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WEEK 03: GENERATION: ALTERNATIVE SOURCES: FUEL CELLS Sections: General | Operation | Systems | Future Introduction4 A fuel cell is a mini power plant that produces electricity without combustion. Chemical energy is converted directly into electrical energy and heat when hydrogen fuel is combined with oxygen from the air. Water is the only by-product. No pollutants are produced if pure hydrogen is used. Hydrogen can be produced from water using renewable solar, wind, hydro or geothermal energy. Hydrogen also can be extracted from anything that contains hydrocarbons, including gasoline, natural gas, biomass, landfill gas, methanol, ethanol, methane and coal-based gas. Definition. A fuel cell is an electrochemical device that continuously converts the chemical energy of a fuel (and oxidant) to electrical energy. The essential difference between a fuel cell and a battry is the continuous nature of energy supply. The fuel and the oxidant, which is usually oxygen, are supplied continuously to a fuel cell from an external source. In a battery, the fuel and the oxidant are contained within, when the contained reactants have been consumed, the battery must be replaced or recharged. The fuel cell uses liquid or gaseous fuels, such as hydrogen, hydrazine, hydrocarbons, and coal gas. The oxidant is usually gaseous oxygen or air. A practical fuel cell power plant, consists of at least three basic subsystems: 1. A power section which consists of one or more fuel cell stacks -- each stack containing may individual fuel cells usually connected in series to produce a stack output ranging from a few to several hundred volts of direct current. This section converts processed fuel and the oxidant into dc power. 2. A fuel subsystem that manages the fuel supply to the power section. This subsystem can range from simple flow controls to complex fuel-processing facility. This subsystem processes fuel to the type required for use in the fuel cell (power section). 3. A power conditioner that converts the output from, the power section to the type of power and quality required by the application. This subsystem could range from a simple voltage control to a sophisticated device that would convert the dc power to an ac power output. In addition, a fuel cell power plant, depending on size, type, and sophistication, may require an oxidant subsystem as well as thermal and fluid management subsystems. Components The important components of the individual fuel cell are: (1) The anode (fuel electrode) must provide a common interface for the fuel and electrolyte, catalyze the fuel exodation reaction, and conduct electrons from the reaction site to the external circuit. (2) The cathode (oxygen electrode) must provide a common interface for the oxygen and the electrolyte, catalyze the oxygen reduction reaction, and conduct electrons from the external circuit to the oxygen electrode reaction site. (3) The electrolyte must transport one of the ionic species involved in the fuel and oxygen electrode reactions while preventing the conduction of electrons which causes short circuit. Sections: General | Operation | Systems | Future
1. The hydrogen dissociates on the catalytic surface of the fuel electrode, forming hydrogen ions and electrons. 2. The hydrogen ions migrate through the electrolyte (and a gas barrier) to the catalytic surface of the oxygen electrode. 3. Simultaneously, the electrons move through the external circuit to the same catalytic surface. 4. The oxygen, hydrogen ions, and electrons combine on the oxygen electrode's catalytic surface to form water. The reaction mechanisms of this fuel cell, in acid and alkaline electrolytes, are shown in the table below.
The net reaction is that of hydrogen and oxygen producing water and electrical energy. As in the case of batteries, the reaction of one electrochemical equivalent of fuel will theoritically produce 26.8 Ah of dc electricity at a voltage that is a function of the free energy of fuel-oxidant reactions. At ambient conditions, this potential is ideally 1.23 Vdc for a hydrogen-oxygen fuel cell. Performance Characteristics of a fuel cell is represented by the current density versus voltage (or polarization) curve. Whereas ideally a single H2-O2fuel cell could produce 1.23 Vdc at ambient conditions, in practice, fuel cells produce useful voltage outputs that are considerably less than the ideal and decrease with increasing load (current density). The losses or reduction in voltage from ideal are referred to as polarization, including: (1) activation polarization represents energy losses that are associated with the electrode reactions; (2) ohmic polarization represents the summation of all the ohmic losses within the cell, including electronic impedances through electrodes, contacts, and current collectors and ionic impedance through the electrolyte; and (3) concentration polarization represents the energy losses associated with mass transport effects, i.e., the performance of an electrode reaction may be inhibited by the inability of reactants to diffuse to or products to diffuse away from the reactio site. The net result of these polarizations is that practical fuel cells produce between 0.5 and 0.9 V dc at currents of 100 to 400 mA/cm² of cell area. Fuel cell performances can be increased by increasing cell temprature and reactant partial pressure. Sections: General | Operation | Systems Classifications Fuel cell systems can take a number of different configurations, depending on the combination of type of fuel and oxidant, whether the fueling is direct or indirect, the type of electrolyte, the temperature of operation.
A. Acid Fuel Cells are characterized by (1) ionic conduction is provided by hydrogen ions or by hydronium ions H3O+; (2) platinum or platinum alloys, usually in very small quantity, are the active electrocatalysts, and (3) carbon, in the form of graphite, is an acceptable material of construction for current collectors, gas separators, etc. Acid fuel cells are of two types: the solid polymer electrolyte and the phosporic acid types. The a) solid polymer electrolyte (SPE) system uses an ion exchange membrane as the electrolyte. This system cannot operate at a temperature greater than 120°C. The b) phosporic acid electrolyte (PAFC) types operate from 150 to 220°C. At lower temperature, phosporic acid is a poor ionic conductor. B. Alkaline Fuel Cells (AFC) are characterized by (1) ionic conduction is provided by hydroxyl (OH-) ions; (2) electrocatalysts used include nickel, silver, metal oxides, spinels (hard crystalline minerals including oxides of Mg, Fe, and Al.) and noble metals (chemically non-reactive with acids or air, oftentines called as precious metals to include: Ruthenium [Ru-44], Rhodium [Rh-45], Palladium [Pd-46], Silver [Ag-47], Osmium [Os-76], Iridium [Ir-77], Platinum [Pt-78] and Gold [Au-79]). and (3) construction materials are carbon, nickel and stainless steel. Alkaline FCs operate at high temperature from 102°C to 250°C using a concentrated 85% weight of potassium hydroxide. C. Molten Carbonate Fuel Cells (MCFC) are characterized by (1) ionic conduction is provided by a carbonate ion; (2) CO2is recycled; (3) Nobel metals are not used, while Nickel and Nickel Oxide (NiO2) are used; and (4) Construction materials include Nickel and ceramics. Molten Carbonate FC uses alkali metal (Li, K, Na) carbonate as electrolyte. Salts function only in liquid phase thus opeate at 600 to 700°C, which is above the melting point of salts. These FCs are usually integrated to a carbonaceous fuel processors, either a reformer or a coal gassifier.
D. Solid Oxide Fuel Cells (SOFC) are characterized by (1) ionic conduction is provided by oxide ions; (2) Cathode used are metal oxides, e.g., praesodymium oxide or indium oxide, and the anode could be nickel or nickel cerment; and (3) Construction materials include ceramics and metal oxides. SOFCs employs a solid, nonporous metal oxide electrolyte which allows ionic conductivity by the migration of oxide ions to the lattice of the crystals. Most common electrolyte is stabilized Zirconia which operates from 900°C to 1000°C. E. Metal Hydride Fuel Cells (MHFC) are experimental fuel cells which employs metal hydride such as calcium hydride, sodium aluminum hydride, iron titanium, lanthanum nickel and oxides or rare-earth metals. These fuel cells store large amounts of H2, thus they exhibit higher-energy densities. Common reactions are as follow:
F. Direct Fuel Cells (DFC) are experimental fuel cells which uses methanol (CH3OH) and hydrazine (N2H4) as liquid fuel, and operates from 1000 to 1200°C.
Sections: General | Operation | Systems | Future Energy DiversityFuel cells can promote energy diversity and a transition to renewable energy sources. Hydrogen, the most abundant element on earth, can be used directly, or a fuel cell system that includes a “fuel reformer” can utilize the hydrogen from any hydrocarbon fuel: natural gas, ethanol, methanol and even gasoline. Hydrogen can also be produced using wind or solar power, or it can be extracted from "novel" feedstocks such as landfill gas or anaerobic digester gas from wastewater treatment plants. Japan’s Asahi brewery is generating some of its own power with a fuel cell running on hydrogen from the methane gas produced in its brewing process Poor energy use is a problem that runs rampant throughout developing economies and must be solved before these economies can truly progress, according to the Department of Energy’s Pacific Northwest National Laboratory. Many factors contribute towards poor energy usage, including inefficient or insecure power generation and transmission capabil-ities, inefficient industrial and manufacturing processes, lack of sufficient infrastructure, and lack of reliable and clean fuel sources. These factors not only affect the economies of the developing countries, but the health and quality of life of the countries’ inhabitants. Fuel cells and other clean, efficient distributed power technologies can help provide a solution. Benefits One of the benefits of using fuel cells in developing countries is environmental. With a global focus on climate change and reduction of greenhouse gases, all countries are being forced to look at ways of expanding their economies while simultaneously decreasing emissions of greenhouse gases. It is possible to improve access to electricity by utilizing clean technologies, thereby reducing the number of people who rely on diesel generators or on wood or oil burning for cooking and heating. Use of fuel cells is a cleaner and far more efficient way of providing both heat and electricity. CogenerationThe cogeneration aspect of fuel cell technology is extremely important for countries with colder climates, such as the developing countries of the former Soviet bloc. A recent article by the Environmental News Network states that Russian families "spend 25 percent of their take-home pay on heat for their apartments." Fuel cell power plants installed at an apartment complex could provide both the electricity for the building as well as heat and hot water for the building’s tenants. A World Bank Policy Paper entitled The World Bank’s Role in the Electric Power Sector notes, "It hasbeen estimated that older power plants in many developfing countries consume from 18 to 44 percent more fuel per kilowatt hour of electricity produced than do plants in OECD countries". Replacing these old plants with highly efficient power generation technology makes sense: you will need less plant to provide the same, probably more, generation capacity. Fuel cells are well-suited to this application: they are modular and scaleable, enabling developers to add to capacity easily as demand grows. The scaleability also enables incremental build-up of capacity where there is minimal funding available for the up-front capital costs of multi-megawatt sized power plants. Another problem noted by the World Bank Policy Paper is the high percentage of electricity that is "lost" during transmission and distribution from a central source. The World Bank estimates that these losses total about 300 billion kilowatt hours per year. For example, in Bangladesh, 31 percent of electricity generated is lost during transmission and distribution. A January 1991 supervision report by the World Bank stated that "payment for electricity reflects only 57 percent of the energy generated" in Bangladesh. Why? Technical inadequacies play a major role, but theft is also a big factor in these losses. The more these countries can place the power source at the site of use, the fewer the losses due to thieves and faulty transmission wires. The sizing of fuel cell technology is flexible so that one can site a fuel cell power plant anywhere, from single-family dwellings to factories, businesses to utilities, remote locations to town centres. When integrated with a fuel reformer, the fuel cell can generate electricity using hydrogen from a wide variety of fuels, at a higher efficiency than simply burning the fuel. Using the reformer, a fuel cell power plant can even capture the energy potential of "novel" fuel sources such as landfill gas, gas from wastewater treatment plants, waste methane from industrial processes, and ethanol from biomass. Hydrogen can also be created using solar- or wind-powered electrolysis of water. Status of Development Industry timelines suggest we will be seeing a wide variety of commercially available fuel cell products within the next two to three years. Natural gas fuel cell power plants are currently available in a modular 200kW size that are "stackable" depending on load. On the way are stationary units as small as 1kW for residential or remote power. Larger fuel cell power plants by Siemens Westinghouse, providing enough power for about 200 homes, may be marketed as early as 2001. Toshiba has also announced that it has begun selling fuel cell power plants that run on gas from sewage sludge. The company hopes to sell about 10 systems a year. 1. Power Matters: Looking to the future of fuel cellsAsian Electricity November-December Vol. 18 no. 08, DMG World Media, UK, 2000, pp. 20,22. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
A simple fuel cell is illustrated at left. Two catalyzed carbon electrodes are immersed in an electrolyte and separated by a gas barrier. The fuel, usually hydrogen, is bubbled across the surface of one electrode while the oxidant, usually oxygen from ambient air, is bubbled across the other electrode. When the electrodes are electrically connected through an external load, the following events occur:
