Behaviour of a concrete wall containing micro‐encapsulated PCM after a decade of its construction

1 Behaviour of a concrete wall containing micro‐encapsulated PCM 1 after a decade of its construction 2 Luisa F. Cabezaa, Lidia Navarroa, Anna Laura Pisellob,c, Lorenzo Olivierid,e, Cesar 3 Bartoloméf, José Sánchezg, Servando Álvarezg, Jose Antonio Tenorioe 4 a GREiA Research Centre, INSPIRES Research Centre, Universitat de Lleida, Pere de Cabrera s/n, 5 25001 Lleida, Spain 6 b CIRIAF—Interuniversity Research Center, University of Perugia, Via G. Duranti 63, 06125 Perugia, 7 Italy 8 c Department of Engineering, University of Perugia, Via G. Duranti 93, 06125 Perugia, Italy 9 d Department of Construction and Technology in Architecture, Escuela Técnica Superior de Arquitectura, 10 Universidad Politécnica de Madrid, Av. de Juan de Herrera 4, 28040 Madrid, Spain 11 e Instituto de Ciencias de la Construcción Eduardo Torroja – CSIC, Serrano Galvache 4, 28033 Madrid, 12 Spain 13 f IECA, Instituto Español del Cemento y sus Aplicaciones, Calle de José Abascal 53, 28003 Madrid, 14 Spain 15 g Grupo de Termotécnia, Universidad de Sevilla, Escuela de Ingenieros Camino de los Descubrimientos 16 s/n. 41092, Seville, Spain 17 18 Abstract 19 20 Today, our society has the duty of reducing the energy consumed in the building sector. A 21 promising technology to achieve this goal is the implementation of thermal energy storage (TES) 22 solutions in buildings envelope. Lately, much literature dealing with the effect of the inclusion of 23 latent heat storage materials in construction materials to provide higher thermal inertia has 24 appeared, mostly focusing on the evaluation of the thermal properties, density, or porosity of these 25 new materials. However, few of them evaluated the long stability properties of the materials with 26 embedded PCM when included in a building, very much needed since the lifetime of a building 27 is about 50 years. Therefore, in this study, an evaluation of a house‐like cubicle of concrete with 28 micro‐encapsulated PCM after a decade of its construction is carried out. The results are compared 29 to the tests done in 2005 concluding that the thermal performance of this cubicle presented no 30 degradation in the PCM effect.31 32 Keywords: building; energy efficiency; thermal energy storage; long lasting performance 33 34 35 36 37 This is an Accepted Manuscript of an article published by Elsevier in Solar Energy, Volume 200, April 2020, available at: https://doi.org/10.1016/j.solener.2019.12.003 Copyright 2019. Intern. Solar Energy Society, Elsevier. En idUS Licencia Creative Commons CC BY-NC-ND 2  1. Introduction 38 39 Buildings use 32% of global final energy demand, and generate 30% of energy‐related CO2 40 emissions, and approximately one‐third of the black carbon emissions. Over 60% of residential 41 and almost 50% of commercial buildings use thermal energy, with higher contribution from water 42 heating in residential buildings and from cooling in commercial ones. The drivers of the heating 43 and cooling energy demand in buildings show a clear tendency where the decrease of the energy 44 demand is a need [1]. 45 46 Thermal energy storage (TES) is one of the highlighted technologies to achieve energy demand 47 decrease in buildings. It is well known that improvements in the building envelope have high 48 influence in the energy demand reduction. The use of phase change materials (PCM) in the 49 building envelope has been widely studied to take profit of the latent heat stored during the phase 50 change. The implementation of PCM as a passive system became a way to provide thermal inertia 51 to the building at a specific temperature, in this case the comfort building temperature range [2]. 52 53 Different methodologies can be used to insert PCM in buildings. Direct incorporation, immersion, 54 vacuum impregnation, encapsulation, shape‐stabilisation, and form stable composites are the 55 identified methods. However, when including PCM into a construction material during the mixing 56 process, such as concrete or cement mortar, the one that prevents the leakage problem is using 57 micro‐encapsulation PCM. This method consists in encapsulating PCM in small particles (1 μm 58 ‐ 1000 μm) with a thin solid shell which is made from natural and synthetic polymers [3]. More 59 details can be found in Marani and Nehdi, 2019 [4]. 60 61 Several studies have been also dedicated to the performance analysis and enhancement of 62 composites made by cement or concrete based components and PCM based inclusions in a variety 63 of ways and for different uses. Most of them are aimed at improving building thermal inertia, 64 since the PCM contribution is able to dampen down thermal waves entering and exiting the 65 building envelope at reasonable cost, as recently demonstrated [5,6]. PCM can be effectively 66 integrated into structural and non-structural elements, which is the reason why several 67 experiments and case studies concern their characterization in walls, ceilings, roofs, windows [7]. 68 In this regard, the reduction of mechanical properties of concrete when PCM is added should be 69 considered; the literature reports a reduction of up to 51% [8]. More details can be found in 70 Berardi and Gallardo, 2019 [9]. Very little research is done in also evaluating the durability of the 71 mortars or concrete after the addition of PCM [10]. 72 73 3  The most typical PCM materials are organic or inorganic. Organic PCM are usually paraffin and 74 fatty acids, while inorganic PCM are usually salt hydrates. This second material are frequently 75 associated to high volume change rates during the thermal transitions and large subcooling, which 76 compromises both their effect and the quality of prediction models which are elaborated in order 77 to better predict their field behaviour, after inclusion into the cement based matrix. Therefore, 78 organic PCM (paraffin mainly) are the most acknowledged from the scientific community and 79 the practical field of application nowadays [11]. Since the large majority of these investigations 80 concern microcapsuled PCM or aggregated microcapsulated PCM, they have been gaining a 81 massive success thanks to a variety of experimental data, now available for verification. In details, 82 several studies have been focused on the variability of concrete mechanical behaviour with 83 varying the quantity of PCM or its thermal performance analysis associated to structural and 84 microstructural reliability of the material. Since microcapsulated additives may be responsible for 85 new voids and possible cracking effects, resulting in the material weak sections. In general, a 86 PCM content higher than 3% may be responsible for more than 50% reduction in the original 87 strength capability (i.e. compression and bending strength) which for sure needs to be taken into 88 account when designing structural components [12,13]. At the same time, given the PCM 89 capability to absorb released heat, they may be helpful in reducing curing-related cracks due to 90 their function of hydration heat absorber. Such promising function may be achieved by means of 91 a relatively smaller quantity of PCM, i.e. less than 2% [14]. Therefore, PCM inclusions may be 92 seen as a promising way to produce lightweight concretes with enhanced thermal mass by 93 improving both the mechanical and thermal performance of building envelope. To ensure 94 durability of the system, the breakage of the microcapsules during concrete production needs to 95 be avoided; to achieve this goal, robust microcapsules have been developed [15]. 96 97 All these analyses were also confirmed by large scale experiment in test-buildings or prototype 98 rooms, also named as cubicles [16,17]. This experiment configuration allowed to estimate the 99 TES capability of composite solutions even when applied to building envelope components and 100 energy systems in operating conditions, with varying climate and occupancy boundaries [18,19]. 101 In this panorama, Cabeza et al. (2007) [13] published an experimental study about two cubicles 102 made of prefabricated concrete panel, one of them was constructed with a conventional concrete 103 and the other one had 5wt.% of micro‐encapsulated PCM. Both cubicles were tested under real 104 summer weather conditions. Results showed that the cubicle with PCM provided more stable 105 temperature conditions in the internal ambient and a delay of the maximum peak temperature. 106 Time-zero experiments are becoming popular for the reasons explained above, i.e. to demonstrate 107 the effect of TES incorporation in more real conditions, e.g. in full scale continuously monitored 108 buildings. At the same time, durability assessment is still missing while studying the scientific 109 literature of the sector, since those few studies concerning TES or PCM performance during the 110 4  course of the time are pretty focused on TES-materials in particular, and not on the composite 111 system when integrated into building envelope or HVAC system [20]. That is the reason why this 112 paper wants to fill this gap by investigating the long-term behaviour of TES-doped system 113 represented by a concrete ceiling slab with PCM-incorporation aimed at stabilizing indoor air 114 temperature, energy saving and improving indoor comfort conditions, following the work 115 presented by Cellat et al. [21]. Therefore, in this paper, the durability of building materials with 116 PCM and the behaviour of the micro‐capsules inside these matrixes are investigated, since they 117 are not usually taken into account in most of the published studies so far. 118 119 However, these properties are considered of significant interest for builders, architects and 120 building engineers, to achieve better acceptance on these new materials. Therefore, the 121 experiment of this study represent a follow up of a previous experiment carried out more than one 122 decade before, in summer conditions, with the precise purpose to observe any evidence of 123 difference or degradation in the concrete with PCM cubicle. Additionally, a critical comparison 124 between the initial performance of the concrete with PCM system and the current status is 125 presented. 126 127 2. Experimental set‐up 128 129 In the experimental setup of Puigverd de Lleida (Spain), two house‐like cubicles made of 130 prefabricated concrete panels of 0.12 m of thickness were used in this study. These cubicles were 131 built in 2005 within a European project framework (MOPCON). Both cubicles have the same 132 dimensions (2.4×2.4×2.4 m) and orientation (N‐S, 0°), but one of them have conventional 133 concrete, and the other one 5wt.% of micro‐encapsulated PCM in the concrete matrix (Figure 1). 134 This fact allows the authors to evaluate and compare the effect of PCM incorporation in these 135 construction systems. The windows are also distributed with the same pattern in both cubicles, 136 having one at the east and west wall, and four windows in the south wall, as well as the metallic 137 door located in the north facade. The monitoring of the cubicles is continuous and it is evaluated 138 evaluating the data in a weekly basis; the results are shown for selected weeks that allow showing 139 the performance of the cubicles in different periods of the year. 140 141 5  Figure 1. Construction of the cubicles with prefabricated concrete panels in 2005 at the experimental 142 setup of Puigverd de Lleida (Spain) 143 144 In this application, the PCM was implemented as a passive cooling system to avoid indoor 145 overheating in summer conditions. For this reason, just three panels of concrete with PCM were 146 installed in the southern and western walls, and in the roof. The selected PCM was a commercial 147 micro‐encapsulated (Micronal DS 5001) with a melting point of 26 °C, and a phase change 148 enthalpy of 110 kJ/kg [22]. 149 150 The cubicles were instrumented to be able to measure and analyse their thermal performance in 151 terms of temperature of internal wall surfaces (East, West, North, South walls, roof and floor) and 152 also external South wall temperature, and indoor temperature and humidity of the cubicle. 153 Moreover, the meteorological station registered outdoor temperature and humidity, global solar 154 radiation over the horizontal plane, wind velocity and direction. All the data were collected every 155 5 minutes and memorized in dedicated data loggers of the facility. 156 157 In order to analyse the performance of the concrete with PCM system after a decade of the cubicle 158 construction, the same experiments from Cabeza et al. (2007) [13] were performed in the facility. 159 During the summer period, the evaluation of both concrete cubicles was done in free floating 160 conditions, where no cooling system was operating, hence the evolution of the temperatures can 161 be compared. On the other hand, the compressive strength was tested in 2005 once the concrete 162 was hardened, following the European standard UNE‐EN 12390‐3 and UNE‐EN 12504‐1. 163 164 165 6    Figure 2. Concrete test samples extracted from both cubicles 166 167 The same mechanical tests were performed to compare the properties of the concrete with PCM 168 after a decade. Therefore, two cylindrical specimens were extracted from the field installation, 169 i.e. one from the conventional concrete cubicle, and the other one from the concrete with PCM 170 cubicle (Figure 2). 171 172 3. Results 173 174 3.1. Thermal analyses comparison 175 176 In the experiments performed during summer, as previously mentioned, free floating conditions 177 were tested in both cubicles. From the whole experimental campaign, key representative results 178 are showed in this study to demonstrate the thermal performance of the concrete with PCM system 179 after a decade. 180 181 The internal air temperature profiles collected in both the cubicles are reported in Figure 3. Since 182 the free floating conditions experiments do not have active cooling supply, both cubicles present 183 difficulties to maintain the ambient temperature inside the comfort zone. However, the internal 184 ambient temperature of the concrete with PCM cubicle is always dampened down thanks to the 185 PCM inclusion and its peaks are always below the ones in the conventional concrete cubicle. In 186 fact, the daily fluctuation of air temperature in the cubicle with PCM is never higher than 4°C and 187 its thermal peaks are lower than the reference cubicle by about 2°C. 188 189 7  190 Figure 3. Outside and internal ambient temperatures in both cubicles in 2016 191 192 By looking at the external surface temperature from the South wall, big differences can be also 193 seen between the two cubicles (Figure 4). Conventional concrete cubicles present relatively higher 194 temperature peaks compared to the concrete with PCM case. Maximum temperatures achieved in 195 the South external surface wall in conventional concrete cubicle are between 34 °C and 39 °C, 196 while in the one with PCM mostly are around 28 °C. Figure 4 shows the very clear PCM effect, 197 which is able to store the heat coming from high temperature peak hours and postpone the thermal 198 peaks and decreasing temperature wave by about 4-6 hours (on July 13th, 14th and 17th). The results 199 corroborated the behaviour of PCM when included in a building material. As stated by several 200 authors [23,24], the phase change temperature of a PCM depends on the heating and cooling rate, 201 therefore in Figure 4 some peaks show that the melting of the PCM is not exactly at 26 ºC as 202 expected, but it appears at the range between 26 ºC and 28 ºC. 203 204 8  205 Figure 4. Surface temperature from the external South wall of conventional concrete and concrete with 206 PCM cubicles (12 – 18 July 2016) 207 208 Another example of the PCM effect can be observed in Figure 5, where the solidification and 209 melting process are clearly reported for both studied periods. Additionally, a delay on the 210 maximum peak temperature is observed in the concrete with PCM cubicle compared to the 211 conventional concrete cubicle, e.g. by about 4-5 hours. From these experiments authors observed 212 that the PCM incorporated in the concrete matrix is correctly working, storing the heat from daily 213 peak hours and releasing it later, once the outdoors temperature is dropping down. When 214 comparing both studied periods, summer 2005 and summer 2016, both periods had similar solar 215 radiation (reaching 1000 W/m2 every day), but the external temperature was higher in 2016 216 (reaching 32-36 ºC during the day and dropping to 10-16 ºC during night) than in 2005 (reaching 217 26-32 ºC during the day and dropping to 12-16 ºC during night). Therefore, the indoor temperature 218 was higher also in 2016. This is why, the difference between the cubicle with PCM and that 219 without PCM was 1-3 ºC in 2005 and reached 5-7 ºC in 2016. These results show how the effect 220 of the PCM is higher with more extreme weather conditions. 221 222 9  223 224 Figure 5. External surface South wall temperature in both cubicles in 2005 and 2016 225 226 The results from the experiments presented in this study were compared to the ones published in 227 the study of Cabeza et al. (2007) [13]. The effect observed in experiments performed in 2005 228 were the same as the ones seen nowadays. As an example, the external surface temperature of the 229 south wall in the concrete with PCM cubicle shows lower peaks compared to the conventional 230 concrete. Also, the solidification and melting process of the PCM added in the concrete are clearly 231 reflected in the external surface temperature. In addition, the authors in Cabeza et al. (2007) [13] 232 concluded that the effect of PCM included in the concrete matrix contributed to a temperature 233 reduction in the internal ambient of the cubicle. 234 235 236 237 238 239