Distributed Fuel Cell Generation - Essay Example

Fuel Cell Introduction: Fuel cell is a device that converts chemical potential energy into electrical energy. In that sense it is like a galvanic cell or secondary batteries. However, the difference lies in the fact that a fuel cell is fed with chemicals which can be seen as fuel and the same is fed continuously and the byproducts are removed continuously. It should be noted that production of electricity by thermal plants is not a very efficient method and is a major source of pollution. In such plants, the chemical energy (heat of combustion) of fossil fuels (coal, gas or oil) is first converted into thermal energy which is used for converting water into high pressure steam. This is then used to run a turbine to produce electricity. Thus there are many intermediate steps each with certain efficiency (always less than 100%) associated with each of them. This is because thermal energy is the worst form of energy with maximum entropy and therefore can never be converted fully into useful work. Now the overall efficiency of chemical potential energy to electricity conversion is a product of efficiency of each of the intermediate steps and therefore is much smaller. The best thermal power plants operate at maximum conversion efficiency of about 40%. Also, the combustion products are highly polluting adding to carbon emission and global warming. Fuel cells on the other hand convert the chemical potential energy directly into electricity and thus have very high conversion efficiency which is typically 70%. Besides, the reaction products are relatively harmless and in many cases completely harmless. Thus fuel cells are being seen as power plants of future and a tangible potential solution for containing the menace of carbon emission and therefore, global warming. These are already into many specialized and high end applications. In this report the basic principle, related developments and current and potential applications of fuel cells are briefly discussed. Basic Principle of Fuel Cell A fuel cell is like a galvanic cell. It directly converts chemical energy into electricity and is highly efficient. It is now possible to make such cells in which reactants are fed continuously to the electrodes and products are removed continuously from the electrolyte compartment. Galvanic cells that are designed to convert the energy of combustion of fuels like hydrogen, methane, methanol, etc. directly into electrical energy are called fuel cells. It has following components 1) Anode 2) Cathode 3) Electrolyte 4) Fuels / Reactants 5) Arrangements for feeding fuels / reactants and extracting electrical current and reaction byproducts. Schematic diagram of a typical fuel cell based on hydrogen and oxygen as fuels is shown in Fig. 1, below. Fig. 1: Schematic Diagram of a Fuel Cell based on Oxygen and Hydrogen as Fuel Such a fuel cell was used for providing electrical power in the Apollo space program. The water vapors produced during the reaction were condensed and added to the drinking water supply for the astronauts. In the cell, hydrogen and oxygen are bubbled through porous carbon electrodes into concentrated aqueous sodium hydroxide solution. Catalysts like finely divided platinum or palladium metal are incorporated into the electrodes for increasing the rate of electrode reactions. The electrode reactions are given below: Cathode Reaction: O2(g) + 2H2O(l ) + 4e– → 4OH–(aq) Anode Reaction: 2H2 (g) + 4OH–(aq) → 4H2O(l) + 4e– Overall reaction: 2H2(g) + O2(g) → 2H2O(l ) Different Types of Fuel Cells Fuels cells are classified on the basis of either the fuel or the electrolyte. Some common types of fuel cells are presented in Table 1, below[1]. Table 1: Different Types of Fuel Cells and Their Uses Type of fuel cell Efficiency Operating Temperature Use Polymer Electrolyte Membrane (PEMFC), sometimes called Proton Exchange Membrane 40% (80% with Cogeneration) 175º F Transportation – cars, buses, boats, trains, scooters, bikes Residential – household electrical power needs Portable – laptop computers, cell phones, medical equipment [2, 3] Direct Methanol (DMFC) 40% 120 - 150º F Portable – cell phones, laptop computers, vacuum cleaners, highway road signs Alkali (AFC) 60% (80% with Cogeneration) 250 - 500º F NASA space program – space vehicles Phosphoric Acid (PAFC) 40% (80% with Cogeneration) 300 - 400º F Landfill/wastewater treatment facilities – To generate power from methane gas [4] Solid Oxide (SOFC) 55% (85% with cogeneration) 1,800º F Commercial – utility power plants, airport terminals, public and commercial office buildings, hotels, hospitals Molten Carbonate (MCFC) 55% (85% with cogeneration) 1,200º F Commercial – utility power plants, airport terminals, schools, office buildings, hotels, hospitals Developments and Potential Applications Though there has been many developments in the field of fuel cells the main focus of these developments have been on increasing the operating temperature, upping the scale and extending the fuels which can be used. One very important development in this direction has been development of Solid Oxide Fuel Cell or SOFC. In solid oxide fuel cells, electrolyte is a hard ceramic, porous metal oxide instead of a liquid. The hard electrolyte normally zirconium or calcium oxide, reduces assembly and maintenance costs. Besides, very high operating temperatures allow for internal reforming of hydrocarbon fuels without the need for precious metal catalysts, which are very costly. Anodes are typically made of nickel and cathodes are made from alloys containing metals such as lanthanum, strontium, cobalt, ferrite, magnetite, or zirconium. Electrons generated at the anode travel to an external load and are returned to the cathode, completing the circuit. While oxygen is gets converted into oxide ion and diffuses through the defect structure in the solid oxide electrolyte. These solid oxide fuel cells are about 60 percent efficient, and operate at temperatures between 1,000 to 1,800º F. The promise of this fuel cell is in providing power at large, stationary power plants. The high temperature of operation provides an opportunity to increase efficiency to 85 percent through cogeneration, by capturing waste heat to generate steam for heating, industrial processing, or in a steam turbine to make more electricity. SOFCs are sulfur-resistant and are not poisoned by carbon monoxide, and so can use coal-derived gases as fuel. Start-up is slow and personnel must be protected from operating temperatures. Due to high operating temperatures, SOFCs are not suitable for small vehicle, residential, or portable applications, but work well as very large stationary units. Large power plants based on a combination of high temperature nuclear reactors and SOFC are being seen as a viable option in nuclear power program of many countries. The basic underlying philosophy is that the heat generated by the nuclear reactor will not be used for steam generation, instead it will be use for cracking hydrogen bearing substances like water, ammonia etc. which will be used as fuel for the SOFC based power plants. This scheme will have much better efficiency than conventional nuclear power plants and hence will utilize the nuclear power in better manner. Conclusions: Fuel cells are very promising power generation device for future. References: [1] www.hhoencyclopedia.com/hho/articles/Common%20Types%20of%20Fuel%20Cells.pdf Retrieved on March 29, 2010 [2] Williamson SS, Emadi A, Shahidehpour M. Distributed fuel cell generation in restructured power systems. IEEE Power Engineering Society General Meeting 2004;2(10):2079–84. [3] Lehman PA, Chamberlin CE, Zoellick JI, Engel RA. “A photovoltaic/fuel cell power system for a remote telecommunications station.” IEEE 28th Photovoltaic Specialists Conference 2000; 1552–5. [4] Spiegel RJ, Preston JL. “Technical assessment of fuel cell operation on anaerobic digester gas at the Yonkers, NY, wastewater treatment plant”. Waste Management 2003;23(8):709–17. Read More