and applications.  37 temperature gradients inside the solid are negligible. This is accomplished if the convective thermal resistance inside the microcapsules is low in comparison to the convective thermal resistance between the microcapsule and its surroundings. The fulfillment of this condition can be analyzed through the Biot number, which should be under 0.1. If the Biot number is calculated for a PCM microcapsule, a value much lower than 0.1 will be obtained. This means that inside the microcapsule the temperature gradients in the PCM are very low and that the conductive resistance can be neglected. When the PCM is microencapsulated, low thermal conductivity ceases to be a problem (Streicher et al. 2005). So, PCM slurries in water can improve heat transfer as a consequence of the relationship area/volume of droplets in the case of emulsions and of microcapsules in the case of slurries, in comparison to systems in which the PCM is macroencapsulated. Besides, the fact of dispersing phase change particles into a fluid can improve heat transfer through convection with respect to water. These slurries can serve either as thermal storage materials or heat transfer fluids. The thermal properties of these slurries are different from those of PCM and the fluid in question, which are essential to evaluate the fluid and the heat transfer characteristics of a system with these slurries. The thermal properties to be discussed are thermal conductivity and convection heat transfer coefficient. The analysis of the different studies regarding the convection heat transfer coefficient is presented in a separate section, due to their extension and importance within this review. The thermal conductivity of diluted dispersions, λd, can be evaluated from Maxwell’s relationship, described in equation 1.1: ) λ λ ·(c λ λ )-λ λ ·(c· k λ λ λ f p f p f p f p f d 12 1 22 --   (eq. 1.1) Where λp is the thermal conductivity of the disperse phase, λf is the thermal conductivity of the continuous phase and c is the volumetric concentration of the disperse phase. As a consequence of the interactions particle/fluid, the effective thermal conductivity is greater than that predicted by this equation, and can be obtained from the correlation of equation 1.2: m p f ePe·c·B λ λ 1 (eq. 1.2) Analysis of microencapasulated phase change material slurries and phase change material emulsions as heat transfer fluid and thermal storage material  38 Where Pep is the Peclet number for the particle. The values of B and m depend on the Peclet number of the particle. In general, the thermal conductivity of phase change particles is not very elevated and must be improved (Charunyakorn et al. 1991). Xuan et al. (2009) developed a new type of heat transfer fluid, consisting of a magnetic slurry with microencapsulated PCM that incorporated the advantages of microencapsulated PCM and of a magnetic fluid. Iron nanoparticles were added to the melamine-urea-formaldehyde capsule surrounding the paraffin. Thermal conductivity was measured through the hot wire method. The measured data pointed out that among all particle components, the content of iron nanoparticles was the dominating factor on thermal conductivity. The thermal conductivity ratio increased linearly with the increase in concentration of iron nanoparticles inside the capsules. Similarly, Ho and Gao (2009) prepared a n-octadecane emulsion with alumina particles. Compared bulk paraffin, the emulsion presented a higher thermal conductivity, depending on the concentration of the alumina particles. This relationship was not linear. Improvements between 2 and 6% were obtained with weight concentrations of alumina between 5-10%. Ho et al. (2010) managed to improve the thermal conductivity of PCM emulsions by adding alumina nanoparticles, to values even better than those of water. Figure 1.11 shows some values of thermal conductivity of dispersions studied in literature. As it was expected, higher PCM mass fractions in dispersion, lower thermal conductivity of the dispersion. Figure 1.11 Thermal conductivity values for different mPCM slurries studied in literature CHAPTER 1. State of technology. Review on PCM emulsions and microencapsulated PCM slurries: Materials, heat transfer studies and applications.  39 1.5 Heat transfer The main objective of the publication by Kasza and Chen (1985) was to point out the benefits of the use of PCM slurries in water, such as the improvement in heat transfer and increase in storage efficiency. Some of the specific benefits mentioned were: 1) Reduction in the temperature difference between source-drain. 2) Increase of the heat capacity of the fluid, as a consequence of the PCM dispersion, giving place to a lower mass flow and therefore a lower pumping consumption. 3) Dynamic use of the PCM. In a conventional system, heat exchange between PCM (static use) and a separated heat transfer fluid is needed to transport heat or cooling. Nevertheless with PCM slurries, thermal storage and the heat transfer fluid are integrated into the PCM slurry. The energy losses of the heat exchange will be annulled, as the necessity for a secondary heat transfer fluid is eliminated. The possible improvement mechanisms for heat transfer are enumerated. Improvement in heat transfer occurs in slurries, with or without phase change. This improvement is substantially greater when considering PCM slurries. Several mechanisms responsible for this improvement were examined, including particle rotation and migration in the flow as well as the role of phase change. 1.5.1 Heat transfer by internal forced convection In the review article published by the author of the present thesis (Delgado et al. 2012), a review of the studies carried out up to the moment about the heat transfer phenomenon is shown (mainly internal convective heat transfer in a tube) and they have been classified according to the type of fluid (mPCM slurry or PCM emulsion), according to the type of study, experimental or numerical, and according to the flow pattern. Also the geometry and the boundary conditions have been taken into account. This compilation presented in the paper expects to complete the subsection of numerical modeling on the heat transfer phenomenon in PCM slurries of the review of Dutil et al. (2011) about PCMs and mathematical modeling. In table I.4, a summary of the studies carried out about the heat transfer by internal forced convection in PCM dispersions can be observed, according to Analysis of microencapasulated phase change material slurries and phase change material emulsions as heat transfer fluid and thermal storage material  40 the flow pattern of the dispersion, the type of study, geometry (circular tube or rectangular channels) and boundary conditions. In total, 7 experimental works and 12 numerical works have been analyzed. From these 12 numerical works, 6 were validated with the same experimental work. According to the analysis accomplished from the review of these papers, no clear conclusions about if these new fluids improve the heat transfer phenomenon by internal forced convection in comparison to water can be deduced. Regarding to other type of geometries, different from the circular tube and from the rectangular channel, Heinz and Streicher (2006) studied the heat transfer water-mPCM slurry in a plate heat exchanger. It was observed that the convective heat transfer coefficient decreased with the concentration of PCM, due to a greater viscosity and lower thermal conductivity. With a 20% concentration it decreased 30% in comparison to water, and with a 40% concentration, a decrease of 40% was observed. In spite of the lower overall heat transfer coefficient, the exchanged power of the plate heat exchanger is approximately the same as that of water. This is due to the higher difference in average temperatures, as result of the higher heat capacity of the slurry. 1.5.2 Heat transfer through natural convection in microencapsulated PCM slurries Heinz and Streicher (2006), in addition to analyze the heat transfer in a heat exchanger, they studied experimentally a 200-liter tank with a mPCM slurry developed by BASF with a melting temperature of 60ºC, with a typical spiral exchanger, inside which water circulated. As the limiting factor for heat transfer is the natural convection of the surface of the exchanger to the storage fluid, it resulted interesting to study the heat transfer coefficient by natural convection. The heat transfer coefficients decreased as phase change process takes place, due to the reduction in the difference of temperatures between the exchanger and the storage fluid. Due to higher viscosities, the heat transfer coefficient through natural convection decreased when the PCM concentration increased in water. Even with the lowest PCM concentration, 20%, the values of the heat transfer coefficients for natural convection were lower than for water. A very similar study was carried out by Diaconu et al. (2010). An experimental study was carried out on the heat transfer phenomenon of natural convection in a tank filled with a microencapsulated RT6 slurry (organic PCM from the manufacturer Rubitherm), where the heat exchanger through which water circulated consisted of a helical copper tube, used as storage tank for solar air conditioning applications. During phase change of PCM, it was observed that CHAPTER 1. State of technology. Review on PCM emulsions and microencapsulated PCM slurries: Materials, heat transfer studies and applications.  47 For the moment there are very few installation examples that make use of PCM emulsions and mPCM slurries. The main application seems to be oriented towards storage tanks combined with chilled ceilings, shifting the cooling production to the night in order to shave the demand peaks, improve COP, and make better use of nocturnal tariffs. Some authors also suggest the use of these new fluids in supply networks. Analysis of microencapasulated phase change material slurries and phase change material emulsions as heat transfer fluid and thermal storage material  Influential factors or parameters Objective magnitudes Influence when the factor increases Positive influence Negative influence Particle diameter Rupture of microcapsules  Rupture pressure of microcapsules decreases, higher number of ruptured capsules. Subcooling Greater probability of existing nucleation agents, and therefore lower subcooling.  Apparent hysteresis  Possible non-equilibrium between PCM and water temperatures, possibility of hysteresis Heat transfer Improvement in convection coefficient.  Stability of emulsions  Creaming speed increases PCM concentration Heat capacity Increase in heat capacity, increase in transported heat.  Pressure drop  Increase of viscosity, increase of pressure loss and pumping work. Up to PCM concentrations of 15-20% the increase is slightly superior to water. Heat transfer Decrease in Stefan number and therefore improvement of convection coefficient.   Increase in viscosity, decrease in turbulence degree, and therefore worsening or convection coefficient.  Decrease of thermal conductivity, occasioning deterioration in heat transfer. Operation temperatures range Heat transfer The operation temperature range must fit with the phase change temperature range, and be the narrowest possible.  Table I.5 Objective magnitudes and influential parameters at the time of selection of a PCM emulsion or mPCM slurry as heat transfer fluid or thermal storage material. CHAPTER 2. Determination of the thermophysical properties of microencapsulated PCM slurries and PCM emulsions.  49 This chapter describes the search for mPCM slurries and PCM emulsions in the commercial market, universities and research institutes. In addition, the thermophysical properties are determined, specifically the EnthalpyTemperature curves and the Thermal ConductivityTemperature curves. In the latter case, it is suggested how to measure the thermal diffusivity in liquids with Laser Flash equipment. 2 Determination of the thermophysical properties of microencapsulated PCM slurries and PCM emulsions Analysis of microencapsulated phase change material slurries and phase change material emulsions as heat transfer fluid and thermal storage material  50 2.1 Introduction An exhaustive search has been undertaken among PCM manufacturers, universities and research institutes to identify those that supply microencapsulated PCM or PCM in slurry form. Eleven manufacturers or research institutes were contacted, only five of whom work with PCM in the form of microcapsules or dispersions. Samples of the products listed in table II.1 were obtained for analysis in the laboratory for the determination of their properties by the GITSE group. Comercial name of the product Manufacturer / Research Institute Type of product Solids content Phase Change Temperature Data supplied by the manufacturer Latent heat (kJ/kg) Thermal conductivity (W/(m·K)) Viscosity (mPa·s) Density (kg/m3) DS 5000 BASF mPCM slurry 42% 26ºC 45 n.a. 200-600 980 DS 5007 BASF mPCM slurry 42% 23ºC 41 n.a. 200-600 980 DS 5001 BASF Microcapsules 26ºC 110 n.a. n.a. DS 5008 BASF Microcapsules 23ºC 100 n.a. n.a. DS 5045 BASF mPCM slurry 40% n.a. n.a. n.a. 226.8 900 Fraunhofer UMSICHT Emulsion 30% 20ºC n.a. n.a. n.a. n.a. Fraunhofer UMSICHT Emulsion 30% 10ºC n.a. n.a. n.a. n.a. MPCM 6D Microtek Laboratories Microcapsules 6ºC n.a. n.a. n.a. MPCM 18D Microtek Laboratories Microcapsules 18ºC n.a. n.a. n.a. MPCM 28D Microtek Laboratories Microcapsules 28ºC n.a. n.a. n.a. Thermusol HD32 Salca Microcapsules 32ºC 150 n.a. n.a. Aero-University of Ljubljana mPCM slurry 30% n.a. n.a. n.a. n.a. Table II.1 Samples gathered in the laboratory Microencapsulated PCM slurries were prepared from the microcapsules and distilled water. Nevertheless, in the course of just a few hours it was observed that these were not stable due to the creaming phenomenon and to the lack of addition of surfactants and thickeners that would stabilize the said slurries. The creaming, as explained in chapter 1, is a process caused by the action of gravity which produces a vertical gradient in the PCM microcapsule concentration, in this case as a consequence of the difference of density with the water, not affecting their size distribution. An example of this separation can be observed in figure 2.1 for the slurries prepared from the PCM microcapsules from two different manufacturers. In the case of the slurry prepared from the Microtek microcapsules, this separation is even more noticeable. Surfactants and thickeners would be needed to solve this problem. CHAPTER 2. Determination of the thermophysical properties of microencapsulated PCM slurries and PCM emulsions.  51 It is quite clear that if the slurry was pumped, this problem would not occur as the forced movement produced by the pump would result in the microencapsulated PCM being in suspension. However, these samples were discarded since during storage this could cause a serious problem. That is to say, the microcapsules from BASF, Microtek Laboratories and Salca were discarded. Figure 2.1 Aspect of the slurries prepared from PCM microcapsules. PCM mass fractions 10, 20 and 30%. Left: BASF manufacturer; Right: Microtek Laboratories.  The analysis was focused on the PCM dispersions which were previously stable, that is to say, on the PCM emulsions and mPCM slurries already supplied in this form. It was decided to analyze the Enthalpy-Temperature curves from the rest of the candidate substances, although among the samples from BASF (DS 5001, DS 5007 and DS 5045) it was decided to analyze just the samples DS 5007 and DS 5045, since the DS 5001 product was very similar to DS 5007 regarding its formulation and development according to the manufacturer’s data, with just a slight change in the phase change temperature. This was not the case with the DS 5045 product which was developed to solve certain problems in these other two previous products. These samples were diluted with water to obtain slurries with different PCM mass fractions. The original DS 5007 slurry had 42% solid contents of PCM and the DS 5045 slurry had 40%. The DS 5007 was diluted with distilled water to obtain slurries with PCM mass fractions of 14, 20 and 30% and the DS 5045 slurry was diluted down to PCM mass fractions of 20 and 35%. Analysis of microencapsulated phase change material slurries and phase change material emulsions as heat transfer fluid and thermal storage material  52 The emulsion from Fraunhofer UMSICHT with a phase change temperature of 20ºC has two variations (with and without thickener) and the emulsion with a phase change temperature of 10ºC is with thickener. All emulsions from the German Institute Fraunhofer UMSICHT have a 30% paraffin mass fraction. A thickener in two of the three samples was added to prevent creaming. The two samples with thickener remain homogeneous but they have a higher viscosity. The sample with a phase change temperature of 20ºC shows the creaming phenomenon. However, in comparison to the other two samples with thickener, it does not show a high viscosity. In the sample with thickener, after two months of storage, deformation of the container was observed. A certain incompatibility with the plastic of the container was revealed, as shown in figure 2.2. Figure 2.2 Emulsion Fraunhofer UMSICHT with thickener. Deformation of the plastic container. One of the main tasks when a latent energy storage system has to be analyzed is the characterization of the thermophysical properties of the PCM that forms the storage system: phase change enthalpy, thermal conductivity and density. All these properties were determined depending on the temperature, in the GITSE group laboratory for the determination of properties. 2.2 Enthalpy depending on the temperature In addition to knowing the phase change enthalpy of each one of the materials, it is important to highlight other characteristics of their behavior in the meltingsolidification cycles. For each material, it is necessary to know the following characteristics:  Temperature range of the phase change. CHAPTER 2. Determination of the thermophysical properties of microencapsulated PCM slurries and PCM emulsions.  53  Subcooling, meaning the decrease in the temperature below its phase change temperature for the formation of the first crystal. The rest of the molecule crystallizes around this first crystal, returning to the phase change temperature. As explained in the previous chapter, this phenomenon can be critical in PCM slurries as a consequence of the small size of the droplet or PCM capsule. Smaller sizes mean a lower propability of finding a nucleating agent that allows solidification to occur.  Hysteresis, meaning the difference in temperature between the phase change from solid to liquid and from liquid to solid. 2.2.1 Equipment for the determination of the Enthalpy depending on the temperature For the determination of the Enthalpy-Temperature curves of the different samples during the phase change, an installation of the T-history method has been used (Zhang et al. 1999). The main criteria for the selection of the method were the sample size and the heating and cooling rate. The sample size in the method of determination must agree with that of the application. This criterion is also very important in the case of PCM slurries, since they are formed by different substances. In real applications the sample size will be in the order of kilograms. For this reason it is considered more suitable to use a bigger sample size. In this case the T-history method has no size limitation, in comparison with the sample size in a DSC which is in the order of 1 miligram. Regarding the heating and cooling rate, the rate in real processes can be slow. In very fast processes it can happen that the succession of states of equilibrium representative of the melting/solidification procces does not take place. At lower rates or slower procceses, the real conditions in which the material is going to work can be reproduced better. The minimum velocity that can be reached in a DSC is in the order of 6ºC/h against 1ºC/h in the T-history method. 2.2.2 Results obtained The different PCM emulsions and mPCM slurries shown in table II.1 were analyzed in the installation of the T-history method illustrated in figure 2.3. The accuracy of the precision balance of the installation is 0.01 mg for measurements of up to 31 g and 0.1 mg for measurements up to 120 g. The sample size of this T-history installation is in the order of 10 cm3. The Analysis of microencapsulated phase change material slurries and phase change material emulsions as heat transfer fluid and thermal storage material  54 temperature sensors for the measurement of the sample temperatures and of the room temperature are 4-wire Pt100. The tolerance of the sensors of the class DIN 1/10 is between 0.03 and 0.08ºC. The record of these temperature measurements in the course of time has been carried out using a datalogger Agilent 34970ª, which has a basic accuracy of 0.004%. Figure 2.3 Installation of the T-history method for the determination of the EnthalpyTemperature curves (Lázaro 2008) The verification of the installation of the T-history method was accomplished from the calibration of the sensors, from the verification of the measurement of temperature and from the verification of the measurement of the enthalpy variation (Lázaro et al. 2006 a). Two pure substances were employed with a constant phase change temperature and known phase change enthalpy (gallium and hexadecane). In the determination of the enthalpy, the difference was lower than 12% in all cases. The Enthalpy-Temperature curves were obtained for both the melting process and the solidification process. These curves are shown in figure 2.4. From all the samples that appear in table II.1, those with similar phase change temperatures according to the manufacturer’s data were analyzed (around 2025ºC), discarding those samples prepared in the laboratory from PCM microcapsules. Given the results obtained, the two samples developed by the Fraunhofer Institute UMSICHT were discarded, both the sample without thickener due to the fast creaming (complete separation in the course of a day) and the sample with thickener, due to the incompatibilities with the plastic container as seen in figure 2.2. The slurry developed by AERO-University Ljubljana was also analyzed and discarded, as this slurry showed a hysteresis CHAPTER 2. Determination of the thermophysical properties of microencapsulated PCM slurries and PCM emulsions.  55 between the melting and the solidification curve of about 10ºC. Thus, the candidate samples for use as heat transfer fluid and as thermal storage material were the two slurries developed by BASF, DS 5007 and DS 5045, the latter showing a higher phase change enthalpy. Their curves can be observed in detail in figure 2.5. Figure 2.4 Enthalpy-Temperature curves for the different PCM emulsions and mPCM slurries analyzed  15 20 25 30 35 0 20 40 60 80 100 120 Enthalpy (kJ/kg) Temperature (ºC)    Analysis of microencapsulated phase change material slurries and phase change material emulsions as heat transfer fluid and thermal storage material  56 Figure 2.5 Enthalpy-Temperature curves of the candidate mPCM slurries 15 20 25 30 35 0 20 40 60 80 100 120 Enthalpy (kJ/kg) Temperature (ºC) DS 5007 14% melting DS 5007 14% solidification DS 5007 20% melting DS 5007 20% solidification DS 5007 30% melting DS 5007 30% solidification DS 5045 20% melting DS 5045 20% solidification DS 5045 35% melting DS 5045 35% solidification Water 2.3 Thermal conductivity The hot wire method is the most commonly used and most suitable for the measurement of thermal conductivity in liquid samples. However in the frame of this Ph.D. a Laser Flash equipment (LFA) has been chosen to carry out the measurements, since this is the instrument that the laboratory of properties determination of the GITSE group has. This equipment allows measurements of thermal diffusivity in both solid and liquid samples. It is manufactured by Netzsch, LFA 457 MicroFlash model and has the following characteristics:  Temperature range: Room temperature-1100ºC  Thermal diffusivity range: 0.01-1000 mm2/s  Heat source: Laser pulse (up to 18 J/pulse)  Heating and cooling rate: from 0.01 K/min to 50 K/min  Measurement of the temperature increase with an infrared detector cooled by N2 liquid  Sample size between 10 and 25.4 mm with a thickness from 0.1 to 6 mm. CHAPTER 2. Determination of the thermophysical properties of microencapsulated PCM slurries and PCM emulsions.  63 Figure 2.9 Signal of the infrared sensor with the empty sampleholder and with water. -5000 0 5000 10000 15000 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 Detector signal (V) Time (ms) Sampleholder with water. Pulse 1 Sampleholder with water. Pulse 2 Sampleholder with water. Pulse 3 Empty sampleholder. Pulse 1 Empty sampleholder. Pulse 2 Empty sampleholder. Pulse 3  Regarding the measurement of the thermal diffusivity during the phase change from solid to liquid, the manufacturer of the equipment suggests measuring the thermal diffusivity of the sample with the Laser Flash and modifying the curve obtained with the DSC in such a way that the energy associated to the phase change is subtracted, by considering the transition as a straight line between the specific heat in the solid and the specific heat in the liquid (interpolated heat). This means ignoring the effect of the increase in the specific heat of the sample in the temperatures range where the phase change takes place. However, the solutions from which the software obtains a value of thermal diffusivity consider that the specific heat does not vary. For this reason, with the objective of obtaining values that allow a comparison of the thermal conductivity of phase change materials, only the measurements in the single-phase states, solid and liquid, have been considered. 2.3.2 Measurement of liquids whose thermal diffusivity is known Although the manufacturer Netzsch sells standards of thermal diffusivity for solids, this is not the case for liquids. For this reason, different liquids whose thermal diffusivity or conductivity is known were measured in the laboratory. Three different liquids were chosen: distilled water, hexadecane and glycerine. These liquids have thermal diffusivity values within the range of mPCM slurries for the temperature range of the application of these fluids. Analysis of microencapsulated phase change material slurries and phase change material emulsions as heat transfer fluid and thermal storage material  64 To measure solid samples with the Laser Flash equipment, a vacuum was first created and then an inert atmosphere of N2. However, when this procedure was carried out for liquids, the vacuum and the pressure reduction in the equipment chamber caused the water (the first liquid tested) to evaporate when reaching the vapor pressure. This was checked by weighing the sample before and after the vacuum. Finally, the vacuum was omitted and a longer time was given for the creation of the N2 atmosphere. During the tests, the amount of sample that was evaporated was minimal, with mass losses lower than 5%, not affecting the measurement. Figure 2.10 shows the values of thermal diffusivity for water under vacuum conditions compared to the thermal diffusivity values given in the literature. Figure 2.10 Thermal diffusivity values of water under vacuum or otherwise. 35.0 35.5 36.0 36.5 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 Measurements-Chamber vacuum Measurements-Without chamber vacuum Literature value Thermal diffusivity (mm2·s) Temperature (ºC)  As already mentioned, the sampleholder used was made of Pt90Rd10. The external surfaces of the sample holder were coated with graphite to increase the amount of energy absorbed and to guarantee that all the parts of the sample had the same absorption. The maximum temperature that this surface reaches can be calculated by energy balance, where the energy supplied by the laser has an almost linear relationship with its voltage. In this manner 17 joules match with the maximum voltage of the equipment, 2978 V, with the transmission filter at 100%. Once the maximum voltage or the maximum temperature is estimated, it is possible additionaly to check if part of the sample has been evaporated, by weighing before and after the measurement. CHAPTER 2. Determination of the thermophysical properties of microencapsulated PCM slurries and PCM emulsions.  65 From the previous study by Coquard and Panel (2009), it was known that a complete filling up of the sample holder was crucial, as well as the correct determination of the thickness of the liquid sample. The sample thickness was obtained from the measurements of the thickness of the sample holder executed by a caliber which has an accuracy of 0.0011 mm. In order to guarantee the complete filling up of the sample holder, the volume of the liquid sample holder was calculated from the geometrical data and the amount of sample was controlled by a micropipette. Previously, the manufacturer’s suggestion had been to use as a sample the amount of liquid that remained adhered to the lid of the sampleholder by surface tension. However, it was observed that this amount did not totally fill the sampleholder. Taking all these considerations into account, thermal conductivity values were obtained for the three liquids tested: water, hexadecane and glycerin. The thermal diffusivity values were obtained from the three-layer model provided by the software of the equipment. The thermal conductivity values are shown in figure 2.11. Figura 2.11 Values of thermal conductivity of liquids in comparison with their reference values 22 24 26 28 30 32 34 36 38 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Water (measured) Water (reference) Hexadecane (measured) Hexadecane (reference) Glycerin (measured) Glycerin (reference) Thermal conductivity (W/(m·K)) Temperature (ºC) These values are the average value of five repetitions executed both for thermal diffusivity and temperature, together the standard deviation of these measurements. In the case of distilled water, the results show a maximum error Analysis of microencapsulated phase change material slurries and phase change material emulsions as heat transfer fluid and thermal storage material  66 of 7.87% and for hexadecane 4.31%. In the case of glycerin, higher errors were obtained, up to 15.38%. The reference values for water, hexadecane and glycerin were taken from the following references, respectively (Incropera and Witt, 1990, Mukhamedzyanov et al. 1963, Perry and Green, 1997). It is interesting that the deviations in the glycerine measurements are lower than in the water and hexadecane measurements. 2.3.3 Measurements of the PCM slurries Figure 2.12 compiles the thermal conductivity values for the DS 5007 slurries with PCM microcapsule mass fractions of 14, 20 and 30% and for the DS 5045 slurry with mass fractions of 20 and 35%. The measurements taken at 20ºC were not considered very reliable, especially for the slurry DS 5007, since even a very small increase in the temperature due to the laser pulse causes the specific heat to change sharply (from previous analyses, the phase change region can be considered between 20 and 24ºC) and this methodology may not be valid given that the specific heat is considered constant in the calculation. It must be pointed out that the increase in the PCM microcapsule mass fraction entails a decrease in the thermal conductivity. This behavior was expected, as the thermal conductivity of paraffin is lower than that of water. Figure 2.12 Thermal conductivity values measured for the candidate slurries DS 5007 and DS 5045 with different PCM microcapsule mass fractions. 18 20 22 24 26 28 30 32 34 36 0.20 0.25 0.30 0.35 0.40 0.45 0.50 DS 5007 14% DS 5007 20% DS 5007 30% DS 5045 20% DS 5045 35% Thermal conductivity (W/(m·K)) Temperature (ºC)  CHAPTER 2. Determination of the thermophysical properties of microencapsulated PCM slurries and PCM emulsions.  67 Specifically, at around 30ºC for a 20% PCM microcapsule mass fraction, the DS 5007 and DS 5045 slurry experienced a reduction of their thermal conductivity in comparison to water by 31.7 and 37.6% respectively. 2.4 Conclusions The Enthalpy-Temperature curves of 5 PCM slurries have been obtained in the laboratory, specifically the curves of 3 mPCM slurries (2 of them with different PCM microcapsule mass fractions) and of 2 PCM emulsions. These curves have been obtained from an installation using the T-history method. Further tests in the case of the slurry from the AERO-University of Ljubljana were rejected because of a hysteresis of about 10ºC. Further tests on the samples developed by Fraunhofer UMSICHT were also rejected because of stratification problems (in the sample without thickener) or problems of incompatibility with the plastic (in the sample with thickener). Regarding the thermal diffusivity measurements, a first approach to a methodology has been established to accomplish measurements with a Laser Flash in the case of measuring mPCM slurries and PCM emulsions at temperatures close to the ambient temperature. The analyses carried out indicate that for the correct measurement of this property the following aspects must be considered:  A vacuum should not be created in the chamber of the Laser Flash so as to avoid the evaporation of the sample.  The amount of sample evaporated during the test must be controlled. It must be checked that the amount of sample that is evaporated is small (mass loss<5%).  The sampleholder must be correctly filled by calculating the volume of the sample from the geometric parameters of the sampleholder.  When applying the corresponding calculation model for obtaining thermal diffusivity values from the data recorded by the infrared sensor, the observation time must be as short as possible to avoid the influence of the sampleholder. After having satisfactorily measured the thermal diffusivity of water and hexadecane with errors below 8%, the thermal diffusivity of DS 5007 and DS 5045 slurries for different mass fractions were measured. As expected, it was observed that when increasing the PCM microcapsule mass fraction, the Analysis of microencapsulated phase change material slurries and phase change material emulsions as heat transfer fluid and thermal storage material  68 thermal conductivity of the slurry decreased due to the lower thermal conductivity of paraffin compared to water. CHAPTER 3. Determination of rheological properties  69 In the previous chapter the enthalpy and the thermal conductivity depending on temperature of mPCM slurries and PCM emulsions compiled in the laboratory have been determined. To complete the characterization, this chapter addresses the rheological characterization of the candidate mPCM slurries. The chapter has been completed with the approach of a methodology for the determination of the viscosity of octadecane in the melted phase and during the phase transition. The methodology proposed can be useful for the determination of the viscosity of other PCMs. These values could be used when modeling natural convection in PCM in melted phase, in traditional thermal energy storage systems with PCMs. 3 Determination of rheological properties Analysis of microencapsulated phase change material slurries and phase change material emulsions as heat transfer fluid and thermal storage material  70 3.1 Introduction or theoretical basis of the measurements to accomplish The accomplished measurements in the present chapter to analyze the rheological behavior of mPCM slurries have been carried out with a control stress rheometer from TA Instruments model AR-G2. To keep the sample at the set temperature, the rheometer has two different configurations: an environmental test chamber or oven and a Peltier plate, allowing working in a temperatures range from -150 to 600ºC. It can provide a heating rate up to 60 K/min. Its control technology of the torque through magnetic bearings in replacement of the traditional air-bearing provides capacities of microstresses, being ideal for samples with very low viscosities. The rheometer has different accessories which allow characterizing a wide range of materials and viscosities. In the laboratory of determination of thermophysical properties from GITSE group, there are cones and plates geometries with different diameters and different angle values for the case of the cone. In addition it has the accessory solvent trap, which avoids the evaporation of the sample. In figure 3.1 an image of the rheometer can be observed. Figure 3.1 Control stress rheometer AR-G2 from TA Instruments. The thecnical specifications are enumarated below:  Minimum torque in oscillatory mode 0.003 µN·m CHAPTER 3. Determination of rheological properties  71  Minimum torque in rotational mode 0.01 µN·m  Torque resolution 0.1 nN·m  Motor inertia 18 µN·m·s  Displacement resolution 25 nrad  Normal force range 0.0005-50 N The measurements accomplished for the viscosity determination of octadecane in melted phase and during the phase transition were made with the control stress rheometer that the research group TAG (Thermische Anlagen und Gebäudetechnik) from the Fraunhofer ISE Institute from Freiburg (Germany) has in its laboratory. These measurements were carried out during the research stay accomplished in this institute. It is a rheometer from Thermo Scientific model Haake Mars II. The sample temperature is controled by a Peltier plate. In these tests a titanium plate with a diameter of 60 mm has been used. The most important technical specifications of this rheometer are named below:  Minimum torque in oscillatory mode 0.05 µN·m  Minimum torque in rotational mode 0.05 µN·m  Torque resolution 0.5 nN·m  Motor inertia 10 µN·m·s  Displacement resolution 12 nrad  Normal force range 0.01-50 N The tests executed in the present chapter are classified in two kinds of tests: 1) rotational or flow tests and 2) oscillatory tests. Rotational tests consist of applying a torque (or stress) and measuring the strain, to obtain in this way viscosity values. The Viscosity-Shear rate curves have been obtained through a shear rate sweep from 0.001 to 1000 s-1. For this purpose, stress has been applied to the sample. The measurement of the viscosity is accomplished when the material has reached the steady state. The stress is increased logarithmically and the process is repeated, providing the flow viscosity curve. The steady state is reached when the variation of the applied stress varied less than 1% during 40 seconds, with a maximum time of 60 seconds. Analysis of microencapsulated phase change material slurries and phase change material emulsions as heat transfer fluid and thermal storage material  72 In oscillatory tests the sample is subjected to an oscillatory strain of low amplitude and the stress that the fluid causes is measured (see in figure 3.2.). According to the lag between the applied strain ( )tω·cos(γγ 0  ) and the measured stress ( )δtω·cos(ττ  0): If δ=0º → elastic solid If δ=90º → fluid purely viscous If 0< δ<90ºC → viscoelastic fluid Figure 3.2 Oscillatory strain (geomtry seen from above) )wt·cos(γ=γ 0 In this manner an elastic module (G’) that would be the elastic or returnable part and a viscous or loss module (G’’), that would be the viscous or non-returnable part are obtained (equation 3.1): ''* iG'GG  (eq. 3.1) In the left image of figure 3.3 the curve obtained in the case of an elastic response is shown, where the oscillatory stress would be )tω(sen·γ·Gτ*0  and the resulting strain )tω(sen·γγ 0 . In the right image of figure 3.3 the curve that would be obtained in the case of a viscous response is shown, the oscillatory stress would be )tω·cos(γ·ω·ηγ·ητ 0   and the strain )δ-tω(sen·γγ 0 . Two types of oscillatory test have been carried out: strain or stress sweeps and frequency sweeps. In the strain or stress sweeps a frequency is set for the test, a strain or stress is applied and the response is measured (stress or strain). In the frequency sweeps, the strain or stress is set, a frequency sweep is executed and the response is measured (stress or strain). Both tests appear drawn in figure 3.4. CHAPTER 3. Determination of rheological properties  79 continuous phase. When the slurry is sheared at very low shear rates, there is no cooperative motion between the microcapsules so that they move in the flow direction, and therefore the viscosity is high. However, when the slurry is sheared at high velocities, the microcapsules start to move from their random distribution towards a situation where layers are formed. In this manner, the average distance between particles decreases in the flow direction and increases in the perpendicular direction. This change in spatial distribution facilitates the movement of the particles and the viscosity drops. This phenomenon is shown in figure 3.11. Figure 3.10 Viscosity-Shear rate for the DS 5007 slurry at a temperature of 27ºC and for the DS 5045 slurry at a temperature of 29ºC 1E-3 0.01 0.1 1 10 100 1000 1E-3 0.01 0.1 1 10 100 DS 5007 14% DS 5007 20% DS 5007 30% DS 5045 20% DS 5045 35% Viscosity (Pa·s) Shear rate (1/s) Shear rate range when pumping fluids  Figure 3.11 Formation of layers under the shear. Shear thinning or pseudoplastic behavior (Barnes 2000) Analysis of microencapsulated phase change material slurries and phase change material emulsions as heat transfer fluid and thermal storage material  80 The Newtonian plateau is reached for the five samples from shear rates about 10 s-1. A flowing fluid would have shear rates within the range of the Newtonian plateau. According to the software “Best fit Viscosity-Shear”, the model that gives the best fitting to the measured values of the complete curve is the Carreau model (Carreau 1972). Equation 3.2 shows this model and table III.1 shows the adjustment coefficients for the DS 5007 and DS 5045 slurries. Together with the adjustment parameters, the standard error calculated by the software of the equipment according to equation 3.3 is shown. The software considers a good fitting when this standard error is lower than 20. 22 1 1 /m 0))γ·k(( η-η η-η    ∞ ∞ (eq. 3.2) 1000 2 2 · Range n )ηη( errordardtanS adjustmentmeasured     (eq. 3.3) Sample η0 (Pa·s) η∞ (Pa·s) k (s) m Standard error DS 5007 14% 13.80 4.89·10-3 288.80 1.02 15.53 DS 5007 20% 10.88 6.45·10-3 315.40 1.06 12.31 DS 5007 30% 6.45 18.32·10-3 82.05 1.03 14.01 DS 5045 20% 6.46 6.14·10-3 239.40 0.98 17.23 DS 5045 35% 28.56 12.54·10-2 201.6 0.89 15.76 Table III.1 Adjustment coefficients according to the Carreau model (Carreau 1972) This model is very similar to the Cross model (Cross 1965), being maybe this one simpler (equation 3.4): m 0)γ·k( η-η η-η    1 1 ∞ ∞ (eq. 3.4) The Carreau model and the Cross model are the same model at very low and very high shear rates and they only differ slightly when 1≈γ·k • . When m tends to 0, it describes a behavior more Newtonian and when m tends to 1, it describes a behavior more pseudoplastic. CHAPTER 3. Determination of rheological properties  81 In the flow curves of the DS 5007 slurry with a PCM microcapsules mass fraction of 14 and 20%, as well as in the flow curve of the DS 5045 slurry with a mass fraction of 20% in figure 3.10, it is observed that the curve starts to increase around shear rates of 200 s-1. This phenomenon is not very usual in slurries. Nevertheless tests in liquids of low viscosity at very high shear rates can cause secondary flows, causing an apparent rise of the viscosity (Barnes 2000). To know which value of viscosity should be taken to make a certain calculation, firstly the shear rate of the fluid should be known. This shear rate would come defined by r u  . In the case of Newtonian fluids the velocities profile for laminar flow and for turbulent flow is defined by equation 3.5 and 3.6 respectively. By differentiating these equations with respect to the radial coordinate, the shear rate profile is obtained. In figure 3.12 on the left these profiles are observed for laminar flow, and on the right for the turbulent flow, in the case of water flowing at a temperature of 25ºC, through a tube with an internal diameter of 10 mm.               2 12 R r ·u·)r(u average (eq. 3.5)            R r ·logf·.f·.·u)r(u DDaverage 11524311 10 (eq. 3.6) Figure 3.12 Velocity and shear rate profile for water under laminar flow (uaverage=0.2 m/s) and under turbulent flow (uaverage =1 m/s) -0.004 -0.002 0.000 0.002 0.004 0.0 0.1 0.2 0.3 0.4 u du/dr Radius (m) u (m/s) 0 40 80 120 160 du/dr (1/s) -0.004 -0.002 0.000 0.002 0.004 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 u du/dr Radius (m) u (m/s) 0 50 100 150 200 250 300 350 400 du/dr (1/s)  If an average shear rate was taken, it is observed that in both cases, this shear rate would be within the Newtonian plateau of the curves of figure 3.10. The viscosity of the samples depending on temperature has been also measured, for a shear rate within the Newtonian plateau (at 100 1/s). In figure Analysis of microencapsulated phase change material slurries and phase change material emulsions as heat transfer fluid and thermal storage material  82 3.13 both the cooling test (40-10ºC) and the heating test (10-40ºC) can be observed, conducted at a heating and cooling rate of 1ºC/min for the DS 5007 slurry with a mass fraction of 30%. It is observed a change of slope in the viscosity around the phase change temperatures (between 22.2 and 24.2ºC, phase change temperatures range according to the curves obtained in the installation of the T-history method). This peak is not observed at the same temperature in the heating and cooling curve, maybe because the heating and cooling rate is too quick and it has not enough time to reach the temperature of the Peltier plate or it may point out a possible phenomenon of hysteresis, as in the T-history curves it was not observed, being the velocity of the test in that case slower. It would be a hysteresis phenomenon due to the measurements conditions. Figure 3.13 Viscosity-Temperature for the DS 5007 slurry with a microcapsules mass fraction of 30%, shear rate=100 1/s 20 25 30 35 0.010 0.015 0.020 0.025 0.030 0.035 Heating curve 1ºC/min Cooling curve 1ºC/min Viscosity (Pa·s) Temperature (ºC)  3.3 Rheological characterization of PCMs for the study of natural convection The incorporation of PCMs in a particular application or system in bulk form or macroencapsulated usually requires a numerical analysis that allows optimization of the system performance. In current models, the natural convection in the PCM is not usually considered. However, various experimental works have shown that this term must be taken into account. When there is not a forced movement in a fluid, a spontaneous movement in the fluid may appear (and therefore convective phenomena) if there are density CHAPTER 3. Determination of rheological properties  83 gradients (due to temperatures gradients) under a gravitational field. Apart from the buoyancy forces, this convective phenomenon called natural convection will depend on viscous forces, which will prevent that movement. For this reason, the determination of the viscosity property is relevant. In TES systems, where PCM is in bulk form or macroencapsulated (no forced movement of the PCM), the natural convection could take place in the liquid phase or during the transition. Natural convection will be more relevant in the melted phase than in the melting phase, due to the increasing values of viscosity during the phase transition. More specifically in macroencapsulated PCM or in the bulk PCM, during melting, heat is transferred to the PCM first by conduction, and later by natural convection, because the thickness of the liquid region increases near the heat transfer surface. Due to the lower themal conductivity of liquid PCM in comparison to solid PCM, the heat transfer by conduction almost becomes negligible when the melting continues, dominating the natural convection in the liquid phase. On the other hand, during solidification, the heat transfer phenomenon is dominated by conduction. In this case, natural convection is important at the beginning and as the time goes, natural convection become almost zero compared to the conduction. The review of Jegadheeswaran and Pohekar (2009) and the chapter 4 of the doctoral thesis of Campos (2012) compile many experimental references that prove this phenomenon of natural convection in latent thermal energy storage systems. In view of the presence of the natural convection phenomenon in TES systems with PCM, and the dependence of the natural convection on the viscosity, this property should be measured. How accurate the viscosity must be determined, it will depend on the error that we want to assume in our response of interest (in this case, the effect of natural convection) and on the specific application (boundary conditions, operation…). In general, once established the response of interest and the error to assume, the precision of this property could be determined according to an uncertainty analysis (Dolado 2011). Arkar and Medved (2005) pointed out the importance of determining PCM properties. They compared the results of their numerical model with experimental results. The comparison confirmed their hypothesis of the importance of the role played by the thermal properties of the PCM, especially in slow running processes. Specifically, they studied the influence of the Heat Analysis of microencapsulated phase change material slurries and phase change material emulsions as heat transfer fluid and thermal storage material  84 capacity-Temperature curve obtained with a DSC for different heating and cooling rates on the results of a TES system with PCM spheres. The first studies to take natural convection into account were those of Sparrow et al. (1978) and Bathelt et al. (1979). To simulate this heat transfer mechanism, some authors (Farid y Husian 1990, Farid et al. 1998, Rieger et al. 1983) considered an effective thermal conductivity (equation 3.7), whose value will depend on the viscosity property: ne Ra·c λ λ (eq. 3.7) Costa et al. (1991) studied numerically the thermal behavior of three PCMs, a paraffin (octadecane) and two metals, confined in a rectangular domain where the natural convection in the fluid and the conduction in the solid were both considered. In the case of the octadecane, the authors pointed out a bad fitting with the experimental results in the upper part. The PCM started to melt at the sides. This melted PCM occupied the upper part of the rectangular domain as a consequence of its lower density, the melting in this zone being faster. The authors think that the discrepancies between the theoretical and experimental results were caused by thermal inertias, systems instabilities, thermal losses, lack of reliable information about the physical properties of the materials, 3D behavior, consideration of constant thermophysical properties, density variations, high calculation time and an important change in the viscosity with the temperature. The same authors in a later publication (Costa et al. 1997) observed discrepancies between their numerical solution and experimental results. These discrepancies could be attributed to the viscosity value taken in the numerical simulation, a constant value of 0.003898 Pa·s. To check its influence, they took the value of viscosity at a higher temperature. The change in the viscosity value caused differences in the melting front. However, the variation of viscosity with temperature did not explain other differences. Due to the dependence of viscosity on natural convection, it is therefore necessary to determine rheological behavior, and specifically the viscosity property dependent on the temperature and even in the phase transition, in order to incorporate these data in natural convection simulations. However, there are few reported studies about the rheological properties of PCMs. In fact, a recent review about the methods of characterization of PCMs, accomplished in the field of the COST Action TU 0802, proves that there is a CHAPTER 3. Determination of rheological properties  85 lack of researchers working in the rheological behavior of PCMs within this framework. Tipvarakarnkoon et al. (2008) analyzed the rheological properties of three commercial coconut fats and undertook a structure analysis during their solidliquid and liquid-solid phase transition. They obtained flow curves (viscosity vs. shear rate) in the liquid phase, using a double gap rotational cylinder. The shear rate was increased from 100 to 1500 1/s in 2 minutes and then decreased from 1500 to 100 1/s in 2 minutes. To evaluate the phase transition, the G’ and G’’ modulus were obtained from oscillatory tests. Specifically, stepwise temperature sweeps were carried out with a cooling rate of 0.5 K/min. Values of tan G’’/G’>>1 corresponded with the liquid phase, values of tan G’’/G’<<1 corresponded with the solid phase and values of tan G’’/G’=1 corresponded with the phase transition. The work presented in this section 3.3 is part of the COST Action TU0802 (Next generation cost effective phase change materials for increased energy efficiency in renewable energy systems in buildings). One of the objectives of this action is to develop standardized methodologies to characterize PCM, in which the importance of rheology for PCM is also identified. This work also represents a contribution to Task 42-Annex 24 of the International Energy Agency (IEA). Specifically, it is included within the development of measuring and testing procedures to characterize new storage materials reliably and reproducibly. The aim of the work presented here is to develop a measurement procedure of the viscosity property. In Task 42-Annex 24, much work has already been accomplished on the standardization procedure for EnthalpyTemperatures curves with DSC. Octadecane has been used for this, and is also used here for viscosity property measurements. Therefore, the objective of the tests that are going to be shown in the following sections is to propose a test methodology, as well as suggestions of the test conditions, that allow to characterize rheologically the PCM in a reproducible way, obtaining reliable values of viscosity depending on the temperature in a temperatures range close to the solid-liquid phase transition. 3.3.1 Materials and methodology Materials As it has just been mentioned, octadecane has been characterized, in spite of the fact that the most part of comercial PCMs of organic nature are blend of Analysis of microencapsulated phase change material slurries and phase change material emulsions as heat transfer fluid and thermal storage material  86 different alkanes. The octadecane to be analyzed has a purity of 98.11%. The octadecane was purchased from the Sasol company, specifically the product Parafol® 18-97. Table III.2 shows the octadecane properties, according to the data supplied by the National Institute of Standards and Technology (NIST). Purity of the sample (%) 98.11 Phase change temperature (melting) (ºC) 27.85 Phase change enthalpy (J/g) 241.66 Specific heat solid phase (298.15 K) (J/(g·K)) 1.91 Specific heat liquid phase (325 K) (J/(g·K)) 2.23 Table III.2 Properties of octadecane Methodology Two types of tests were conducted to analyze the rheological behavior of the PCM: 1) rotational test and 2) oscillatory tests. These types of tests were already explained in section 3.1. The Viscosity-Shear rate curves or rotational tests have been obtained through a shear rate sweep from 0.001 1/s to 1000 1/s. The stress is increased logarithmically. The steady state is reached when the variation of the applied stress varies less than 1% during 40 seconds, with a maximum time of 60 seconds. For the oscillatory tests, three types of test were carried out: strain or stress sweeps, frequency sweeps and temperature sweeps. The frequency sweeps here presented cover a frequency range from 0.01 Hz to 1 Hz and were carried out within the viscoelastic region. Stress sweeps were previously carried out from 0.01 to 100 Pa at a frequency of 1 Hz. The permanence within the viscoelastic region has to be guaranteed above 1 Hz. This 1 Hz frequency is considered sufficient since above 1 Hz the measurements may present considerable inertia. In spite of the correction of the software, it is advisable not to take into account measurements at high frequencies. Besides, in this analysis measurements at low frequencies are of interest since the PCM is at rest. The obtaining of the elastic module G’ and the viscous module G’’ allows to obtain a value of complex viscosity according to equation 3.8: CHAPTER 3. Determination of rheological properties  87 22              ω ''G ω 'G η* (eq. 3.8) 3.3.2 Results Since the PCM is at rest in the TES system, a rotational sweep was initially considered for determining the viscosity at very low shear rates. However, this approach was quickly rejected because it was observed during the tests that when the geometry turns at a specified velocity, it may cause the crystals formed to break, influencing the results in this mode (this is only one of the phenomena that could cause erroneous measurements in rotational experiments). Measurements in oscillatory mode were thus proposed. First of all, a stress or strain sweep has to be undertaken to determine the linear viscoelastic region of the octadecane. This region has to be determined for different frequencies and for different temperatures. Regarding to the frecuencies, this must not be too high, to avoid inertia problems, and considering that lower frequencies mean higher linear viscoelastic region. The range of interest relates to temperatures within the phase transition and in the liquid state. Figure 3.14 shows the results derived from these stress sweeps at 1 Hz. Figure 3.14 Stress sweep at different temperatures (melted phase and transitio phase). Frequency=1 Hz. Gap ∼0.5mm. 0.01 0.1 1 10 100 1E-4 1E-3 0.01 0.1 1 10 100 1000 10000 100000 1000000 G'' at 29°C G'' at 27.8°C G' at 27.6°C G'' at 27.6°C Stress (Pa) G' (Pa) 1E-4 1E-3 0.01 0.1 1 10 100 1000 10000 100000 1000000 G''(Pa)  Analysis of microencapsulated phase change material slurries and phase change material emulsions as heat transfer fluid and thermal storage material  88 At 29°C and 27.8ºC the G’ module is not visible in the graph because the PCM is completely melted and its elastic part is too low for the rheometer to obtain a value. At 27.6°C (during the phase transition), it can observed that at very low stresses (up to 1 Pa approximately) the measurements show considerable noise. From liquid to solid, when the transition takes place, the material starts to form molecular bonds, what it means an abrupt increase in the G’ module due to the fast transition, leading to an increase in the complex viscosity (see figure 3.15 to support this fact). With high values of viscosity, low stresses will cause even lower strains. These strain values that are going to be measured by the sensor of the rheometer will be low, they might be close to the resolution of the displacement sensor of the rheometer, 12 nrad in this case. If the values obtained from measurement with low stresses are discarded, it can be seen that the linear viscoelastic region reaches values up to 100 Pa. Once the linear viscoelastic region was determined, the “oscillatory temperature steps” procedure was carried out both for the melting curve and for the solidification curve. The procedure was done using temperature steps of 0.1ºC. Since the sample needs an equilibrium time to reach the Peltier plate temperature, a stabilization time of 60 seconds was chosen. The sample was loaded at its liquid phase. The gap, which is the distance between the Peltier plate and the geometry, was adjusted so that the sample formed a meniscus. Once the sample was placed between the Peltier plate and the geometry, the value of the normal force present was fixed as a reference (contact force exerted on the sample). When the PCM changed phase, the rheometer thus adapted the gap between the Peltier plate and the geometry in such a way as to maintain the same normal force value, since with the phase transition the sample changes its volume and this would affect the normal force. For this reason, plate geometry was chosen in order to be able to adapt the gap. Figure 3.15 shows the obtained results. According to this graph, the octadecane changes from liquid to solid at 27.327.4°C and from solid to liquid in the temperature range 27.6-28°C. Figure 3.16 shows how the rheometer changed the gap during the phase transition to maintain the same normal force reference value. CHAPTER 3. Determination of rheological properties  95 Figure 3.22 Influence of the applied stress (within the linear viscoelastic region) on the Complex viscosity-Temperature curves. Gap=0.4 mm. Heating rate=0.5ºC/min. 26 27 28 29 1E-3 0.01 0.1 1 10 100 1000 10000 100000 1000000 (Pas) T(°C) 10 Pa 1 Pa 0.1 Pa Figure 3.23 Influence of the frequency on the Complex Viscosity-Temperature curves. Shear stress=1 Pa. Gap=0.5 mm. Heating rate=0.5ºC/min. 27 28 29 1E-3 0.01 0.1 1 10 100 1000 10000 100000 1000000 (Pas) T(°C) f=1 Hz f=0.1 Hz f=0.01 Hz 3.3.4 Methodology proposed for the determination of the viscosity Tests for the determination of viscosity of PCMs must be carried out in oscillatory mode. The steps to be executed with a plate as geometry and a Peltier plate as temperature controller are as follows: Analysis of microencapsulated phase change material slurries and phase change material emulsions as heat transfer fluid and thermal storage material  96 1) Strain or stress sweep at different temperatures (both in the liquid phase and during the phase transition). A frequency about 1 Hz is advisable. With this step, the linear viscoelastic region will be determined. With melted PCM, lower frequencies should perhaps be used. 2) Frequency sweeps in the molten and in the phase transition region. The frequency sweep must be carried out at a stress or strain within the linear viscoelastic region (defined previously in step 1). In any case, it would not be advisable to use high stress or strain as this could affect the measurements. It is important to work far from the minimum angular displacement value of the rheometer. This frequency sweep will provide information about the rheological behavior of the sample in the molten and in the phase transition region (Newtonian or non-Newtonian, if the Cox-Merz rule is fulfilled). If the sample is Newtonian, step 3 may be tested at any frequency. Frequencies below 1 Hz are recommended to avoid problems of inertia in the case of measuring with control stress rheometers, especially with melted PCM. 3) Once the linear viscoelastic region is determined and the frequency test executed at different temperatures, an oscillatory temperature ramp test (or by steps) must be executed (both for melting and solidification). If the PCM is non-Newtonian, the test will be carried out at 0.01 Hz and if it is Newtonian, 1 Hz will be sufficient. Low gaps must be used to avoid temperature gradients in the sample. Different gaps should be tested. When the results do not vary, an appropriate gap will have been found. Different heating and cooling rates should be tested, and the rate for which the Complex viscosity-Temperature curve does not change should be selected. In addition, the normal force during the test must be controlled. This must be always the same. The rheometer will adapt the gap as a consequence of the volume change during the phase transition. 4) To know if the complex viscosity values can be extrapolated as shear viscosity values, since the phase transition cannot be measured with steady state flow, the curve in steady state flow η-   and the curve of the frequency sweep η*-ω in the liquid state must be compared. If these values fit within the uncertainty range previously defined from a viscosity standard of the same order of magnitude of the PCM to analyze, the complex viscosity values will be taken during the phase transition as shear viscosity values, as if the steady state flow curve had been obtained (Cox-Merz rule). If these curves do not fit well, the complex viscosity values may not be considered as shear viscosity values. CHAPTER 3. Determination of rheological properties  97 Notes about the methodology: - The procedure described above is for working with plate geometry and a Peltier controller. The plate is not necessarily the most appropriate geometry, due to the shear gradient across the sample. However, a cone does not allow the normal force to be controlled since the gap is set by the geometry itself. Regarding the temperature controller, it would be interesting to use an “environmental test chamber” in order to provide more realistic results for PCMs with phase transition temperatures well above or below room temperature, since this controller would avoid the temperature gradient in the sample. However, the air flow in the test chamber can cause the melted PCM to leave the geometry due to its low viscosity. Also, the problem may be solved with the combination of a Peltier Plate and an upper heated plate. - If the PCM during its phase transition or in its liquid phase is not a Newtonian fluid, or if the measurements with the tested frequency present considerable inertia, it is necessary to test the oscillatory temperature ramp at very low frequencies, about 0.01 Hz (PCM at rest). The problem in this case is that the rheometer needs at least one period to obtain a value of complex viscosity. If the frequency is 0.01 Hz, the rheometer would need at least 100 seconds. If the heating and cooling rate is, for example, 0.2°C/min, in these 100 seconds the temperature will vary by about 0.34°C. The measurement would not therefore be a correct measurement due to the inconstant temperature. In this case, precise measurements at set temperatures should be carried out. For this reason, a temperature steps procedure is considered to be better than a temperature ramp. 3.4 Conclusions 3.4.1 Conclusions about the rheological characterization of microencapsulated PCM slurries and PCM emulsions The methodology and results of the rheological characterization of mPCM slurries studied in the framework of this doctoral thesis by a control stress rheometer has been presented. Besides the behavior models of these mPCM slurries for different PCM microcapsules mass fraction have been obtained. As example, it is mentioned that both slurries, DS 5007 and DS 5045, for a 20% PCM microcapsule mass fraction, reach in their Newtonian plateau a viscosity six times higher than of the water. Analysis of microencapsulated phase change material slurries and phase change material emulsions as heat transfer fluid and thermal storage material  98 3.4.2 Conclusions about the rheological characterization of PCMs for the study of natural convection From the results obtained of the tests, a first approach of methodology has been proposed for the measurement of viscosity of PCMs during its melted phase and during the phase transition. These values will be able to be used in numerical simulations of thermal energy storage systems with PCM, in the case of that natural convection plays an important role in the heat transfer phenomenon. CHAPTER 4. Analysis of stability and compatibility of thermal energy storage systems with PCM  99 This chapter analyzes the stability and compatibility both of traditional TES systems where PCM is macroencapsulated and of systems where PCM is in the form of a mPCM slurry or PCM emulsion. The physical stability of mPCM slurries and PCM emulsions with regard to possible problems of stratification or creaming and with regard to the possible rupture of PCM microcapsules in suspension under thermal-mechanical cycles is discussed. Possible phenomena of microbial contamination are also studied. The compatibility of different low temperature PCMs with spherical plastic capsules is evaluated, as well as possible corrosion phenomena of certain metallic alloys when coming into contact with PCM slurries. 4 Analysis of stability and compatibility of thermal energy storage systems with PCM Analysis of microencapsulated phase change material slurries and phase change material emulsions as heat transfer fluid and thermal storage material  100 4.1 Physical stability of microencapsulated PCM slurries and PCM emulsions 4.1.1 Problems of stratification or creaming As mentioned in the first chapter, one of the main issues to be tackled in mPCM slurries and PCM emulsions is their lack of stability, since stratification or creaming problems tend to occur. Stirring could be adopted as a solution to this problem; however, this stirring could destroy the thermal stratification. Generally, the destabilization process can take weeks and even months. To predict the physical stability of PCM slurries without having to wait so long, a methodology well known in the food and pharmaceutical field has been applied. This methodology basically consists of measuring samples with a rheometer in oscillatory mode. These measurements can be related to measurements visually obtained of the destabilization process of creaming, specifically with the creaming percentage over time. In this way, the dominant parameters relating to destabilization can be obtained which manufacturers can modify in order to improve the physical stability of PCM slurries, or to predict such destabilization processes. For this analysis, measurements in oscillatory mode were performed with the control stress rheometer AR-G2 from TA Instruments whose characteristics have been detailed in chapter 3. The theoretical basis of these measurements has already been explained in section 3.1. The results of the oscillatory tests can be related to the destabilization processes observed in the sample DS 5007 for different PCM microcapsule mass fractions. Specifically, it has been attempted to relate these oscillatory results for the DS 5007 slurries with PCM microcapsule mass fractions of 14, 20, 30 and 42% with the creaming height observed in these samples in the course of time. To date, this study has not been developed with the DS 5045 sample. Such a destabilization process was not observed in this sample. It is suggested that such an analysis could be the subject of future work since the results may be used to set objective parameters for the sample to avoid destabilization processes. Firstly, a strain sweep was carried out at a frequency of 1 Hz on each sample to determine the linear viscoelastic region. The results are shown in figure 4.1. CHAPTER 4. Analysis of stability and compatibility of thermal energy storage systems with PCM  101 Figure 4.1 Strain sweeps for the four samples of DS 5007; Temperature=27ºC; f=1 Hz 0.01 0.1 1 10 1E-3 0.01 0.1 1 10 DS 5007 14% DS 5007 20% DS 5007 30% DS 5007 42% G' (Pa) Strain (-)  Once this region was determined, a strain value within the linear viscoelastic region has been selected, and a frequency sweep has been carried out. The frequency sweeps made in this case cover a frequency range from 0.005 Hz to 100 Hz. The values obtained at very high frequencies were very different, possibly as a consequence of the high inertia. However, the analysis of interest is in the G’ and G’’ moduli at low frequencies, since the structural stability of these slurries is expected to be analyzed when they are at rest during storage (long periods being the situation of PCM dispersions at rest during storage). The most extensive G’ and G’’ moduli of real examples of structured liquids are shown in figure 4.2. The exact values of the G’ and G’’ moduli and their position in the frequency scope will change, but their qualitative overall behavior will be as shown in the figure, provided that such a wide frequency range can be determined. Rheometers usually work within the range from 0.01 to 100 rad·s-1 and can see just two of the zones described in the figure. Analysis of microencapsulated phase change material slurries and phase change material emulsions as heat transfer fluid and thermal storage material  102 Figure 4.2 Oscillatory response for real systems (Barnes 2000) If the frequency sweeps are compared (figure 4.3) to the typical oscillatory response of structured fluids (figure 4.2), it is observed that the test covers the “plateau” and “transition” zones only. The “rubbery” or “plateau” zone is the region where elastic behavior is more predominant. Whereas it seems that there is a plateau in many cases, in fact there is always a slight increase in the G’ modulus with the frequency. The G’’ modulus is always lower than the G’ modulus, but sometimes it can be significant. When the slope of G’- ω is low, the G’’ value decreases when increasing ω up to a minimum where it rises again. The lower the slope of the G’- ω curve, the deeper is the valley of G’’ (Barnes 2000). Figure 4.3 Frequency sweeps for the four DS 5007 samples: Temperature=27ºC; Strain=0.1 0.01 0.1 1 10 100 1E-3 0.01 0.1 1 10 100 DS 5007 14% G' DS 5007 14% G'' DS 5007 20% G' DS 5007 20% G'' DS 5007 30% G' DS 5007 30% G'' DS 5007 42% G' DS 5007 42% G'' Angular frequency (rad/s) G' (Pa) 1E-3 0.01 0.1 1 10 100 G'' (Pa)  CHAPTER 4. Analysis of stability and compatibility of thermal energy storage systems with PCM  103 From these G’- ω and G’’-ω curves, information in qualitative terms about the microstructure of the slurry can be obtained. In fact, the G’ value for the frequency ω where G’’ shows a minimum (“plateau” zone) is related to the structural stability of the system. With higher G’ values, the stability improves. In figure 4.3 it is observed that at very low frequencies, the G’ modulus remains constant. However, a minimum in the G’’-ω curve is not observed so clearly, only for the slurry with a 42% PCM microcapsule mass fraction, maybe because it is located at lower ω values than the tested ω values. As G’ remains constant, this value has been taken for the other three curves. The value of tan(δ)=G’’/G’ has also been taken, which gives additional information about stability. These values are collected in table IV.1. It is noticeable that the G’ and the G’’ module for the slurry with 20% PCM microcapsules is higher compared to 30% PCM microcapsules. Different samples were analyzed without obtaining significant differences. PCM microcapsule mass fraction in suspension G' module tan(δ) 14% 0.0324 0.1785 20% 0.0700 0.3461 30% 0.0480 0.2322 42% 0.9680 0.2438 Table IV.1 Values obtained from the frequency sweeps for the study of the structural stability of DS 5007 slurries. It is observed that at low frequencies the values of tan δ are low, about 0.25. This means that the elastic part of the slurry is much greater than the viscous part (as observed in figure 4.3), i.e. interaction between the particles that form the slurry is very strong. These forces between particles will promote their aggregation and are able to cause phenomena of flocculation or coalescence (TA Instruments 2011). It has been attempted to relate the values measured by the rheometer with the destabilization processes observed in the mPCM slurries, specifically with the creaming process. In a highly stable system, where Brownian motion is negligible for the micron sized particles, the action of gravity causes the PCM particles with a lower density than water to move upward, causing a rise in the PCM microcapsule concentration in the upper part. An increase in the average rapprochement between the particles is thus caused to such an extent that attractive forces predominate. The creaming caused in this manner is a creaming of low volume. The small particles occupy the cavities that the bigger Analysis of microencapsulated phase change material slurries and phase change material emulsions as heat transfer fluid and thermal storage material  104 particles leave, forming a compact packaging. These kinds of slurries are called deflocculated slurries and they are characterized by a long creaming time, a small volume of creaming and a muddy aspect of the water. It is difficult to redisperse the particles in suspension. On the other hand, there are slurries where the stability is insufficient, and the aggregation of particle groups or floccules occurs before creaming. The creaming volume is greater, the creaming time is shorter, the water is clear and it is easy to re-disperse the slurry. These kinds of slurries are called flocculated slurries (Gerbino 2011). The sedimentation rate of very diluted slurries of rigid and spherical particles without interaction follows the Stokes law. However, the sedimentation or creaming of more concentrated slurries is a more complex process. The relationship between the observed creaming phenomenon and the measured rheological parameters has been evaluated and established as follows. Four calibrated and graduated test tubes with a volume of 10 ml were prepared and the height or volume of the creaming part over time was observed, from the graduation of the tubes. These test tubes have been verified according to the ISO Standard 4788:2005. The average volume at 20ºC is 10.016 ml, with a standard error of 0.021 ml. During the first days, the measurements were taken every 60 minutes. After these first days, the interval of data gathering was every 24 hours. Figure 4.4 shows the test tubes on the seventh day and figure 4.5 shows the creaming percentage over time. Figura 4.4 Creaming observed in the four samples at t=10080 minutes. The creaming percentage has been calculated by using equation 4.1: CHAPTER 4. Analysis of stability and compatibility of thermal energy storage systems with PCM  111 Figure 4.13 DS 5045 sample observed with an environmental SEM. Top left image: sample with a 40% mass fraction. Top right image: sample with a 35% mass fraction pumped during 5 weeks. Lower left image: sample with a 35% mass fraction pumped during 2 weeks. Lower right image: sample with a 25% mass fraction pumped during 4 weeks. The top left image of figure 4.13 shows the DS 5045 non-pumped slurry diluted down to 10 times. Damage in its morphology is not distinguished. The microcapsules seem to have folds. The top right image shows the same slurry with a 35% mass fraction diluted down to 20 times, after being pumped in the installation during five weeks (having experienced in the order of 8000 meltingsolidification cycles). It appears that the spheres, especially those of bigger size, are more damaged and that some of them are even broken. Regarding the size, no difference is observed. The lower left image shows the same previous slurry but having been pumped for just two weeks (having experienced in the order of 1500 melting-solidification cycles). For its observation, this sample was diluted down to 30 times. It seems that some spheres are damaged and that some of them are even broken. Again, no significant changes in the particle size are observed. The sample observed in the lower right image is the DS 5045 slurry with a mass fraction of 20% after having been pumped during 4 weeks (in the order of 4000 melting-solidification cycles). This sample was diluted down to 30 times. It seems that some spheres are considerably damaged and some of them are broken. The damage seems similar to the top right sample. They appear creased and are also open or broken. Analysis of microencapsulated phase change material slurries and phase change material emulsions as heat transfer fluid and thermal storage material  112 4.1.3 Microbial contamination As the manufacturer points out in the manual for handling and storage of dispersions, which can be consulted in Appendix I, possible microbial contamination was observed in a sample that had been stored in the laboratory during 12 months (figure 4.14). To prevent possible contamination that represents a risk for human health (such as Legionella or Aspergillus, in spite of the fact that the slurry is not sprayed in the possible applications), a culture in a non-selective medium was made in order to study the possible growth of any microorganism. The culture was developed by the Department of Microbiology, Preventative Medicine and Public Health at the University of Zaragoza. It was observed that the product, as suspected, was contaminated. It had filamentous fungi in a concentration of approximately 10 cfu/50 µl (colony-forming unit), and yellowish colonies that could correspond to environmental bacteria, such as sarcinas or yeasts, with a concentration of 40 cfu/50 µl, which may have helped to decompose the product. The possibility of Aspergillus fungus and Legionella bacterium has been dismissed. The need for a biocide is evident, since the installation is not sterile and the product allows the growth of microorganisms. Figure 4.14 DS 5007 contaminated after a storage period of 12 months. Culture in nonselective medium. 4.2 Analysis of compatibility 4.2.1 PCM-plastic compatibility As mentioned in the preamble, part of this thesis is framed within a R&D project carried out with a private company operating in the heating and air conditioning sector in Spain. This company develops spherical capsules of plastic. The compatibility of these spherical capsules with different low temperature PCMs has been studied for their possible subsequent application in solar cooling systems. CHAPTER 4. Analysis of stability and compatibility of thermal energy storage systems with PCM  113 It is known from previous studies (Lázaro et al. 2006 b, Castellón et al. 2011) that PCMs can migrate through plastics or that PCMs can absorb water through the plastic wall in hydrophilic PCMs. Different commercial PCMs within a phase change temperature range for thermal energy storage at low temperatures have been analyzed, with the focus on solar cooling applications. Specifically, the inorganic nature of the product Climsel C7 has been analyzed together with the organic nature of the products RT9, RT6 and EPS A8 from Rubitherm and EPS, respectively. The experimental methodology adopted to analyze the compatibility of the plastic spheres with the different PCMs is based on the work of Lázaro et al. (2006 b), based in turn on the standard ISO 175:1999 Plastics, Methods of Test Determination of the Effects of Immersion in Liquid Chemicals. In this article four types of plastic were tested: high density polyethylene (HDPE), low density polyethylene (LDPE), polyethylene terephthalate (PET) and polypropylene (PP). The use of LDPE was discarded because it showed a higher mass variation (migration processes) and big deformations in the plastic encapsulation. PET was recommended as an encapsulation material. If water absorption was taken into account, PP was also considered advisable. In this case, tests must start from zero since the material of the spherical capsules is not known. A sample of 100 ml was placed in each capsule and a fridge was used to carry out the melting and solidification cycles that they would experience in a real installation (figure 4.15). The liquid phase is more unfavorable since in this state the migration process of the PCM is promoted. Figure 4.15 Spherical capsules analyzed in the compatibility study with PCMs. Analysis of microencapsulated phase change material slurries and phase change material emulsions as heat transfer fluid and thermal storage material  114 The experimental methodology is explained below: 1. Place 100 ml of previously agitated liquid PCM in the spherical plastic capsule. 2. Weigh the spherical capsule containing the PCM. This is done with a Mettler Toledo precision balance with an accuracy of 0.1 mg. 3. Place the spherical capsules with their corresponding PCMs in the lower part of the fridge (position 6) during 3 hours (solidification). 4. Next, place the spherical capsules with their corresponding PCMs in the upper part of the fridge (position 1) during 3 hours (melting). The thermal cycles thus take place in a similar way to the cycles in a real application. 5. These steps are repeated and the capsules checked regularly to observe possible deformations in the plastic or other phenomena. 6. The mass measurements are taken at room temperature to avoid deviations due to temperature variation. Absorbent paper is used to remove the PCM that may have migrated and the particles deposited in the walls. Each capsule is then weighed. This measurement is carried out weekly. The mass loss is calculated as: Δm=m(t0)-m(t) (eq. 4.3) Δm(%)=[m(t0)-m(t)]/m(t0) (eq. 4.4) Figure 4.16 shows the mass loss of the spheres with the different PCMs. Although in the spheres with organic PCM (EPS A8, RT6, RT9) it is observed that the PCM has a tendency to migrate through the plastic, in the case of the inorganic PCM this mass loss is not observed. However, the possible water absorption should be analyzed, which could counteract a possible mass loss. A sharp change in the slope of the mass loss is observed in the case of organic PCMs EPS A8 and RT6 from around 325 days. After 392 days, the organic PCMs EPS A8, RT6 and RT9 lost 0.422, 0.322 and 0.123% of mass, respectively. The inorganic PCM CLIMSEL C7 gained a mass of 0.017%. CHAPTER 4. Analysis of stability and compatibility of thermal energy storage systems with PCM  115 Figure 4.16 Mass loss of the plastic spheres over time for different PCMs 0 50 100 150 200 250 300 350 400 0.0 0.1 0.2 0.3 0.4 Mass loss (%) Time (s) EPS A8 RT6 RT9 CLIMSEL C7  Although not carried out in the framework of this thesis, future work could include an analysis of the possible problems of rupture of the plastic spherical capsules by thermal fatigue and weight. When they are arranged in a tank, the first layer of spheres has to support the weight of the rest of the spheres. 4.2.2 Corrosion phenomena of different metallic alloys when in contact with microencapsulated PCM slurries and PCM emulsions Corrosion can be defined as the chemical or electrochemical reaction of a metal or alloy with its environment and the subsequent deterioration of its properties. Corrosion is due to the action of electrochemical batteries, where the metal is affected by the dissolution in the anode region. Thus the process does not affect the entire metallic surface identically, as there is no attack in the cathode region. This electrochemical corrosion takes places when metallic materials are in contact with environments of electrolytic conductivity, in particular with water, saline solutions, or simply the humidity in the atmosphere and the ground. The following basic characteristics can be established in a process of electrochemical corrosion:  It occurs in the presence of an electrolyte.  It takes place at moderate temperatures (lower than 100-150ºC).  It is located in the anode region. Analysis of microencapsulated phase change material slurries and phase change material emulsions as heat transfer fluid and thermal storage material  116  The movement of the electrons takes place from the anode to the cathode through the metal.  The circuit is closed by the electrolyte through the charge transport by means of the ions.  The most usual corrosion products are hydroxides that are formed in the electrolyte, although they can later adhere to the metallic surface introducing a certain solid barrier effect between the metal and the aggressive environment, complicating the subsequent corrosion. These hydroxides are usually later transformed into oxides in the presence of oxygen. The corrosion of metallic materials in contact with water usually fits the general model: Anode region: Me ↔ Men+ + enCathode region: O2 + 2H2O + 4e- ↔ 4OHTherefore, the presence of oxygen is required so that the electrochemical process can take place. There is usually a proportional relationship between the corrosion velocity and the oxygen concentration present in the electrolyte. The amount of salts in the water, that has a significant influence on its conductivity, also determines its aggressiveness (Otero 1997). In closed water systems, corrosion can be non-existent when the oxygen is used up by the cathode reaction during the first stages. From a certain time, the lack of cathode reactive avoids the electrochemical phenomenon of corrosion. Also, hard waters are less corrosive than soft waters (Otero 1997). There are different types of corrosion. The most important types include general or uniform corrosion and galvanic corrosion. This can occur when different metals, with different redox potential, are joined electrically in the presence of an electrolyte. Regarding localized corrosion, where the metal loss occurs in localized regions, microbiological corrosion is significant, where microorganisms (that are generally found in aqueous media) act as an accelerant of the localized corrosive process. These biological organisms present in the water act on the metal surface, accelerating the corrosion phenomenon through different processes. Since PCM slurries have an aqueous basis, and water plays a crucial role in the corrosion phenomenon of metals acting as an electrolyte, an experimental analysis has been conducted to study the possible oxidation of the metals that will be in contact with PCM slurries in possible future applications. CHAPTER 4. Analysis of stability and compatibility of thermal energy storage systems with PCM  117 Several studies have already been carried out relating to the corrosion of metals in contact with PCMs, specifically in contact with hydrated salts. Porisini (1988) examined the corrosion phenomenon in different metals (stainless steel, carbon steel, copper and aluminum alloys) that contained four commercial salts. Stainless steel was the most resistant metal against corrosion. In several works by Cabeza et al. (2001 a, 2001 b, 2001 c, 2002), the corrosion rate in aluminum and copper was evaluated when in contact with molten hydrated salts. They observed that aluminum tends to show corrosion by pitting when it is in contact with chlorides, forming Al(OH)3. However, aluminum showed a strong resistance to corrosion in the case of sodium acetate. Copper was resistant to calcium chloride, but not to sodium acetate. Farrell et al. (2006) studied the corrosion phenomenon in the salts Climsel C18 and EPS E17 on copper samples (UNS C38600) and aluminum (UNS A92024) used in heat exchangers in the air conditioning field. They estimated a mass loss of 0.8 and 0.145 g/(m2·h), respectively. In the case of samples with aluminum, the sample showed a corrosion rate by pitting when in contact with Climsel C18 of 7·10-5 g/(m2·h). In the case of EPS E17, the mass loss was not significant, although the metallographic observations showed localized corrosion by pitting with a diameter of the pitting from 2.5 to 30 µm. In the case of galvanic corrosion, when combining the copper samples and the aluminum alloy, they also observed localized corrosion by pitting, concluding that the use of copper heat exchangers with aluminum fins would not be suitable. Nagano et al. (2004) conducted corrosion tests on different metals (copper, carbon steel, brass and two different types of stainless steel, UNS S30400 and UNS S31600) when in contact with magnesium nitrate hexahydrate, and when the aluminum is also in contact with a salt blend Mg(NO3)2·6H2O+10% MgCl2·6H2O. After 90 days the mass loss of each test plate was measured, observing a more severe corrosion for copper and carbon steel. The stainless steel S31600 and the aluminum showed better results. García-Romero et al. (2009) also conducted a study to analyze the corrosion phenomenon on four types of aluminum alloys (UNS A92024, UNS A93003, UNS A96063 and A91050) when in contact with commercial salts based on Glauber’s salt (Na2SO4·10H2O). The results showed that the alloy UNS A92024 was not compatible with this material due to the extensive formation of NaAlCO3·(OH)2, when in contact with air. The other three alloys were compatible with Glauber’s salt. Analysis of microencapsulated phase change material slurries and phase change material emulsions as heat transfer fluid and thermal storage material  118 Oró et al. (2012 a) also studied the corrosion effect on different metals and polymers when in contact with PCMs for low temperature applications. The PCMs used were Climsel C18 from Climator and E21 from Cristopia, the latter combined with a thickener. In addition, they developed six additional PCMs by their own formulation. The results showed that the use of copper and carbon steel in containers should be avoided. The use of stainless steel UNS S31600 was recommended. In the case of PCM slurries, there are no corrosion studies to date. It was decided to carry out such an analysis because a phenomenon of localized corrosion was observed in the thermostatic bath that contained PCM slurries. In the corrosion analysis, the corrosion rate has been studied in terms of mass loss of the metal in relation to the initial mass per square meter on an hourly basis. To accomplish this analysis, the standard G1 of the American Society for Testing and Materials has been followed. This standard is a guide to how to prepare the plates for the tests, how to clean them after the tests and how to evaluate the damage caused by the corrosion. The test plates of the metals to be analyzed were prepared according to the standard. These metals can frequently be found in thermal installations or thermal equipment. Specifically, test plates of the following materials were prepared:  Stainless steel UNS S30400  Stainless steel UNS A31600  Aluminum UNS A96061  Copper UNS C70600 These test plates were partially immersed or completely immersed in the DS 5045 slurry, and in the DS 5007 slurry contaminated by microorganisms, to study possible microbiological corrosion. Three test plates were placed in each beaker containing the mPCM slurry at room temperature. The stainless steel and aluminum test plates had dimensions of 100x25 mm with a thickness in a range from 1 to 3 mm depending on the material. The copper samples were in the form of tubes rather than plates or slabs. mPCM slurries will probably be in contact with copper when they are pumped in thermal installations. The height of these copper test tubes has been chosen so that the surface in contact with the slurry is the same as that for the stainless steel and aluminum test plates. Figure 4.17 shows the beakers containing the mPCM slurries and the test tubes CHAPTER 4. Analysis of stability and compatibility of thermal energy storage systems with PCM  119 and plates, together with a parafilm to simulate the air tightness conditions that will be present in later applications. Figure 4.17 Corrosion tests. Test specimens immersed in beakers that contain the slurry without contamination and the slurry with microbiological contamination. Aluminum Stainless steel 316 Stainless steel 304 Copper DS 5045 totally immersed DS 5045 partially immersed DS 5007 contaminated totally immersed DS 5007 contaminated partially immersed All the metallic samples were cleaned with acetone and polished with sandpaper to eliminate rougher zones. They were then washed with distilled water and dried in an oven. Once cleaned and dried, their dimensions were measured with a digital caliper (in mm with 2 decimals) and they were weighed with a precision balance (in g with 4 decimals, balance accuracy 0.1 mg). The cleaning of the corrosion products on the specimens was done by mechanical cleaning and chemical cleaning. The cleaning process must remove only the corrosion products and not the metal. For this reason special care must be taken. Mechanical processes such as scrubbing with scrubbers, scrapers or brushes remove the more embedded corrosion products but they can also remove part of the base material. This method must be used when chemical methods are unable to completely remove the corrosion. The chemical cleaning methods consist of the immersion of the specimens in a specific solution, with the minimum dissolution possible of the base metal. In Analysis of microencapsulated phase change material slurries and phase change material emulsions as heat transfer fluid and thermal storage material  120 table IV.3 the solutions used for the corrosion cleaning for each metal are shown. Material Chemical solution Aluminum and aluminum alloys 50 ml of phosphoric acid (specific gravity 1.69) 20 g of chromium trioxide Water to make 1000 ml Copper and copper alloys 500 ml of hydrochloric acid (specific gravity 1.19) Water to make 1000 ml Stainless steel 100 ml of nitric acid (specific gravity 1.42) Water to make 1000 ml Table IV.3 Chemical solutions used for the removal of the corrosion products Figure 4.18 shows the results obtained after 867 test hours, specifically the average corrosion rate in terms of mass variation per surface unit on an hourly basis. Figure 4.18 Results of the corrosion tests on the aluminum and copper specimens. Results for the stainless steel samples are not shown because corrosion phenomena were not observed after the test. The copper and aluminum samples show slight corrosion phenomena. The aluminum samples show a more significant corrosion rate. This is more noticeable in the specimens CAPÍTULO 8. Conclusiones y trabajo futuro 223 Respecto al estudio de corrosión, ninguna de las probetas de acero inoxidable analizadas se ha oxidado. Si bien las probetas de aluminio y cobre evaluadas en el marco de esta tesis sí que se han oxidado, esta oxidación ha sido poco significativa, siendo más apreciable en el caso del aluminio. Sin embargo ambos materiales podrían ser utilizados puesto que presentan buena resistencia a la corrosión. 8.1.5 Resultados sobre la transferencia de calor y mecánica de fluidos Se ha diseñado, validado y puesto en marcha una instalación experimental, especialmente diseñada para el estudio de suspensiones y emulsiones de PCM. La instalación experimental permite el estudio de viabilidad técnica de estos fluidos cuando circulan por elementos típicos de instalaciones térmicas, el estudio de transferencia de calor por convección interior y la medida de pérdida de carga en su sección de ensayo. En el proceso de validación, se ha validado la medida de pérdida de carga, la medida del flujo de calor y la medida de la temperatura de pared. En el caso de la validación de la medida de la temperatura de pared, se observaron ligeras diferencias entre la temperatura medida y la temperatura calculada teóricamente. Para corregir estas desviaciones se ha desarrollado un modelo empírico de corrección, obteniendo con este modelo una desviación en la temperatura de pared de 0.24ºC. Así la instalación experimental permite obtener los coeficientes de transferencia de calor por convección forzada interior de tipo local, con una incertidumbre de en torno al 5-10%. Se ha analizado el comportamiento de dos suspensiones de PCM, DS 5007 y DS 5045, con distintas fracciones másicas de PCM microencapsulado. El procedimiento de análisis fue en todos los casos el mismo. En primer lugar se comprobó que se cumplía el balance de energía, para descartar la posibilidad de que las microcápsulas de PCM se hubiesen depositado por los diferentes componentes de la instalación. Una vez verificado el cumplimiento del balance de energía, se llevaron a cabo las medidas de pérdida de carga y se analizó su comportamiento como fluido caloportador. Se observó en las dos suspensiones analizadas que para una misma energía térmica transportada, el trabajo de bombeo era menor que en el caso del agua. Respecto al estudio de la transferencia de calor, se observó que para obtener mejores resultados que para el caso del agua, el rango de temperaturas de operación se debía ajustar al rango de temperaturas de cambio de fase. Se observó una disminución en la temperatura de pared y una mejora en el coeficiente de convección forzada interior en comparación con el agua. En el caso de la suspensión DS 5007, Análisis de suspensiones y emulsiones de materiales de cambio de fase como fluido caloportador y material de almacenamiento térmico  224 suspensión con la que se pudo realizar un análisis más profundo, se observó que la suspensión con un 20% es la que presentaba un mejor comportamiento térmico para su uso como fluido caloportador, obteniendo una mejora en el coeficiente de convección mayor del 45% respecto del agua. Se ha observado que para mayores caudales másicos o mayores velocidades, la mejora del coeficiente de transferencia de calor por convección es menor. Este fenómeno se ha explicado a partir de los patrones de flujo de desarrollo térmico. A mayor caudal másico, mayor es la longitud de la región de entrada térmica, por lo tanto, para una posición dada, la fracción de microcápsulas de PCM fundida en esa sección es menor, aprovechando en menor medida el calor latente de fusión de las microcápsulas en suspensión. La menor mejora del fenómeno de transferencia de calor al incrementar la concentración de microcápsulas de PCM del 20 al 30% (disminución del 30 y del 70% para los caudales másicos de 20 y 50 kg/h respectivamente) también se puede atribuir al aumento de la viscosidad, y a la disminución de la conductividad térmica, disminuyendo así el grado de turbulencia y deteriorando la transferencia de calor hacia la región del núcleo de flujo respectivamente. Se ha observado en algunos casos, como para un mismo caudal másico, se obtiene tanto para el agua como para algunas de las suspensiones de PCM bajo ciertas condiciones de operación un coeficiente de convección forzada interior muy similar. Sin embargo aunque este coeficiente no mejora, se logra mantener una temperatura en el fluido más estable, en este caso en la suspensión de PCM, y como consecuencia una temperatura de pared también menor, en comparación con el agua. 8.1.6 Comportamiento térmico y técnico de sistemas de almacenamiento de energía térmica A partir de simulaciones numéricas, se ha comparado el comportamiento térmico de diferentes sistemas de almacenamiento térmico de baja temperatura: un sistema con esferas de PCM, un sistema con una suspensión de PCM y un sistema con agua. Se ha observado que el depósito con la suspensión de PCM y el depósito con esferas de menor diámetro presentan ventajas en cuanto a términos de potencia y densidad energética respecto al depósito de agua, y al almacenamiento en esferas de mayor diámetro. El depósito con la suspensión de RT6 es el que presenta una mayor densidad energética, un 35% mayor que la del sistema con esferas de 0.03 m de diámetro. Es capaz de mantener la temperatura de salida del agua por debajo CAPÍTULO 8. Conclusiones y trabajo futuro 225 de los 10ºC hasta 10 horas, frente a las 4 o 6 horas del sistema con agua y del sistema con esferas de 0.03 m de diámetro, respectivamente. En cuanto a la pérdida de carga, son el sistema de almacenamiento en agua y en suspensión de PCM los que presentan valores más elevados, hasta 100 veces superior al caso del depósito con esferas. 8.2 Difusión de resultados Durante el desarrollo de esta tesis se ha realizado un esfuerzo importante en acudir a foros científicos nacionales e internacionales para conocer el estado de arte y difundir los resultados más relevantes obtenidos. Se detallan en la tabla VIII.1 las principales contribuciones a la difusión de resultados en esta tesis, así como respecto del proyecto de I+D en el que se enmarca su desarrollo, y trabajos realizados junto con otros miembros del grupo GITSE dentro de la línea de investigación de sistemas de almacenamiento de energía mediante PCMs. Análisis de suspensiones y emulsiones de materiales de cambio de fase como fluido caloportador y material de almacenamiento térmico  226 Tema Revistas científicas internacionales Conferencias internacionales Estado del arte de suspensiones y emulsiones de PCM  Renewable and Sustainable Energy Reviews 16 (1) (2012), pp. 253-273  8 th IIR Conference on Phase-Change Materials and Slurries for Refrigeration and Air Conditioning, 2009, Karlsruhe (Alemania) Análisis experimental de suspensiones de PCM  Applied Thermal Engineering 36 (2012), pp. 370-377  1 artículo enviado a Applied Energy en proceso de revisión (enviado en Abril 2013)  11th International Conference on Thermal Energy Storage, EFFSTOCK, 2009, Estocolmo (Suecia) Propiedades  Thermochimica Acta 548 (2012), pp. 81-87. Artículo fruto de la estancia de investigación en el Instituto Fraunhofer ISE.  1 artículo de revisión sobre caracterización de PCMs en el marco del proyecto en colaboración europea eCost Action, TU 0802, preparado para enviar a revisión.  10th IIR Conference on Phase-Change Materials and Slurries for Refrigeration and Air Conditioning, 2012, Kobe (Japón) Estabilidad física de suspensiones de PCM  Artículo aceptado en International Journal of Refrigeration.  12th International Conference on Energy Storage, INNOSTOCK, 2012, Lérida (España) Intercambio PCMAgua  Energy and Buildings 47 (2012), pp. 458-466  12th International Conference on Energy Storage, INNOSTOCK, 2012, Lérida (España)  2 nd International Conference on Sustainable Energy Storage, 2013, Dublin (Irlanda) Intercambio PCMAire  Energy Procedia 30 (2012),pp. 225234  SHC 2012, International Conference on Solar Heating and Cooling for Buildings and Industry, San Francisco (EEUU)  2 nd International Conference on Sustainable Energy Storage, 2013, Dublin (Irlanda) PCMs de bajo coste  Eurosun, International Conference on Solar Heating, 2010, Graz (Austria)  12th International Conference on Energy Storage, INNOSTOCK, 2012, Lérida (España)  Congreso Internacional de Ingeniería Química de la ANQUE, 2012, Sevilla (España). Tabla VIII.1 Difusión de los resultados más relevantes en revistas científicas y conferencias internacionales. CAPÍTULO 8. Conclusiones y trabajo futuro 227 8.3 Líneas futuras Como continuación del trabajo aquí presentado se pretende desarrollar las siguientes tareas:  Estudio experimental de la convección natural en un depósito con suspensiones de PCM microencapsulado en la instalación desarrollada por Álvaro Campos Celador en el marco de su tesis doctoral para el estudio de sistemas de almacenamiento de energía térmica de tipo latente (“Integration of latent thermal energy storage systems in the design and operation of residential cogeneration plants”). Esta línea se desarrollará en el proyecto financiado por el proyecto CICYT del Plan Nacional de I+D+i ENE2011-28269-C03-01.  Obtención de una correlación experimental del número de Nusselt para la suspensión DS 5007 con distintas fracciones másicas.  Rediseño de la instalación experimental objeto del capítulo 5 para el análisis de transferencia de calor bajo condiciones de flujo turbulento.  Colaboración para la puesta en marcha del densímetro y analizador termomecánico DM 40 y TMA/STDA841e. Planteamiento de una primera metodología para la obtención de la curva Densidad-Temperatura en PCMs.  Estudio de las propiedades termofísicas, propiedades reológicas, fenómeno de transferencia de calor, mecánica de fluidos, estabilidad física de muestras nuevas de suspensiones de PCM microencapsulado.  Medida de la viscosidad según la metodología propuesta de diversos PCMs para su implementación en simulaciones numéricas, en aplicaciones fuera del ámbito de esta tesis.  Medidas de viscosidad y coeficiente de expansión volumétrica del PCM RT6 para poder analizar el efecto del a convección natural en las simulaciones numéricas presentadas en el capítulo 7.  Desarrollo del proyecto de investigación de análisis de suspensiones de PCM microencapsulado en aplicaciones de refrigeración solar y para la nivelación de la curva de demanda energética, aprovechamiento de la tarifa nocturna y disminución de la potencia a instalar, en instalaciones de refrigeración. Seleccionada en la convocatoria de 2013 de las Ayudas Análisis de suspensiones y emulsiones de materiales de cambio de fase como fluido caloportador y material de almacenamiento térmico  228 a la Investigación en Energía y Medio Ambiente de la Fundación Iberdrola.  Solicitud de un proyecto europeo del VII Programa Marco bajo el call EeB.NMP.2013-1 Nanotechnology for mutifunctional lightweight construction materials.  Participación en la extensión por tres años más de la Task 42 de la Agencia Internacional de la Energía. Encargada de la parte de reología del grupo de caracterización: Test and Characterization.  Estudio de sistemas de almacenamiento termoquímico, mediante la participación en un proyecto de I+D+i con la empresa BSH para aplicaciones en electrodomésticos. Bibliographic references 229 Bibliographic references Literature Adine, H.A. & El Qarnia, H. 2009, "Numerical analysis of the thermal behaviour of a shell-and-tube heat storage unit using phase change materials", Applied Mathematical Modelling, vol. 33, no. 4, pp. 2132-2144. Ahuja, A.S. 1975, "Augmentation of Heat Transport in Laminar-Flow of Polystyrene Suspensions .1. Experiments and Results", Journal of Applied Physics, vol. 46, no. 8, pp. 3408-3416. Ali, M.E. 1998, "Laminar natural convection from constant heat flux helical coiled tubes", International Journal of Heat and Mass Transfer, vol. 41, no. 14, pp. 2175-2182. Ali, M.E. 1994, "Experimental investigation of natural convection from vertical helical coiled tubes", International Journal of Heat and Mass Transfer, vol. 37, no. 4, pp. 665-671. Ali, S. 2001, "Pressure drop correlations for flow through regular helical coil tubes", Fluid Dynamics Research, vol. 28, no. 4, pp. 295-310. Alkan, C., Sarı, A., Karaipekli, A. & Uzun, O. 2009, "Preparation, characterization, and thermal properties of microencapsulated phase change material for thermal energy storage", Solar Energy Materials and Solar Cells, vol. 93, no. 1, pp. 143-147. Alvarado, J.L., Marsh, C., Sohn, C., Vilceus, M., Hock, V., Phetteplace, G. & Newell, T. 2006, "Characterization of supercooling suppression of microencapsulated phase change material by using DSC", Journal of Thermal Analysis and Calorimetry, vol. Volume 86, no. Number 2, pp. 505509. Alvarado, J.L., Marsh, C., Sohn, C., Phetteplace, G. & Newell, T. 2007, "Thermal performance of microencapsulated phase change material slurry in turbulent flow under constant heat flux", International Journal of Heat and Mass Transfer, vol. 50, no. 9-10, pp. 1938-1952. Arkar, C. & Medved, S. 2005, "Influence of accuracy of thermal property data of a phase change material on the result of a numerical model of a packed Analysis of microencapsulated phase change material slurries and phase change material emulsions as heat transfer fluid and thermal storage material  230 bed latent heat storage with spheres", Thermochimica Acta, vol. 438, no. 12, pp. 192-201. ASTM G1-03 (2011) Standard Practice for Preparing, Cleaning, and Evaluating Corrosion Test Specimens. Barnes, H.A. 2000, A handbook of elementary rheology, Institute of NonNewtonian Fluid Mechanics. University of Wales. Bathelt, A.G., Viskanta, R. & Leidenfrost, W. 1979, "An experimental investigation of natural convection in the melted region around a heated horizontal cylinder", Journal of Fluid Mechanics, vol. 90, pp. 227-239. Bédécarrats, J.P., Castaing-Lasvignottes, J., Strub, F. & Dumas, J.P. 2009, "Study of a phase change energy storage using spherical capsules. Part II: Numerical modelling", Energy Conversion and Management, vol. 50, no. 10, pp. 2537-2546. Bédécarrats, J.P., Strub, F., Falcon, B. & Dumas, J.P. 1996, "Phase-change thermal energy storage using spherical capsules: performance of a test plant", International Journal of Refrigeration, vol. 19, no. 3, pp. 187-196. Blumm, J. & Opfermann, J. 2002, "Improvement of the mathematical modeling of flash measurements", High Temperatures-High Pressures, vol. 34, no. 5, pp. 515-521. Blumm, J., Lindemann, A. & Min, S. 2007, "Thermal characterization of liquids and pastes using the flash technique", Thermochimica Acta, vol. 455, no. 1–2, pp. 26-29. Cabeza, L.F., Illa, J., Roca, J., Badia, F., Mehling, H., Hiebler, S. & Ziegler, F. 2001 a, "Immersion corrosion tests on metal-salt hydrate pairs used for latent heat storage in the 32 to 36ºC temperature range", Materials and Corrosion, vol. 52, no. 2, pp. 140-146. Cabeza, L.F., Illa, J., Roca, J., Badia, F., Mehling, H., Hiebler, S. & Ziegler, F. 2001 b, "Middle term immersion corrosion tests on metal-salt hydrate pairs used for latent heat storage in the 32 to 36ºC temperature range", Materials and Corrosion, vol. 52, no. 10, pp. 748-754. Bibliographic references 231 Cabeza, L.F., Roca, J., Illa, J., Badia, F., Mehling, H., Hiebler, S. & Ziegler, F. 2001 c, "Corrosion Experiments on Salt Hydrates used as Phase Change Materials in Cold Storage", International Energy Agency (IEA), ECES IA Annex 17 Workshop, Lleida (Spain). Cabeza, L.F., Roca, J., Nogués, M. & Mehling, H. 2002, "Immersion corrosion tests on metal-salt hydrate pairs for latent heat storage in the 48 to 58ºC temperature range", Materials and Corrosion, vol. 53, pp. 902-907. Campos, A. 2012, “Integration of latent thermal energy storage systems in the design and operation of residential cogeneration plants”, Ph.D. Thesis. Universidad del Pais Vasco. Cape, J.A. & Lehman, G.W. 1963, "Temperature and finite pulse-time effects in the flash method for measuring thermal diffusivity", Journal of Applied Physics, vol. 34, no. 7, pp. 1909-1917. Carreau, J.P. 1972, "Rheological Equations from Molecular Network Theories", Transactions of the Society of Rheology, vol. 16, no. 1, pp. 99-127. Carslaw, H.S. & Jaeger, J.C. 1986, Conduction of Heat in Solids, Oxford Science Publications. Castellón, C., Martorell, I., Cabeza, L.F., Fernández, A.I. & Manich, A.M. 2011, "Compatibility of plastic with phase change materials (PCM)", International Journal of Energy Research, vol. 35, no. 9, pp. 765-771. Charunyakorn, P., Sengupta, S. & Roy, S.K. 1991, "Forced convection heat transfer in microencapsulated phase change material slurries: flow in circular ducts", International Journal of Heat and Mass Transfer, vol. 34, no. 3, pp. 819-833. Chen, B., Wang, X., Zeng, R., Zhang, Y., Wang, X., Niu, J., Li, Y. & Di, H. 2008, "An experimental study of convective heat transfer with microencapsulated phase change material suspension: Laminar flow in a circular tube under constant heat flux", Experimental Thermal and Fluid Science, vol. 32, no. 8, pp. 1638-1646. Chen, B., Wang, X., Zhang, Y., Xu, H. & Yang, R. 2006, "Experimental research on laminar flow performance of phase change emulsion", Applied Thermal Engineering, vol. 26, no. 11-12, pp. 1238-1245. Analysis of microencapsulated phase change material slurries and phase change material emulsions as heat transfer fluid and thermal storage material  232 Chen, S., Chen, C., Tin, C., Lee, T. & Ke, M. 2000, "An experimental investigation of cold storage in an encapsulated thermal storage tank", Experimental Thermal and Fluid Science, vol. 23, no. 3-4, pp. 133-144. Chidambaram, L.A., Ramana, A.S., Kamaraj, G. & Velraj, R. 2011, "Review of solar cooling methods and thermal storage options", Renewable and Sustainable Energy Reviews, vol. 15, no. 6, pp. 3220-3228. Cho, Y.I., Choi, I. & Lorsch, H.G. 1991, "A novel concept for heat transfer fluids used in district cooling systems", Progress Report. Development of advanced low-temperature heat transfer fluids for district heating and cooling. The U.S. Department of Energy. Cho, K. & Choi, S.H. 2000, "Thermal characteristics of paraffin in a spherical capsule during freezing and melting processes", International Journal of Heat and Mass Transfer, vol. 43, no. 17, pp. 3183-3196. Choi, E. 1993, “Forced convection heat transfer with water and phase-change material slurries: turbulent flow in a circular tube”, Ph.D. Thesis. Drexel University Choi, E., Cho, Y.I. & Lorsch, H.G. 1994, "Forced convection heat transfer with phase-change-material slurries: turbulent flow in a circular tube", International Journal of Heat and Mass Transfer, vol. 37, no. 2, pp. 207. Choi, E., Cho, Y.I. & Lorsch, H.G. 1991, "Effects of emulsifier on particle size of a phase change material in a mixture with water", International Communications in Heat and Mass Transfer, vol. 18, no. 6, pp. 759-766. Choi, M. & Cho, K. 2001, "Effect of the aspect ratio of rectangular channels on the heat transfer and hydrodynamics of paraffin slurry flow", International Journal of Heat and Mass Transfer, vol. 44, no. 1, pp. 55-61. Churchill, S.W. 1983, Heat Exchanger Design Handbook. Free convection around immersed bodies. Hemisphere Publishing Corp., New York. Clark, L.M. & Taylor, R.E. 1975, "Radiation loss in the flash method for thermal diffusivity", Journal of Applied Physics, vol. 46, no. 2, pp. 714-719. Bibliographic references 239 Lorsch, H.G., Murali, K. & Cho, Y.I. 1997 a, "Improving thermal and flow properties of chilled water-Part 1: Material Selection and Instrument Calibration", ASHRAE Transactions, vol. 103, no. 1, pp. 188-197. Lu, W. & Bai, F. 2004, "A new model for analyzing laminar forced convective enhanced heat transfer in latent functionally thermal fluid", Chinese Science Bulletin, vol. 49, no. 14, pp. 1457-1463. Lu, W. & Tassou, S.A. 2012, "Experimental study of the thermal characteristics of phase change slurries for active cooling", Applied Energy, vol. 91, no. 1, pp. 366-374. Manlapaz, R.L. & Churchill, S.W. 1981, "Fully-developed laminar convection from a helical-coil", Chemical Engineering Communications, vol. 9, no. 1-6, pp. 185-200. Manual of Handling and storage of polymer dispersions. BASF. Medrano, M., Yilmaz, M.O., Nogués, M., Martorell, I., Roca, J. & Cabeza, L.F. 2009, "Experimental evaluation of commercial heat exchangers for use as PCM thermal storage systems", Applied Energy, vol. 86, no. 10, pp. 20472055. Mehling, H. & Cabeza, L.F. 2008, Heat and cold storage with PCM. An up to date introduction into basics and applications, Springer, Heat and Mass Transfer. Monllor Pérez, P. 2007, Caracterización de microencapsulados aplicados sobre materiales textiles, Ph.D. thesis. Universidad Politécnica de Valencia. Moreno Botella, R. 2006, Curso de Reología Aplicada. Viscoelasticidad, 23-24 Febrero 2006, Universidad de Zaragoza Mukhametzyanov, G.K., Usmanov, A.G. & Tarzimanov, A.A. 1963, "Determinations of the thermal conductivity of liquid saturated hydrocarbons", Izv. Vyssh. Ucheb. Zaved. Neft. Gaz, vol. 6, no. 9, pp. 7579. Nagano, K., Ogawa, K., Mochida, T., Hayashi, K. & Ogoshi, H. 2004, "Performance of heat charge/discharge of magnesium nitrate hexahydrate and magnesium chloride hexahydrate mixture to a single vertical tube for a Analysis of microencapsulated phase change material slurries and phase change material emulsions as heat transfer fluid and thermal storage material  240 latent heat storage system", Applied Thermal Engineering, vol. 24, no. 2–3, pp. 209-220. Naphon, P. & Wongwises, S. 2006, "A review of flow and heat transfer characteristics in curved tubes", Renewable and Sustainable Energy Reviews, vol. 10, no. 5, pp. 463-490. Oró, E., Miró, L., Barreneche, C., Martorell, I., Farid, M.M. & Cabeza, L.F. 2012 a, "Corrosion of metal and polymer containers for use in PCM cold storage", Applied Energy, In Press, Corrected Proof. Oró, E., Gil, A., Miró, L., Peiró, G., Álvarez, S. & Cabeza, L.F. 2012 b, "Thermal Energy Storage Implementation Using Phase Change Materials for Solar Cooling and Refrigeration Applications", Energy Procedia, vol. 30, pp. 947956. Otero, E. 1997, “Corrosión y degradación de materiales”, Editorial Síntesis. Parker, W., Jenkings, R.J., Butler, C.P. & Abbot, G.L. 1961, "Flash Method of Determining Thermal Diffusivity, Heat Capacity, and Thermal Conductivity", Journal of Applied Physics, vol. 32, no. 9, pp. 1679-1684. Perry, R.H. & Green, D.W. 1997, Perry’s Chemical Engineers’ Handbook, Mc Graw Hill. Pimenta, T.A. & Campos, J.B.L.M. 2012, "Friction losses of Newtonian and nonNewtonian fluids flowing in laminar regime in a helical coil", Experimental Thermal and Fluid Science, vol. 36, pp. 194-204. Pollerberg, C. & Dötsch, C. 2006, "Phase Changing Slurries in cooling and cold supply networks", 10th International Symposium on Disctrict Heating and Cooling, 3-5 September, 2006, Hannover (Germany). Porisini, F.C. 1988, "Salt hydrates used for latent heat storage: Corrosion of metals and reliability of thermal performance", Solar Energy, vol. 41, no. 2, pp. 193-197. Rady, M. 2009, "Thermal performance of packed bed thermal energy storage units using multiple granular phase change composites", Applied Energy, vol. 86, no. 12, pp. 2704-2720. Bibliographic references 241 Rao, Y., Dammel, F., Stephan, P. & Lin, G. 2007, "Convective heat transfer characteristics of microencapsulated phase change material suspensions in minichannels", Heat and Mass Transfer, vol. Volume 44, no. 2, pp. 175186. Rao, Y., Dammel, F., Stephan, P. & Lin, G. 2006, "Flow frictional characteristics of microencapsulated phase change material suspensions flowing through rectangular minichannels", Science in China, vol. 49, no. 4, pp. 445-456. Rieger, H., Projahn, U., Bareiss, M. & Beer, H. 1983, "Heat transfer during melting inside a horizontal tube", Journal of Heat Transfer, vol. 105, no. 2, pp. 226-234. Rogers, G.F. & Mayhew, Y.R. 1964, "Heat transfer and pressure loss in helically coiled tubes with turbulent flow", International Journal of Heat and Mass Transfer, vol. 7, no. 11, pp. 1207-1216. Roy, S.K. & Avanic, B.L. 1997, "Laminar forced convection heat transfer with phase change material emulsions", International Communications in Heat and Mass Transfer, vol. 24, no. 5, pp. 653-662. Roy, S.K. & Avanic, B.L. 2001 a, "Laminar forced convection heat transfer with phase change material suspensions", International Communications in Heat and Mass Transfer, vol. 28, no. 7, pp. 895-904. Roy, S.K. & Avanic, B.L. 2001 b, "Turbulent heat transfer with phase change material suspensions", International Journal of Heat and Mass Transfer, vol. 44, no. 12, pp. 2277-2285. Royon, L., Perrot, P., Guiffant, G. & Fraoua, S. 1998, "Physical properties and thermorheological behaviour of a dispersion having cold latent heat-storage material", Energy Conversion and Management, vol. 39, no. 15, pp. 15291535. Royon, L. & Guiffant, G. 2008, "Forced convection heat transfer with slurry of phase change material in circular ducts: A phenomenological approach", Energy Conversion and Management, vol. 49, no. 5, pp. 928-932. Sabbah, R., Farid, M.M. & Al-Hallaj, S. 2009, "Micro-channel heat sink with slurry of water with micro-encapsulated phase change material: 3D- Analysis of microencapsulated phase change material slurries and phase change material emulsions as heat transfer fluid and thermal storage material  242 numerical study", Applied Thermal Engineering, vol. 29, no. 2-3, pp. 445454. Salaün, F., Devaux, E., Bourbigot, S. & Rumeau, P. 2008, "Development of a precipitation method intended for the entrapment of hydrated salt", Carbohydrate Polymers, vol. 73, no. 2, pp. 231-240. Sarı, A., Alkan, C., Karaipekli, A. & Uzun, O. 2009, "Microencapsulated noctacosane as phase change material for thermal energy storage", Solar Energy, vol. 83, no. 10, pp. 1757-1763. Schalbart, P., Kawaji, M. & Fumoto, K. 2010, "Formation of tetradecane nanoemulsion by low-energy emulsification methods", International Journal of Refrigeration, vol. 33, no. 8, pp. 1612-1624. Schmidt, E.F. 1967, "Wärmeübergang und Druckverlust in Rohrschbugen", Chemie Ingenieur Technik, vol. 39, no. 13, pp. 781-789. Schmidt, M. 2008, "Phase Change Materials-latent heat storage for interior climate control. BASF Micronal" Energiforum Danmark. Seban, R.A. & McLaughlin, E.F. 1963, "Heat transfer in tube coils with laminar and turbulent flow", International Journal of Heat and Mass Transfer, vol. 6, no. 5, pp. 387-395. Shibutani, S. 2002, "PCM-micro Capsule Slurry Thermal Storage System for Cooling in Narita Airport", Proceedings of 3rd Experts meeting and Workshop of IEA Annex 17, Tokyo (Japan). Sparrow, E.M., Schmidt, R.R. & Ramsey, J.W. 1978, "Experiments on the Role of Natural Convection in the Melting of Solids", Journal of Heat Transfer, vol. 100, no. 1, pp. 11-16. Streicher, W., Cabeza, L.F. & Heinz, A. 2005, Inventory of Phase Change Materials. A report of IEA Solar Heating and Cooling programme –Task 32 “Advanced storage concepts for solar and low energy buildings” Report C2 of Subtask C. Su, J., Huang, Z. & Ren, L. 2007 a, "High compact melamine-formaldehyde microPCMs containing n-octadecane fabricated by a two-step coacervation Bibliographic references 243 method", Colloid & Polymer Science, vol. Volume 285, no. 14, pp. 15811591. Su, J., Ren, L. & Wang, L. 2005, "Preparation and mechanical properties of thermal energy storage microcapsules", Colloid & Polymer Science, vol. 284, no. 2, pp. 224-228. Su, J., Wang, L. & Ren, L. 2007 b, "Synthesis of polyurethane microPCMs containing n-octadecane by interfacial polycondensation: Influence of styrene-maleic anhydride as a surfactant", Colloids and Surfaces A: Physicochemical and Engineering Aspects, vol. 299, no. 1-3, pp. 268-275. Sug Lee, J. & Ogawa, K. 1974, "Pressure drop through packed beds", Journal of Chemical Engineering of Japan, vol. 27, no. 5, pp. 691-693. TA Instruments 2011, Seminario de Reología y Viscoelasticidad, 7-8 Junio, Madrid. Tadros, T. 2004, "Application of rheology for assessment and prediction of the long-term physical stability of emulsions", Advances in Colloid and Interface Science, vol. 108-109, pp. 227-258. Tallmadge, J.A. 1970, "Packed bed pressure drop-an extension to higher Reynolds numbers", AIChE Journal, vol. 16, no. 6, pp. 1092-1093. Tipvarakarnkoon, T., Blochwitz, R. & Senge, B. 2008, "Rheological properties and phase change behaviors of coconut fats and oils", Annual Transactions of the Nordic Rheology Society vol. 16. Trp, A., Lenic, K. & Frankovic, B. 2006, "Analysis of the influence of operating conditions and geometric parameters on heat transfer in water-paraffin shell-and-tube latent thermal energy storage unit", Applied Thermal Engineering, vol. 26, no. 16, pp. 1830-1839. Tumuluri, K., Alvarado, J.L., Taherian, H. & Marsh, C. 2011, “Thermal performance of a novel heat transfer fluid containing multiwalled carbon nanotubes and microencapsulated phase change materials”, International Journal of Heat and Mass Transfer, vol. 54, pp. 5554-5567. Wakao, N. & Funazkri, T. 1978, "Effect of fluid dispersion coefficients on particle-to-fluid mass transfer coefficients in packed beds: Correlation of Analysis of microencapsulated phase change material slurries and phase change material emulsions as heat transfer fluid and thermal storage material  244 sherwood numbers", Chemical Engineering Science, vol. 33, no. 10, pp. 1375-1384. Wang, X. & Niu, J. 2009, "Performance of cooled-ceiling operating with MPCM slurry", Energy Conversion and Management, vol. 50, no. 3, pp. 583-591. Wang, X., Niu, J., Li, Y., Wang, X., Chen, B., Zeng, R., Song, Q. & Zhang, Y. 2007, "Flow and heat transfer behaviors of phase change material slurries in a horizontal circular tube", International Journal of Heat and Mass Transfer, vol. 50, no. 13-14, pp. 2480-2491. Wei, J., Kawaguchi, Y., Hirano, S. & Takeuchi, H. 2005, "Study on a PCM heat storage system for rapid heat supply", Applied Thermal Engineering, vol. 25, no. 17–18, pp. 2903-2920. White, C.M. 1929, "Streamline flow through curved pipes", Proceedings of the Royal Society A: Mathematical, physical & Engineering Sciences, vol. 123, pp. 645-663. World Energy Outlook 2012. International Energy Agency. Executive Summary. Wu, S. & Fang, G. 2011, "Dynamic performances of solar heat storage system with packed bed using myristic acid as phase change material", Energy and Buildings, vol. 43, no. 5, pp. 1091-1096. Xing, K.Q., Tao, Y.X. & Hao, Y.L. 2005, "Performance evaluation of liquid flow with PCM particles in microchannels", Journal of heat transfer, vol. 127, no. 8, pp. 931-940. Xuan, Y., Huang, Y. & Li, Q. 2009, "Experimental investigation on thermal conductivity and specific heat capacity of magnetic microencapsulated phase change material suspension", Chemical Physics Letters, vol. 479, no. 4-6, pp. 264-269. Yamagishi, Y., Sugeno, T., Ishige, T., Takeuchi, H. & Pyatenko, T. 1996, "An evaluation of microencapsulated PCM for use in cold energy transportation medium", Energy Conversion Engineering Intersociety Conference-IECEC, vol. 3, pp. 2077-2083. Bibliographic references 245 Yamagishi, Y., Takeuchi, H., Pyatenko, A.T. & Kayukawa, N. 1999, “Characteristics of Microencapsulated PCM Slurry as a Heat-Transfer Fluid”, AICHe Journal, vol. 45, no. 4, pp. 696-707 Yang, R., Xu, H. & Zhang, Y. 2003, "Preparation, physical property and thermal physical property of phase change microcapsule slurry and phase change emulsion", Solar Energy Materials and Solar Cells, vol. 80, no. 4, pp. 405416. Zeng, R., Wang, X., Chen, B., Zhang, Y., Niu, J., Wang, X. & Di, H. 2009, "Heat transfer characteristics of microencapsulated phase change material slurry in laminar flow under constant heat flux", Applied Energy, vol. 86, no. 12, pp. 2661-2670. Zhang, Y. & Faghri, A. 1995, "Analysis of forced convection heat transfer in microencapsulated phase change material suspensions", Journal of Thermophysics and Heat Transfer, vol. 9, no. 4, pp. 727-732. Zhang, G.H. & Zhao, C.Y. 2011, "Thermal and rheological properties of microencapsulated phase change materials", Renewable Energy, vol. 36, no. 11, pp. 2959-2966. Zhang, P., Ma, Z.W. & Wang, R.Z. 2010, "An overview of phase change material slurries: MPCS and CHS", Renewable and Sustainable Energy Reviews, vol. 14, no. 2, pp. 598-614. Zhang, X.X., Fan, Y.F., Tao, X.M. & Yick, K.L. 2004 a, "Fabrication and properties of microcapsules and nanocapsules containing n-octadecane", Materials Chemistry and Physics, vol. 88, no. 2-3, pp. 300-307. Zhang, X., Fan, Y., Tao, X. & Yick, K. 2005, "Crystallization and prevention of supercooling of microencapsulated n-alkanes", Journal of colloid and interface science, vol. 281, no. 2, pp. 299-306. Zhang, X., Xiao-Ming, T., Kit-Lun, Y. & Xue-Chen, W. 2004 b, "Structure and thermal stability of microencapsulated phase-change materials", Colloid & Polymer Science, vol. 282, no. 4, pp. 330-336. Zhang, Y., Jiang, Y. & Jiang, Y. 1999, “A simple method, the T-history method, of determining the heat of fusion, specific heat and thermal conductivity of Analysis of microencapsulated phase change material slurries and phase change material emulsions as heat transfer fluid and thermal storage material  246 phase-change materials”, Measurement Science and Technology, vol. 10, pp. 201-205. Zhao, Z., Hao, R. & Shi, Y. 2008, "Parametric analysis of enhanced heat transfer for laminar flow of microencapsulated phase change suspension in a circular tube with constant wall temperature", Heat Transfer Engineering, vol. 29, no. 1, pp. 97-106. Zou, D., Feng, Z., Xiao, R., Qin, K., Zhang, J., Song, W. & Tu, Q. 2010, "Preparation and flow characteristic of a novel phase change fluid for latent heat transfer", Solar Energy Materials and Solar Cells, vol. 94, no. 12, pp. 2292-2297. Web sites Climator (fecha de acceso 8 de marzo de 2013) http://www.climator.com/en/home/ COST Action TU0802 (fecha de acceso 8 de marzo de 2013) http://www.cost.eu/domains_actions/tud/Actions/TU0802 EPS-Environmental Process Systems LTD (fecha de acceso 8 de marzo de 2013) http://www.epsltd.co.uk/pcm.htm Eurostat. European Commission. Energy production and imports (fecha de acceso 8 de marzo de 2013) http://epp.eurostat.ec.europa.eu/statistics_explained/index.php/Energy_producti on_and_imports IEA Task Annex 42 24 (fecha de acceso 8 de marzo de 2013) http://task42.iea-shc.org/ Microcápsulas de Microtek Laboratories (fecha de acesso 8 de marzo de 2013) http://www.microteklabs.com/micropcm.html Micronal PCM website (fecha de acceso 8 de marzo de 2013) http://www.micronal.de/portal/basf/ien/dt.jsp?setCursor=1_290798 National Institute of Standards and Technology website (fecha de acceso 8 de marzo de 2013) Bibliographic references 247 http://www.nist.gov/index.html Producto Thermusol de Salca BV (fecha de acesso 8 de marzo de 2013) http://www.microteklabs.com/micropcm.html Rubitherm (fecha de acceso 8 de marzo de 2013) http://www.rubitherm.de/english/index.htm Sasol Company-Parafol paraffins (fecha de acceso 8 de marzo de 2013) http://www.sasoltechdata.com/MarketingBrochures/PARAFOL.pdf Analysis of microencapsulated phase change material slurries and phase change material emulsions as heat transfer fluid and thermal storage material  248 APPENDIX II. Technical specifications of the equipments and calibration certificates  255 APPENDIX II. Technical specifications of the equipments and calibration certificates                Analysis of microencapsulated phase change material slurries and phase change material emulsions as heat transfer fluids and thermal storage material   256   KROHNE Ca libration Certifi cate - Kalibrier¿ertifikat - Certificat d'éta lonnage DIN 55 350-18-4.2.2 Typerfyp/Type Sales Order / VK-A!ftrág / Comñande de venle Senal Number / Senennumrner / N!méro de série Tag Nr¡mber / fagnur¡¡mer i ReÉÉre OPTIMASS TOOO 506 410001609 c 1 000000021 022m 27ú857t203o707 Water/Wasser/Eau 0.035% Cal¡brat¡on Method / Kalibr¡ermethode / Méthod€ d étalonnage The calrbrahon was perto¡med In mass flow rgs us¡ng werghrng scales |n start / stop operatron All we€hrng sc€¡es are pe¡rodically calrbrated by l¡lernationaily accredited laboratories Die Kallbrierung wurde an lV!assedurchflussstánde¡ mrl Waagen im Start / Slop-Belrieb durchgefuhd Alle Waagen werden regelmáBrg durch ¡nternal¡on¿l akkredrtene Prüflabofe kalrbriert. L étalonnage a elé réalisé sur un banc utilisanl des pesons de référence avec plusaeura pesées success¡ves Tous les pesoñs sont contrólés eéaod|quement par d6s laboratgrres Internatronaux accredrtés Test Equipment Oata / Kalibr¡erstand / Données du banc d'étalonnage Serial Number / Serien nummer ,/ ñuméro de série CaIbratron flurd / KaIbrierflüssrgkert / Flurde d élalonnage lJncerlarnly 1 Messunsrcherheit / Incerlitude Calibration Results / Kal¡brierergebnis / Résultats d étalonnage Set Flow rate gewáhtter Ourchfluss (kg/h) Measured Mass gernessene Masse Masse mesurée (ks) ActualMass tatsáchliche Masse lflasse réelle (Kg) Deviel¡on Abwe¡chuñg Ec€ rt % 157 4.50 4.122 0.069 0.138 723001 '10.52634 12.€614€ 7.29893 10.51906 12.64401 Cal¡brat¡on Oata / Kal¡b.ierdaten / Données d'étalonnage CF1 19.2 CF6: 86.991142 cF11 56.636711 CF15: 0.0000000 CF21: 0.0000000 CF26 0.0000000 DCFI 2 DCF6 0.0000000 cF2 502.95 cF7 1381.3405 cF12 -153.71669 CF17: 0.0000000 CF22: 0.0000000 cF27 0.0000000 DCF2 998.89819 DCFT 1.0000000 CF3 2m.19 cF8 -2u.5B€34 cF13 -252.01645 cF18 0.0000000 CF23: 0.0000000 DCF3 1.0000000 DCFg 74.013733 CF4 154.50858 cF14 418.9673 cF19 -3.0490146 cF24 0.0000000 DCF4 79.1,16706 DNlO PN.lo to D|N25O1 ER3.3.5_ s/N: 10174384 2.2.0 S/N: 10193077 201043-27 CF5 1¡1003.025 CF 1 5 59.,10924'1 cF20 5.m93|08 CF25: 0 DCFs O Additional Oata / Zusahdaten / Données complémentaires Procéss Connecirons / Prozessanschlüssen / Raccords process Eleclronrc Revisroñ / Elektronrk Revtsion / Version électron¡qle Sensor Electronics / Sensorelektronik / Electronioue de mesure Calibration Date / Ka|br¡erdatum / Date d ételonnaoe ¡rd.r rj1r. , esi !¡lJe !!¡s s r.ál-.re KRONNE Lrrl Rurhdlúd ttrv.. Pr* Fñ h.rdfir €s*, ur!¡rñgavq¡g}! ¡¡d0'¡b tJr¡85a8 f¡t. +.¡,tlo) 193 rr)ale  Entre la entrada, la medida y la salida -relés, analógica, RS 485 ó RS 232 Datos técnicos Pág. 4 Información del aparato Capítulo 1 Alimentación auxiliar Valor nominal: Margen de frecuencia: Consumo: Visualizador Resolución del conversor A/D c.c.: Método de conversión: Tiempo de conversión: Núm. de conversiones por muestra: Precisión de la medida: Margen de medida: Sobrecarga de tensión: Sobrecarga de corriente: Consumo de tensión: Consumo de corriente: Aislamiento Tensión de prueba: Test de pulsaciones: -9999 ... 9999 7 segmentos 4 dígitos de 14 mm de altura Color rojo Indicación exceso de escala: "----" 8 leds de indicación Punto decimal programable Ciclo de presentación: 500 ms -9999 ... 9999 Circuito de entrada Salida analógica Tipos de salidas: Impedancia de carga: Tiempo de respuesta: Ripple - RMS: Clase de precisión: Especificación del error: Coeficiente de temperatura: Condiciones ambientales Características generales Normativas de diseño Temperatura de almacenamiento: Temperatura de trabajo: Características de los relés Intensidad nominal c.a.: Intensidad máxima c.a.: Tensión nominal: Tensión máxima (VDE 0435): Potencia máxima de conmutación de una carga resistiva: Resistencia de aislamiento 500V: Aislamiento contacto-bobina: Aislamiento contacto-contacto: Esperanza de vida mecánica: Esperanza de vida eléctrica: Características de los relés Intensidad nominal c.a.: Intensidad máxima c.a.: Tensión nominal: Resistencia de aislamiento 500V: Aislamiento contacto-bobina: Aislamiento contacto-contacto: Esperanza de vida mecánica: Esperanza de vida eléctrica: Dimensiones: Peso: Material de la caja: Indice de protección: : D Datos técnicos Pág. 5 Información del aparato Capítulo 1 UC040T-H Refrigerador de circulación con máquina de enfriamiento refrigerada por aire y bomba de circulación (de plástico ). Carcasa y recipiente de expansión abierto a la atmósfera con evaporador (Refrigerador) soldado en cobre de acero inoxidable. Con medidor digital de nivel. Para applicaciones cerradas externamente. Con protección ajustable contra sobretemperatura según DIN12876. Caso especial acetona y poliglicol: La bomba de plástico no es resistente contra acetona y poliglicol (dependiente del fabricante). Se recomienda usar agua con glysantin o etilenglicol como protección anticongelante. Se puede suministrar un plástico adecuado de mayor resistencia por solicitud y a costo adicional. CC-Pilot: Controlador ultra moderno con la nueva tecnología innovativa E-grade para funcionalidades extendidas sin cambiar de controlador. Un código de activación es ingresado vía el panel de control y la probada tecnología Pulg & Play para servicio profesional. La brillante pantalla TFT muestra todos los datos relevantes de proceso. Interfaz de usuario amigable: Las funciones auto-explicativas están listadas en orden alfabético en cada uno de los idiomas seleccionados. Los idiomas disponibles son: alemán, inglés, francés, italiano, español y ruso. Easy Control: es virtualmente idéntica a la de los Unistat. La función ampliar (zoom) permite que los valores sean leídos a distancia. Resolución del indicador en la versión básica de 0,1K. Límites de valor de ajuste, alarma acústica y óptica, función automática en caso de fallo de la red eléctrica. Sensor calibrable, control por medio de la interfase RS232 y ComG@te Namur (opcional) por ej. Para conexión a un sistema de control de proceso, como control remoto por medio de un cable de datos. La funcionalidad puede ser extendida en cualquier momento mediante el código de activación con E-grade (opcional). E-grade “Exclusive”: Función gráfica, resolución del indicador de 0.01K, programador con 3 programas cada uno con 5 pasos, control de modo de temperatura (interno, proceso), TAC (True adaptive Control - Control Adaptativo Verdadero), regulador automático interno de optimización y cascada de control, función de rampa. E-grade “Profesional: Función de administrador, programador con 100 pasos divisibles entre 10 programas, http://www.huber-online.com/html/produkte/detail_druck.html?&art=1... 1 de 3 24/08/2010 10:15 Rango de temperatura de trabajo -10...100 °C Indicador de temperatura digital Ajuste de temperatura digital Sensor de temperatura interno Pt100 Conexión de sonda externa Pt100 Interfaz analógica E/S (via ComG@te)0/4-20mA o 0-10V Clasificación de seguridad Clase III / FL Potencia calorífica 2 kW Potencia de enfriamiento a 15°C 4 kW a 0°C 2.5 kW a -10°C 1.5 kW Máquina frigorífica refrigerada por aire, libre de CFC y H-CFC Refrigerante R507 Cantidad de refrigerante 2.2 kg Bomba de circulación: B Descarga máx. 27 l/min Presión de descarga máx. 3,0 bar Descarga a 0,2 bar 22 l/min Descarga a 0,5 bar 20 l/min Descarga a 1.0 bar 17 l/min Descarga a 2.0 bar 10 l/min Descarga a 3.0 bar 5 l/min Conexión de bomba 3/4" Volumen de llenado min. 3.5 l. Volumen de expansión 3.5 l. Dimensiones A x L x Alto 500x552x1451 mm Peso neto 138 kg Suministro de energía trifasíca 400V 3~N 50Hz Consumo máx. de corriente trifásica 8.5 A Fusible (trifásico) 3x10 A Temperatura ambiental min. 5 °C Temperatura ambiental máx. 40 °C control externo por medio de un sensor Pt 100 (opcional) NLR (non-linear ramping - Rampeo no-lineal) para ciclos de temperatura no lineales, segundo valor nominal, el cual puede ser activado en caso de una alarma bajo condiciones predefinidas, calibración de más puntos para el sensor del regulador. Para E-grade “Professional” se requiere contar con E-grade “Exclusive”. 3-2-1 garantia - Requiere registro UC040T-H Número de pedido: 3014.0003.04 Grupo de descuento : 3 Precio : 8270 EUR (* Precio ex fábrica en Offenburg, Alemania ) Accesorios y periféricos: Conector de manguera3/4"*, , Tapa para tanque de expansión*, Válvula de paso o cierre, Mangueras de conexión, , ComG@te. http://www.huber-online.com/html/produkte/detail_druck.html?&art=1... 2 de 3 24/08/2010 10:15 * equipo estándar Lecturas de salida válidas con temperatura ambiente de 20°C Nos reservamos el derecho de cambios técnicos y de errores sin previo aviso Última actualización : 24-08-2010 http://www.huber-online.com/html/produkte/detail_druck.html?&art=1... 3 de 3 24/08/2010 10:15  É 3 = !.i-t .!- o = , :-::- :-:.-- :--_-_ : :. l-=.\ . --:'.- --l \ APPENDIX III. Numerical models of simulation in EES of TES systems  271 APPENDIX III. Numerical models of simulation in EES of TES systems                Analysis of microencapsulated phase change material slurries and phase change material emulsions as heat transfer fluid and thermal storage material   272   File:G:\Tesis Doctoral\Anexos\Modelo en EES depósito esferas.EES 15/03/2013 12:53:10 Page 1 EES Ver. 9.215: #3470: For use only by students and faculty in the Departamento de Ingenieria Mecanica Universidad de Za {Datos del tanque y de las esferas} R=0,022 {radio de la esfera de PCM} npisos=6+1 {nº de pisos calculado previamente por geometría} Dtanque=0,24 {diámetro del tanque} Ltanque=0,55 {altura del tanque} Vesferas=ntotal_esferas*4*pi#*R^3/3 {volumen que ocupan las esferas} Vtanque=pi#*(Dtanque/2)^2*Ltanque {volumen del tanque} porosidad=1-Vesferas/Vtanque {porosidad del tanque} nesferas_prim_piso=19 {número de esferas en la primera fila de cada piso calculado previamente por geometría} nesferas_piso=31 {número de esferas en cada piso calculado previamente por geometría} ntotal_esferas=31*npisos {número total de esferas en el depósito} masa_PCM=Vesferas*densidad {masa de PCM} {Propiedades del PCM} landa_PCM=0,2 {conductividad del PCM} densidad_PCM=800 {densidad del PCM} {Propiedades del agua} densidad_agua=Density(Water;T=13;P=100) {densidad del agua} landa_agua=Conductivity(Water;T=13;P=100) {conductividad termica del agua} visc_agua=Viscosity(Water;T=13;P=100) {viscosidad dinámica del agua} Cp_agua=Cp(Water;T=13;P=100) {Calor específico del agua} magua=densidad_agua*vel_media_intersticial*porosidad*pi#*Dtanque^2/4 {caudal másico del agua} vel_media_intersticial=0,01 {velocidad media intersticial del agua} vel_media=vel_media_intersticial*porosidad {velocidad media del agua} {Curva del calor especifico del PCM} Cps=3000 hm=132000 sigma=0,85 Tm=6,7 {Discretización espacial en las esferas} nx=20 {nº de nodos espaciales en la esfera de PCM} Duplicate i=0;nx r[i]=i*R/nx end {Tabla paramétrica para el tiempo} At=30 {incremento temporal} tiempo=(fila-1)*At Duplicate j=1;npisos Duplicate i=0;nx Tant[i;j]=Tablevalue('Evol_temp';fila-1;#T[i;j]) end end {Nodo central} Duplicate j=1;npisos Duplicate i=0;0 (landa_PCM/r[i+1])*(T[i+1;j]-T[i;j])*4*pi#*(r[i+1]/2)^2=densidad_PCM*(Cps+(hm/sigma/6,28^0,5)*2,71^(-0,5*((Tant[i;j]-Tm)/ sigma)^2))*(T[i;j]-Tant[i;j])/At*4/3*pi#*(r[i+1]/2)^3 end end {Nodo intermedio} Duplicate j=1;npisos Duplicate i=1;nx-1 4*pi#*landa_PCM*((T[i+1;j]-T[i;j])/(r[i+1]-r[i])*((r[i+1]+r[i])/2)^2-(T[i;j]-T[i-1;j])*((r[i]+r[i-1])/2)^2/(r[i]-r[i-1]))=(densidad_PCM/At) *(Cps+(hm/sigma/6,28^0,5)*2,71^(-0,5*((Tant[i;j]-Tm)/sigma)^2))*(T[i;j]-Tant[i;j])*(4/3)*pi#*(((r[i+1]+r[i])/2)^3-((r[i]+r[i-1])/2)^ 3) end end File:G:\Tesis Doctoral\Anexos\Modelo en EES depósito esferas.EES 15/03/2013 12:53:10 Page 2 EES Ver. 9.215: #3470: For use only by students and faculty in the Departamento de Ingenieria Mecanica Universidad de Za {Nodo exterior} Duplicate j=1;npisos Duplicate i=nx;nx -4*pi#*landa_PCM*(T[nx;j]-T[nx-1;j])*(((r[nx]+r[nx-1])/2)^2)/(r[nx]-r[nx-1])+hconv*(Tagua[j]-T[nx;j])*4*pi#*r[nx]^2=( densidad_PCM*(Cps+(hm/sigma/6,28^0,5)*2,71^(-0,5*((Tant[i;j]-Tm)/sigma)^2))*(T[nx;j]-Tant[nx;j])/At)*(4/3)*pi#*(r[nx]^3-(( r[nx]+r[nx-1])/2)^3) end end {Cálculo del coeficiente de convección esfera - agua} Rep=densidad_agua*vel_media_intersticial*porosidad*2*R/visc_agua Pr=Prandtl(Water;T=13;P=100) Nu=2+1,1*(6*(1-porosidad))^0,6*Rep^0,6*Pr^(1/3) Nu=hconv*2*R/landa_agua {Balance de energía} Duplicate j=1;npisos nesferas_piso_calculo[j]=if(j;npisos;31;31;31) -magua*Cp_agua*(Tagua[j]-Tagua[j-1])=nesferas_piso_calculo[j]*hconv*(Tagua[j]-T[nx;j])*4*pi#*R^2 q[j]=nesferas_piso_calculo[j]*hconv*(Tagua[j]-T[nx;j])*4*pi#*R^2 end potencia=sum(q[j];j=1;npisos) {Cálculo de la pérdida de carga por unidad de longitud según distintas correlaciones} AP_ergun/Ltanque=(150*0,001*vel_media*(1-porosidad)^2/(2*R)^2/porosidad^3)+1,75*1000*vel_media^2*(1-porosidad)/( 2*R)/porosidad^3 AP_tallmadge/Ltanque=1000*vel_media^2/(2*R)*((150*(1-porosidad)^2/Rep/porosidad^3)+4,2*(1-porosidad)^1,166*Rep^(- 1/6)/porosidad^3) AP_sug_lee_ogawa/Ltanque=1000*vel_media^2/(2*R)*(6,25*(1-porosidad)^2/porosidad^3*(29,32*Rep^(-1)+1,56*Rep^(-n) +0,1)) n=0,352+0,1*porosidad+0,275*porosidad^2 AP_kuerten/Ltanque=1000*vel_media^2/(2*R)*(25/4*(1-porosidad)^2/porosidad^3*(21*Rep^(-1)+6*Rep^(-0,5)+0,28)) File:G:\Tesis Doctoral\Anexos\Modelo en EES depósito suspensión.EES 15/03/2013 13:21:40 Page 1 EES Ver. 9.215: #3470: For use only by students and faculty in the Departamento de Ingenieria Mecanica Universidad de Za {Datos del tanque} h_tanque=0,55 {altura del tanque} Dtanque=0,24 {diámetro del tanque} Vtanque=pi#*(Dtanque/2)^2*h_tanque {volumen del tanque} m_PCM=densidad_PCM*Vtanque_pcm {masa de suspensión de PCM} densidad_PCM=960 {densidad de la suspensión de PCM} Vserpentin=pi#*(Dserp/2)^2*Lserp {volumen del serpentin} Vtanque_pcm=Vtanque-Vserpentin {volumen del tanque ocupado por la suspensión de PCM} densidad_fluido=1000 {densidad del agua} caudal_fluido=0,003 {caudal másico de agua} Asecciontubo=pi#*(Dserp_int/2)^2 {sección del tubo} caudal_fluido=v_fluido*densidad_fluido*Asecciontubo {velocidad media del agua} Dserp=0,016 {diámetro exterior del serpentín} Lserp=3,7 {longitud del serpentín} Dserp_int=0,014 {diámetro interior del serpentín} Dhelice=0,14 {diámetro de la helice} {Curva del calor específico de la suspensión de PCM} Cps=3649 hm=59400 sigma=0,85 Tm=6,7 Nodos_x=8 {número de nodos de discretización del tanque} Ax=h_tanque/Nodos_x A=pi#*Dserp*(Lserp/Nodos_x) {área de transferencia de calor del serpentín} Rcond=(Dserp/2/k_('Copper'; 30))*ln(Dserp/Dserp_int) {resistencia de conducción del cobre} {Tabla paramétrica para el tiempo} tiempo=(fila-1)*At At=30 {incremento temporal} Duplicate i=1;Nodos_x Tpcm_ant[i]=Tablevalue('Evol_temp';fila-1;'Tpcm[i]') Tfluido_ant[i]=Tablevalue('Evol_temp';fila-1;'Tfluido[i]') end {Balance de energía} Duplicate i=1;Nodos_x -caudal_fluido*Cp_fluido[i]*(Tfluido[i]-Tfluido[i-1])=Uext[i]*A*(Tfluido[i]-Tpcm[i]) (m_PCM/Nodos_x)*(Cps+(hm/sigma/6,28^0,5)*2,71^(-0,5*((Tpcm_ant[i]-Tm)/sigma)^2))*(Tpcm[i]-Tpcm_ant[i])/At=Uext[i]* A*(Tfluido[i]-Tpcm[i]) q[i]=Uext[i]*A*(Tfluido[i]-Tpcm[i]) {calor intercambiado en cada piso del tanque} {Coeficiente de convección natural (a partir de los datos del agua)} Cp_agua[i]=1000*Cp(Water;T=Tpcm_ant[i];P=100) densidad_agua[i]=Density(Water;T=Tpcm_ant[i];P=100) dif_agua[i]=Conductivity(Water;T=Tpcm_ant[i];P=100)/densidad_agua[i]/Cp_agua[i] visc_cin_agua[i]=Viscosity(Water;T=Tpcm_ant[i];P=100)/Density(Water;T=Tpcm_ant[i];P=100) coef_vol[i]=VolExpCoef(Water;T=Tpcm_ant[i];P=100) Ra_fluido[i]=9,81*coef_vol[i]*(Tfluido_ant[i]-Tpcm_ant[i])*Lserp^3/dif_agua[i]/visc_cin_agua[i] Nu_ext[i]=0,802*(Ra_fluido[i])^0,278 Nu_ext[i]=hconv_ext[i]*Lserp/Conductivity(Water;T=Tpcm_ant[i];P=100) hconv_ext_pcm[i]=-1,7974*hconv_ext[i]+1180,4 Rconv_ext_PCM[i]=1/hconv_ext_pcm[i] {Coeficiente de convección interior} Cp_fluido[i]=1000*Cp(Water;T=Tfluido_ant[i];P=100) Re_fluido[i]=4*caudal_fluido/pi#/Dserp_int/Viscosity(Water;T=Tfluido_ant[i];P=100) Pr_fluido[i]=Prandtl(Water;T=Tfluido_ant[i];P=100) Nu_int[i]=(2,153+0,318*Dean[i]^0,643)*Pr_fluido[i]^0,177 {Valida para De entre 20 y 200; Pr entre 0,7 y 175; relación entre Dserp/Dhel entre 0,0267 y 0,0884} Dean[i]=Re_fluido[i]*((Dserp)/(Dhelice))^0,5 Nu_int[i]=hconv_int[i]*Dserp_int/Conductivity(Water;T=Tfluido_ant[i];P=100) File:G:\Tesis Doctoral\Anexos\Modelo en EES depósito suspensión.EES 15/03/2013 13:21:40 Page 2 EES Ver. 9.215: #3470: For use only by students and faculty in the Departamento de Ingenieria Mecanica Universidad de Za Rconv_int[i]=1/hconv_int[i] {Coeficiente global de TQ} 1/Uext[i]=Rconv_ext_PCM[i]+Rcond+Rconv_int[i]*Dserp/Dserp_int end potencia=sum(q[i];i=1;Nodos_x) {potencia intercambiada} {Cálculo de la pérdida de carga} Re=(4*caudal_fluido/3,14/Dserp_int/Viscosity(Water;T=13;P=100)) {Reynolds calculado para una temperatura del agua de 13ºC} De=Re*(Dserp_int/Dhelice)^0,5 {número de Dean} f=16/Re {Factor de fricción en tubos rectos} Re_transicion=20000*(Dserp_int/Dhelice)^0,32 {Reynolds de transición de laminar a turbulento} AP/Lserp=f_ito*1000*v_fluido^2/2/Dserp_int {Pérdida de carga} {Factor de fricción en régimen laminar en serpentines} {Correlación White para laminar e isotermo} f_white=f*(1-(1-(11,6/De)^0,45)^2,2)^(-1) {Correlación Ito para laminar e isotermo} f_ito=num_ito/den_ito num_ito=344*(Dhelice/Dserp_int)^(-0,5) den_ito=(1,56+log10(Re*(Dhelice/Dserp_int)^(-0,5)))^5,73 Re_ito_min=13,5*(Dhelice/Dserp_int)^0,5 Re_ito_max=2000*(1+13,2*(Dhelice/Dserp_int)^(-0,6)) {Correlación Manlapaz and Churchill para laminar e isotermo. Exponente 0 depende del número de Dean} sum1=(1-0,18*(1+(35/He)^2)^(-0,5))^0 sum2=((1+(Dserp_int)/(3*Dhelice))^2)*He/88,33 f_manlapaz_churchill/f=(sum1+sum2)^0,5 He=De/(1+(pitch/3,14/Dhelice)^2)^0,5 pitch=0,037 APPENDIX IV. Substances analyzed  277 APPENDIX IV. Substances analyzed                 Analysis of microencapsulated phase change material slurries and phase change material emulsions as heat transfer fluid and thermal storage material   278       