Here is a term paper on ‘Fuel Cells‘ for class 9, 10, 11 and 12. Find paragraphs, long and short term papers on ‘Fuel Cells‘ especially written for school and college students.
Term Paper on Fuel Cells
Term Paper # 1. Introduction to Fuel Cells:
A fuel cell is a device in which the chemical energy is converted directly into electrical energy and heat without combustion. The chemical energy is the free energy of the reactants employed. The basic feature of the fuel cell is that the fuel and its oxidant are combined in the form of ions rather than neutral molecules.
Fuel cells are different from conventional batteries in that they consume reactant from an external source, which must be replenished—a thermodynamically open system. By contrast batteries store electrical energy chemically and hence represent a thermodynamically closed system. The first practical fuel cell was demonstrated in 1959 by Francis T. Bacin and J.C. Frost of Cambridge University.
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In conventional steam power plants the chemical energy of the fuel is converted into heat energy by burning and the heat energy is, then, converted into electrical energy. The efficiency of this conversion process is limited by the limitations of Carnot cycle. In fuel cells the chemical energy of the reactants is converted into electrical energy as an isothermal process.
Thus heat is not involved in the conversion process and high conversion efficiency is possible. Another reason for the interest in fuel cells is that their efficiency and cost per kW of power are independent of size (or rating) of the fuel cell. This advantage makes the prospects of fuel cells very attractive as portable power plants for space crafts, locomotives etc.
The other advantages of fuel cells are:
(i) The unit is lighter and smaller and requires little maintenance because of absence of mechanical parts.
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(ii) They cause little pollution and little noise.
(iii) No overhead line is required.
(iv) Fuel can be used more effectively than in a central power plant.
(v) A fuel cell gives a few times more electrical energy per unit weight as compared to a turbo-generator or storage battery.
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(vi) They can become remarkable home units.
(vii) A variety of fuels such as methane, ethane, ethylene, acetylene, propane, butane, benzene, methanol, ammonia, hydrazine, LPG, biogas or coal gas can be used.
Theoretically a fuel cell should be capable of generating electricity very efficiently. However the development costs are very high. It is necessary to work at high temperatures or high pressures or use costly catalysts for the reaction to take place at a speed in order to give high current densities required for an economic plant. Other drawbacks of the fuel cells are low voltage and low service life.
Inspite of the limitations and drawbacks of fuel cells, development works of fuel cells are in progress at many places in USA and other countries. They are likely to have their own place in generation of electrical energy in the near future and will have revolutionary effect in spreading electricity in remote and rural areas of the world.
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The development of fuel cells will be specially beneficial to India for supply of electrical energy to irrigation pumping sets in the villages and remote areas as the fuel cells as supply source of electricity will not require transmission lines for which a lot of money is needed. Indian scientists are, therefore, exploring the development and utilisation of fuel cells.
Fuel cells can be manufactured as small or as large as required for a particular power application. Presently, there are micro fuel cell that are of the size of a pencil eraser and generates few mW of power while there are others large enough to provide large amount of power. The power output of fuel cells is fully scalable by varying the cross-sectional area of each cell to provide desired current and by stacking multiples cells in series to provide the required voltage.
A fuel cell normally contains two electrodes separated by an electrolytic solution. A fuel reactant, usually hydrogen or carbon monoxide is fed into one porous electrode and oxygen or air is fed into the other porous electrode. The electrodes should be capable of passing through both fuel and electrolyte and also to conduct electrons to the terminal. The electrodes must contain a chemical catalyst that breaks the fuel compound into atoms so that they are more reactive.
The most commonly used catalysts are platinum and sintered nickel. The electrodes should neither have pores of too large size to cause bubbles of fuel gas nor of too small size to cause insufficient contact between the reactant and the electrolyte. The electrolyte solution must be highly permeable to either a H+ or OH– ion which is produced as an intermediate product at one of the electrodes. The same ion is transferred through the electrolyte to the other electrode where it combines with the other reactant.
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The working of a fuel cell using hydrogen and oxygen is explained below:
When two permeable nickel electrodes are immersed in a well conducting electrolyte (say a solution of H2SO4 or KOH), negative electrode is fed with hydrogen, bubbled around it through the solution and positive electrode is fed with oxygen and the electrodes are connected together through an external circuit, then for every molecule of hydrogen consumed, two electrons pass from negative to positive electrode, where they react with absorbed oxygen.
The operation of fuel cell can be summarized as follows:
Reaction at -ve electrode is 2H2 → 4H+ + 4 e–
Every hydrogen molecule brought to the electrode surface is dissociated into two atoms by virtue of the catalytic properties of the surface. These enter the solution as hydrogen ions leaving behind two electrons which pass through the external circuit to the positive electrode. Reaction at positive electrode is O2 + 4H+ + 4 e– → 2H2O.
The oxygen supplied to the positive electrode reacts with hydrogen ions from the electrolyte and the electrons to give water. Thus water is the waste product of the cell. For the above case the electrolyte is acidic and the intermediate ion is H+.
In case of alkaline electrolyte (a typical 40% KOH solution), the intermediate ion will be OH– and the chemical reaction in the cell will be as follows:
At cathode 2H2 + 4OH– → 4H2O + 4 e–
At anode 2HzO + O2 + 4e– → 4OH–
So, when the electrolyte is acidic, water is formed at the anode and when the electrolyte is alkaline, it is formed at cathode.
If the electrodes are on open circuit, negative charges accumulate at hydrogen electrode. These negative charges attract potassium ions, K+ of the electrolyte producing a double layer. Similarly the loss of electrons from the oxygen electrode results in a layer of positive charges which in turn attracts hydroxyl ions, OH– from the electrolyte and forms a double layer. These electrical double layers build up at the electrodes until the potentials are such that they inhibit any further reaction between the electrolyte and the fuel gases.
An open-circuit voltage of 1.23 V at one atmospheric pressure at 25°C is developed. If the circuit is closed through an external load resistance, the electrons flow from the hydrogen electrode, through the external circuit to the oxygen electrode and take part in the reactions as mentioned above. The movement of electrons constitutes a current flowing through the external load circuit. The electrons movement is from the hydrogen electrode to oxygen electrode. Thus hydrogen electrode serves as cathode and the oxygen electrode as anode.
Fuel cells can be adopted to a variety of fuels by changing the catalyst, but ‘hydrox’ fuel cells using hydrogen and oxygen as fuel are the most efficient and most highly developed cells. Hydrogen can be obtained from natural gas, by catalytic cracking of ammonia or hydrocarbons or as a by-product from some processes.
Oxygen may be obtained from the air or from decomposition of peroxides. A fuel cell power plant generally also contains a reformer and an inverter. The reformer uses chemical processes to convert the fuel to form that can be utilised by the cell while an inverter is used for converting output direct current into alternating current.
A single ‘Hydrox’ fuel cell can produce an emf of 1.23 volts at one atmospheric pressure and 25°C, as already mentioned. However, it is possible to create useful potentials of 100 to 1,000 volts and power level of 1 kW to 100 MW by connecting a number of cells in series-parallel combination. The current depends upon the physical size of the cell. The output of the fuel cell varies directly with pressure, so to increase the cell output, the gas pressure is raised. The optimum size of the cell at present is about 0.027 cubic metre per kW.
Hydrox cells are of two type’s namely – low temperature cell and high pressure cell.
Fuel cells are particularly suited for low voltage and high current applications. Apollo Astronauts going to the Moon used fuel cells to convert hydrogen and oxygen to electricity. The power cells were located in the Apollo service module and provided the primary power source to operate life, support communication, guidance and other electrical system.
One type of fuel cell considered suitable for fuel cell power generation system is phosphoric acid fuel cell that operates at a temperature of about 190°C. The fuel used in this cell is high calorific value gas (with methane as the principal constituent), oxidizer is air, electrolyte is phosphoric acid and electrodes are made of carbon catalyzed by platinum. The fuel cell voltage is 0.7 V, current density 200 mA/ cm2 and expected life 10,000 hours.
Term Paper # 2. Choice of Fuel for Fuel Cells:
The choice of fuel for a fuel cell is governed by cost, availability, volume, transportability, etc.
Amongst the fuels used in a fuel cell, hydrogen is the most important. This is because hydrogen and oxygen are capable of releasing more energy per unit weight than most
other oxidizer combinations. Such fuel cells are widely used in spacecraft power supplies.
Hydrocarbons (such as methane, ethane, acetylene, benzene etc.) are less reactive than hydrogen, much more difficult to oxidize and their by-products are usually undesirable.
Compromise fuels (such as methanol, ammonia, hydrazine etc.) have reactivity in between that of hydrogen and the hydrocarbons. They are easy to use. Hydrazine is highly reactive at normal temperatures and does not require any catalyst. Hydrazine is used in fuel cells employed in military systems and submarines though it is very costly and poisonous. Ammonia is much cheaper than hydrazine, is readily available and easier to handle but its reactivity is low. Fuel cells using ammonia are considered quite suitable for specialized remote, low power applications.
Different fuels with voltage at 25°C are given below in tabular form:
Various types of fuel cells with their operating temperature are given below in tabular form:
Term Paper # 3. Advantages and Limitations of Fuel Cells:
Fuel cells have the following advantages:
1. Fuel cells have high efficiency (exceeding 50%) in full- load as well as part-load operation. These are potentially ideal sources of power generation. Efficiencies of the order of 40% have already been achieved. An overall efficiency of more than 80% can be achieved in cogeneration plants if heat generated in the fuel cell can also be utilized in addition to electrical energy produced.
2. No pollution emissions. When fuel cells are used for power generation and transport sectors, NO2 will be reduced by 50% to 90% and CO2 by 50% in comparison to present conventional technologies.
3. Water is bi-product of reaction. This is a useful product in space and remote applications.
4. The noiseless operation of fuel cells makes them quite suitable for military and other strategic applications. This is because of absence of moving parts.
The main limitations hindering the growth of fuel cells are:
1. High capital cost of fuel cells is the main limitation against commercialization. However, this limitation can be overcome by development of new materials as the materials used presently are very costly. Search for new applications and markets can help in reducing the costs by taking the advantage of scale of production.
2. Short life span of fuel cells due to heavy corrosion of electrodes. This can be overcome again by new material technologies.
A 50 kW fuel cell stack has been developed by Plug Power LLC, Latham, New York. It generates electrical energy from different fuels like gasoline, ethanol, methanol and natural gas. A fuel processor has been developed to convert raw fuel into hydrogen, which is supplied to a fuel cell for generation of electrical engineering. The carbon dioxide removal system has also been developed. The technology can be used for powering vehicles and in homes and apartments to provide heat and electricity.
A fuel cell of 1.5 kW capacity has been developed in Australia to meet the power requirements of a house.