FABSTRACT. Technical Overview of Fuel Cell Systems: How Computer Simulation is Used to Reduce Design Time

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1 W H I T E P A P E R FABSTRACT Fuel cells offer the means for the conversion of chemical energy in hydrogen rich fuels (fossil and renewable) directly to electricity without having to generate thermal energy as an intermediate step. In a fuel cell, the fuel and oxidizer, usually the oxygen in air, react electrochemically to produce water and heat. The entire fuel cell system includes the fuel cell and a number of additional components. These components are used to process the primary fuel into a hydrogen-rich gas stream, to remove sulfur and carbon monoxide, to efficiently store and retrieve hydrogen fuel upon demand, and to manage the water produced by the fuel cell. The objective is to have an overall system with high energy conversion efficiency while maximizing the power to volume ratio for the power plant. This white paper takes a closer look at the current challenges in each of several fuel cell systems. Whenever appropriate, references to Polymer Electrolyte Membrane (PEM), Direct Methanol Fuel Cell (DMFC), Solid Oxide Fuel Cell (SOFC), and Solid Acid Fuel Cells (SAFC) will be made. INTRODUCTION Technical Overview of Fuel Cell Systems: How Computer Simulation is Used to Reduce Design Time Fuel cells offer distinct advantages over contemporary energy storage and conversion devices such as batteries, internal combustion engines, and gas/steam turbines. Fuel cells win out over batteries because of their continuous supply of power - the fuel cell never depletes as long as fuel is provided. Battery life, size and weight can also raise issues for mobile electronics such as cell phones, laptops, portable military equipment, and automotive applications. Hossam Metwally, PhD, ANSYS, Inc. William Wangard, PhD, ANSYS, Inc. Fuel cell power plants emit much less pollution than conventional power plants. The principle product of the reaction is pure water, which is produced as a liquid in low temperature cells and as water vapor in high temperature cells. The generation of pollutants such as NO x and SO x, produced from fuel reforming steps needed to convert the primary fuel into hydrogen, are dramatically lower. As a system, a fuel cell power plant will generate CO 2 as long as the primary fuel contains carbon. Fuel cell systems also offer noise and vibration advantages over conventional internal combustion power plants. The lower operating temperatures of the fuel cell provide a small thermal signature which is of interest in military applications, where stealth is necessary. WP120 1

2 CHALLENGES There are many challenges facing the widespread adoption of fuel cell technology. Below are issues that can be addressed by numerical simulation: Fuel Preparation - Purification and desulphurization Hydrogen Production - Reforming and purification from CO if needed Fuel Cell Operation - Hydrogen management - Water management - Thermal management - Air management FUEL PROCESSING Apart from generating hydrogen through electrolysis or thermal splitting of water, reforming of hydrocarbon fuel is expected to be the main hydrogen source for years to come. There are several reforming methods. In order of popularity, they are: Steam Methane Reforming (SMR) Partial Oxidation (POX) Auto-Thermal Steam Reforming Fuel reforming is a process used to convert a readily-available hydrocarbon fuel into a hydrogen-rich fuel stream. It requires a catalyst and a source of heat. Inside the reformer, a catalytic reaction strips the hydrogen molecules from the fuel and generates CO 2 and small amounts of CO. In low and moderate temperature fuel cell systems, the CO must be removed from the gas stream (to very low concentrations), as it tends to poison the catalyst inside the cells, rendering them useless. Steam Methane Reforming About 95% of the hydrogen generated in the US is produced by steam methane reforming1. In this process, high temperature steam (700 C to 1000 C) reacts with methane in the presence of a catalyst in an endothermic reaction (requires heat) to generate hydrogen and carbon dioxide as follows: CH 4 (methane) + H 2 O (steam) + HEAT CO (carbon monoxide) + 3 H 2 (hydrogen) This reaction is then followed by another reaction (called water shift) where CO reacts with steam to generate more hydrogen and carbon dioxide as follows: CO + H 2 O (steam) CO 2 (carbon dioxide) + H 2 (hydrogen) + HEAT These two reactions convert the methane and steam into carbon dioxide, but not completely. A significant amount of carbon monoxide exists after the reforming step that must be removed in all low temperature fuel cell systems. Also, the resulting reaction when combining the above two reactions is endothermic, which means that heat energy must be WP120 2

3 supplied. This energy is usually provided by the combustion of the primary fuel source, which negatively affects the system efficiency. It is interesting to note that large scale steam methane reforming is already a mature technology, but the renewed interest is in small distributed reforming (e.g. in gas station-like facilities) as well as on-board reforming for automotive applications. Partial Oxidation In partial oxidation, a small amount of oxygen is injected into the natural gas stream, but it is not enough to completely oxidize the hydrocarbons to CO 2 and water. With less than the sufficient (stoichiometric) amount of O 2 available for the reaction, the reaction products contain primarily H 2 and CO (and N 2 if air is used), and a relatively small amount of CO2 and other compounds. Subsequently, in a water-gas shift reaction, CO reacts with water to form CO 2 and more H2. Partial oxidation is an exothermic process, i.e. it releases heat. Because of that, it is typically a much faster process than steam reforming and requires a smaller reactor vessel. However, this process initially produces less hydrogen per unit of the input fuel than is obtained by steam reforming of the same fuel. Auto-Thermal Steam Reforming Similar to partial oxidation, a small amount of O 2 is injected and it reacts with the hydrocarbon, carbon, CO, or even H 2. The trick is to provide just enough O 2 to provide the heat required to reach the auto thermal condition (i.e. no external heat needed). General Reforming Challenges Regardless of the reforming technique involved, the following are typical engineering challenges for reforming: Increasing Fuel Processing Efficiency Reducing the amount of energy supplied to the process per amount of H 2 produced is of course a primary objective. To deal with this challenge, it may be easier to consider it as being composed of two folds; the first being chemical control of the process and the second being a thermal management problem. Controlling the chemical reaction environment in the reformer is crucial since, as mentioned before, there are several mechanisms of reaction that the H2 rich fuel may follow. For example, while a certain surface reaction on the catalyst is desirable and generates H2, coking, i.e. the generation of carbon which may be deposited on the catalyst surface, is certainly not. In general, reaction rates are dependent on local concentration of the different reactants and local flow and temperature conditions. Thus, reactions may be controlled, and prioritized, by controlling concentration, flow and thermal conditions inside the reformer. Moreover, reformers typically have a very high surface area to volume ratio and possibly a complex internal layout. In this case, ensuring uniform flow of reactants on the complex catalyst surface is not a straightforward task. Thermal management, on the other hand, is another means of controlling the reactions inside the reformer. Whether the reaction is exothermic or endothermic, providing/extracting the required amount of heat and maintaining the desired temperature field over the complex catalyst surface is certainly another challenge. Detailed computer modeling through computational fluid dynamics (CFD) is successfully being used to model the three dimensional variation in flow, heat transfer and reacting species inside the reformer 2,3. These simulations may be used to optimize the operating conditions as well as geometrical layout of the reformer at the design stage. WP120 3

4 Catalyst Material, Loading and Consumption Catalysts used for reforming may be Ni, Mg, or K based materials supported on alumina (Al 2 O 3 )/silica structure. There is a continuous desire however to find new catalysts which may be cheaper while still providing the chemical characteristics. In addition, increasing the catalyst loading (packing density) may be favorable from a chemical reaction point of view, but certainly will impose higher pressure drop on the overall system and will affect the local flow. The catalytic behavior of the different materials is usually obtained experimentally for simple catalyst layout. This information may then be fed into a CFD model that describes the actual reformer in more detail to investigate how the catalyst would behave under real operation. The effect of different catalyst loading may be similarly investigated. WP120 4

5 Alternatively, instead of fixed bed catalysts, fluidized catalysts may also be used4,5. Fluidized catalysts can provide additional flexibility for regenerating the catalyst material if it experiences coking or even sulfur poisoning. There, the catalyst is loaded or coated on support particles. Challenges faced when dealing with fluidized beds in general are to find the proper operating conditions for fluidization and to prevent or minimize particle contamination in exhaust gas. While small particle size promotes fluidization, it also makes it easier for the solid particles to get entrained in the gas flow and contaminate the exhaust gas. Detailed computational fluid dynamics studies have proven to be a useful in analyzing this complex multiphase (gas-solid, some times even gas-liquid-solid) problem while also predicting the solid particle loading at the end of the reformer. Other Pre- and Post-Reforming Treatments When reforming natural gas, sulfur has to be removed prior to the reformer otherwise the catalyst will be poisoned. Sulfur gas is what gives natural gas its odor and is added to the gas for safety reasons. Typically, zeolites are used as desulpherizers and act as a filter upstream of the reformer. The challenge here is to ensure the uniform exposure of the zeolites to the gas and to determine when the filter has to be refreshed. Similarly, after reforming, H 2 gas has to be separated from CO, and CO 2 generated by the reforming reactions. Different types of fuel cells have varying degrees of sensitivity towards CO and CO 2 impurities. For example, PEMFC will be poisoned by CO, whereas SOFC can utilize CO as an additional fuel. In cases where nearly pure H 2 is needed, pressure swing adsorption (PSA) may be used to get rid of CO and CO 2. This is done by letting the gas flow over a bed of zeolites or carbon particles that adsorb CO and CO 2. Eventually, the bed gets saturated. To regenerate it, the pressure of the chamber is lowered and the adsorbed gas is released. For continuous operation, two chambers are usually operating in parallel; one adsorbing the impurities, while the other is being regenerated. FUEL CELL TYPES There are several fuel cell types that differ in construction, material and operating conditions. The three most popular, currently, are Polymer Electrolyte Membrane (PEM), Direct Methanol Fuel Cell (DMFC), and Solid Oxide Fuel Cell (SOFC) A relatively new technology is the Solid Acid Fuel Cell (SAFC). In the next section, the construction, operation and some of the most popular challenges of each will be discussed. WP120 5

6 Polymer Electrolyte Membrane Fuel Cell (PEM) PEM fuel cells have the potential, and are currently being tested, for transportation applications and small auxiliary power units (APU). Their advantages are low temperature operation (60 C to 100 C) which makes them ideal for quick start up. They require a complex fuel processing system however and are extremely sensitive to fuel impurities. Because of their low temperature operation, water management represents a specific challenge to this type of fuel cell. This issue will be covered later in the paper. Construction and Operation A typical PEM fuel cell consists of the following components: Bipolar plates or current collectors. Usually made of graphite or coated metal, the purpose of the bipolar plate is to transfer electrons between adjacent fuel cells and to deliver the electrons to the catalyst zone. In addition, bipolar plates provide a physical barrier to the fuel and oxidizer streams of adjacent fuel cells in a stack. The transport of electrons is modeled by solving the potential field within the bipolar plate. As a result, the current density over the plate can be provided through simulation. Changing the type of the bipolar plate and its effects on the resulting current density can be mimicked through changing its electron transport properties. Flow channels. These are channels or grooves cut into the anode and cathode sides of the bipolar plates that provide a low resistance path for oxidizer and fuel to reach the catalytic zone. The plates also provide a path for the products of the chemical reaction to leave the cell. The ideal channel pattern supplies fuel and oxidizer uniformly to the surface of the catalytic zone. Computer modeling through the use of computational fluid dynamics (CFD) is often used to assess the performance of the channels in terms of the degree of uniformity of fuel/oxidizers they are able to supply. Pressure drop across the channels is also investigated by CFD. Porous gas diffusion layer or (GDL). The GDL is a region on the cathode and anode sides of the fuel cell sandwiched between the bipolar plate/flow channels and the catalyst zone. It is made of a porous, electrically conductive material, usually graphite fibers. It allows the gas in the channel to reach the entire surface of the catalytic zone. The gas flows through the pores of the material. Electrons must also reach the catalytic zone. They travel from the bipolar plates through the carbon fibers and travel to the catalytic zone. On the cathode side, the GDL material is treated to ease the transport of liquid water into the flow channel, where the gas flows push the water out of the cell. Modeling can provide the effect of local water content on the diffusion process as well as the possible entrainment (electro-osmotic drag) of water. Catalytic zone. This is a complex zone in which the electrolyte, carbon fibers, and gaseous reactants and catalyst material (platinum or platinum alloy) meet, allowing the electrochemical reaction to occur. The catalyst material is expensive so its use is minimized. However, since the electrochemical surface reaction only occurs on the catalyst surface, the more of the surface area that is present, the more effective the use of the catalyst is. Membrane. The membrane is a material that permits the transport of hydrogen ions from the anode to the cathode, where they meet with oxygen and electrons to form water. The membrane also serves to separate the fuel and oxidizer streams. The membrane must be hydrated to remain conductive. CFD analysis of the membrane is a primary challenge in the modeling of fuel cells, due to the complex physics of water transport. WP120 6

7 The performance of a fuel cell is the result of complex interactions between species transport, fluid flow, heat transfer, electrochemical reaction, potential field transport and the resulting current density. This makes the design of the fuel cell a challenging task since it involves this complex level of multi-physics. Another complexity arises from the fact that the materials used in the fuel cell are usually novel materials. Their properties and the resulting variation with operating conditions (temperature and/or relative humidity) are not fully understood or cataloged - especially diffusion properties of species/current/potential. For example, the diffusivity of protons through the membrane is dependent on water content, i.e. relative humidity, which directly affects the performance of the fuel cell. The relative humidity, on the other hand, is a function of temperature. In general, this means that testing for material behavior on a fundamental level is essential. PEM Operational Changes Water management In PEM, water has two sources. The first is external through humidifying the fuel stream to maintain the required humidity level in the membrane. The second is the internal generation as a result of the electrochemical reaction at the cathode. Inside the cell, water is transported through the membrane in three ways: Electro-osmotic drag from the anode to the cathode via protons Back diffusion due to concentration difference By pressure drop in the entire stack (convective transport) At high current densities, where electro-osmotic drag is high, if the inlet gases are not well humidified, the membrane will get dehydrated and hence its ability to conduct protons will decrease. As a result, the rate of hydrogen electrons transport will diminish and the fuel cell performance will suffer. Another important issue arises because PEM operates at lower temperatures, less than 100 C, if the amount of water generated at the cathode exceeds that of 100% relative humidity, condensation will occur. Liquid water will then block the pores between the GDL and the membrane on the cathode side and thus prevent more oxidizer from reaching the catalyst surface. The performance of the cell will suffer. Another interesting issue is if considerable amount of water condenses inside the oxidizer channels. Again, this liquid water will block parts of the channels, increasing the pressure drop considerably while reducing the amount of gas flowing in the channels. As a result, not enough oxidant will be provided to the catalyst, reducing the amount of fuel consumed and thus the total cell power. Water management becomes even more of a problem at subfreezing ambient conditions. This has directed the US Department of Energy to fund research on finding new membranes whose performance is less sensitive to temperature and humidity 9. Flow channel design As mentioned above, the primary task of the channels in the bipolar plates is to deliver a uniform distribution of reactants (fuel and oxidizer) to both sides of the membrane/catalyst interface. This is necessary to reach a certain level of available pressure head. Uniform distribution of reactants on MEA will result in a uniform current density (A/cm2). Nonuniform current density results in non-uniform temperature variation and the existence of local hot and cold spots. Hot spots result in lifetime and reliability degradation whereas cold spots may result in local condensation and, again, GDL pores blockage. Several channel layouts exist including: straight, serpentine, converging/diverging, all of which are continuously being examined. WP120 7

8 It is evident that finding the optimum operating conditions for a fuel cell will require a lot of testing; fundamental testing to characterize the material properties and cell testing to determine the cell performance under various conditions. Computational fluid dynamics (CFD) has already made substantial contributions in cell design and optimization. WP120 8

9 Direct Methanol Fuel Cells (DMFC) Construction and Operation While the components may sound similar to PEM fuel cells (anode, cathode, membrane and flow channels), the main difference in DMFC is that they use liquid methanol as fuel. At the anode, methanol and water combine to form carbon dioxide, hydrogen protons and electrons according to the following reaction: CH 3 OH + H 2 O CO 2 + 6H + + 6e - Water has to be supplied at the anode for the reaction to complete. This is done either through supplying the methanol-water mixture to the anode or relying on the back diffusion (osmosis) of water from the cathode reaction (described below). 1.5 O H + + 6e - 3H 2 O Advantages, Disadvantages, and Challenges Since DMFC consumes liquid methanol, the complexities for producing (e.g. reforming) and storing gaseous hydrogen are eliminated. However, it requires higher catalyst loading (platinum loading) than PEM. Also, fuel crossover (when a part of the fuel is able to escape through the membrane without reacting), results in reduced efficiency. This is the fault of the membrane material. DMFC cannot use the same membrane as PEM (NAFION) because of methanol. Water management is also an issue. The challenge is how to provide water mixed with the methanol at the anode which is just enough for the reaction to complete while still having a concentrated fuel mixture to provide operation for a longer period of time. Toshiba is working on an example to overcome this hurdle 7. The combined advantages and disadvantages of DMFC make it an ideal candidate for portable electronics (cell phones, laptops etc.) where the power required is relatively small. It is argued that although DMFC are less efficient than PEM, their simple design and independence on pure hydrogen may expedite its market acceptance. Solid Oxide Fuel Cells (SOFC) Construction and Operation Similar to any fuel cell, SOFC consists of a solid electrolyte sandwiched between the anode and cathode. The cell assembly may be tubular8 or flat. However, the electrochemical reactions are the same. WP120 9

10 Advantages, Disadvantages, and Challenges SOFC has the unique ability to handle CO mixed with hydrogen. It even accommodates internal reforming which enables it to directly work off methane, propane or even diesel fuel9. Also, since it operates at elevated temperatures (600 C to 1,000 C), its exhaust may be further utilized for power or heat generation. High temperature operation, however, makes it a challenge to operate SOFC intermittently. WP120 10

11 Other SOFC challenges are similar to PEM where flow and thermal management of an entire cell stack/bundle is not trivial. For example, ensuring a uniform airflow through all tubes of a tubular design is essential to make sure that each cell is adequately fed and performs as desired. Also, the possible partial mixing of spent fuel (containing water vapor) with fresh fuel to prepare for internal reforming represents another interesting challenge 10. As described above, steam reforming has also its own set of problems. Solid-Acid Fuel Cells (SAFC) Construction and Operation Solid-acid fuel cells (SAFCs) utilize an anhydrous, nonpolymeric proton-conducting electrolyte that can operate at temperatures higher than the saturation temperature of water. The components of this cell are similar to that of the PEM, but simpler since the GDL, especially on the cathode side, does not have the complications related to the transport of liquid water. Advantages Due to the lack of liquid water in the cell, the solid-acid fuel cell has many advantages over the PEM. Some of these are as follows: Water management is much simpler Freeze protection is much greater Less performance sensitivity to the humidity of the anode and cathode streams Disadvantages The power density of SAFCs is substantially lower than that of PEM fuel cells. As of 2005, SAFC is reported to have maximum power densities of 415 W/cm2, compared to the typical 1000 W/cm2 power density for PEM fuel cells 11. Modeling Needs The technical needs of modeling SAFCs include the prediction of pressure drop in the flow channels, temperature distribution and performance, in addition to flow distribution to the stack. Additionally, it may be necessary to model the fuel processing steps to develop an understanding of the system performance. CONCLUSIONS The challenges involved in the development of fuel cell systems range from novel material development to system (or stack) performance optimization. In this overview, a sampling of the fuel preparation (reforming) and cell/stack performance optimization challenges were discussed. To truly understand fuel cell performance, engineers need to conduct analyses to visualize issues associated with gas/liquid flow, electrochemical reaction and thermal management. While experimentation is needed to quantify material behavior (e.g. porous diffusivities), additional cell/stack level testing is also needed to assess the system's overall performance. Detailed computational fluid dynamics (CFD) can help reduce and guide the cell/stack level testing and thus aid in the fuel cell system development process. Ultimately, an optimized fuel cell design that offers superior power generation, reliability and durability can be attained much faster by incorporating computer modeling in the design process. WP120 11

12 1 DOE website, 2 Surface reaction in catalytic tubes: 3 Reactor tubes example: (fluidized bed example) 5 Gera D., Syamlal M., and O'Brien T.J., Hydrodynamics of Particle Segregation in Fluidized Beds, International Journal of Multiphase Flow, Vol. 30, pp , April, Eldrid C., Shahnam M., Prinkey M., Dong Z., 3D Modeling of Polymer Electrolyte Membrane Fuel Cells, 1st International Conference on Fuel Cell Science, Engineering & Technology, 4/21/2003, Transaction of ASME. 7 Toshiba website, 8 Siemens Westinghouse office site, 9 DOE report, 10 Technical Note, Fluent Inc., 11 Uda T. and Haile S.M., Thin-Membrane Solid-Acid Fuel Cell, Electrochemical and Solid-State Letters, Vol 8, No. 5, pp A245-A246 (2005). ANSYS, Inc. Southpointe 275 Technology Drive Canonsburg, PA U.S.A ansysinfo@ansys.com Toll Free U.S.A./Canada: Toll Free Mexico: Europe: eu.sales@ansys.com ANSYS, Inc. All Rights Reserved. WP120 12