Comparison of energy storage technologies for application of unmanned electrical aerial vehicles

Comparison of Energy Storage Technologies for applications of Unmanned Electrical Aerial Vehicles INEGI André Filipe Rosário de Brito Lhamas Relatório do Projecto Final / Dissertação do MIEM Orientador no INEGI: Dr. Nuno Correia; Eng. João Barbosa Orientador na FEUP: Prof. Dr. António Torres Marques Faculdade de Engenharia da Universidade do Porto Mestrado Integrado em Engenharia Mecânica Julho 2013 Comparison of energy storage technologies for applications of UEAV Page ii André Lhamas, July 2013 Comparison of energy storage technologies for applications of UEAV André Lhamas, July 2013 Page iii Aos meus Pais. Comparison of energy storage technologies for applications of UEAV Page iv André Lhamas, July 2013 Comparison of energy storage technologies for applications of UEAV André Lhamas, July 2013 Page v Comparação de tecnologias de armazenamento de energia para aplicações de veículos aéreos não tripulados eléctricos (UEAV) Resumo O trabalho relatado nesta dissertação visou projectar a redução de massa numa célula de combustível de membrana de permuta iónica (PEM FC) através do uso de materiais compósitos nas placas terminais (end plates), como alternativa ao metal. Uma vez que as placas de terminais, assim como outros componentes de fixação, representam uma fracção significativa da massa global do sistema, esta redução resulta numa maior potência e energia específicas, podendo possivelmente tornar os sistemas PEM FC mais interessantes em aplicações onde o peso se apresenta como um factor crítico. Além da redução de massa, é também discutida a deformação na placa. Foi demonstrado na literatura que grandes deformações nas placas de terminais podem levar a influências negativas no desempenho da célula de combustível. Esta dissertação perspectiva soluções construtivas alternativas que são capazes de reduzir significativamente o peso, assim como alcançar valores de deformação mais baixos do que as placas de fim metálicas, aumentando assim a eficiência global do sistema. O sistema de célula de combustível foi estudado para o caso de uma aeronave com 210 kg de peso máximo de descolagem (MTOW), possuindo um requisito de cerca de 20 kW de potência máxima. Neste caso, a massa disponível para o sistema propulsivo foi de 70 kg. Através do uso de uma análise por elementos finitos, o conceito com o melhor desempenho entre os conceitos sugeridos foi seleccionado. Posteriormente, foram realizados cálculos com vista à determinação da energia específica do sistema resultante. Dessa forma, tornou-se possível uma comparação com os conceitos de placas de fim actualmente usados. Além disso, foi também efectuada a comparação dos sistemas referidos com um sistema propulsivo convencional, compreendendo um motor de combustão interna (ICE). O novo conceito de placa de fim permitiu um aumento significativo na energia específica (28,3%), ainda que tenha sido também provado a incapacidade actual de as células de combustível igualarem sistemas ICE convencionais, mesmo que, tal como é demonstrado, ter também provado que existe espaço amplo para melhoria. Comparison of energy storage technologies for applications of UEAV Page vi André Lhamas, July 2013 Comparison of energy storage technologies for applications of UEAV André Lhamas, July 2013 Page vii Abstract The work reported by this dissertation aimed to engineer a reduction in mass in a proton exchange membrane fuel cell (PEM FC) by using composite materials as an alternative to metals in the end plate. Since end plates, along with other fixation components, account for a significant fraction of the global system weight, this reduction results in increased specific power and specific energy, possibly making PEM FC systems more interesting in applications where weight is critical. Besides mass reduction, plate deformation is discussed. Large strains in the end plate have been shown to negatively influence fuel cell performance. This dissertation reports insights into alternative construction solutions that are able to significantly reduce weight, as well as achieve lower strain values than metal end plates, increasing overall system efficiency. The fuel cell system was studied for the case of a 210 kg maximum take-off weight (MTOW) airplane with a maximum power requirement of c. 20kW for take-off. In this case the available mass for the propulsive system was 70 kg. Through the use of a finite element analysis, the best performing amongst the suggested concepts was selected. Thereafter, calculations for determining specific energy of the resulting system were accomplished, making a comparison with the currently used end plate concepts possible, as well as a comparison with a conventional propulsive system, comprising an internal combustion engine (ICE). The new endplate concept allowed a significant increase in specific energy (28,3%), albeit also proving that fuel cells are not yet capable of matching conventional ICE systems, even if, as is shown, there is ample room for improvement. Comparison of energy storage technologies for applications of UEAV Page viii André Lhamas, July 2013 Comparison of energy storage technologies for applications of UEAV André Lhamas, July 2013 Page ix Agradecimentos Agradeço ao meu Orientador Professor Doutor António Torres Marques toda a disponibilidade, assistência e sugestões ao longo do projecto. Além disso, agradeço a sua promoção de contactos que teve resultados muito positivos para o trabalho. Agradeço ao meu Orientador Doutor Nuno Correia toda a disponibilidade, amizade e apoio demonstrados no decorrer do trabalho, apesar das exigências da sua profissão. De referir que, muitas das sugestões conceptuais presentes neste trabalho foram sua sugestão, sendo que lhe fico por isso muito grato. Agradeço ao meu Orientador Engenheiro João Barbosa por toda a disponibilidade, companheirismo, e ânimo dado no decorrer da dissertação, assim como por me ter amavelmente mostrado a componente prática de construção aeronáutica. Agradeço toda a orientação dada pelo Professor Doutor José Esteves e pela sua total disponibilidade na resolução das mais diversas dúvidas que lhe fui apresentando. Agradeço ao Doutor António Melro e ao Engenheiro José Cerqueira do INEGI pelo esclarecimento de dúvidas e sugestões dadas. Agradeço ao Cristiano as largas horas que passou comigo a resolver problemas críticos no software Abaqus, assim como todas as cruciais sugestões que fez. Agradeço-lhe também o seu companheirismo e boa disposição no decorrer do meu estágio no INEGI. Um enorme obrigado à minha família e amigos, pela motivação ao longo de todo este percurso académico. Agradeço aos meus colegas da FEUP e do INEGI toda a amizade e bons momentos vividos no decorrer deste semestre. Um obrigado a todos aqueles que de alguma forma contribuíram para a realização deste estudo. Comparison of energy storage technologies for applications of UEAV Page xvi André Lhamas, July 2013 Figure 35: Normal plate dimensions. Positioning of holes and contact surface with graphite block (inside rectangle) ............................................................................................................ 59 Figure 36: Normal plate with Solid Works rendering.............................................................. 59 Figure 37: Pre-curvature plate curvature in the y-z plane ........................................................ 60 Figure 38: Pre-curvature plate curvature in the x-z plane ........................................................ 60 Figure 39: Pre-curvature plate sketched as surface .................................................................. 61 Figure 40: Conventional Shoe Box cover ................................................................................ 61 Figure 41: Shoe Box concept with Solid Works rendering ..................................................... 62 Figure 42: Placement of supports in shoe box face ................................................................. 62 Figure 43: Dimensions of the support and epoxy adhesive ..................................................... 63 Figure 44: General dimensions of the Inertia plate .................................................................. 63 Figure 45: Front view of Inertia plate ...................................................................................... 64 Figure 46: Back view of the Inertia plate ................................................................................. 64 Figure 47: Reinforcement plate general dimensions ................................................................ 64 Figure 48: Reinforcement plate Solid Works render ............................................................... 64 Figure 49: The UAV Engines Limited AR741 Wankel engine, [36] ...................................... 67 Figure 50: Finite Element Geometry and Mesh of an applied concept ................................... 70 Figure 51: Representation of the model with applied loads (yellow arrows) and boundary conditions (blue and orange triangles) ..................................................................................... 71 Figure 52: Abaqus Composite Layup Manager ....................................................................... 77 Figure 53: Example of tested assembly; Plate on the left, reinforcement in the middle, and graphite block to the right ........................................................................................................ 79 Figure 54: Double pre-curvature plate; displacements disabled (orange triangles) in centre (U1,U2; x and y) and side (U2; y) ........................................................................................... 80 Figure 55: Loads (yellow arrows) and fixation (orange and blue triangles) seen from above 80 Figure 57: Top edge path on the graphite block ...................................................................... 83 Figure 58: Bottom edge path on the graphite block ................................................................. 83 Figure 59: Displacement distribution in graphite block with pre-curvature end plate ............ 84 Figure 60: Von Mises Stress distribution in graphite block with pre-curvature end plate ...... 84 Figure 61: Placement of path for the graphite block, considering the pre-curvature plate ...... 84 Figure 62: Displacement of each concept of the plate model with respect to the x-direction . 86 Figure 63: Deflection of each concept of the plate model with respect to the x-direction ...... 88 Figure 64: Available energy of the considered FC systems with different EP materials, as well as ICE system ........................................................................................................................... 96 Comparison of energy storage technologies for applications of UEAV André Lhamas, July 2013 Page xvii Figure 65: Specific energy of the considered FC systems with different EP materials, as well as ICE system ........................................................................................................................... 96 Figure 66: Filament Winding concept assembly Solid Works rendering ............................... 102 Figure 67: General dimensions of the FW box....................................................................... 103 Figure 68: FW tube fibre orientation ...................................................................................... 105 Figure 69: FW half stack assembly in Abaqus ....................................................................... 105 Figure 72: Specific energy of the considered FC systems, as well as the FW concept and the ICE system .............................................................................................................................. 109 Figure 73: Dependency of the band gap, temperature and efficiency, [40] ........................... 126 Figure 74: Variation of output with insulation for representative sub-arrays, [39] ................ 127 Figure 75: Epitaxial stacks of multi-junction solar cells, [39] ............................................... 128 Figure 76: Force balance for an aircraft in steady level flight................................................ 133 Figure 77: Schematic of an Alkaline fuel cell operation, [14] ............................................... 136 Figure 78: Schematic of PAFC operation, [14] ...................................................................... 138 Figure 79: Schematic of MCFC operation, [14] ..................................................................... 139 Figure 80: Schematic of a SOFC operation, [14] ................................................................... 141 Figure 82: Schematic of the spray-up process, [30] ............................................................... 143 Figure 83: Schematic of the filament-winding process, [30] ................................................. 144 Figure 84: Pultrusion, [30]...................................................................................................... 144 Figure 85: Schematics of the RTM process, [30] ................................................................... 145 Comparison of energy storage technologies for applications of UEAV Page xviii André Lhamas, July 2013 Comparison of energy storage technologies for applications of UEAV André Lhamas, July 2013 Page xix Table of Tables Table 1: Features of low and high temperature fuel, [12] ........................................................ 18 Table 3: Fuel Cell Reactions and the Corresponding Nernst Equations, [19] .......................... 24 Table 4: Ideal Voltage as a function of Cell Temperature, [19] ............................................... 24 Table 5: Summary results of the assessment for Type III (T3) and Type IV (T4) single and dual-tank compressed hydrogen storage systems, [20] ............................................................ 31 Table 6: Compliance matrixes of isotropic (left) and transversely isotropic (right) materials. 41 Table 7: Categorization for UAS, UVS International .............................................................. 46 Table 9: Proton Motor 8 kW features, [35] .............................................................................. 53 Table 10: Metal End Plate mass and mass ratio to FC stack total mass for aluminium and steel .................................................................................................................................................. 56 Table 11: Composite ply thickness, stacking sequence and resulting plate thickness ............. 57 Table 12: Flexural Stiffness (EI), curvatures (R) of the best performer composite plate tested in [18] ....................................................................................................................................... 60 Table 13: Conceptual plates ..................................................................................................... 65 Table 14: Technical specifications of the selected engine - UAV Engine Limited AR741 Wankel engine, [36] ................................................................................................................. 67 Table 15: Component Geometrical sketch procedure .............................................................. 72 Table 17: Mechanical Properties of the considered isotropic materials, and respective mass density ....................................................................................................................................... 76 Table 18: Toray M40J Fibre Properties .................................................................................... 76 Table 19: Mechanical Properties of the considered orthotropic material, and mass density ... 77 Table 20: Carbon/epoxy stacking sequence and ply thickness for the considered composite parts .......................................................................................................................................... 77 Table 21: Contact description and representation .................................................................... 78 Table 22: Equivalent force per hole for the metal and composite end plates ........................... 81 Table 26: Stress and displacement distribution in the graphite block's face for every tested concept ...................................................................................................................................... 89 Table 27: PM 8 kW, PM 4 kW and proposed combination masses ......................................... 93 Table 28: Mass ratio of 2 end plates and determination of the mass of the remaining components ............................................................................................................................... 93 Table 29: Total mass for the steel and CFRP Plate 4 and 8 kW FC stack ............................... 93 Table 30: Total stack mass for aluminium, steel and CFRP .................................................... 94 Table 32: Contained energy and specific energy for the aluminium, steel and CFRP FC stacks, according to the gravimetric targets of the DOE ...................................................................... 95 Comparison of energy storage technologies for applications of UEAV Page xx André Lhamas, July 2013 Table 33: Contained energy and specific energy for the aluminium, steel and CFRP FC stacks, according to the gravimetric targets of the DOE, considering 60% overall efficiency ........... 95 Table 36: Contained energy and specific energy of the ICE propulsive system, considering 30% overall system efficiency ................................................................................................. 96 Table 37: Comparison Specific Energy ICE-CFRP FC ........................................................... 96 Table 38: Comparison Specific Energy CFRP FC-Al FC ....................................................... 96 Table 39: Part description for FW model............................................................................... 103 Table 40: Material definition for the used parts in the FW model ......................................... 104 Table 41: Carbon/epoxy stacking sequence and ply thickness .............................................. 105 Table 42: Mass of the parts used in the FW model ................................................................ 106 Table 43: Determination of remaining mass for the FW 8 kW stack mass ........................... 107 Table 44: Determination of remaining mass for the fw 4 kW stack mass ............................. 107 Table 45: Equivalent mass for 2 EP and FC stack mass for the 8 kW and 4 kW considering the FW model ......................................................................................................................... 107 Table 47: Calculus of the available mass for fuel (H2) for the FC propulsive system in the FW model ...................................................................................................................................... 108 Table 48: Contained energy and specific energy for the aluminium, steel and CFRP FC stacks, according to the gravimetric targets of the DOE for the FW model ...................................... 108 Table 49: Contained energy and specific energy for the aluminium, steel and CFRP FC stacks, according to the gravimetric targets of the DOE, considering 60% overall efficiency for the FW model ............................................................................................................................... 108 Table 50: Efficiency of the different types of PV .................................................................. 129 Table 51: Specific strength of rotor materials, [29] ............................................................... 130 Table 52: SFC DMFC Development ..................................................................................... 137 Table 53: AMI Portable SOFC Development ........................................................................ 140 Table 54: Manufacturing Process selection criteria, [30] ...................................................... 142 Table 55: Applications by segment ........................................................................................ 147 Comparison of energy storage technologies for applications of UEAV André Lhamas, July 2013 Page xxi List of Acronyms AFC Alkaline Fuel Cell AUV Autonomous Underwater Vehicle BoP Balance of Plant CFRP Carbon Fibre Reinforced Polymer CHP Combined Heat and Power CVT Continuously Variable Transmission DMFC Direct Methanol Fuel Cell DOE Department of Energy EM Electric Motor EP End Plate FC Fuel Cell FW Filament Winding GDL Gas Diffusion Layer GFRP Glass Fibre Reinforced Polymer ICE Internal Combustion Engine IST Instituto Superior Técnico LAETA Laboratório Associado para Energia, Transportes e Aeronáutica MCFC Molten Carbonate Fuel Cell MEA Membrane Electrode Assembly MTOW Maximum Take-off Weight NATO North Atlantic Treaty Organization PAFC Phosphoric Acid Fuel Cell PEM Proton Exchange Membrane PM Proton Motor PV Photovoltaic RTM Resin Transfer Moulding SOFC Solid Oxide Fuel Cell UAV Unmanned Aerial Vehicle UAS Unmanned Aerial System UBI Universidade da Beira Interior UV Unmanned Vehicle Comparison of energy storage technologies for applications of UEAV Page xxii André Lhamas, July 2013 Comparison of energy storage technologies for applications of UEAV André Lhamas, July 2013 Page 1 1. Introduction 1.1 INEGI INEGI was originally formed by the Department of Mechanical Engineering of the Faculty of Engineering of the University of Porto (FEUP) in 1986 and it still holds that connection to the departments of Mechanical Engineering and of Industrial Engineering and Management that constitute its strongest links to scientific and technical knowledge and training and expertise. In its 26 year existence INEGI developed and consolidated a strong partnership with the Industry in Research, Development and Innovation (R&D+I) projects. Today INEGI’s R&D+I projects under contract with the Industry account for more than 60% of its turnover. As a private non-profit association and a public service entity (Entidade de Utilidade Pública) INEGI is positioned as an agent with responsibilities for the development of the Economy and Society contributing towards the development and consolidation of a competitive model based on knowledge and technological density in-and-of products, processes and technological based innovation. Currently the Institute has 62 Associated Institutions that represent all spheres of activity from the University of Porto to Industrial Associations in relevant fields of INEGI’s activity as well as Private Companies and Public Institutions. INEGI is governed by a board which is formed by three representatives from private industrial associates and two representatives from the University of Porto thereby ensuring its positioning as an institution oriented towards the economic and social valorisation of R&D results, knowledge and technology. The Board reports to the General Assembly of the Private and Public Associates. INEGI’s organizational structure relies on around three pillars of activity in different technology readiness levels, maintaining a strong competence matrix element:  Research  Innovation and Technology Transfer (ITT)  Consulting and Services At its base, INEGI’s structure is formed by different scientifically and technologically specialized Units which support research activity, ITT and consulting. Most significantly ITT and Consulting are especially directed towards the development of solutions for private companies. This organizational structure is specialized in the development and innovation projects of high technological complexity that require the integration of multidisciplinary knowledge and competences. INEGI is part of a Research Unit of the Associated Laboratory for Energy, Transportation and Aeronautics (LAETA), which also counts as members the Mechanical Engineering Institute – IST, the Mechanical Engineering Institute - FEUP, the Centre for Aerospace Sciences – IST, Comparison of energy storage technologies for applications of UEAV Page 2 André Lhamas, July 2013 the Industrial Aerodynamics Institute at the University of Coimbra and the Aeronautics and Astronautics Research Center of the University of Beira Interior. INEGI owns a diverse set of facilities and equipment in order to support its activity, namely laboratories (some of which are accredited laboratories), experimental and development laboratories (capable of producing components and pre-series) and an extensive engineering software catalogue. Software available at INEGI ranges from CAD (including CATIA), FEM (including ABAQUS), CAM, casting simulation, sheet metal forming, as well as other tools that support INEGI’s work on Wind Energy consulting, such as WAsP and WindFarmer for atmospheric studies and GIS (ArcGis). INEGI’s staff is composed of 170 people, 80% of whom are graduates or post-graduates. This number is complemented with c. 50 academic researchers from the Faculty of Engineering of the University of Porto. Comparison of energy storage technologies for applications of UEAV André Lhamas, July 2013 Page 3 1.2 The Electric Long Endurance UAV Project and the NATO Unmanned Vehicle Study Group This dissertation was carried out in the context of an internship that took place at INEGI, within the scope of the “Electric Long Endurance Unmanned Aerial Vehicle (ELE-UAV)” project jointly proposed by IST, UBI and INEGI teams of Laboratório Associado para a Energia, Transportes e Aeronáutica (LAETA) for the development of a small sized aircraft intended for civilian applications. The multidisciplinary project aims to put together the special skills of each participating member in a broad range of fields of study concerning subjects such as Aerodynamics, Propulsion, Mechanical Project, Structures & Materials, Non-Destructive Inspection, Aerodynamic Project, Optimization, Avionics, Aerial Control and Transportation, among others. Those areas are expected to experience sustained development, so that a sharing network of research, skills, and knowledge can be created within the LAETA. The sharing of knowledge and technical expertise is expected to enable scientific research, within its own specific time and budget limitations, as well as solving of actual practical problems that come up during its implementation. Furthermore, INEGI is currently participating in a NATO study group for the implementation of fuel cell systems in Unmanned Vehicles (UVs), being that INEGI is solely focused on the Unmanned Aerial Vehicles (UAV) part. This dissertation also contributed to solving problems in the on-going work of that study group. The initial objectives of this project were to provide with a State-of-the-Art gathering of electrical propulsive technologies for high endurance, followed by a proper analysis and comparison of each, as well as a practical assignment of physically collaboration in the multidisciplinary construction of the aforementioned aircraft. Comparison of energy storage technologies for applications of UEAV Page 10 André Lhamas, July 2013 Figure 3: Low-temperature absolute thermoelectric power for copper and lead, [10] For practical industrial applications, however, alloys are used as thermocouples rather than pure metals. Figure 4 shows the absolute thermoelectric power for some alloys (chromel and alumel) and pure metals (platinum and copper) that are commonly used in thermocouples in different combinations and with other metals and alloys, being platinum the third reference material in thermoelectricity. Also, since thermoelectric power for platinum and alumel is negative, their curves are shown as . Figure 4: High-temperature absolute thermoelectric power for copper, platinum, chromel and alumel, [10] As for the photovoltaic conversion, it is a technology that has experienced a fast development and growing usage over the past few years and has proven to be a practical and viable method for conversion of the solar electromagnetic energy to electricity. Even though this type of conversion cannot be strictly considered a heat engine, its efficiency can be modelled using Comparison of energy storage technologies for applications of UEAV André Lhamas, July 2013 Page 11 the Carnot theory, which will not be addressed in this report. Further discussion over this technology is available in the annexes (see Annexe E). 2.1.2 Direct Conversion Direct conversion systems are based on the direct conversion of chemical energy to electrical energy. These energy conversion systems cannot see their efficiency modelled through the Carnot theory, since they do not comprise the step of heat release previously seen for Heat conversion engines. As seen from Figure 1, these systems are categorized into batteries and fuel cells. The principle of operation of batteries, the most common and widely used direct conversion system, is based upon the oxidation/reduction reactions that take place within the device which leads to the degradation of the electrodes. They can be further divided into primary and secondary. In the first, the electrodes are consumed irreversibly, making them discarded after use. In the latter, however, the electrode materials are regenerated during recharging. It is possible to make infinite combinations of fuel and oxidants as electrode material for battery. The total available energy for both kinds of batteries is, therefore, proportional to the amount of electrode material contained in the device. The electrodes in Fuel cell systems are, however, not consumed. Instead they provide the site in which the chemical reactions between fuel, oxidant and transported ions, driven by catalysers, will take place as long as the reacting chemicals, i.e., fuels and oxidants, are fed. The total energy available in a fuel cell system is related to the amount of reactant stored in such a system, a feature than can be compared to an Internal Combustion Engine (ICE). Comparison of energy storage technologies for applications of UEAV Page 12 André Lhamas, July 2013 2.2 Efficiencies of Conversion Processes 2.2.1 Heat Engines and the Carnot Cycle efficiency limitation As previously mentioned, a heat engine is defined by the four requirements below. It should be noted that, although it is possible to model the efficiency of the Photovoltaic and Seebeck effects through the Carnot Theory, they are not considered heat engines. 1. Receives heat from a high-temperature source (e.g., coal furnace, nuclear reactor); 2. Converts part of this heat to work (e.g., by a turbine); 3. Rejects the remaining waste heat to a low-temperature sink (e.g., atmosphere, river); 4. Operates on a thermodynamic cycle. Some heat engines do not follow the requirement of operating on a thermodynamic cycle. Such is the example of internal combustion engines and gas turbines, for instance, since the employed working fluid is continuously replaced, the resulting combustion gases are exhausted and the input of new volume of air takes place to prepare the following cycle. The steam power plant however follows perfectly the mentioned requirements, since it receives heat from an external combustion chamber, extracts work through a turbine and rejects the heat to a condenser. Since for a cyclic process, the initial and final states are identical, the First Law relation involves only the heat input and work output terms, as in Equation 2.2. Equation 2.2 Thusly, one can conclude that the net work performed by the system equals the net heat flow that enters it, see Equation 2.3. Equation 2.3 Since at least two thermal reservoirs are involved in the cycle of a heat engine, with heat entering the system from the high-temperature reservoir and heat exiting the system to the low-temperature reservoir, one can define that the net work, , in Equation 2.4 is the difference between the heat input, , and heat output, , of the system. Equation 2.4 The thermal efficiency, , of a heat engine is determined by the amount of work converted from the amount of energy input into the system, as in Equation 2.5. Equation 2.5 Combining Equation 2.4 and Equation 2.5, one can reach the following conclusion regarding thermal efficiency, as in Equation 2.6. Equation 2.6 The conclusion drawn from the previous equation is that the smaller the ratio between the heat output and the heat input, the higher the efficiency. The Carnot cycle involves four reversible processes, as depicted in Figure 5 with a piston in a cylinder. The idealized cycle of Carnot deemed all the processes to be conducted in a Comparison of energy storage technologies for applications of UEAV André Lhamas, July 2013 Page 13 reversible manner, since that would be the way to achieve the maximum possible work in a heat engine. The four stages of the reversible Carnot cycle:  (1-2): isothermal expansion  (2-3): adiabatic expansion  (3-4): isothermal compression  (4-1): adiabatic compression Figure 5: The four stages of the reversible Carnot cycle, [9] Since heat addition and heat rejection are performed reversibly and isothermally, and considering Equation 2.7, Equation 2.8 can be used to determine the efficiency of the cycle. Equation 2.7 Equation 2.8 Equation 2.9 is the heat addition step (1-2), and Equation 2.10 is the heat rejection step (3-4). Equation 2.9 Equation 2.10 One observes from Figure 2 that during the reversible adiabatic processes, steps (2-3) and (41), the entropy remains the same. The shaded area in the figure representing the net work, , is shown to be the difference between the heat input, , and the heat output, . Since the entropy terms in Equation 2.9 and Equation 2.10 are identical, i.e.: Equation 2.11 Equation 2.12 is the result of the substitution of Equation 2.11 into Equation 2.6, the thermal efficiency of the Carnot cycle and the maximum possible conversion efficiency for any heat engine, since all of the processes are reversible. Equation 2.12 It is then determined that the maximum thermal efficiency of a thermodynamically reversible heat engine depends upon the ratio of low and high temperatures in the thermodynamic cycle. Since the low temperature is usually fixed (ambient temperature), the efficiency is determined by the highest temperature in the cycle: the higher the temperature, the higher the efficiency. Comparison of energy storage technologies for applications of UEAV Page 14 André Lhamas, July 2013 2.2.2 Conversion efficiency of Direct Conversion systems The efficiency of a Fuel cell can be determined according to two definitions. The first, based on the First Law of Thermodynamics, can be used as ground of comparison with thermal heat engines. Like in the previously demonstrated Carnot efficiency, it compares the work produced by the cell with the fuel's heating value. The second, based on the Second Law of Thermodynamics, compares the cell's actual performance with the maximum work it could develop. To make an analogy with Heat engines, it would be as comparing the actual work of the engine with its Carnot efficiency. First Law Efficiency In an electrochemical cell, the system operates at constant temperature (isothermal process), i.e., the products of the reaction leave at the same temperature as the reactants, assuring that more of the chemical energy of the reactants is converted to electrical energy, instead of being consumed to raise the temperature of the products. Since the process is isothermal, the Carnot efficiency cannot be applied. Instead, the maximum work for an electrochemical cell, , is equal to the change in the Gibbs function (or Gibbs energy), , between products and reactants (see Equation 2.13). Equation 2.13 In this case, the work is performed by the movement of electrons through a difference in electrical potential, , and is related to the charge, , of the electrons moving through a potential difference, , resulting in Equation 2.14. Equation 2.14 In the previous equation, is the number of electrons transferred per mole of fuel and F is the charge carried by a mole of electrons, which is Faraday's number ( ). The First Law efficiency, i.e., the maximum thermal efficiency of an electrochemical cell is, as in heat engines, a ratio relating the net work of the system, yet the heat input, , is substituted for the higher heating value (HHV), resulting in Equation 2.15. Equation 2.15 It is in the equilibrium condition in which no current is being drawn from the cell, the open circuit voltage, , that the maximum thermal efficiency of an electrochemical cell is obtained. For a hydrogen-oxygen fuel cell, the value of the open-circuit voltage, at and , equals (this value can be determined by relating Equation 2.13 and Equation 2.14 and by using the tabulated Gibbs energy data). Substituting the value of results in the maximum thermal efficiency for the previously mentioned conditions (see Equation 2.16). Equation 2.16 The inefficiency's cause is related to the entropy generated from the chemical reactions. Comparison of energy storage technologies for applications of UEAV André Lhamas, July 2013 Page 15 Taking into account an ambient temperature of , the value of the high temperature in the Carnot cycle would have to be to match the 83% efficiency of Equation 2.16, as demonstrated in Equation 2.17: Equation 2.17 The reversible work for an electrochemical cell that uses hydrogen and oxygen is compared to the reversible work of a heat engine in Figure 6. The change in the Gibbs energy of the reaction decreases with the rise of temperature. Analysing Equation 2.13, one is able to see that that situation will result in a decrease in the maximum work output of the fuel cell with the rise of temperature. Figure 6: The reversible work produced by a H2/O2 fuel cell is greater than that of a Carnot engine at temperatures below 950 K. At higher temperatures, the Carnot engine is able to convert more of the HHV of H2 (285,840 kJ/mol) into work, [9] The Gibbs energy of the formation of water vapour is at the previously mentioned conditions and and decreases to at . As seen from Figure 6, the reversible work of the heat engine, using the HHV of hydrogen as the source of heat, increases with temperature. That is due to the fact that the Carnot cycle efficiency increases as well. The minimum temperature for which more reversible work is drawn from the combustion of hydrogen in a heat engine, rather than by direct conversion via fuel cell is, as seen in Figure 6, ). Thermal efficiency of car engines is usually calculated in terms of power, meaning the heat input written as a rate according to the flow rate of fuel. An attempt to do the same in the calculation of the efficiency of a fuel cell, leads to the appearance in the equation of a factor related to the completeness of the combustion of fuel consumed to produce an electrical current. The analogous concept to completeness of combustion in direct conversion systems is fuel utilization, a measure of the fuel consumed to produce an electrical current, leading to the equation of the electrical efficiency, detailed in Equation 2.18. Equation 2.18 In Equation 2.18, is the current in Amperes and is the flow rate of fuel in mol/sec. As seen from the previously mentioned equation, the inverse of the electrical efficiency is the Comparison of energy storage technologies for applications of UEAV Page 16 André Lhamas, July 2013 fuel stoichiometry. The fuel stoichiometry is in fact the relation between the amount of fuel fed and the amount required by the cell to provide the demanded electrons. If hydrogen is in fact used as fuel, resulting in a current efficiency of 83% and considering that one mole of hydrogen contains two moles of electrons , that means that 83% of the hydrogen is converted to electricity, while the remaining 17% either leave the cell without reacting or having reacted non-electrochemically, i.e., not contributing its electrons to the cell current). Second Law Efficiency "The Second Law efficiency, , of an energy conversion device indicates its degree of reversibility, comparing the actual work against the maximum work potential", [9] (see Equation 2.19). The performance of an actual heat engine, for instance, would be the actual work performed by the engine divided by the work produced by a Carnot cycle engine. In a way, it is like comparing the actual work of an engine with the maximum it could provide. Equation 2.19 Considering that, in a fuel cell, the thermal efficiency becomes voltage efficiency, as in Equation 2.20. Equation 2.20 The latter equation shows voltage efficiency to be a comparison between the actual voltage, , with the maximum possible voltage, . The actual meaning of this inefficiency applied to a Fuel cell represent is its losses (polarization) due to irreversibility. These phenomena will be further discussed in Section 2.3.4. Comparison of energy storage technologies for applications of UEAV André Lhamas, July 2013 Page 17 2.3 Fuel Cell The basic principle of operation of a fuel cell consists in reversing water electrolysis to generate energy (electricity and heat) and water from hydrogen and oxygen. It was first discovered in 1839 by William Grove and remains unchanged until today. "A fuel cell is an electrochemical "device" that continuously converts chemical energy into electric energy (and some heat) for as long as fuel and oxidant are supplied.", [9] Like mentioned previously, fuel cells, unlike batteries, continuously generate electricity, as long as fuel is supplied. The fuel, like previously demonstrated, does not go through any type of combustion, resulting in a quiet process, pollution free (again, depending on FC type and manner to which fuel is obtained) and increased efficiency when compared with combustion processes. The theoretical specific energy of hydrogen and oxygen combined in an electrochemical reaction is [11]. However, when the mass of fuel and oxidant storage tanks, as well as the mass of the fuel cell itself is taken into account, the overall specific energy density is reduced to values no greater than - still several times higher than that of a battery. According to the Manwearable Study Group Report [12], there are three main markets for fuel cell technology: stationary power, transportation power and portable power. 2.3.1 Types of fuel cells The primary classification of a fuel cell is related to the kind of electrolyte used. That classification leads to the knowledge of the kind of chemical reactions that take place in the cell and other factors, such as, [13]:  the kind of catalysts required;  the temperature range in which the cell operates;  the fuel required;  other. 2.3.1.1 Reactant type Fuel As the reducing agent, fuel cells are able to use hydrogen, methanol, methane, carbon monoxide , and other organic substances, as well as some inorganic reducing agents (e.g., hydrogen sulphide, hydrazine) Oxidizing agent As the oxidizing agent, fuel cells can use pure oxygen, air oxygen, hydrogen peroxide and chlorine. Versions with other exotic reactants have also been proposed, [12]. 2.3.1.2 Electrolyte type Fuel cells can use liquid electrolytes and solid electrolytes. Within the liquid electrolytes one can distinguish aqueous solutions of acids, alkalis and salts, and molten salts. The most common solid electrolytes are ionically conducting organic polymers, and inorganic oxide compounds. Comparison of energy storage technologies for applications of UEAV Page 18 André Lhamas, July 2013 Besides serving as separators, i.e., keeping reactants from reaching the wrong electrode space, solid electrolytes also have the advantage of reducing the danger of leakage of liquids from the cell. A liquid leakage in a fuel cell device may be a serious problem, since it may lead to corrosive interaction with the construction materials. 2.3.1.3 Working temperature One distinguishes low-temperature fuel cells, those having a working temperature of no more than ; and high-temperature fuel cells, over . Low-temperature fuel cells include membrane-type fuel cells, alkaline fuel cells and phosphoric acid electrolyte. Hightemperature fuel cells include fuel cells with molten carbonate (working temperature to ) and solid-oxide fuel cells (working temperature above ). In recent years, interim-temperature fuel cells with a working temperature in the range to have been introduced. These include certain varieties of solid-oxide fuel cells developed more recently. The temperature ranges are stated conditionally. The following table compares the different features between low temperature and high temperature fuel cells: Table 1: Features of low and high temperature fuel, [12] Low temperature FC High temperature FC Generally incorporate precious metal electrocatalysis to improve performance Increased operating temperature reduces the need for expensive electrocatalysts Require a relatively pure supply of hydrogen as a fuel (e.g. PEM catalysts are poisoned by carbon monoxide; AFCs damaged by carbon dioxide). This usually means that a fuel processor is required to convert primary fuels such as natural gas Fuel flexibility: can be operated on a range of hydrocarbon fuels; Benefits in Combined Heat and Power (CHP) generation Exhibit fast dynamic response and short start-up times Exhibit long start-up times and are sensitive to thermal transients Available commercially May require expensive and exotic construction materials to withstand the operating temperature, particularly in the balance of plant (piping, heat exchangers, etc.) Reliability and durability may be a concern 2.3.2 Overall Comparison Below is compiled a table resuming the main features of the different types of fuel cells previously discussed. Comparison of energy storage technologies for applications of UEAV André Lhamas, July 2013 Page 19 Table 2: Fuel cell Type Comparison, [14] Fuel cell Type Common Electrolyte Operating Temperature Typical Stack Size Efficiency Applications Advantages Disadvantages Alkaline (AFC) Aqueous solution of potassium hydroxide soaked in a matrix 10-100 kW 60% Military Space Cathode reaction faster in alkaline electrolyte, leads to high performance Low cost components Sensitive to CO2 in fuel and air Electrolyte management Polymer Electrolyte Membrane (PEM) Perfluoro sulfonic acid , Typically < 1 kW100 kW 60% transportation Backup power Portable power Distributed generation Transportation Specialty vehicles Solid electrolyte reduces corrosion & electrolyte management problems Low temperature Quick Start-up Expensive catalysts Sensitive to fuel impurities Low temperature waste heat Phosphoric Acid (PAFC) Phosphoric acid soaked in a matrix 400kW 100 kW module 40% Distributed generation Higher temperature enables CHP Increased tolerance to fuel impurities Pt catalyst Long start up time Low current and power Molten Carbonate (MCFC) Solution of lithium, sodium, and/or potassium carbonates, soaked in a matrix 45-50% Electric utility Distributed generation High efficiency Fuel flexibility Can use a variety of catalysts Suitable for CHP High temperature corrosion and breakdown of cell components Long start up time Low power density Solid Oxide (SOFC) Ytrria stabilized zirconia 1kW2MW 60% Auxiliary power Electric utility Distributed generation High efficiency Fuel flexibility Can use a variety of catalysts Solid electrolyte Suitable for CHP & CHHP Hybrid/GR cycle High temperature corrosion and breakdown of cell components High temperature operation requires long start up time and limits Direct Methanol (DMFC) Nafion <40% Portable electronic systems of low power, running for long times Technical maturity High energy density Fast start-up Commercially available Easy to carry and handle Simple to use Toxic fuel Higher MEA cost (may limit cost reduction facing other technologies) Limited power density Exhaust moisture Comparison of energy storage technologies for applications of UEAV Page 26 André Lhamas, July 2013 Equation 2.24 Where is the limiting current. The available cell voltage is the difference between the available potential in the cathode with the one at the anode, subtracted by resistance losses, as in the following equation. Equation 2.25 One must then have to take into account the other two mentioned losses occurring separately in both anode and cathode. The total polarization at the electrodes is the sum of activation, , and concentration polarization, , as demonstrated in the following equations: Equation 2.26 Equation 2.27 The effect of polarization is to shift the potential of the electrode to a new value : Equation 2.28 For the anode (Equation 2.29) and cathode (Equation 2.30): Equation 2.29 Equation 2.30 Substituting Equation 2.29 and Equation 2.30 into Equation 2.25: Equation 2.31 Or, since , then: Equation 2.32 From the equation above, it is possible to observe the decrease in cell voltage with the increase in current due to losses by electrode and ohmic polarizations. Ideal is that approaches as best as possible. That aim can be reached either by modifications in fuel cell design, such as improvement in electrode structures, better electrocatalysts, more conductive electrolyte, thinner cell components, etc., or by modifying the fuel cell operating conditions, i.e., higher gas pressure, higher temperature, change in gas composition to lower the gas impurity concentration. Nevertheless, complications regarding stability and durability arise within a fuel cell system when operating at higher temperature or pressure. Any modification in operation conditions must have the basic limitations of the working type into account. 2.3.5 Fuel Cell Performance Variables By analyzing the changes in the Gibbs free energy caused by variations in temperature and pressure, it is possible to know the effect of the variation of the mentioned parameters on the ideal potential, . Comparison of energy storage technologies for applications of UEAV André Lhamas, July 2013 Page 27 Equation 2.33 Equation 2.34 The reversible potential of the fuel cell decreases with an increase in temperature by 0,84 mV/ºC (Fuel Cell Handbook), since the entropy change for the reaction is negative (assuming reaction product is liquid water). However, for the same reaction, the reversible potential increases with an increase in pressure, since the volume change is negative. Figure 13 shows the practical effect of temperature on the voltage of some types of early life fuel cells, along with the variation of the reversible potential of fuel cells. The cell voltages of PEMFCs, PAFCs and MCFCs show a strong dependence on temperature, with the PEMFC exhibiting a maximum, but also PAFCs, MCFCs and PAFC show an increase in operating voltages, while the reversible potential decreases with increasing temperature. Figure 13: Dependence of the Initial Operating Cell Voltage of Typical Fuel Cells on Temperature, [19] 2.3.6 Balance of Plant The Balance of Plant (BoP) comprises all remaining infrastructural systems, components and structures necessary for the fuel cell operation. Since this is such a vast field of discussion and mentioning all necessary components for all the possible types of a fuel cell system would result in a large workload, [12] defines five functional areas within the BoP, as seen in Diagram 1. Comparison of energy storage technologies for applications of UEAV Page 28 André Lhamas, July 2013 Diagram 1: BoP subsystems, [12] Diagram 2: BoP subsystems expanded, [12] According to [20], significant BoP components include a primary pressure regulator, solenoid control valves, fill tube/port, and pressure gauge/transducer. A schematic for an automotive application based on the requirements defined in the draft European regulation "Hydrogen Vehicles: On-board Storage Systems" and US Patent 6.041.762 is given in Figure 14. Comparison of energy storage technologies for applications of UEAV André Lhamas, July 2013 Page 29 Figure 14: On-board compressed hydrogen storage system schematic, [20] 2.3.7 Fuel and Oxygen Storage An area fundamental to a FC system is the fuel and oxygen storage. The fuel storage issue is fundamental to the objective at hand. For that matter this will be properly discussed, while the oxygen storage will only be briefly discussed in the following sections. Fuel Storage According to [21], there are three leading options: 1. Compressed hydrogen gas storage: this system comprises storing compressed pure hydrogen as a compressed gas in a high-pressure cylinder. The gas can be stored at ambient temperature, not requiring costly thermal insulation. 2. Metal hydride storage: Hydrides are reversible hydrogen compounds formed by the absorption of hydrogen under moderate pressures and low temperatures. The storage tank contains powdered metals that absorb hydrogen and release heat when the hydrogen is forced into the tank under pressure. The hydrogen is released from the compound when the pressure is reduced and heat is applied. 3. Onboard methanol reformer system: This fuel storage type required the vehicle to have a methanol storage tank, a steam reformer to produce hydrogen gas and the rest of the components are the same as hydrogen powered fuel cell vehicles. 4. Cryogenic storage: although not considered in [21], there is the possibility of cooling the hydrogen to extremely low temperatures (-253ºC; 6-350 bar), thus liquefying the hydrogen. In a liquid state, hydrogen is denser, thus containing more stored energy, however tank insulation required to prevent hydrogen loss considerably increases weight, [22]. Comparison of energy storage technologies for applications of UEAV Page 30 André Lhamas, July 2013 Figure 15: Comparison of compressed gas vs. metal hydride options, [23] Development on Compressed Hydrogen Storage Tanks According to [24], there are four categories into which high-pressure tanks can be classified:  Type I: all metal tanks;  Type II: metal tanks (metal liner hoop) wrapped with resin-impregnated filament winding in a circumferential direction;  Type III: tanks are made of composite materials (fibreglass; carbon fibre) with a metal liner (aluminium; steel), i.e., the inside facing acting as barrier.  Type IV: Composite tanks with a polymer liner (mostly thermoplastic polymers, of the polyethylene or polyamide families). As seen from Figure 16, Type III and IV storage tanks with composite material composition can withstand higher pressure per mass, and thus store more Hydrogen. Filament winding is the used process for Types II, III and IV storage tanks, for it offers the best fibre/matrix interface and therefore the best stiffness/weight ratios, [25]. The process comprises the placement of fibre bundle onto a rotating removable mandrel in the same orientation as the load that it is intended to withstand. Composite parts for axisymetric structures, such as high pressure storage tanks, can be highly optimized through the use of this process, resulting in a greater performance factor, as seen from Figure 16. Figure 16: Comparative analysis of the performance factor of various tank types, [24] Comparison of energy storage technologies for applications of UEAV André Lhamas, July 2013 Page 31 Compressed Hydrogen storage tank systems assessment A technical assessment of compressed hydrogen storage tank systems for automotive applications was conducted by [20]. The study comprised the study of storage systems with design pressures of and for Type III and IV storage systems with singleand dual tank systems and compares each parameter result with the Department of Energy (DOE) 2010 and 2015 targets. The summary results of the assessment are given in Table 5. Table 5: Summary results of the assessment for Type III (T3) and Type IV (T4) single and dual-tank compressed hydrogen storage systems, [20] Since this assessment is intended for an automotive application, the two most important parameters that can be extracted from this table are the System gravimetric capacity, (weight of usable hydrogen divided by total onboard tank system weight), and System volumetric capacity in mass of per system cubic meter . Since this assessment was made for automotive applications, the mentioned cost parameters are only used for means of comparison, for they do would have to be recalculated if a UAV application is being considered. Regarding and the system volumetric capacity, the first conclusion drawn from this assessment is that the one-tank solution is more attractive than the two-tank solution regarding the two mentioned parameters, having identical Ownership and Fuel cost. Furthermore, the 2010 target of is reached in Type IV tanks for both pressures ( and ) and for both tank systems (singleand dual-tank), while the 2015 target for the same parameter is only reached for the Type IV, , one-tank solution. Regarding System volumetric capacity, however, assessment shows the parameter's failure in reaching both 2010 and 2015 targets in all given system possibilities. Oxygen Storage As previously mentioned, there are some types of FC applications that require the storage of oxygen for the system operation, whether it is because the system requires pure oxygen or because it leads to increased system efficiency. From [23], the available options for an autonomous underwater vehicle (AUV) that can be transposed to UAV are:  Compressed gas vessel;  Low-temperature liquid oxygen tank;  Hydrogen peroxide storage. Figure 17 provides with a comparison of the required weight and volume of the storage system, so as to store 16,7 kg of oxygen. Comparison of energy storage technologies for applications of UEAV Page 32 André Lhamas, July 2013 Figure 17: Comparison of oxygen storage options (16,7 kg Oxygen), [23] There is also a possibility for UAVs that cannot be transposed to AUV, which is the use of a compressor. Since PEM FC systems do not require a feed of pure oxygen, a compressor may be a suitable manner of providing oxygen at the required pressure to the system, without having to be concerned with heavy storage unit systems. Comparison of energy storage technologies for applications of UEAV André Lhamas, July 2013 Page 33 2.4 Internal Combustion Engine Internal combustion engines distinguish themselves for the high specific energy of its hydrocarbon and hydrogen fuels. Especially when compared with other energy storage systems, such as batteries. Despite having low thermal efficiencies as previously demonstrated (sometimes as low as 5%), the specific energy of hydrocarbon fuels can be as high as , and about as considering the required system mass, [26]. If this is compared to the and for alkaline and lithium batteries, respectively, one reaches the conclusion that ICEs can still be far superior to battery-motor electrical systems in many applications. Figure 18 compares the pure and system specific energy of some storage systems, such as compressed hydrogen at 700 bar and gasoline. Figure 18: Pure and System Specific Energy of storage systems, [26] Taking into account the flight consumption pattern of an UAV, a combination of ICE with an electric motor (EM), i.e., a hybrid system would be interesting. The hybrid system configurations that follow can also be thought of for applications where the Fuel Cell is used as the main propulsive system. For that matter, this issue will be further discussed. 2.4.1 Hybrid-electric Propulsion System In a hybrid propulsion system, two or more power sources are combined together to increase the efficiency of the vehicle. There are several possibilities, regarding configuration of hybrid systems, the most commonly used being:  Series  Parallel  Power-Split A study carried out by [27] demonstrated a significant decrease in energetic consumption for a UAV with hybrid configuration when compared to a four-stroke gasoline powered UAV: 52% and 22% less energy use for a one-hour and three-hour intelligence, surveillance or reconnaissance mission, respectively), [28] A brief description of the mentioned configurations will be performed below. Series Features: Comparison of energy storage technologies for applications of UEAV Page 34 André Lhamas, July 2013  EM is the sole component providing power to the mechanical drive train, leaving the Internal Combustion Engine (ICE) as an auxiliary power unit with the role of driving the EM to propel the aircraft, through the Generator and then Battery Advantages:  This configuration enables the ICE to operate at optimum torque and speed range, regardless of the driving conditions. Limitations:  Considerable energy conversion losses, since the mechanical energy of the ICE must be firstly converted to electrical energy and then back to mechanical energy through the EM to power the propeller, resulting in an overall system efficiency reduction.  Another limitation that arises from this configuration is the fact that the EM and Battery generally need to be bigger to meet peak power demands, even if the ICE can be smaller, since it only has to meet average power demands. Furthermore, the Generator needs also to be taken into account, resulting in a significant weight penalty and making this configuration unsuitable for small UAVs. See Figure 19. Figure 19: Hybrid Series configuration, [28] Parallel Features:  Commonly used in some of today's automobile hybrid vehicles (e.g., Honda Insight, Civic and Accord). Advantages:  Redundancy that comes from the ability of propelling the aircraft either by ICE alone, EM alone, or both simultaneously if required. In fact, this configuration ensures a combination of high endurance, acoustic quietness, high reliability, and redundancy. The latter being a very important feature in UAV applications. Limitations:  The ICE does not operate continuously in its most efficient region, naturally leading to decreased efficiency. A way of mitigating this effect is to integrate a Continuously Variable Transmission (CVT), rather than a conventional transmission. Doing so will Comparison of energy storage technologies for applications of UEAV André Lhamas, July 2013 Page 35 however raise difficulties in torque control and in maximizing efficiency when different or combined power sources are required. See Figure 20. Figure 20: Hybrid Parallel Configuration, [28] Power-Split Features:  Configuration currently in use on hybrid automobiles such as the Toyota Prius and comprises a planetary gear used to transfer the power generated by the ICE and/or EM to propel the UAV, therefore comprising no direct connection between the various power plants and the mechanical drive train. Advantages:  It combines the various power sources in a more efficient manor, thus reducing fuel usage. Limitations:  Cost and control are disadvantages which must be properly considered. See Figure 21. Figure 21: Hybrid Power split configuration with Planetary Gear, [28] Comparison of energy storage technologies for applications of UEAV Page 42 André Lhamas, July 2013 For that matter, the stiffness matrix in plane stress state is denoted by . The constitutive equations are written as: Equation 2.55 Where: Equation 2.56 Macromechanical behaviour of a laminate Now thinking of the macromechanical system, a brief description on how to determine the elastic properties of a laminate follows. Firstly, laminate coordinate system definition is in order. As seen in Figure 27, the laminate coordinate system is constituted by the axes , , and . The and axes define the plane of the laminate, and the axis is thus normal to this plane. Figure 27: Laminate coordinate system xyz, [33] In order to take into account the contribution of each layer to the laminate, there must be a distinction between them. In order to do so, it is first necessary to establish a layer numbering convention [33], as depicted in Figure 28, where the number of layers of the laminate is , the thickness of the laminate, . Furthermore, the top layer is considered to be layer number 1, and the bottom one, layer number . Moreover, the coordinates of the top and bottom surfaces of a given 'th layer are and , respectively. Figure 28: Layer and layer interface numbering convention for laminates, [33] Comparison of energy storage technologies for applications of UEAV André Lhamas, July 2013 Page 43 So as to consider the orientation of each layer, notation for that parameter is also required. Figure 29 depicts the notation for layer orientation, where is the rotation angle in the plane. Figure 29: Rotation of layer axes notation, [33] Remembering the stiffness matrix of plane stress in the 123 coordinate system, now comes the time to define a similar matrix considering the xyz-coordinate system, as well as taking into account layer orientation, for each layer. The result is the layer stiffness matrix, , and can be obtain through the transformation equation below: Equation 2.57 Where is the transformation matrix for the layer . Strain-Displacement Relations The laminate stress-strain relations can be stated in terms of stiffnesses as in Equation 2.58. Equation 2.58 Where N and M are the in-plane forces (forces per width corresponding to the stress state in the laminate) and moments (defined in a similar way to the previous; moment effect on the laminate stresses), respectively. The and are the laminate strains and curvature, respectively. The , and in the relation above are the in-plane, coupling and flexural stiffness matrices, respectively, and can be determined as demonstrated in Equation 2.59, Equation 2.60, and Equation 2.61. Equation 2.59 Equation 2.60 Comparison of energy storage technologies for applications of UEAV Page 44 André Lhamas, July 2013 Equation 2.61 The three matrices can be combined into a 6 by 6 stiffness matrix of the laminate. The laminate stress-strain relations can also be stated in terms of compliances, which leads to Equation 2.62. Equation 2.62 Where, in turn, , and are the in-plane, coupling, and flexural compliance matrices of the laminate. The combined compliance matrix is the inverse of the 6 by 6 stiffness matrix, as in Equation 2.63. Equation 2.63 Normalized stiffness and compliance matrices In order to reach the in-plane engineering constants of a laminate, one has to achieve the normalized in-plane, coupling, and flexural, , compliance matrices of the laminate, which can be written in terms of the normalized stresses, as in Equation 2.64. Equation 2.64 So as to obtain the normalized matrices, the normalization procedure for each of the matrices is as follows: Laminate Engineering Constants Finally, attainment of the in-plane engineering constants of a laminate can be performed using the analogy between the ply compliance matrix (see Table 6) and the normalized in-plane compliance matrix of a laminate. Comparison of energy storage technologies for applications of UEAV André Lhamas, July 2013 Page 45 2.6 UAV review Increased development of UAV systems took place in the end of the 20th Century, focusing on military applications. Surveillance, reconnaissance and detection were foreseen as the main applications for UAVs, however, over the past few years, a broad range of possible missions for UAV systems has been made possible, following a wide array of available sizes with maximum take-off weights (MTOWs) ranging from nano-scale (0,025 kg) to considerable scales of 10.000 kg. Research conducted by Frost & Sullivan, mentioned in the European Commission report [34], shows an increase of 1 000 to 5 000 in the number of UAVs deployed globally between the years of 2004 to 2008. The greatest contributor to this growth were the United States (US), whose budget and current need is larger than any other country or region in the world, [34]. Forecasted future investment on military Unmanned Aerial Systems (UAS) by the EU and the US is show in Chart 1. Chart 1: Expenditure on Military UAS (Europe and the USA), 2007-2016, [34] Until recently, military applications, such as the previously foreseen mentioned ones, as well as support and intervention in theatres of war, have been the developing drive. However, the technology has become increasingly interesting for civilian and commercial purposes, such as forest fire surveillance, law enforcement (e.g., border control, coast guard), as well as research and monitoring. 2.6.1 UAV Categorization According to UVS international, UAS are categorized according to performance parameters, such as the following:  MTOW: The maximum take-off weight is the maximum weight for which all airworthiness, i.e., safe flight, requirements are ensured for the aircraft to take-off.  Payload: defined by the cargo (e.g., sensors; cameras) capacity of the aircraft  Endurance: parameter defined as the maximum flight time;  Speed: cruise, loiter or maximum speed impact on both the time to station and the hourly acquisition rate (the area a UAS can swift in one hour), assuming the sensor package does not limit performance.  Range: Range is the maximum distance the aircraft can travel between take-off and landing, and it comes as a function of variation of aircraft weight (fuel), engine specific fuel consumption and aerodynamic configuration. Comparison of energy storage technologies for applications of UEAV Page 46 André Lhamas, July 2013 UVS International constructed a categorization of UAS, according to the previously performance features, given in Table 7. Table 7: Categorization for UAS, UVS International UAS Categories Acronym Endurance (h) Range (km) Flight Altitude (m) MTOW (kg) Tactical Nano Nano <1 <1 100 <0,025 Micro Micro 1 <10 250 <5 Mini Mini <2 <10 300 <30 Close Range CR 2 to 4 10 to 30 3000 150 Short Range SR 3 to 6 30 to 70 3000 200 Medium Range MR 6 to 10 70 to 200 5000 1250 Medium Range Endurance MRE 10 to 18 >500 8000 1250 Low Altitude Deep Penetration LADP 0,5 to 1 >250 50 to 9000? 350 Low Altitude Long Penetration LALE >24 >500 3000 <30 Medium Altitude Long Endurance MALE 24 to 48 >500 14000 1500 Strategic High Altitude Long Endurance HALE 24 to 48 >2000 20000 12000 Special Purpose Unmanned Combat Aerial Vehicle UCAV 2 1500 10000 10000 Lethal LETH 3 to 4 300 4000 250 Decoy DEC <4 0 to 500 5000 250 Stratospheric STRATO >48 >2000 20k to 30k tbd Exo-Stratospheric EXO tbd tbd >30000 tbd Space SPACE tbd tbd tbd tbd 2.6.2 Civil application of UAVs According to Frost & Sullivan [34], the European civilian UAV market is currently small. However it is expected to grow significantly over the next ten years, as seen from Chart 2. Comparison of energy storage technologies for applications of UEAV André Lhamas, July 2013 Page 47 Chart 2: European civil and commercial UAV market. Total Market by vertical segment, 2008-2020, [34] Furthermore, development is expected to occur within different governmental and commercial segments, as depicted in List 1. List 1: UAV application by market segment, [34] Government •Law enforcement (Police, Civil Security) •Border security •Coastguard Fire Fighting and Emergency Services •Forest fires •Other major incidents •Emergency rescue (e.g., Mountain rescue) Energy Sector •Oil and gas industry distribution infrastructure •Electricity grids / distribution networks Agriculture Forestry and Fisheries •Environmental monitoring •Crop dusting •Optimising use of resources (water, fertilizers) Earth Observation and Remote Sensing •Climate monitoring •Aerial photography mapping and surveying •Seismic events •Major incident and pollution monitoring Communications and Broadcasting •VHALE platforms as proxy-satellites •MALE / S/MUAS as short-term, local communications coverage Comparison of energy storage technologies for applications of UEAV Page 48 André Lhamas, July 2013 2.7 Goal of this Dissertation Having gathered the necessary information regarding UAV state of the art propulsive systems, and withdrawn the required conclusions, it is now possible to formulate an hypothesis to address the objective proposed in Section 1.: assess the feasibility, from a technological perspective (i.e., not considering costs), of a Fuel Cell based propulsive system for a UAV. The hypothesis of this dissertation is formulated, taking into account six considerations regarding UAV propulsion that follow. 1. Considering that the efficiency of a Fuel cell system is almost 2 times bigger than that of an ICE and that a PEM FC shows a 60% efficiency (confirmed by the PM catalogue: >52% efficiency, as in [35]), along with the 95% efficiency of the Launch Point Halbach Array Motor. a) ; ; b) ; 2. Considering that the Specific Energy contained in a H2 vessel at 700 bar reaches 33,3 kWh/kg, according to [26]; 3. Considering a possible Electric Motor (EM) with a weight of 3 kg. This assumption occurred because of the 7 HP Launch Point Halbach Array EM (available in Annexe D). The EM achieves 7 HP 5,2 kW weighing only 0,645 kg. Since Launch Point does not provide EM with the required power, an assumption of 3 kg mass was assumed it the EM were to achieve 20 kW; 4. Considering the implementation as the propulsive system of an Unmanned Aircraft with a Maximum Take-off Weight (MTOW) of 210 kg; 70 kg of which are available to the propulsive system. The power required for take-off is about 20 kW. This information was based on previous projects done by INEGI on an aircraft of similar dimensions and MTOW; 5. Considering that, as mentioned in Table 5, the DOE targets for system gravimetric capacity, wt%, i.e., the targets for stored mass of hydrogen per total system storage mass are as follows: a) The wt% in 2015 should be 5,5%; b) The ultimate target for the wt% is 7,5%; 6. Finally, considering the total FC mass, as well as End Plate mass weight in the total stack mass. It be should noted that, no Fuel Cell systems catalogues with power of 20 kW were found online. An assumption of using 2 Proton Motor 8 kW, combined with a 4 kW, resulting in 20 kW in total power was made ( . Stack total mass is then 38,4 kg , assuming an extra weight of 2,5 kg of BoP; results in 40,9 kg. The assumption of using 2 systems of 8 kW, combined with 1 of 4 kW, will naturally result in increased weight when facing one with 20 kW, since a larger system is expected to have lesser mass than a combination of smaller systems. Furthermore, one must take into account that the relative mass weight of the end plate in the total FC stack is 19% according to [3]. Since mass decrease improvements are expected in the bipolar plates, as already mentioned, the end plates are expected to have a greater significance in weight Comparison of energy storage technologies for applications of UEAV André Lhamas, July 2013 Page 49 distribution in the FC stack. Section 3 has further development on the reason for which this FC system was chosen. From the six previous considerations, the following assumptions can be made: 1. Ratio of FC/ICE efficiencies: ; Combining Bullets 2. and 5.:  2. (Specific Energy of 33,3 kWh/kg) and 5. (Gravimetric capacity, i.e., ratio of Hydrogen mass per mass of storage, wt%) o H 2 stored at 700 bar: 33,3 kWh/kg; o wt% (2015) = 5,5%: 1,832 kWh/kg (see Table 5); o wt% (ultimate) = 7,5%: 2,498 kWh/kg (see Table 5);  3. (EM) e 6. (FC weight) From consideration 3. that foresees an Electric Motor with the required power of 20 kW to weigh 3 kg and consideration 6. that establishes the mass ratio in a FC. o 3 kg (EM) + 38,4 kg (FC) + 2,5 kg (FC_BoP);  4. (Aircraft: Available weight for propulsion) o ; o   In the following calculation, the achieved energy can also be seen as if it was powering the propeller mechanical drain with roughly 12 kW for 5,5 h: . Therefore, considering the 66,555 kWh of available energy, for the 70 kg mass of the propulsive system, , a Specific Energy of is achieved, as in Equation 2.65. Equation 2.65 However, this specific energy was obtained considering the mass of the currently used End Plates. The objective of this dissertation is then to ascertain whether the introduction of Composite Materials in the end plates can result in a sufficient mass reduction, in order to significantly increase the specific energy of the concerned FC propulsive system, and provide a comparison with a conventional type of fuel, such as a gasoline ICE propulsive system. The specific energy of gasoline is , which is clearly lower than of Hydrogen stored at 700 bar. However, its gravimetric capacity is c. , i.e., the fuel mass over fuel storage system mass in gasoline systems is far greater than that of hydrogen, according to [26], causing it to have a far superior specific energy when considering the real practical system. Fuel Cell systems require a stabilizing element for power peak stages during flight, such as take-off. That can be achieved through battery hybridization, i.e., the implementation of batteries for peak power instants, as shown from the literature review in Section 2.4. This fact was not taken into account on the hypothesis for this aircraft. This consideration would imply further study in the battery area, which was not possible to achieve. Furthermore, if Comparison of energy storage technologies for applications of UEAV Page 50 André Lhamas, July 2013 considered, the increased weight caused by the introduction of batteries, could possibly result in rendering FC systems unfit for current existing FC systems. Nevertheless, this could be avoided if a different take-off mechanism were to be considered. Such a mechanism would provide the necessary energy for take-off to the aircraft, making unnecessary the use of heavy batteries for the take-off power peak. The mechanism could be for instance a catapult or a tow system, the latter resembling current glider aircrafts. Comparison of energy storage technologies for applications of UEAV André Lhamas, July 2013 Page 51 3. Conceptual Development This third section of the dissertation discusses the conceptual development path taken. It provides a description of the concepts, and all the required assumptions to proceed to the stage that follows. The sketching of small to medium sized parts, as is the case of the parts dealt in this dissertation, on computer aided design is often easier if the length units are in millimetres. However, that fact naturally has its implications in many other dimensional properties, as e.g. mass coming in tonnes, rather than kilograms. For that reason, consistency in units is required and the commonly designated "MPa" unit system was used. Table 8 shows the required units to consider for several properties when using millimetres as a dimension for length. Table 8: Table of consistent units Property or Load SI (International System of Units) "MPa" unit system Mass Length Time Temperature Velocity Acceleration Force Moment Pressure Density Comparison of energy storage technologies for applications of UEAV Page 58 André Lhamas, July 2013 Figure 34: Composite Insert Quantity of Holes in Plate An issue that immediately came up was the number of plate holes for screw fixation. An uneven number of screws on each of the top and bottom sides, leading to the existence of plate fixation in the middle, is thought to assist in decrease stress and strain in the middle, though undermining composite strength. For that matter, all suggested composite concepts come with 10 holes (5 on top, 5 on the bottom), instead of the 8 holes of the metal plate. Normal Plate This plate was the first composite design thought upon and tested. Its name is due to its basic design (rectangular plate with similar dimensions to the metal end plate). Figure 35 depicts the dimensions of the normal plate, with hole positioning, as well as the contact surface of the normal plate with the graphite block (inner rectangle). It should be noted that hereinafter this will be the basic geometry of every composite concept, with the exception of the Inertia plate. Comparison of energy storage technologies for applications of UEAV André Lhamas, July 2013 Page 59 Figure 35: Normal plate dimensions. Positioning of holes and contact surface with graphite block (inside rectangle) Figure 36 depicts a Solid Works rendering of the Normal plate, having a thickness of 10 mm. Figure 36: Normal plate with Solid Works rendering Pre-curvature This concept came up as a suggestion from Professor Dai Gil Lee from the Korea Advanced Institute of Science and Technology, as he had already performed work on this area, and with a similar concept. Comparison of energy storage technologies for applications of UEAV Page 60 André Lhamas, July 2013 An end plate with a pre-made curvature is seen as a possible way to resolve the non-uniform stress issue, for once the clamping force is performed, the plate is expected to deform and apply a uniform pressure to the fuel cell stack. A pre-curvature theoretical calculus should have been made prior to plate design, however, the calculus was said to be too demanding, as well as too time consuming. Since there is already conclusive literature on this issue, for an end plate with similar dimensions (100x200) to the ones being considered, a decision for the radius dimension was taken based on the results of the mentioned paper. The plate that achieved the biggest uniformity was a plate comprising two types of composite materials: glass fibre-epoxy and carbon fibre-epoxy. Its details are shown in Table 12. Table 12: Flexural Stiffness (EI), curvatures (R) of the best performer composite plate tested in [18] Composite End Plate Glass/epoxy Carbon/epoxy EI R (m) Stacking sequence Thickness (mm) Stacking sequence Thickness (mm) 10,5 4,5 3782 10,52 The curvature is of 10,52 mm. For that matter, a curvature of 10 mm was chosen to the design of the pre-curvature plate used in this report. Figure 37 and Figure 38 depict the curvature in the plane, and plane, respectively. Figure 37: Pre-curvature plate curvature in the y-z plane Figure 38: Pre-curvature plate curvature in the x-z plane Comparison of energy storage technologies for applications of UEAV André Lhamas, July 2013 Page 61 Figure 39 depicts a Solid Works render of the Pre-curvature plate. Figure 39: Pre-curvature plate sketched as surface Shoe Box This concept was thought upon as a good strategy to create a rigid cover. It was named because its basic concept consists of a conventional shoe box cover, as shown in Figure 40. Figure 40: Conventional Shoe Box cover This conceptual plate would have a carbon fibre supports, connecting the face to each of the perpendicular side faces for increased rigidity. These supports would be adhered to each surface through the use of an epoxy resin. Figure 41 is the result of a Solid Works rendering to demonstrate the thought-of concept. Comparison of energy storage technologies for applications of UEAV Page 62 André Lhamas, July 2013 Figure 41: Shoe Box concept with Solid Works rendering Supports The whole assembly then consists of the Shoe Box cover, along with the adhesive contact and the supports. There are 8 supports separated by 28,5 mm along each horizontal face on the bottom and upper side, and 5 supports separated by 21,25 mm located on both vertical edges. Figure 42 shows the positioning of the supports in the shoe box face. Figure 42: Placement of supports in shoe box face Figure 43 depicts the remaining dimensions of the support and adhesive. Comparison of energy storage technologies for applications of UEAV André Lhamas, July 2013 Page 63 Figure 43: Dimensions of the support and epoxy adhesive Inertia Plate The inertia plate was named as so because it intended to decrease deflection through increase in moment of inertia. It comprises a centred face with the same dimensions of the other suggest composites, however the sides protrude. This was a big increase in volume, and with it, mass. Figure 44 depicts the general dimensions of the Inertia plate. Figure 44: General dimensions of the Inertia plate Figure 45 and Figure 46 depict a front and a rear view, respectively, of the Inertia plate. Comparison of energy storage technologies for applications of UEAV Page 64 André Lhamas, July 2013 Figure 45: Front view of Inertia plate Figure 46: Back view of the Inertia plate Reinforcement Plate The reinforcement plate was conceived to be located between the suggested plates and the graphite block, so as to provide extra rigidity, as well as distribute stress more evenly across the graphite block's contact surface. Figure 47 depicts the reinforcement plate's general dimensions. Figure 48 depicts a Solid Works render of the reinforcement plate with the used thickness (10 mm). Figure 47: Reinforcement plate general dimensions Figure 48: Reinforcement plate Solid Works render Comparison of energy storage technologies for applications of UEAV André Lhamas, July 2013 Page 65 3.2.3 Conceptual Plates Table 13 comprises a summary of the applied concepts. Table 13: Conceptual plates Component Metal End Plate Steel End Plate Aluminium End Plate Shoe Box Assembly Shoe Box Face and Cover Adhesive for Shoebox Comparison of energy storage technologies for applications of UEAV Page 66 André Lhamas, July 2013 Support Double Pre-curvature Inertia Plate Normal Reinforcement Comparison of energy storage technologies for applications of UEAV André Lhamas, July 2013 Page 67 3.3 Clamping pressure assumption The assumed clamping pressure was taken from the literature review. Work on the design of end plates for a 5 kW PEM FC, an assumption of 1,2 MPa for the clamping pressure was found suitable in that instance for the selected MEA. In fact, for safety testing purposes, a 1,5 MPa pressure was tested in the mentioned paper, [2]. In this case, having no information regarding the clamping pressure of the concerned system, and since the rated power is greater, a pressure of 1,5 MPa was assumed. 3.4 Selected ICE system In order to perform a proper comparison, an ICE had to be chosen. There is, in the UAV industry, great concern over durability and reliability of engines. For that matter, engines with a higher rated power than that of aircraft requirements are often chosen, since working in a lower power than the rated one, results in increased reliability, though not working in the ideal state, leads to increased consumption. For this case of 20 kW required for propulsion, the suggestion was to select an engine with about 30 kW of power. For that purpose, the UAV Engines Limited AR741 Wankel engine, a Single rotor Wankel-type spark ignition engine with 28,3 kW of power at 7800 RPM was chosen, as seen in Figure 49. Figure 49: The UAV Engines Limited AR741 Wankel engine, [36] Table 14 comprises the technical specifications of the selection engine. Table 14: Technical specifications of the selected engine - UAV Engine Limited AR741 Wankel engine, [36] Technical Specifications Chamber size 208 Maximum Power Output at 7800 RPM 37,95 HP 28,3 kW Fuel Type Mogas regular grade , leaded or unleaded, or AVGAS 100LL Vibration Nominally zero radial vibration Comparison of energy storage technologies for applications of UEAV Page 74 André Lhamas, July 2013 Table 16: Used materials for each component and part Part/Assembly Material Metal End Plate Steel End Plate Steel Aluminium End Plate Aluminium Shoe Box Assembly Shoe Box Face and Cover CFRP Shoebox Adhesive Epoxy Support CFRP Double Pre-curvature CFRP Comparison of energy storage technologies for applications of UEAV André Lhamas, July 2013 Page 75 Inertia Plate CFRP Normal CFRP Reinforcement CFRP Graphite Block Graphite Block Assumption Material For this model, only elastic analysis were performed, since no plastic behaviour for the mentioned materials is foreseen with an applied pressure of 1,5 MPa. In fact, CFRP and the Graphite assumption are brittle; therefore do not possess plastic behaviour even if nonlinearity can be seen in delamination/damage propagation of composites. For that matter, only the elastic mechanical properties of each material were considered. As shown in Section 2, unidirectional composite materials, unlike steel and aluminium e.g., are not isotropic and it is necessary to distinguish the mechanical property description of each. Regarding material selection and assumption, for the considered steel and aluminium materials, no specific catalogue material was selected. Since there was no clear idea on what to consider for the graphite block, an assumption was made regarding its mechanical properties and mass density, based in a comparison with common graphite The assumption was performed considering that graphite has the following properties.   Comparison of energy storage technologies for applications of UEAV Page 76 André Lhamas, July 2013  , The graphite block, i.e., the combination of all the components within the clamped cells, was assumed to have . The assumption made regarding its mechanical properties was that, since mass density ratio ( is roughly 1 to 2, then its mechanical properties would also be divided two fold (see Table 17). Table 17 and Table 19 comprise the mechanical properties for the modelled isotropic and orthotropic materials, respectively. Table 17: Mechanical Properties of the considered isotropic materials, and respective mass density Isotropic Material Young Modulus, E (GPa) Poisson Coefficient, Mass density, Steel 210,00 0,3 7800 Aluminium 70,00 0,3 2400 Epoxy 3,66 0,36 1200 Graphite Block Assumption Material 5,50 0,3 1000 The chosen CFRP was the Toray M40J (data sheet available in Annexe A). Table 18 displays the properties of the fibre employed in the composite M40J. Table 18: Toray M40J Fibre Properties Fibre Properties Fibre Tensile Modulus 377 GPa Fibre Density 1770 kg/m3 Composite Properties Composite Tensile Modulus 230 GPa Tensile Strain 1,1% Fibre volume 60% Matrix Toray 250ºF Epoxy Resin Considering that the M40J CFRP has 60% fibre volume, i.e. , then recalling Equation 2.41and Equation 2.48, the values for and should be c. 227 GPa and 11 GPa, respectively, suggesting that the 230 GPa of Composite Tensile Modulus in Table 18 are the result of an approximation. Since no data was available regarding shear modulus: , , ; and Poisson ratios: , and , these parameters had to be assumed. Recalling Table 6:    Comparison of energy storage technologies for applications of UEAV André Lhamas, July 2013 Page 77  The following parameters were assumed to have the following values:    Thus, resulting in . Composite mass density can also be obtained through the rule of mixtures, as in Equation 4.1. Equation 4.1 All parameters, except for the mass density of the matrix, , are known. Since no data is provided by the manufacturer on this matter, is assumed to be after consultation of similar matrixes in the software ESAComp. As previously mentioned, Table 19 comprises the mechanical properties, along with the mass density of the employed CFRP. Table 19: Mechanical Properties of the considered orthotropic material, and mass density Non Isotropic Material (GPa) (GPa) (GPa) (GPa) (GPa) (GPa) CFRP 227 11 11 0,25 0,25 0,25 5 5 4,4 1542 Table 20 describes the stacking sequence, ply thickness, and resulting plate thickness for the considered carbon/epoxy composite material used. Table 20: Carbon/epoxy stacking sequence and ply thickness for the considered composite parts Carbon/epoxy composite Stacking Sequence Ply thickness (mm) Resulting plate thickness (mm) 0,200 10 For the composite section assignment, the Abaqus Composite Layup Manager was used. This manager enables the definition of several parameters for each ply of the laminate, such as the intended region, material, ply thickness, coordinate system, fibre orientation (Rotation Angle), and number of integration points, as seen in Figure 52. This Manager can be applied in either Shell or Solid Elements. As for Shell Elements, the Manager allows the offset to be performed in the location best preferred by the user: Top, Middle, Bottom or User defined surface. Figure 52: Abaqus Composite Layup Manager Comparison of energy storage technologies for applications of UEAV Page 78 André Lhamas, July 2013 4.1.3 Contact definition Contact definition is an important aspect of a numerical model where two or more components are, or come into contact throughout the simulation. The contact regions of the model can be defined as the regions where contact between parts takes place. There were two types of contact definition considered for both considered models:  Through the Interaction Manager, a Surface-to-Surface contact was selected, having considered "Normal Behaviour", Finite Sliding and allowed separation after contact in the Contact Property options. This type of contact definition is most suitable for contact of materials that have friction between each other, having always been used in the contact definition between the Graphite Block and the Reinforcement Plate or End Plate that followed.  Through the Constraint Manager, a Tie type constraint was defined. This type of contact definition was used always between the Composite End Plate concepts and reinforcements, as well as between the Shoe Box face and adhesive and support. A Position Tolerance of 0,1 mm was considered. Having defined the interaction properties, Abaqus then asks to proceed to select the surfaces that come into contact defining each as slave and master surfaces. The contact interactions are defined in Table 21. Table 21: Contact description and representation Contact Description Representation Interaction between Graphite Block and End Plates Tie between Plates and Reinforcement Tie between Shoe Box side face and Adhesive Comparison of energy storage technologies for applications of UEAV André Lhamas, July 2013 Page 79 Tie between Shoe Box centre face and Adhesive Tie between Adhesive and support Figure 53: Example of tested assembly; Plate on the left, reinforcement in the middle, and graphite block to the right 4.1.4 Load and External connections definition This part of the section describes the approach taken to loads and fixation. Independently of the model, null displacements in the directions 1 and 2 ( and ) were performed in a centre point of each part, as well as null displacement in direction 2 in a side point of each part, in order to prevent unintended torsion. This situation is exemplified in Figure 54. The orange triangles represent the central and side fixations. Comparison of energy storage technologies for applications of UEAV Page 80 André Lhamas, July 2013 Figure 54: Double pre-curvature plate; displacements disabled (orange triangles) in centre (U1,U2; x and y) and side (U2; y) As mentioned previously (see Figure 51), the clamping force is applied in the holes, and the fixation is performed in the graphite block symmetry face, as seen from an above view in Figure 55. Also seen in Figure 55, the axis is normal to the CFRP block. Stress and displacement results will be obtained in the mentioned axis. Figure 55: Loads (yellow arrows) and fixation (orange and blue triangles) seen from above The calculus of the equivalent force, , calculated knowing the assumed pressure, , and the area, , onto which it would be applied, i.e., the area of the contact surface of the graphite block with the end plate. Remembering Section 3.3, the assumed pressure, taken from [2], is as follows: Comparison of energy storage technologies for applications of UEAV André Lhamas, July 2013 Page 81 and are then: The obtained force must then be divided by the number of holes present in the plate. This differs from the metallic end plate to the composite suggested ones, since the metallic end plate has 8 holes, while the composite has 10. Since each hole contains 4 points, the force must then be divided by 4. Table 22: Equivalent force per hole for the metal and composite end plates Number of holes (N) Equivalent force per point (N) Metal end plate 8 3186 796,5 Composite end plate 10 2548,8 637,2 It should be noted that, in reality, the reaction force of each screw is not the same for all. In fact, the reaction force should be bigger on the centre holes than in the side holes. A different, and perhaps more accurate, approach would be to consider the pressure to be applied in the face of the graphite block located on the symmetry plane. The fixation could be performed by a tool present in Abaqus, which consists of rigidly fixing reference points in the holes, such as if they were springs, being that the rigidity of the spring would be the one of the screw/fixating element. 4.1.5 Discretization Perhaps one of the most important and influential aspects in the construction of a numerical model is finite element discretization. Quality of results is strongly influenced by the type of elements employed, as well as finite element mesh quality. Solid (C3D8) and shell (S4) elements were used, always with reduced integration, hence the "R". Table 23 describes the type of element, quantity of nodes, and quantity of elements for the meshed parts. Comparison of energy storage technologies for applications of UEAV Page 82 André Lhamas, July 2013 Table 23: Type of Element, Quantity of Nodes and Elements for each part Component Type of Element Quantity of Nodes Quantity of Elements Metal End Plate Steel End Plate Solid C3D8R 37671 31587 Aluminium End Plate Shoe Box Assembly Shoe Box Face and Cover Solid C3D8R 6654 4200 Adhesive For Shoe Box Solid C3D8R 576 245 Support Shell S4R 1793 1696 Double Pre-curvature Shell S4R 852 753 Inertia Plate Solid C3D8R 17772 14100 Normal Shell S4R 4140 3936 Reinforcement plate Shell S4R 521 456 Graphite Block Solid C3D8R 102846 95160 The accuracy of the results from a FEA is directly related with the number of elements used. However, an increase in element number leads to an increased computational cost, and for that matter, any mesh refinement has to be properly considered by the user. In this sense, it would be wise to increase element number in the parts/areas of the model where the gathering of results will take place, thus leading to greater productivity. Partition The part partition process is a division of geometrically complex parts, containing holes, and/or parts resultant from merge of two or more parts. It is an essential procedure in order to generate a good quality structured mesh and which can be considerably time consuming. Abaqus offers a "Auto-partition" tool, however it rarely performs the partition in the most efficient way. For that reason, all of the partitions performed in the tested parts were manual partitions, that were often very complex and demanding tasks. Such was the example of the pre-curvature plate shown in Figure 56, for not only were the holes a demanding problem to solve, but also the fact that the surface had a double curvature, leading to a very time demanding task. Comparison of energy storage technologies for applications of UEAV André Lhamas, July 2013 Page 83 Figure 56: Pre-curvature plate partition 4.1.6 Results The higher values of stress and displacement were generally found in the upper and lower edges of the graphite block. For that matter, using the "path" tool on Abaqus, the collection of nodes on those edges was established, so that the displacement could be then gathered for each point so as to be analysed and to create a plot. The top and bottom edge paths can be seen in Figure 57 and Figure 58, respectively. Figure 57: Top edge path on the graphite block Figure 58: Bottom edge path on the graphite block There was however the exception of the pre-curvature plate. Its maximum displacement and stress were not found in the upper nor lower edges, as seen in Figure 59 and Figure 60, which show the displacement and stress distribution on the graphite block's face. For that matter its path was covered elsewhere, as shown in Figure 61. The higher displacement values are negative. This is obviously related to approach taken to model construction, i.e., the application of force in the holes, naturally results in negative displacement, considering the employed coordinate system. For that matter, the areas in blue in Figure 59 and Figure 61 are the places with the biggest displacement, contrary to Figure 60, where red stands for the location with the highest stress. Comparison of energy storage technologies for applications of UEAV Page 90 André Lhamas, July 2013 No Reinforcement 10 mm reinforcement Top edge Note: the colours in the displacement column are inverted due to negative displacement, i.e., blue stands as maximum displacement, while red minimum. Comparison of energy storage technologies for applications of UEAV André Lhamas, July 2013 Page 91 5. Results Discussion 5.1 End Plate Numerical Model results As was expected, the steel EP has the smallest deflection, however three of the composite plates (Inertia Plate, Shoe box and Pre-curvature) concepts succeed in achieving smaller deflection than the Aluminium end plate. Of those three, the one that succeeds in having the smaller mass is the pre-curvature, as seen previously in Table 25. The best composite performer in this model is considered to be the pre-curvature plate with reinforcement. Although it did not achieve the lowest values of stress and deflection, the difference between these results and the best performer, the Inertia Plate, is not very significant, and more importantly, its mass is significantly lower than the Inertia Plate: 0,776 kg to 2,482, little more than 3 times lower. Furthermore, its stress-deflection results are clearly better than the aluminium plate. Moreover, looking at the stress-displacement figures provided by Abaqus in Table 26, one can see that the pre-curvature plate has a configuration that is completely unlike all the others. It has apparently managed to distribute stress differently, especially out from the corners which now have a similar stress to the centre, unlike the other concepts, where the critical stress is in the corners. A significant stress concentration (reddish areas) can be seen in what seems to be an ellipse, in which the biggest stress value is of c. 5,9 MPa, against the 5,7 MPa of the Inertia Plate and 4,1 MPa of the steel end plate. A study on cell performance would have to be made in order to ascertain which of the stress distributions result in the best performance, however, having a similar value of stress as its competitors only placed in a more centred region rather than in the block's corners, the pre-curvature plate has apparently the most uniform stressdisplacement distribution of all the concepts studied. Within the concepts without a reinforcement plate, only the Inertia was selected, the others had far worse results than the average in such a way that it did not make sense to consider them, for it would harm the comparative analysis. Another interesting observation is that the introduction of the reinforcement plate in the assembly successfully managed to mitigate graphite block deflection caused by the bottom holes. As seen in Figure 63, the aluminium plate curve has two irregular parts of the curvature, which seem to be created by the holes, for they are positioned close to the locations of the holes. On the remaining components, this does not seem to occur with such a profound impact. Lack of experience in Abaqus modelling, along with short time for the necessary work dictated composite inserts could not to be considered in the models, however their use would Comparison of energy storage technologies for applications of UEAV Page 92 André Lhamas, July 2013 possibly reduce composite plate stress and deformation for the reasons previously mentioned, although having a negative impact on system mass. While the chosen CFRP has a relatively high Young Modulus (230 GPa), a higher modulus Carbon Fibre could also have been chosen. The employment of such a material would also have resulted in decreased strain results. The results on the suggested composite concepts regarding stress and strain can then be seen as the worst case scenario; however they are not when considering mass. As previously mentioned, the introduction of composite inserts would result in an increase in mass. Comparison of energy storage technologies for applications of UEAV André Lhamas, July 2013 Page 93 5.2 Comparison with other propulsive systems 5.2.1 Contained Energy and Specific Energy determination This section discusses the calculations required to determine the specific energy of the metal end plate systems as well as the ones suggested from the composite concepts. The aim is to propose a comparison between them, and lastly a comparison with a typical ICE gasoline system will be attempted. The objective is to determine the feasibility of the Fuel Cell system with the proposed modifications. Only the pre-curvature plate concept, the composite plate best performer, will be considered for the reasons previously stated. Table 27: PM 8 kW, PM 4 kW and proposed combination masses FC stack Mass (kg) PM 4 kW 9,2 PM 8 kW 14,6 38,4 Since aluminium was assumed to be the material of the Proton Motor 8 and 4 kW FC systems, the calculus to determine the mass of the remaining components within the stack will be made taking into account the aluminium end plate masses. Table 27 provides the mass for the aluminium FC with one, and two end plates, along with the end plate mass ratio, i.e., the mass of the plate divided by total stack mass. Furthermore, it provides the mass of the remaining components within the stack. Table 28: Mass ratio of 2 end plates and determination of the mass of the remaining components 1 EP mass (kg) 2 EP mass (kg) EP 8 kW mass ratio (%) EP 4 kW mass ratio (%) Remaining mass 8 kW (kg) Remaining mass 4 kW (kg) Aluminium Concept mass (kg) 2,005 4,010 27,47 43,59 10,59 5,19 The total stack masses for the steel and CFRP are obtained by adding the remaining masses calculated before, with the mass of both end plates. The resulting total stack masses of the steel and CFRP end plate systems are given in Table 29. Table 29: Total mass for the steel and CFRP Plate 4 and 8 kW FC stack End Plate Concepts 1 EP mass (kg) 2 EP mass (kg) FC stack mass 8 kW (kg) FC stack mass 4 kW (kg) Steel 5,794 11,588 22,178 16,778 CFRP 0,776 1,552 12,142 6,742 Thus, the total stack mass for each end plate material, along with the mass ratio of each for both 8 and 4 kW stacks, as in Table 30. Comparison of energy storage technologies for applications of UEAV Page 94 André Lhamas, July 2013 Table 30: Total stack mass for aluminium, steel and CFRP 8 kW mass (kg) EP mass ratio 8 kW (%) 4 kW mass (kg) EP mass ratio 4 kW (%) Aluminium 14,600 27,47 9,200 43,59 Steel 22,178 52,25 16,778 69,07 CFRP chosen 12,142 12,78 6,742 23,02 Having determined the FC stack mass for each of the considered materials, it becomes possible to estimate the mass of the global system, with the aim of determining the available mass for fuel, . Knowing that the available mass for propulsion established in the hypothesis section must contain every required component/sub-system for the propulsion, wherein the FC case comes as follows: the mass of the electric motor, , the FC stack mass, , the balance of plant mass, along with the , as in Equation 5.1. Equation 5.1 Being that the is the only unknown parameter, one can easily determine it for each material case, as shown in Table 31. Table 31: Calculation of the available mass for fuel for the FC propulsive system Available propulsion mass (kg) EM mass (kg) FC stack mass (kg) BoP mass (kg) Available mass for Fuel (kg) Aluminium 70 3 38,4 2,5 26,100 Steel 61,134 3,366 Chosen CFRP 31,026 33,474 Recalling the gravimetric targets of the DOE for a 700 bar vessel, stated in the hypothesis in Section 2.7, it is now possible to determine the contained energy by multiplying the available mass for fuel by the specific energy of each of the gravimetric goals, as seen in Equation 5.2 and Equation 5.3. Equation 5.2 Equation 5.3 Then, recalling Equation 2.65, it is possible to determine the specific energy by dividing the result for the total propulsive system mass, as shown in Equation 5.4 and Equation 5.5. Equation 5.4 Equation 5.5 Comparison of energy storage technologies for applications of UEAV André Lhamas, July 2013 Page 95 The results for the contained energy considering both gravimetric targets, as well as Specific energy, for the three considered materials are shown in Table 32. Table 32: Contained energy and specific energy for the aluminium, steel and CFRP FC stacks, according to the gravimetric targets of the DOE Energy contained in H2 5,5% (kWh) Specific Energy 5,5% (kWh/kg) Energy contained in H2 7,5% (kWh) Specific Energy 7,5% (kWh/kg) Aluminium 47,802 0,683 65,185 0,931 Steel 6,165 0,088 8,407 0,120 CFRP chosen 61,308 0,876 83,601 1,194 Table 33 shows the resulting values for the same parameters seen in Table 32, considering a 60% overall efficiency for the FC propulsive system. Table 33: Contained energy and specific energy for the aluminium, steel and CFRP FC stacks, according to the gravimetric targets of the DOE, considering 60% overall efficiency FC system, considering 60% efficiency Energy contained in H2 5,5% (kWh) Specific Energy 5,5% (kWh/kg) Energy contained in H2 7,5% (kWh) Specific Energy 7,5% (kWh/kg) Aluminium 28,681 0,410 39,111 0,559 Steel 3,699 0,053 5,044 0,072 CFRP chosen 36,785 0,525 50,161 0,717 In order to make a comparison with an ICE, the same computational procedure as before is required to be performed. Recalling Equation 5.1, the calculation of the available mass for fuel (gasoline), , only now one has to consider the ICE mass, , the generator mass, , as in Equation 5.6. Equation 5.6 Again, resolving considering as unknown, results in the values in Table 34. Table 34: Determination of the available mass for fuel (gasoline) for the ICE propulsive system Available Propulsion mass (kg) ICE mass (kg) Generator mass (kg) Available mass for fuel (kg) 70 10,7 3,5 55,8 Considering the fuel (gasoline) to have a gravimetric specific energy of 8 kWh/kg, as stated in the hypothesis, and multiplying it by the available mass for fuel, as in Equation 5.7. Equation 5.7 The calculus of the specific energy of the ICE is depicted in Equation 5.8. Equation 5.8 The results are shown in Table 35. Comparison of energy storage technologies for applications of UEAV Page 96 André Lhamas, July 2013 Table 35: Contained energy and specific energy of the ICE propulsive system ICE propulsive system Energy contained in Fuel (kWh) Specific Energy (kWh/kg) 446,4 6,377 Finally, considering an overall system efficiency of 30% results in: Table 36: Contained energy and specific energy of the ICE propulsive system, considering 30% overall system efficiency ICE system, considering 30% efficiency Energy contained in Fuel (kWh) Specific Energy (kWh/kg) 133,920 1,913 Finally, having calculated the available and specific energy for all system, it is possible to make a comparison between the four. Figure 64 and Figure 65 comprise a comparison between the available energy and specific energy, respectively, of the analysed systems. Figure 64: Available energy of the considered FC systems with different EP materials, as well as ICE system Figure 65: Specific energy of the considered FC systems with different EP materials, as well as ICE system The steel end plate achieves an insignificant specific energy, proving that it was definitely not suitable for the considered stack power design. The ICE system is by far the one that achieves the biggest specific energy, having almost 2,7 times more than the CFRP end plate stack, as seen in Table 37. Table 37: Comparison Specific Energy ICE-CFRP FC Specific Energy (Wh/kg) Ratio ICE/FC ICE 1,913 2,667 CFRP EP FC 0,717 Nevertheless, the CFRP succeed in achieving a 28,30% increase in specific energy, facing the used aluminium plate, as seen in Table 38. Table 38: Comparison Specific Energy CFRP FC-Al FC Specific Energy (Wh/kg) Ratio CFRP EP/Al EP CFRP EP FC 0,717 1,283 Aluminium EP FC 0,559 0 20 40 60 80 100 120 140 Available Energy (kWh) Aluminium EP FC Steel EP FC CFRP EP FC ICE 0 0,5 1 1,5 2 2,5 Specific Energy (kWh/kg) Aluminium EP FC Steel EP FC CFRP EP FC ICE Comparison of energy storage technologies for applications of UEAV André Lhamas, July 2013 Page 97 The Specific Energy, along with the available energy, in the ICE system is far greater (almost 2,7 fold) than that of the fuel cell. Even if with significant mass reduction, the Fuel Cell cannot yet achieve a sufficient result to match the conventional ICE system. However, significant mass reduction, and subsequent increase in Specific Energy was achieved. In fact, the increase in Specific Energy and Available Energy was of . Even though no assumption or result validation was performed, it has been proven that the suggested composite material concepts can bring a very interesting improvement to FC systems. Note that there were several critical assumptions made in this dissertation. Result validation requires a validation of those assumptions. 30% efficiency was considered for the ICE system, however the de-rating of the ICE engine, i.e., the selection of a higher rated power engine, so as to obtain reliability and durability would result in increased consumption, thus reducing efficiency. Perhaps, considering a smaller efficiency would be more accurate, and would realistically shorten the gap between FC and the ICE system. Comparison of energy storage technologies for applications of UEAV Page 98 André Lhamas, July 2013 Comparison of energy storage technologies for applications of UEAV André Lhamas, July 2013 Page 99 6. Conclusions and Future Work 6.1 Conclusions The prediction and awareness of the structural behaviour of parts with relatively complex geometries, used in fuel cell systems, was performed through the use of finite element numerical modelling. The outcome enabled the selection of the assumed best performing concept regarding mass, as well as stress-deflection uniformity within a group of previously suggested concepts. The selected concept was thereafter used in the calculation of the specific energy to determine whether this modification would make feasible, merely from a technological perspective, an implementation of a Fuel Cell system in an unmanned aerial aircraft. The best performing concept managed to achieve a considerable improvement in specific energy facing current FC end plate concepts, however it also showed that FC systems are not yet viable as the propulsive system of an UAV when compared to currently used ICE systems. Nevertheless, this improvement is hoped to have shown that there is significant room for improvement, for this area of study appears to have been somewhat forgotten when it comes to fuel cell research. There are however several implications regarding this study. There were a significant number of assumptions made throughout this project that influenced the obtained results. Nevertheless, there is the awareness that, with such limited time, and since some of those assumptions had to be made due to the limited availability of critical technical information, work on this subject could not have been done differently. Having only been made commercially available recently, it is still a considerably arduous task to obtain that detailed information regarding fuel cell systems online or through any other source, for that matter. If implementation of the concepts here proposed were to be considered, all of the assumptions made would have to be thoroughly confirmed with the selected FC manufacturer, or with a proper expert on the subject. In fact, having failed in finding a suitable FC system for the required power, which in turn led to the necessity of selecting a combination of several smaller systems, decreased significantly the chance of proving FC viability facing conventional systems. It can be naturally assumed that the mass sum of a combination of smaller systems, far exceeds the mass of the single equivalent system. One critical assumption made early in this report, was not to consider the battery mass. As mentioned previously in this report, FC systems intended for transportation applications require a stabilizing element which can be achieved through hybridization, i.e., implementation of batteries for peak power instants. This was not considered in the hypothesis, or in the performed calculations. Even though the battery system would not have Comparison of energy storage technologies for applications of UEAV Page 106 André Lhamas, July 2013 red surface) is applied on the graphite block symmetry face. The filament winding tube is fixed in the edges of the symmetry plane (orange and blue triangles), and U1 and U2 ( and ) displacements are limited on the top and bottom sides (orange triangles), as seen in Figure 70. Figure 70: Load and boundary conditions in the FW model Results This part describes the results and attempts a comparison with the previously presented concepts. Table 42 describes the mass of the parts used in this model. Table 42: Mass of the parts used in the FW model Part Mass (kg) Filament Winding 0,450 Composite reinforcement 0,249 Composite cover 0,712 Total assembly 1,412 Since this approach only considers half of the model, the total mass reached in Table 42 must be multiplied by 2, thus reaching a total mass of 2,823 kg. It should be noted that this mass exceeds the 1,552 kg mass of the 2 pre-curvature EPs, however in this case, the mass of fixation components is nonexistent. If considering that this solution leads to non necessity of the fixating elements, then a significant mass reduction can be achieved. The mass weight of the fixating elements is 17%, according to [5]. Recalling Figure 9, the mass reduction should be as in Equation 6.1 and Equation 6.2. Comparison of energy storage technologies for applications of UEAV André Lhamas, July 2013 Page 107 In order to make a comparison with the end plate models, the sum of masses of the parts considered in the FW model is hereon named two end plate equivalent mass (2 EP equivalent mass). As mentioned previously, Table 45 comprises the FC stack mass for the FW concept for both 8 and 4 kW systems. Table 45 comprises the equivalent mass, as well as the total 4 and 8 kW FC stack mass. Recalling Table 28, both stack masses were obtained through the sum of equivalent mass with the assumed remaining mass. In order to determine the remaining mass for both stacks, the performed calculations were to subtract the assumed remaining mass for the aluminium end plate stack with 17% of the total stack mass (see Equation 6.1 and Equation 6.2) for both 8 and 4 kW systems, as in Equation 6.3 and Equation 6.4, respectively. Equation 6.1 Equation 6.2 Equation 6.3 Equation 6.4 Table 43 and Table 44 comprise the mass of the remaining components in both FW 8 kW and 4 kW systems, respectively. Table 43: Determination of remaining mass for the FW 8 kW stack mass PM 8 kW mass (kg) (kg) Remaining mass 8 kW Remaining mass fw 8 kW 14,6 2,482 10,590 8,108 Table 44: Determination of remaining mass for the fw 4 kW stack mass PM 4 kW mass (kg) (kg) Remaining mass 4 kW (kg) Remaining mass fw 8 kW (kg) 9,2 1,564 5,190 3,626 As mentioned previously, Table 45 comprises the FC stack mass for the FW concept for both 8 and 4 kW systems. Table 45: Equivalent mass for 2 EP and FC stack mass for the 8 kW and 4 kW considering the FW model End Plate Concepts 2 EP equivalent mass (kg) FC stack mass 8 kW (kg) FC stack mass 4 kW (kg) FW concept 2,653 10,761 6,279 Table 46 comprises the mass of the two end plate equivalent mass, as well as its relative weight in the stack. Comparison of energy storage technologies for applications of UEAV Page 108 André Lhamas, July 2013 Table 46: EP equivalent mass ratio for the 8 and 4 kW stacks EP equivalent mass ratio 8 kW (%) EP equivalent mass ratio 4 kW (%) FW concept 24,65 42,25 Considering the FC stack mass to be the sum of two 8 kW stacks andone of 4 kW, it is possible to achieve total stack mass, and available mass for fuel, as in . Table 47: Calculus of the available mass for fuel (H2) for the FC propulsive system in the FW model Available propulsion mass (kg) EM mass (kg) FC stack mass (kg) BoP mass (kg) Available mass for Fuel (kg) FW concept 70 3 27,800 2,5 36,700 In likeness of before, and considering the calculations performed previously through Equation 5.2 and Equation 5.3, one can obtain the contained energy and specific energy considering both DOE targets, see Table 48. Table 48: Contained energy and specific energy for the aluminium, steel and CFRP FC stacks, according to the gravimetric targets of the DOE for the FW model Energy contained in H2 5,5% (kWh) Specific Energy 5,5% (kWh/kg) Energy contained in H2 7,5% (kWh) Specific Energy 7,5% (kWh/kg) FW stack 67,215 0,960 91,657 1,309 Again, considering an overall 60% efficiency for the FC system, the intended comparing parameters are reached, see Table 49. Table 49: Contained energy and specific energy for the aluminium, steel and CFRP FC stacks, according to the gravimetric targets of the DOE, considering 60% overall efficiency for the FW model FC system, considering 60% efficiency Energy contained in H2 5,5% (kWh) Specific Energy 5,5% (kWh/kg) Energy contained in H2 7,5% (kWh) Specific Energy 7,5% (kWh/kg) FW stack 40,329 0,576 54,994 0,786 Reaching a value of 0,786 kWh/kg, this attempt of modelling the proposed FW concept achieved greater specific energy than that of the aluminium EP FC, but has also achieved one greater than the previously suggested CFRP EP FC, see Figure 71 and Figure 72. Comparison of energy storage technologies for applications of UEAV André Lhamas, July 2013 Page 109 Figure 71: Available energy of the considered FC systems, as well as the FW concept and the ICE system Figure 72: Specific energy of the considered FC systems, as well as the FW concept and the ICE system This concept is still of a very basic nature, however it has proven that it may be an interesting alternative to FC stack fixation. Naturally, solutions for all the implications mentioned at the beggining of this chapter, and for any other that could come up, would have to be properly developped. 0 20 40 60 80 100 120 140 Available Energy (Wh) Aluminium EP FC Steel EP FC CFRP EP FC ICE FW 0 0,5 1 1,5 2 2,5 Specific Energy (Wh/kg) Aluminium EP FC Steel EP FC CFRP EP FC ICE FW Comparison of energy storage technologies for applications of UEAV Page 110 André Lhamas, July 2013 Comparison of energy storage technologies for applications of UEAV André Lhamas, July 2013 Page 111 7. References [8, 9, 12, 13, 19, 21, 23, 28, 29, 38-45] 1. Defense, O.o.t.S.o.D.-D.o. Unmanned Aerial Vehicles Roadmap 2000-2025. 2001. 2. Asghari, S., M.H. Shahsamandi, and M.R. Ashraf Khorasani, Design and manufacturing of end plates of a 5 kW PEM fuel cell. International Journal of Hydrogen Energy, 2010. 35(17): p. 9291-9297. 3. Rungsima Yeetsorn, M.W.F.a.C.T., A Review of Thermoplastic Composites for Bipolar Plate Materials in PEM Fuel Cells, Nanocomposites with Unique Properties and Applications in Medicine and Industry. 2011: InTech. 4. Wang, X., Y. Song, and B. Zhang, Experimental study on clamping pressure distribution in PEM fuel cells. Journal of Power Sources, 2008. 179(1): p. 305-309. 5. Mawdsley, J.R., et al., Composite-coated aluminum bipolar plates for PEM fuel cells. Journal of Power Sources, 2013. 231(0): p. 106-112. 6. Li, X. and I. Sabir, Review of bipolar plates in PEM fuel cells: Flow-field designs. International Journal of Hydrogen Energy, 2005. 30(4): p. 359-371. 7. Middelman, E., et al., Bipolar plates for PEM fuel cells. Journal of Power Sources, 2003. 118(1–2): p. 44-46. 8. Scott, J.H., The Development of Fuel Cell Technology for Electric Power Generation: From NASA's Manned Space Program to the "Hydrogen Economy". Proceedings of the IEEE, 2006. 94(10): p. 1815-1825. 9. Hoogers, G., Fuel Cell Technology Handbook. 2003: Boca Raton [etc.] : CRC Press. 10. Drebushchak, V.A., Universality of the emf of thermocouples. Thermochimica Acta, 2009. 496(1–2): p. 50-53. 11. Barbir, F., T. Molter, and L. Dalton, Efficiency and weight trade-off analysis of regenerative fuel cells as energy storage for aerospace applications. International Journal of Hydrogen Energy, 2005. 30(4): p. 351-357. 12. Organization, N.A.T.O.-S.a.T., Manwearable Power Study Group. 13. Bagotsky, V.S., Fuel Cells: Problems and Solutions. 2 ed. 2012: John Wiley & Sons. 14. Energy, U.S.D.o.E.-E.E.R. Fuel Cells - Types of Fuel Cells. 2011; Available from: http://www1.eere.energy.gov/hydrogenandfuelcells/fuelcells/fc_types.html. 15. Haile, S.M., Fuel cell materials and components. Acta Materialia, 2003. 51(19): p. 5981-6000. 16. Lin, C.-W., et al., Dynamic mechanical characteristics of five elastomeric gasket materials aged in a simulated and an accelerated PEM fuel cell environment. International Journal of Hydrogen Energy, 2011. 36(11): p. 6756-6767. Comparison of energy storage technologies for applications of UEAV Page 112 André Lhamas, July 2013 17. Kim, M., et al., Bipolar plates made of plain weave carbon/epoxy composite for proton exchange membrane fuel cell. International Journal of Hydrogen Energy, 2012. 37(5): p. 4300-4308. 18. Yu, H.N., et al., Composite endplates with pre-curvature for PEMFC (polymer electrolyte membrane fuel cell). Composite Structures, 2010. 92(6): p. 1498-1503. 19. Laboratory, H.U.S.D.o.E.N.E.T., Fuel Cell Handbook. Reprinted from the 2000 edition ed. 2005: Honolulu : University Press of the Pacific, cop. 2005. 20. Hua, T.Q., et al., Technical assessment of compressed hydrogen storage tank systems for automotive applications. International Journal of Hydrogen Energy, 2011. 36(4): p. 3037-3049. 21. Ananthachar, V. and J.J. Duffy, Efficiencies of hydrogen storage systems onboard fuel cell vehicles. Solar Energy, 2005. 78(5): p. 687-694. 22. Energy, U.S.D.o.E.-E.E.R. Hydrogen Storage. Ful Cell Technologies Program, 2011. 23. Najib, W.B., Fuel Cell Powered AUVs; An Overview, 2005, INEGI - Unidade de Materiais Compósitos; Universidade de Aveiro. 24. CEA, C., Hydrogen, new energy technologies - Onboard storage of hydrogen. The Hydrogen Pathway, 2004-2005(No. 50/51). 25. Francescato, P., et al., Comparison of optimal design methods for type 3 high-pressure storage tanks. Composite Structures, 2012. 94(6): p. 2087-2096. 26. Klell, M., Handbook of Hydrogen Storage. Storage of Hydrogen in the Pure Form. 2010: WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. 27. FG, H., Neural network control of a parallel hybrid-electric propulsion system for asmall unmanned aerial vehicle, 2005, University of California: USA. 28. Hung, J.Y. and L.F. Gonzalez, On parallel hybrid-electric propulsion system for unmanned aerial vehicles. Progress in Aerospace Sciences, 2012. 51(0): p. 1-17. 29. Ruddell, D.A., WP Report - Storage Technology Report ST6: Flywheel 2003, Investire-Network. 30. Mazumdar, S.K., Composites Manufacturing : Materials, Product, and Process Engineering. 2002: CRC PRESS. 31. Jones, R.M., Mechanics of Composite Materials. Second Edition ed. 1999. 32. Cambridge, U.o., Mechanics of Fibre Reinforcemend Composites - Stiffness of long fibre composites, 2004. 33. ESAComp, Theoretical Background of ESAComp Analyses, M. Palanterä, Editor 1999, HELSINKI UNIVERSITY OF TECHNOLOGY. 34. Frost&Sullivan, STUDY ANALYSING THE CURRENT ACTIVITIES IN THE FIELD OF UAV, 2007, EUROPEAN COMMISSION - ENTERPRISE AND INDUSTRY DIRECTORATE-GENERAL. 35. Group, P.M.S.p., PM 200 Hydrogen Fuel Cell, P.M.F.C. GmbH, Editor 2010. 36. Limited, B.M. Developing Unmanned Aircraft Systems and Design Software. 2012; Available from: http://www.barnardmicrosystems.com/L4E_wankel.htm. 37. Systems, D.D. Abaqus Unified FEA. 2013; Available from: http://www.3ds.com/products-services/simulia/portfolio/abaqus/latest-release/. 38. Blomen, L.J.M.J., Mugerwa, Michael N., Fuel Cell Systems. 1993. 39. El Chaar, L., L.A. lamont, and N. El Zein, Review of photovoltaic technologies. Renewable and Sustainable Energy Reviews, 2011. 15(5): p. 2165-2175. 40. Fazelpour, F., et al., Considerable parameters of using PV cells for solar-powered aircrafts. Renewable and Sustainable Energy Reviews, 2013. 22(0): p. 81-91. Comparison of energy storage technologies for applications of UEAV André Lhamas, July 2013 Page 113 41. Guimarães, S.M.d.A.L., Estudo do Desempenho de Células de Combustível de Membrana de Permuta Iónica, in Faculdade de Engenharia da Universidade do Porto2006, Universidade do Porto: Porto. 42. Larminie, J.D., Andrew, Fuel cell systems explained. 2nd ed ed.: Chichester : John Wiley & Sons, cop. 2003. 43. Parida, B., S. Iniyan, and R. Goic, A review of solar photovoltaic technologies. Renewable and Sustainable Energy Reviews, 2011. 15(3): p. 1625-1636. 44. Pires, I.A.E., ANÁLISE E PROJECTO DE UMA CÉLULA DE AVIÓNICOS PARA O UAV ALFA, 2009, Academia da Força Aérea. 45. Thomas, C.E., Fuel cell and battery electric vehicles compared. International Journal of Hydrogen Energy, 2009. 34(15): p. 6005-6020. 46. Mair, W.A., Aircraft Performance. 1996: Cambridge University Press. Comparison of energy storage technologies for applications of UEAV Page 114 André Lhamas, July 2013 Comparison of energy storage technologies for applications of UEAV André Lhamas, July 2013 Page 115 Annexes Comparison of energy storage technologies for applications of UEAV Page 122 André Lhamas, July 2013 Annexe D: Launch Point Halbach Array EM Comparison of energy storage technologies for applications of UEAV André Lhamas, July 2013 Page 123 Comparison of energy storage technologies for applications of UEAV Page 124 André Lhamas, July 2013 Annexe E: Performed Literature review Comparison of energy storage technologies for applications of UEAV André Lhamas, July 2013 Page 125 1. Photovoltaic Cells The solar cell working principle is based on the Photovoltaic (PV) effect. Sunlight, when striking a light absorbing material present within the cell structure, imparts enough energy to some electrons to raise their energy level and thus free them. Those free electrons are then used to produce voltage through a built-in potential barrier, and used to drive current through a circuit. A cell is grouped into modules and multiple modules can be wired together to form an array. Generally, the amount of electricity produced increases with the increase of the area of a module or array. Photovoltaic modules and arrays produce direct-current (DC) electricity. Any required voltage and current combination can be achieved by connecting in both series and parallel electrical arrangements. The rated unit for these systems is the peak kilowatt (kWp), which represents the amount of electric power that a system is expected to deliver when the sun is directly overhead on a clear day. A defining property of a PV system is its band-gap, measured in electro-volts (eV). The band-gap is the energy gap an electron must cross in order to be promoted from the valence band to the conduction band. Also, the wider and the larger the band-gap of PV cell is, the greater is the output production voltage, since it absorbs more light. In cells with a smaller band-gap, however, the result is larger current, yet smaller output voltage. According to Review of photovoltaic technologies, there are currently four major types of PV technology: crystalline, thin film, compound and nanotechnology. A solar cells' efficiency is dependent upon many environmental features and weather parameters such as humidity, wind speed, dust, temperature, shading, tilt angle, azimuth plane angle, sun intensity, etc. High relative humidity and temperature, and dust are undesirable factors in a solar cell, since they decrease efficiency. Wind can lead to increased efficiency since it causes convection heat transfer, thus lowering humidity and temperature, yet dust lifting by wind may lead to shading and efficiency decrease. As seen from Figure 73, a higher efficiency is reached for lower temperatures, independently of the solar cell type. In fact, the part of a photovoltaic energy which is not converted into electricity will appear in the heat form of energy in solar cells and elevates temperature of the cell, thus leading to greater inefficiency. Comparison of energy storage technologies for applications of UEAV Page 126 André Lhamas, July 2013 Figure 73: Dependency of the band gap, temperature and efficiency, [40] 1.1 Silicon Crystalline Structure Having been the first generation of PV, crystalline structure hasn't however become obsolete. Instead it has been constantly developed in terms of capability and efficiency. Within this type of cell, the sub-divisions of monoand poly-crystalline and emitter wrap through (EWT) will be further discussed. 1.1.1 Mono-Crystalline PV cells The fact of being the most cost effective cell, naturally leads to being the most commonly used system in the market with an 80% share, nowadays. Yet, its efficiency development finds great limitation due to the fact that with its material (silicon), energy produced decreases with higher wave lengths. Furthermore, radiation with higher wave length leads to decreased efficiency caused by greater thermal dissipation and resulting heating. A efficiency of 20,4% for a module of this type of cells has been recently determined by the National Renewable Energy Laboratory (NREL). Comparison of energy storage technologies for applications of UEAV André Lhamas, July 2013 Page 127 1.1.2 Poly-Crystalline PV cells This technology is becoming more attractive due to its manufacturing cost, despite having a lower efficiency (15%) than mono-crystalline. Another advantage of producing multi-silicon is to decrease the flaws in metal contamination and crystal structure. 1.2 Thin film technology This type of technology consists of applying thin layers of semiconductor material to a solid backing material, such as glass or stainless steel substrate. That allows great savings in the amount of semiconductor material required for each cell when compared to silicon wafers, thus lowering the cost of production of PV cells. Common material used in thin-film cells are Gallium arsenide , copper, cadmium telluride , indium diselenide and titanium dioxide . As a result of light trapping and black surface passivation with optimum silicon thickness, this technology is said to achieve 19% efficiency. One can distinguish four types within Thin-film cells, which are: amorphous silicon cell, thin polycrystalline silicon on a low cost substrate, the copper indium diselenide/cadmium sulphide hetero-junction cell, and the cadmium telluride/cadmium suplhide hetero-junction cell. 1.2.1 Amorphous silicon Unlike crystalline silicon, the silicon atoms in this technology are randomly located from each other, leading to a higher band-gap (1,7 eV) than crystalline silicon (1,1 eV). The larger band -gap allows a-Si cells to absorb the visible part of the solar spectrum more strongly than the infrared portion of the spectrum. There are several variations in this technology where substrates can be glass or flexible stainless steel, tandem junction, double and triple junctions, and each one has a different performance. Figure 74: Variation of output with insulation for representative sub-arrays, [39] 1.2.2 Cadmium telluride or cadmium sulphide/cadmium telluride According to Review of Photovoltaic technologies, Cadmium telluride has long been known to have the ideal band-gap (1,45 eV) with a high direct absorption coefficient for a solar absorber material and recognized as a promising photovoltaic material for thin-film solar cells. Furthermore, recent developments in their manufacturing process have lead to capability to reduce manufacturing cost and have made this technology extremely competitive. An issue Comparison of energy storage technologies for applications of UEAV Page 128 André Lhamas, July 2013 that needs further development regarding this PV cell is the toxicity of cadmium and related environmental issues. 1.3 Concentrator This type of PV cell focus sunlight onto a small area of photovoltaic cells and it does so by using a large area of lenses or mirrors. 1.4 Compound Semi-conductor This type comprises a stack of crystalline layers with different band gaps that are tailored to absorb most of the solar radiation, i.e., the various cells, each one with different band-gaps, are tuned to use the full spectrum, leading to increased efficiency. Gallium arsenide /indium gallium phosphide multi-junction devices have reached the highest efficiency of 39% with NREL recently announcing a record 40,8% from a metamorphic triple-junction solar cell. A schematic is shown in Figure 75. Figure 75: Epitaxial stacks of multi-junction solar cells, [39] 1.5 Nanotechnology The introduction of nanoscale components can lessen limitations seen in other PV technologies, thanks to their ability to control the energy band-gap, providing flexibility and inter-changeability in addition to enhancing the probability of charge recombination. Devices such as nanotubes, quantum dots and hot carrier solar cells are types of nanotechnology structures that can be used for increased efficiency in solar cells. Type Efficiency (%) Crystalline silicon (single-crystal) PV cell/module 20 Crystalline silicon (cast) PV 14 Comparison of energy storage technologies for applications of UEAV André Lhamas, July 2013 Page 129 Crystalline silicon (ribbon) 13 Thin film (amorphous silicon) 8 Thin film other (special material CdTe and CIGS) 12 Concentrator PV (photovoltaic) 38 Table 50: Efficiency of the different types of PV 2. Kinetic Energy Systems Flywheels are devices capable of storing kinetic energy through a rotating mass (rotor). The stored energy is dependent upon the mass, inertia and rotational speed of the rotor. Energy is transferred to the flywheel by applying an accelerating torque to it, increasing rotational speed. In contrast, energy is released from the device by applying a decelerating torque to a mechanical load. 2.1 Stored Energy Equation Equation E.1 shows the way to determine the stored energy of a spinning flywheel with angular velocity and polar moment of inertia, . Equation E.1 Equation E.2 Equation E.3 Equation E.4 As Equation E.4 demonstrates, higher stored energy can be more easily achieved through high angular velocity, rather than higher mass. Given that, Equation E.5 defines the maximum angular velocity the flywheel can take, according to its tensile strength for a flywheel with mass concentrated at the rim at radius, . Equation E.5 With the maximum angular velocity, , for a maximum tensile strength, , being: Equation E.6 Thus defining the maximum the maximum stored energy, , for a given material: Equation E.7 Comparison of energy storage technologies for applications of UEAV Page 130 André Lhamas, July 2013 From the last expression, one concludes that a higher stored energy can be achieved through the use of materials which combine high tensile strength with low density. compares the previously mentioned parameters with the theoretical maximum specific energy of each and reaches the conclusion that composite materials are the most suitable for kinetic energy storage through high speeds. Table 51: Specific strength of rotor materials, [29] Density Strength Theoretical maximum specific energy Steel (AISI 4340) 7800 1800 32 Alloy (AlMnMg) 2700 600 31 Titanium (TiAl6Zr5) 4500 1200 37 GFRP Glass Fibre Reinforced Polymer (60% vol E-glass) 2000 1600 111 CFRP Carbon Fibre Reinforced Polymer (60% vol HT Carbon) 1500 2400 222 Taking into account that the specific energy per unit mass (Equation E.8), , in is: Equation E.8 Or, as a function of tensile strength and density, one reaches the theoretical maximum specific energy, and in , as in Equation E.9: Equation E.9 2.2 Rotor Bearing Being a mechanical energy storage system, minimum losses and maintenance are fundamental to achieve greater stored energy, making rotor bearing design a key aspect in Kinetic Energy Storage systems. Some commonly used examples of high specification bearings are, [29]:  Combination of ceramic ball bearings in a steel race, with magnetic lift to increase bearing life substantially;  Low stiffness, self balancing concept. Bearing system consisting of a passive magnetic bearing at the top, and a low loss pivot bearing at the bottom of the vertical axis;  High-temperature superconductor (HTS) bearing with extremely low rotational loss. These bearing can reach a reduction in rotational drag more than two orders of magnitude lower than that of mechanical bearings. 2.3 Power Interface The power interface of a KES system includes the motor/generator, a variable-speed power electronics converter, and a power controller, [29]: Comparison of energy storage technologies for applications of UEAV André Lhamas, July 2013 Page 131  A high speed permanent magnet machine, integrated with the rotor, is usually employed as the motor/generator, also usually known as integrated synchronous generator (ISG).  A pulse width modulated (PWM) bi-directional converter using insulated-gate bipolar transistor (IGBT) technology is usually used for the power electronics interface. Depending on the application requirements, the converter may be single stage (flywheel ISG a.c. d.c. bus), or double stage (flywheel ISG a.c. d.c. bus a.c. network), according to the application requirements.  A power controller is also required to monitor the flywheel and control the power flow. 2.4 Final Specific Energy and Power While the theoretical specific energies achieved for composite materials in are quite attractive when compared to other storage energy devices such as fuel cells and batteries, the calculation takes only into account the flywheel itself. The complete kinetic energy storage system, i.e., flywheel hub and all of the required electrical and mechanical parts can lead to at least factor of 10 reduction in the previously calculated specific energy, leading to a somewhat disappointing specific energy, especially when compared with the mentioned alternatives. As for power density, it depends mainly upon the flywheel hub, electrical machine and power electronic interface. Figures for specific power for these systems as high as have been quoted. 2.5 Manufacturing Methods of Composite rotors Two basic methods can be used for the manufacture of flywheels in composite materials: filament-winding as well as the resin transfer moulding process (RTM), the latter leading to inferior material properties, having however the possibility of cheap mass production. The description of both fabrication methods can be found in Section 0.