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Fuel Cells Fuel cells provide a range of critical benefits that no other single power technology can match. A fuel cell converts the chemical energy of hydrogen and oxygen directly to produce water, electricity, and heat. They are therefore inherently clean and efficient and are uniquely able to address the issues of environmental degradation and energy security. They are also safe, quiet and very reliable. Fuelled with pure hydrogen, they produce zero emissions of carbon dioxide, oxides of nitrogen or any other pollutant. Even if fuelled with fossil fuels as a source of hydrogen, noxious emissions are orders of magnitude below those for conventional equipment. They offer significant improvements in energy efficiency as they remove the intermediate step of combustion and mechanical devices such as turbines and pistons. Unlike conventional systems, they operate at high efficiency at part load. Also, unlike conventional plants, their high efficiency is not compromised by small sizes. High efficiency saves fuel and reduces CO2 emissions. Fuel cell power plants have demonstrated unprecedented reliability and durability that is significantly better than conventional equipment. The absence of combustion and moving parts means that fuel cells can run continuously for long periods before servicing and they are far less prone to breakdown. They promote energy security and will assist the transition to renewable energy sources. Fuel cells can use hydrogen derived from a variety of sources, including natural gas and coal, and renewables such as biomass or, through electrolysis, wind and solar energy. Fuel cells offer utilities the opportunity to provide customers with an added value energy service that is not subject to the same competitive or regulatory pressures as exist for conventional electric supply and will be able to do so at overall lower cost. Principle of operation of a typical fuel cell: When hydrogen is fed into a fuel cell, a catalyst on the anode converts the gas into negatively charged electrons (e-) and positively charged ions (H+). The electrons (e-) flow through an external load to the cathode. The hydrogen ions (H+) migrate through the electrolyte to the cathode where they combine with oxygen and the electrons (e-) to produce water. Individual cells produce a small voltage. They are arranged in 'stacks' to provide the required level of power. Fuel Cells have emerged as one of the most promising technologies for the power source of the future. The fuel cell is an electrochemical device that converts energy into electricity and heat without combustion. A fuel cell consists of a cathode (negatively charged electrode), an anode (positively charged electrode), an electrolyte and an external load. The anode provides an interface between the fuel and the electrolyte, catalyzes the fuel reaction, and provides a path through which free electrons are conducted to the load via the external circuit. The cathode provides an interface between the oxygen and the electrolyte, catalyzes the oxygen reaction, and provides a path through which free electrons are conducted from the load to the oxygen electrode via the external circuit. The electrolyte acts as the separator between hydrogen and oxygen to prevent mixing and, therefore, preventing direct combustion. It completes the electrical circuit of transporting ions between the electrodes. Fuel Cell is similar to a battery as it has electrodes, an electrolyte and positive and negative terminals. But it is different from a battery in that it does not release energy stored in the cell. Instead the energy is converted into a hydrogen laden fuel directly into electricity. The use of fuel cells does not emit pollutants. Working As shown in the Figure PEM-Fuel Cell, two catalyzed carbon electrodes are immersed in an acid electrolyte and separated by a membrane. As hydrogen and oxygen enter into the fuel cell, hydrogen and oxygen bubbles are formed across the surfaces of the fuel and oxygen electrodes, respectively. The hydrogen reacts with the catalyzed electrode and forms hydrogen ions and electrons. The hydrogen ions move across to the surface of the cathode passing through the electrolyte. The electrons flow through the external circuit to the cathode. The oxygen, hydrogen ions and free electrons combine on the surface of cathode to form water. Fuel Cell System The Figure shows the block diagram of a typical fuel cell system. It consists of three basic sections: Fuel Processor Fuel Cell Inverter The Fuel Processor filters the fuel and converts hydrocarbon based fuel to hydrogen laden gas which is supplied to the Fuel Cell. The filtering of the fuel is necessary as small amounts of sulfur compounds may cause a drastic drop in power production. The fuel cell is the heart of fuel cell system. In this section the chemical reactions responsible for producing electricity take place. The fuel cell converts the mixture of fuel (Hydrogen laden Gas) and air into direct current (DC). Later the inverter converts the output power of the fuel cell (DC) into the power required by the various applications (AC). The fuel cells are not heat engines but significant amount of heat is produced in a fuel cell system which can be used to produce steam or converted into electricity using turbines. This process is called Heat Recovery. Types of Fuel Cells: Different types of fuel cells are as follows: Phosphoric Acid Fuel Cell (PAFC): The PAFC uses phosphoric acid as an electrolyte. This is the most commercially developed fuel cell. It is being used in applications such as hospitals, hotels, offices, schools, etc. It can also be used in larger vehicles such as buses, etc. The efficiency of PAFC is 40%. The operating temperature range of PAFC is 400 degrees F. As phosphoric acid is a poor ionic conductor at low temperatures and other materials such as carbon, platinum, etc. used in the fuel cell may become unstable at higher temperatures so PAFCs do not work efficiently beyond this temperature range. Advantages: Stable electrolyte Ability to highly concentrate phosphoric acid Efficient anode performance Disadvantage: Inefficient cathode performance Proton Exchange Membrane Fuel Cell (PEMFC) The mechanism of PEMFC is same as PAFC. They differ in that PEMFCs operate at relatively low temperatures (about 200 degrees F). They have high power density and can vary their output quickly to meet shifts in power demand. Molten Carbonate Fuel Cell (MCFC): The MCFC uses an alkali metal carbonate (Li, Na, K) as the electrolyte. However an alkali metal carbonate must be in the liquid phase to function as an electrolyte. This cell operates at higher temperature of about 1200 degrees F. The high operating temperature is required to achieve sufficient conductivity of the electrolyte. The MCFC is referred to as a second generation fuel cell after PAFCs. MCFCs differ from PAFCs because of their higher operating temperature and nature of the electrolyte. The higher operating temperature of MCFCs provides the opportunity for achieving higher overall systenm efficiencies and greater flexibility in the use of available fuels. The high operating temperature, however, impose limitations and constraints on choosing materials suitable for long life time operations. Solid Oxide Fuel Cell (SOFC): SOFC uses solid, nonporous metal oxide electrolytes. The metal electrolyte normally used in manufacturing SOFCs is stabilized Zirconia. This cell operates at a higher temperature of about 1800 degrees F. This high operating temperature allows internal reforming, promotes rapid kinetics with nonprecious materials and produces high quality heat. The solid state character of SOFC components implies that there is no restriction on the cell configuration. It is possible to shape the cell according to the criteria such as overcoming design or application issues. Power generating efficiencies of SOFCs could reach 60%. Alkaline Fuel Cell (AFC): AFC uses alkaline potassium hydroxide as the electrolyte. These cells can achieve power generating efficiencies of upto 70%. For different types of fuel cells refer to Figure-Types of Fuel Cells Advantages of Fuel Cells: Environmental Benefits: Fuel cells are considered an excellent alternative energy resource from the environmental point of view. Fuel cells are quiet and produce negligible emissions of pollutants. Efficiency: Different types of fuel cells have varied efficiencies. Depending on the type and design of fuel cells, efficiency ranges from 40 to 60%. Alkaline fuel cells can even achieve power generating efficiencies of upto 70%. Fuel Availability: The primary fuel source for the fuel cell is hydrogen which can be obtained from natural gas, coal gas, methanol, and other fuels containing hydrocarbons. Fuel cells are electrochemical devices that operate using hydrogen and oxygen gases and produce electricity, heat, and water. The electrochemical reaction in fuel cells is similar to that in batteries. Fuel cells are substantially better for the environment than batteries however, and have a considerably longer life-span. Fuel Cell Advantages In general, no environmentally friendly energy source has the reliability, versatility, and efficiency of fuel cells. Fuel cells offer the means to produce power more efficiently and with less pollution than any other power solution. The following summarizes fuel cells' primary advantages 1. Substantial reliability - Partly due to the absence of moving parts, fuel cells can achieve reliability measures of 99.9999%, or 1,000 times higher reliability than conventional power sources. In this regard, fuel cells will almost entirely eliminate the power outages currently seen with combustion-driven generators and other sources of electrical power. 2. Use of renewable fuels - Fuel offer clear advantages in their use of hydrogen and oxygen. Hydrogen is the most abundant element in the universe and can be obtained from a variety of sources including natural gas, coal gas, methanol, and other fuels containing hydrocarbons. Oxygen used in fuel cells can be obtained from pure oxygen, or in some cases, simple air intake. Both oxygen and hydrogen are non-polluting gases in the context of fuel cells. 3. Environmentally clean - Fuel cells are almost entirely pollution free and have the potential to eliminate polluting emissions in commercial applications. Current industrial fuel cells generates as little as one ounce of pollution for every 1,000 kWh of power produced, compared with the estimated 25 pounds generated by typical combustion-driven power plants. 4. Broad versatility - Fuel cells' versatile design and considerable efficiency allow for broad market appeal, with suitable applications in the transportation, industrial, commercial, residential, and portable markets. Basic Reactions Fuel cell designs differ in the components and materials they use to generate electricity.
Approximate Word count = 6740
Approximate Pages = 27
(250 words per page double spaced)
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