BKCHAP14-2011 - Chapter 14 FUEL CELLS 14.1 Introduction...

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14-1 Chapter 14 FUEL CELLS 14.1 Introduction 14.2 Basic Operation 14.3 Advantages and Disadvantages 14.4 Potential Applications 14.5 Possible Fuels for Fuel Cells 14.6 Classification 14.7 Some Important Types of Fuel Cells 14.7.1 Alkaline Fuel Cell (AFC) 14.7.2 Phosphoric Acid Fuel Cell (PAFC) 14.7.3 Polymer Electrolyte Membrane Fuel Cell (PEMFC) 14.7.4 Direct Methanol Fuel Cell (DMFC) 14.7.5 Molten Carbonate Fuel Cell (MCFC) 14.7.6 Solid Oxide Fuel Cell (SOFC) 14.8 Comparison of Fuel Cell Technologies 14.9 Ideal Efficiency 14.10 VI Characteristics of Fuel Cell 14.11 Fuel Cell Power Plant 14.12 Development Stages and Future Trends 14.13 Environmental Effects 14.14 Problems 14.15 References 14.1 Introduction A fuel cell is an electrochemical energy conversion device that converts chemical energy of a fuel directly into electrical energy. Since the conversion process is direct without involving combustion of fuel, the process is clean, quiet and highly efficient – nearly twice as compared to conventional process. It has no moving parts. Its continuous operation requires supply of fuel and oxidant and removal of water vapor, spent fuel, spent oxidant, inert residue and heat. It is known as cell because of some similarities with a primary cell. Like a primary cell it has two electrodes and an electrolyte between them and produces dc output. However, in a fuel cell, active material is generally supplied from outside, unlike conventional cell where it is contained inside the cell. Main exhaust of a fuel cell, if pure hydrogen is used as fuel (and pure oxygen as oxidant), is water vapor, which is not a pollutant. In case of hydrocarbon fuel, carbon dioxide is also produced. However, the amount of carbon dioxide produced is much less as compared to that when electricity is produced through burning of hydrocarbon fuel. If air is used as oxidant, nitrogen (spent oxidant) is also present in the exhaust. No other pollutant such as particulate matter, NO X and SO X are produced. Some amount of heat is also produced, which can be easily dissipated to atmosphere or used locally for heating purpose. Unlike conventional thermal power plants where a substantial quantity of cooling water is required, large
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14-2 scale cooling water is not required in case of fuel cell. As conversion of chemical energy of fuel to electrical energy occurs directly without intermediate thermal stage, the efficiency of conversion is not limited by thermodynamic laws (Carnot cycle) as in the case of conventional thermal power plants. The efficiency of a practical fuel cell may be around 50 - 60%, even without cogeneration. Waste heat from cells can also be harnessed, boosting the efficiency still further. The average cell voltage is typically about 0.7 V (on rated load) and several cells may be connected in series to increase the voltage. The current depends on the electrode area and can be increased by connecting several cells in parallel. Thus modules of different sizes can be constructed by series-parallel combination of required number of cells. This combination is known as a
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BKCHAP14-2011 - Chapter 14 FUEL CELLS 14.1 Introduction...

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