Here is a compilation of essays on ‘Fuel Cells’ for class 8, 9, 10, 11 and 12. Find paragraphs, long and short essays on ‘Fuel Cells’ especially written for school and college students.
Essay on Fuel Cells
Essay Contents:
- Essay on the Introduction to Fuel Cells
- Essay on the Principle of Fuel Cell Operation
- Essay on the Performance Analysis of Fuel Cells
- Essay on the Polarization in Fuel Cells
- Essay on the Types of Fuel Cells
- Essay on the Advantages and Limitations of Fuel Cells
- Essay on the Applications of Fuel Cells
Essay # 1. Introduction to Fuel Cells:
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If an electric current is passed through a dilute solution of an acid or an alkali by means of two platinum electrodes, hydrogen is produced at the anode and oxygen is evolved at the cathode. If this process is reversed by removing the power supply and connecting the two electrodes through a suitable resistance, the presence of hydrogen at one electrode and oxygen at the other will produce a small current in the external circuit, water being produced as a by-product.
This reverse process of electrolysis is the essence of the fuel cell technology as chemical energy stored in hydrogen and oxygen has been combined to produce electricity.
Unlike other heat engines, fuel cells do not have moving parts. Therefore, the fuel cell will be quieter and should require less maintenance and attention in operation. The manufacturing costs should also be less, as close tolerances as in pistons are not required. Because the fuel cells convert chemical energy directly to electrical energy, the conversion efficiency can be much higher; they may be cheaper to operate, and there will be less heat produced, resulting in small radiator and exhaust systems.
Although much of the interest in the fuel cells is due to their efficient use of fuel, there are considerable pollution control advantages to be gained as well. Because the fuel reacts electrochemically rather than by combustion with air, no nitrogen oxides are formed. For the same reason, emission of unburned and partly burned gaseous and particulate products are essentially nil.
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There is relatively little thermal pollution because less heat is lost as waste. Energy conversion via fuel cells, therefore, represents one of the best ways to achieve this goal, because it is possible, simultaneously, to obtain more work and less pollution from a rupee’s worth of fuel with a fuel cell than any other energy conversion device.
With this impressive list of advantages, it is perhaps surprising that fuel cells have not been more readily accepted. Cheap, practical and long-lasting devices are still not available in the market. Fuel cells have not been widely used or developed as terrestrial energy sources because of their high costs (partly due to expensive catalysts) and short operating life time (due primarily to degradation of electrode materials). The early development of fuel cells for electrical power generation was overshadowed by the steam turbine generation and for mobile applications, it could not compete with internal combustion engines.
The biggest boost to fuel cell development has been space power application where high power density and low weight are important. Fuel cells have been an integral part of the Gemini and Apollo manned space flight systems. It is only recently that some of the problems of high cost and short life time may be overcome and fuel cell batteries may be commercially viable within short period.
Hydrogen and oxygen were selected as reactants for the fuel cells used for spacecraft power supplies because of relatively high reactivity of hydrogen. Presently, hydrogen is a relatively expensive fuel. The reactants for commercial fuel cells should be as cheap and readily available as possible; for example, air as oxidant and natural gas or petroleum derivatives as fuel. As coal gasification technology matures, very satisfactory feed stream for fuel cell power plant will be available.
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The output of fuel cell is low voltage dc power, cells may be connected in various series and parallel arrangements to get whatever voltage and power are desired, and highly efficient inverters are available for conversion to ac.
Fuel cells have been proven as practical power sources in certain specific applications, such as space missions and remote site operations. The development of fuel cells for widespread commercial applications is now underway. Although advantages of fuel cells over other heat engines could swing the balance in favour of fuel cells, the relatively high development costs involved means that substantial markets must be sought by finding new applications.
City life depends upon continuous and reliable power supply for comfortable living, entertainment, efficient commutation, communications, commercial and industrial activities in a pollution-and noise-free environment. The application of fuel cells for individual houses, apartments, commercial buildings, street lighting, industrial estates, communication systems, electric vehicles for rail and road transport can be studied so that total power requirements of a city can be met on fuel cell batteries only.
Essay # 2. Principle of Fuel Cell Operation:
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The following three stages of energy conversion are required in a steam power plant:
1. Conversion of chemical energy of fuel into thermal energy by combustion and heat transfer in a boiler.
2. Conversion of thermal energy into mechanical energy of rotation in steam turbine.
3. Conversion of mechanical energy into electrical energy in an electric generator.
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Fuel cells convert the chemical energy of fuel directly into electrical energy by an electrochemical process. Low voltage dc power is produced by using hydrogen or natural gas as fuel. The schematic diagram of a H2 – O2 fuel cell is shown in Fig. 11.1.
The working of a fuel cell may be explained with reference to the hydrogen- oxygen fuel cell using aqueous electrolyte. It consists of two porous metal electrodes with the electrolyte between their inner faces kept stirred by recirculation.
The porous electrodes have a matrix of coarse pores, coated on electrolyte side with a layer of material having finner pores. The reactant gases are supplied to the electrodes as shown in Fig. 11.1. The pressure differential between the gases and the electrolyte is sufficient to displace the liquid from the coarse pores, but not from the fine pores.
In order to obtain a large area of electrode- electrolyte interface (at which electron transfer takes place), the gases must either diffuse through a layer of electrolyte on the surface of the electrodes, or migrate along the surface beneath the electrolyte. Similarly the ions must diffuse through the electrolyte trapped in the fine pore layer, which is not stirred by recirculation.
Platinum and other precious metals are used as electrodes in the fuel cells for military and space applications. Platinum being expensive, porous nickel electrodes and porous carbon electrodes are generally used in fuel cells for commercial applications. A catalyst is included in the electrode to expedite the reactions. The best electro-chemical catalysts are finely divided platinum deposited on electrode material. Other cheaper catalysts like nickel for hydrogen and silver for oxygen are used where possible.
The chemical reactions taking place at the two electrodes are as follows:
(i) Hydrogen Electrode (Anode):
2 H2 = 4H+ + 4e
AH+ + 4OH– = 4 H2O + 4e
(ii) Oxygen Electrode (Cathode):
O2 = 2 O–
2 O– + 2 H2O + 4 e = 4 OH–
(iii) Overall Cell Reaction:
2 H2 + O2 = 2 H2O
The H2 molecules break up into H+ ions at the anode. These H+ ions combine with OH– ions to form water and release electrons at the anode. The electrons travel to the cathode through external circuit. At the cathode the two oxygen atoms combine with the four electrons arriving by the external circuit and two molecules of water (out of the four molecules produced at the anode) to form 4 OH– ions.
These OH– ions migrate towards the anode and are consumed there- The electrolyte remains invariant. This invariance is a critical feature of fuel cell electrolyte and requires that the composition of the electrolyte must not change appreciably as the cell operates. In other words, ions removed by the reaction at one electrode must be replaced one for one at the other electrode.
The most difficult problem in the design of a fuel cell is to obtain sufficient fuel electrode-electrolyte reaction sites in a given volume. A large effective surface is achieved by a porous electrode structure and surface tension forces are utilized to get reasonable contact stability.
Essay # 3. Performance Analysis of Fuel Cells:
The theoretical maximum electromotive force (EMF) developed in a fuel cell can be calculated with the help of Gibbs free energy as follows:
where,
ΔG = Change in Gibbs free energy in the reaction [J/mol]
n = No. of electrons per mole of fuel which take part in the reaction (for hydrogen, n = 2)
F = Faraday’s constant = 96487 coulombs/mole.
The change in the Gibbs free energy in a chemical reaction is given by:
ΔG = ΔN – TΔS [J/mol]
where,
ΔH = change of enthalpy of total reaction [J/mol]
T ΔS – Amount of heat absorbed during a reversible process at constant temperature [J/mol]
The voltage of fuel cell depends upon temperature and pressure. For a H2-O2 fuel cell at a pressure of 1 atmosphere, the value of E at 25°C is 1.23V and only 1.15V at 200°C. If pressure increases, the fuel cell voltage also increases.
The maximum efficiency is achieved in a reversible fuel cell:
where,
ΔG = Change in Gibbs free energy (equal to – 237.14 kJ/mol)
ΔH = Change in enthalpy (equal to – 285.83 kJ/mol) by formation of 1 mole of liquid water from H2 and O2 at 1 atmosphere and 25°C.
The maximum work per mole of H2 (reactant) or per mole of H2O (product) is given as:
Wmax = ΔGR – ΔGH2o [kJ/mol]
ΔGR = O kJ/mol for reactants.
The electromotive force (EMF) is given by:
The overall efficiency of fuel cell is its thermal efficiency and performance factor.
ƞoverall = ƞth × (Loss factor)
The power output of a reversible fuel cell:
where,
ṁ = mass flow rate of hydrogen [kg/s]
MH2 = 2.016 kg/mol = Molar mass of hydrogen.
The actual electrical power output is:
The rate of heat released which can be utilized
Performance Characteristics of H2– O2 Fuel Cell:
The Carnot efficiency of any heat engine increases with the source temperature for a given sink temperature.
The heat source temperature T1 if varied from 400K to 1400K, the performance curve in plotted is Fig. 11.2. The heat sink temperature T0 = 300K. If the fuel cell temperature is equal to source temperature, the efficiency decreases linearly with temperature. The theoretical efficiency of H2 – O2 fuel cell as function of temperature is plotted in Fig. 11.2.
The relationship between current and voltage and power density for H2 – O2 fuel cell are plotted in Fig. 11.3.
Essay # 4. Polarization in Fuel Cells:
There is a significant drop in the voltage in a fuel cell with the increase of current density. This energy loss is called polarization or over potentials.
There are three types of polarization as shown in voltage-current curve:
I. Activation Polarization:
This is related to the activation energy barrier for the electron transfer process at the electrode. A drop of potential at the electrode gives a greater driving force to overcome this energy barrier and thus allows more current to flow.
This polarization may be reduced by:
(i) Using better electrode catalysts,
(ii) Increasing surface area of electrodes, and
(iii) Raising the operating temperature.
II. Resistance (Ohmic) Polarization:
The internal resistance of a fuel cell consists of:
1. Electrode resistance,
2. Bulk electrolyte resistance, and
3. Interface contact resistance between electrode and electrolyte.
The reduction in internal resistance is the main design criterion for low resistance polarization.
This can be reduced by:
(i) Using more concentrated electrolyte,
(ii) Closer spacing of electrodes, and
(iii) Increase of operating temperature.
III. Concentration Polarization:
This can be divided into two parts:
1. Electrolyte Side Polarization:
This is due to slow diffusion of ions into the electrolyte.
2. Gas-Side Polarization:
This is due to slow diffusion of reactants into porous electrode.
This loss can be reduced by increasing temperature.
The no-load or open circuit voltage (e.m.f),
E = V + Vp
where,
V = Operating voltage at a given current density.
Vp = Polarization loss.
All the losses in a fuel cell may be included under voltage efficiency,
Ƞv = V/E
For an ideal fuel cell, Ƞv = 100%
Most of the energy converters convert heat energy into electricity. The efficiency is defined as the ratio of work output and heat input.
The work output of energy converter operating at the thermodynamic reversible potential of the cell is the free energy of the cell.
ΔWmax = -ΔG = -nFE
The heat input is the enthalpy change of the reaction = ΔH.
The ideal efficiency of a fuel cell:
Ƞideal = ΔG/ΔH
= -nFE/ΔH
Generally ΔG is quite close to ΔH and hence efficiency of a fuel cell would be close to unity under ideal conditions.
Essay # 5. Types of Fuel Cells:
Many types of fuel cells have been developed for different applications & various technical specifications are given in Table 11.1.
The power conversion rates depend upon the type of fuel and fuel cell. Some typical values are given in Table 11.2.
The pictorial representations of various types of cells are shown in Fig. 11.3(a).
i. Proton Exchange Membrane Fuel Cells:
The electrolyte is a thin polymer film. The electrodes are made of porous carbon with platinum catalyst.
The reaction at the anode:
H2 → 2H+ + 2e–
The polymer membrane allows protons (H+) to pass through. The free electrons (e–) pass through the external circuit and reach cathode.
The reaction at the cathode:
2e– + 2H+ + ½ O2→ H2O
Water is produced.
The total reaction:
H2 + ½ O2→ H2O
The PEM fuel cells operate at 95°C and have high power density. The output can be varied very quickly. These are very suitable for transportation applications.
The main applications are:
i. Light duty vehicles,
ii. Cogeneration (CHP) plants,
iii. Power supply for buildings, and
iv. Rechargeable batteries for video cameras.
Fuel Cell Vehicles:
i. Bus developed by Ballard Power System, Canada.
ii. XCELLSIS Fuel Engine Inc. (USA), 1998.
iii. Fuel cell vehicle at Munich Airport, 2000 by Daimler/Chrystler’s NECAR-4 fuel cell vehicle.
iv. Mercedes-Benz A class fuel-cell powered cars, 2004 at $18000.
v. It is expected that by 2020, 25% of cars will be run by fuel cells.
ii. Solid Oxide Fuel Cells:
The electrolyte is ceramic membrane with Yttruim Stablized Zirconia (YSZ). The electrodes are made of porous metal-ceramic. These operate at high temperature of 800-1000°C. No catalyst is needed.
The anode reaction:
H2 + O2-→ H2O + 2e–
or CO + O2- → CO2 + 2e–
The cathode reaction:
½ O2 + 2e– → O2-
The total reaction:
H2 + ½ O2 → H2O + 2e–
or CO + ½ O2 → CO2
The capacity of up to 250 kWe is used in cogeneration plants with natural gas as fuel as well as industrial power plants. Efficiencies up to 60% can be attained. Some SOFC systems are being developed in Europe and Japan.
iii. Alkaline Fuel Cells:
These fuel cells have low weight, low operating temperature (90°C), and high efficiency (60%). These are very attractive for space and military applications. The main drawbacks are extremely high cost of the order of 100,000 DM/kW.
iv. Molten Carbonate Fuel Cells:
The electrolyte is liquid solution of lithium and sodium/potassium carbonates soaked in matrix. The operating temperature is 650°C and attainable efficiency is 60%. Fuel utilization factor is 85% in cogeneration plants. The power plants of 2 MW capacities have been tested with hydrogen, natural gas and other gaseous fuels.
Anode Reaction:
H2 + CO32- = H2O + CO2 + 2e-
Cathode Reaction:
½ O2 + CO2 + 2e– = CO32-
Total Reaction:
H2 + 1/2O2 + CO2 = H2O + CO2
v. Direct Methanol Fuel Cells:
A polymer membrane is used as electrolyte
Anode reaction:
CH3OH + H2O → CO2 + 6H+ + 6e–
Cathode Reaction:
6H+ + 6e– + 1.5 O2 → 3H2O
Total Cell Reaction:
CH3OH + 1.5O2 → CO2 + 2H2O
The cell efficiency can be 40% at operating temperature of 50 to 90°C. The anode catalyst can draw hydrogen directly from liquid methanol without reforming.
vi. Phosphoric Acid Fuel Cells:
These are commercially most developed fuel cells. These operate at 160°C to 220°C. The efficiency is above 40% for electricity production but when used in cogeneration plants, the efficiency can increase to 85%.
These cells are suitable for autonomous power plants as well as for cogeneration plants in the capacity range from a few kilowatts to 10 MW. These can be employed for residential, commercial, institutional buildings and also for small factories. These have been also successfully used for hospitals, nursing homes, hotels, office buildings, schools and central power plants in USA.
The capital cost of phosphoric acid fuel cells in the power range of 200 kW is 4500 DM/kW when operated on natural gas. For widespread use of this fuel cell the cost has to be reduced to 1300 DM/kW and life span has to be increased from the present 15000 hours to 40,000 hours.
vii. Regenerative Fuel Cells:
Water can be split into hydrogen and oxygen by the use of solar energy. The gases H2 and O2 can be used as fuel and oxidant for producing electricity. Water produced is re-circulated for electrolysis. These fuel cells can be used for energy storage also.
Essay # 6. Advantages and Limitations of Fuel Cells:
Fuel cells have the following advantages:
1. These cells have high efficiency (> 50%) in full-load and 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 electricity.
2. No pollution emissions. When fuel cells are used and transport sectors, NOx will be reduced by 50% to 90% and CO2 by 50% in comparison to present conventional technologies.
3. Water is a by-product of reaction. This is a useful product in space and remote applications.
4. There is no moving part and operation of a fuel cell is noiseless. This can be a great advantage for military and other strategic applications.
Limitations:
The main limitations hindering the growth of fuel cells are:
1. High capital cost of fuel cells is the main limitation against commercialization. This limitation can be overcome by new material developments as the present material being used is very costly. Search for new applications and markets can help to reduce the costs by taking advantage of scale of production.
2. Low life span of fuel cells due to heavy corrosion of electrodes. This can be overcome again by new material technologies.
Example 1:
Find the reversible voltage for hydrogen-oxygen fuel cell, having the following reaction:
Solution:
Essay # 7. Applications of Fuel Cells:
The applications of fuel cells may be discussed under the following groups:
1. Central power generation.
2. Cogeneration plants.
3. Automotive vehicles.
4. Miscellaneous applications.
1. Central Power Generation:
The fuel cell converts chemical energy directly into electrical energy. Therefore, power generation is the most natural application of fuel cells. For this application, fuel cells must compete with steam turbine, which are remarkably efficient devices with efficiency approaching 40 percent at rated load. However, demand for electrical energy is far from constant.
Over the course of a year, the actual output of a power plant may vary by a factor of four, and the daily variation in load can be almost a factor of three. To adjust to this changing demand, either the large base load plants must sometimes operate at part load, or smaller cycling or peaker units must be used during periods of high demand. Either way efficiency suffers or pollution increases. On the other hand, the fuel cell system not only has a greater efficiency at full load, but this efficiency is retained and even increases as load diminishes, so that inefficient peaking/ standby gensets may not be needed.
A fuel cell system, unlike a steam turbine need not be big to be efficient. This characteristic, taken together with two others, low emissions and capability of operation on a variety of fuels, allows fuel cell system to be operated almost anywhere. A small power plant for a community can be operated on the optimum fuel available locally with nearly the same efficiency achieved by a large central power plant.
An electric supply company of a large metropolis can disperse a number of generators throughout its area and match capacity to local demand, substantially reducing the expense and other problems associated with transmission and distribution of electricity. Costs and other problems involved with local distribution of electrical energy are likely to be greater with the adoption of underground lines in urban areas.
The flow diagram of a hydrocarbon fuel cell for power generation is shown in Fig. 11.4.
Efficiency:
Fuel cells have high conversion efficiency especially at low power levels as they are not subject to Carnot limitations on efficiency. It is a one-step process without moving parts and both mechanical and heat losses are absent. This indicates that these systems have the potential for very high reliability and silent and unattended operation.
The hydrocarbon fuel cells operate at much the same efficiency in the 100 kW range as large multi-megawatt units see Fig. 11.5.
Efficiency may be further improved to about 55 percent if pure hydrogen is used in place of processed fossil fuels, and again to 60 percent when oxygen is additionally substituted for air. The efficiency of the fuel cell increases as the power level is decreased to about 40 percent load.
Pollution:
In conventional power plants, a considerable amount of NOx, SO2, H-C and particulates are emitted due to combustion of fuel. Fuel cells emit mostly nontoxic and harmless air, CO2, water vapour and small heat as exhaust. Thermal pollution of waterways is not a problem because fuel cells are air cooled and are not dependent on a source of water supply.
There are few restrictions on site locations. Air pollutants are reduced by a factor of more than 10 over conventional systems. The environmental impact of fuel cells is compared with code requirements of conventional systems in Table 11.3.
Scale:
Fuel cell efficiency is insensitive to size. Scale-up in power output is accompanied by interconnecting fuel stacks (modules) due to the limit to the size of thin electrodes. There is a saving in cost due to mass production of identical components for scale up.
The fuel cell is a very simple device with no moving parts except a few fuel and coolant pumps. Controls are simple and automatic.
Modularity:
Fuel cell units can be added to a power plant system, incrementally over a period of time and built rapidly. There is no need to tie up considerable capital in unused initial capacity which is a very serious drawback of large power plants with long construction periods.
Versatility:
Fuel cells have inherent adaptability towards a broad range of applications. The direct nature of electrochemical process minimizes the effects of both scale and load level on operating characteristics.
The power plant modular construction and ease with which units can be linked permits matching of capacity to demand. Increased capacity can be added quickly and simply. Because the power plant is air cooled, it is not dependent on cooling water availability and thus has few location restrictions. It has the capacity to operate on a variety of fuels as given in Table 11.2.
Small-scale fuel cell power units can be used with advantage for planning the complete power requirements of a city. For individual houses, apartments, commercial buildings and industrial sites, independent on-site power units could meet power demands from a kW to several kWs. They can be integrated into total energy packages that not only generate electricity, but provide control of temperature, humidity and cleanliness.
They can also decentralize the generation of electricity and trim the expenses of transmission over networks by serving as substations. Fuels can be processed on site or supplied to individual power units through underground pipelines.
2. Co-Generation Units:
Co-generation presents an efficient way of utilising our limited energy resources because the same fuel source is used simultaneously to produce two forms of useful energy including electricity and heat. Cogeneration applications are process and site specific for a given industry or a commercial building. The cogeneration potential is illustrated in the Fig. 11.7.
The phosphoric acid fuel cell can be used for a cogeneration system. The block diagram for such a plant is shown in Fig. 11.8.
It is capable of operating on a variety of fuels, including natural gas, light distillates, propane and coal-derived synthetic fuels. Synthetic fuels include hydrogen/carbon monoxide/methane mixture and methanol that can be refined, reformed and/or shifted to produce hydrogen.
The cogeneration system incorporates fuel cell stacks, turbo-compressor for supply of pressurized air to the cell cathodes, and for recovery of pressure and heat energy from the cell exhaust streams, a desulphurizer for removal of sulphur from raw fuel, a steam reformer for conversion of hydrocarbon fuels to hydrogen and carbon monoxide, shift converters for reaction of carbon monoxide and water to produce hydrogen and carbon dioxide, and a heat recovery system for generation of process steam for industry and reformer steam from waste heat transferred from the cell.
The overall process in the fuel cells consists of the continuous-electrochemical reaction of hydrogen in the fuel stream and oxygen from air to produce electric power and by-product hot water and heat. Electric efficiency of the system, after parasitic power, is in the range of 0.38 to 0.41. If process steam generated at 6 bars is not sufficient for the industry, supplementary firing may be used.
3. Mobile Units for Automotive Vehicles:
Fuel cells may provide practical low-pollution vehicles with useful performance and range. Batteries, fuel cells and combination of both have been tried for electrically powered vehicles. The most practical systems will probably be hybrid power plants in which batteries provide peak power and the fuel cells act as charging units during low power periods.
In transport industry, high efficiency and low pollution make the fuel cell attractive. Fuel cells are expected to meet the requirements of high energy density handily since the amount of energy available is determined by the size of fuel tank. A fuel cell powered vehicle can have a good range without re-fuelling, and can be re-fuelled rapidly, just as can present day internal combustion engines.
Fuel cell systems of adequate performance can be built to propel local trains for the city. It would be very smooth and quiet, virtually pollution-free, and could operate on conventional fuels. The fuel cell could replace the conventional diesel engine/generator set directly or small fuel cells with a motor could be fitted to each wheel.
It is possible to meet the high power density criteria for large buses and trucks. To propel a vehicle of weight comparable to an intermediate car with speeds and accelerations usable in present traffic conditions (almost instant starting characteristic and very intermittent use), it is probably necessary to achieve a power density of about 220 W/kg, which is equivalent to 6 kg/hp. It may be possible to meet that goal by hybridising a fuel battery with one of several high power storage devices such as one of new generation flywheels.
The criteria of high power density are more difficult to achieve for small personal cars. Changed driving patterns of decreased speed and acceleration requirements can help to introduce low power vehicles. In case of a new city, roads can be planned with multi-lane and un-level crossing to mitigate the necessity for high power densities.
4. Miscellaneous Applications:
A 50 kW fuel cell stack has been developed by Plug Power LLC, Latham, New York. It produces electricity from many fuels such as gasoline, ethanol, methanol and natural gas. A fuel processor has been developed to convert raw fuel into hydrogen, which is fed into a fuel cell to produce electricity. The carbon dioxide removal system was also 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 capacities has been developed in Australia to meet the power requirements of a house.