New Approach to Fuel Cell Technology
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02-10-2010, 09:33 AM
Fuel cell may be defined as “an electro chemical device for the continuous conversion of the free energy change in a chemical reaction to electrical energy”.
Fuel cell systems offers clean and efficient energy production and are currently under intensive development by several manufacturers for both stationary and mobile applications. Fuel cells found its first application during the NASA Apollo moon landing program in the late 1960s. It was a logical choice as an energy source. It had no moving parts, was compact in size, and was founded with an unlimited budget. Powered simply by hydrogen gas, the fuel cell produced dc power, pure water as its exhaust, and heat. The dc supply charged the spacecraft batteries. The water was used by the crew and the heat generated by the fuel cell was rejected to the void of space. At that time a brief study was conducted as to how fuel cells could be used to power homes in the same manner as spacecrafts. The study was very short lived but the dream of powering automobiles and houses using fuel cells was born in earnest.
Fuel cells, until recently a curiosity largely confined to the space program, are emerging as a valuable clean and efficient generator of electricity. A number of companies are developing fuel cells for use in stationary applications. Most of the current applications for fuel cells utilize natural gas as a fuel. In certain states, such as New York and Connecticut, fuel cells operating on natural are recognized by the states as a renewable energy source. Recently, however, fuel cells, mostly phosphoric acid, have been shown to operate well on renewable biogas fuels, such as anaerobic digester gas (ADG) produced at wastewater treatment plants as well as landfill gas (LFG) and gas produced at beer breweries.
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03-03-2011, 04:55 PM
Vineeth Kumar Reddy. N
Fuel cells ppt.doc (Size: 355 KB / Downloads: 75)
Fuel Cell Technology
This paper provides a survey of fuel cell technology and application. A description of fuel cell operating principles is followed by a comparative analysis of the current fuel cell technology together with issues concerning various fuels. Appropriate applications for current and perceived potential advances of fuel cell technology are discussed.
Keywords: Fuel cell, Eco friendly, Hydrogen and oxygen as fuels, Water bi-product, Efficiency, Applications of Fuel Cells
In order to move towards a sustainable existence in our critically energy dependent society there is a continuing need to adopt environmentally sustainable methods for energy production, storage, and conversion. The use of fuel cells in both stationary and mobile power applications can offer significant advantages for the sustainable conversion of energy. Benefits arising from the use of fuel cells include efficiency and reliability, as well as economy, unique operating characteristics, and planning flexibility and future development potential. By integrating the application of fuel cells, in series with renewable energy storage and production methods, sustainable energy requirements may be realized.
2. FUEL CELL FUNDAMENTALS
A fuel cell is conventionally defined as an “electrochemical cell which can continuously convert the chemical energy of a fuel and an oxidant to electrical energy by a process involving an essentially invariant electrode-electrolyte system” . For a hydrogen/oxygen fuel cell the inputs are hydrogen (fuel) and oxygen (oxidant) and the only outputs are dc power, heat, and water. When pure hydrogen is used no pollutants are produced, and the hydrogen itself can be produced from water using renewable energy sources such that the system is environmentally benign. In practice hydrogen is the best fuel for most applications. In addition to hydrogen some fuel cells can also use carbon monoxide and natural gas as a fuel. In these reactions, carbon monoxide reacts with water producing hydrogen and carbon dioxide, and natural gas reacts with water producing hydrogen and carbon monoxide, the hydrogen that is produced is then used as the actual fuel.
2.2 Electro Chemistry
The basic physical structure of all fuel cells consists of an electrolyte layer in contact with an anode and cathode electrode on either side of the electrolyte. The electrolyte provides a physical barrier to prevent the direct mixing of the fuel and the oxidant, allows the conduction of ionic charge between the electrodes, and transports the dissolved reactants to the electrode. The electrode structure is porous, and is used to maximize the three-phase interface between the electrode, electrolyte and the gas/liquid, and also to separate the bulk gas phase and the electrolyte. The gas/liquid ionization or de-ionization reactions take place on the surface of the electrode, and the reactant ions are conducted away from or into the three-phase interface . A schematic representation of a fuel cell with the reactant/product gases and the ion conduction flow directions through the cell In theory a fuel cell is capable of producing an electric current so long as it supplied with fuel and an oxidant. In practice the operational life of the fuel cell is finite and fuel cell performance will gradually deteriorate over a period of time as the electrode and electrolyte age. However, because fuel cells operate with no moving parts, highly reliable systems are achieved
The thermal efficiency of the fuel cell can be defined as the percentage of useful electrical energy produced relative to the heat that would have been obtained through the combustion of the fuel (enthalpy of formation). In the ideal case, the maximum efficiency (or thermodynamic efficiency) of a fuel cell operating irreversibly can be expressed as the percentage ratio of Gibbs free energy over the enthalpy of formation, that is,
Where G is change in Gibbs free energy and H is the enthalpy of formation of the reaction. For the hydrogen/oxygen fuel cell the thermodynamic efficiency limit at the higher heating value (HHV) is equal to 83%
In practice the efficiency of the fuel cell can be expressed in terms of the percentage ratio of operating cell voltage relative to the ideal cell voltage as
Where Vcell is the actual voltage of the cell and Videal is the voltage obtained from Gibbs free energy in the ideal case. The 0.83 is from the thermodynamic limit (HHV).
The main advantages of fuel cells are:
Efficiency - Fuel cells are generally more efficient than combustion engines as they are not limited by temperature as is the heat engine.
Simplicity - Fuel cells are essentially simple with few or no moving parts. High reliability may be attained with operational lifetimes exceeding 40,000 hours (the operational life is formally over when the rated power of the fuel cell is no longer satisfied) [3-5].
Low emissions - Fuel cells running on direct hydrogen and air produce only water as the byproduct.
Silence - The operation of fuel cell systems are very quiet with only a few moving parts if any. This is in strong contrast with present combustion engines.
Flexibility - Modular installations can be used to match the load and increase reliability of the system.
The principal disadvantages of fuel cells, however, are the relatively high cost of the fuel cell, and to a lesser extent the source of fuel. For automotive applications a cost of US$10 to $50 per kW and an operation life of 4000 hours are required in order to compete with current internal combustion engine technology. For stationary combined heat and power systems a cost of US$1000 per kW and an operation life of 40,000 hours is required [5,6]. The current cost of a fuel cell system is around US$3000 per kW for large systems with additional costs required for the heat exchanger in the combined heat and power systems. The cost of fuel cells will be brought down with mass manufacture and costs of US$100 per kW have been predicted as the production of fuel cells expand over the following few years.
3. FUEL CELL CLASSES
There are five primary classes of fuel cells, identified by their electrolyte, which have emerged as viable systems . Although the most common classification of fuel cells is by the type of electrolyte used, there are always other important differences as well. Each fuel cell class differs in the materials of construction, the fabrication techniques, and the system requirements. The potential use for different applications is inherent in the main characteristics of each fuel cell class .
Solid Oxide (SOFC): The solid oxide fuel cell operates between 500-1000C. The electrolyte in this fuel cell is a solid, nonporous metal oxide and the charge carriers are oxygen ions. The electrolyte always remains in a solid state adding to the inherent simplicity of the fuel cell. The solid ceramic construction of the cell, can minimise hardware corrosion, allows for flexible design shapes, and is impervious to gas crossover from one electrode to the other. Due to the high temperature operation, high reaction rates are achieved without the need for expensive catalysts and also gases such as natural gas can be internally reformed without the need for fuel reforming. Unfortunately the high operating temperature limits the materials selection and a difficult fabrication processes results. In addition the ceramic materials used for the electrolyte exhibit a relatively low conductivity, which lowers the performance of the fuel cell.
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