Molten carbonate fuel cell MCFC

This is a high temperature fuel cell operating at about 650°C. The electrolyte in this case is an alkaline mixture of lithium and potassium carbonates which becomes liquid at 650°C and is supported by a ceramic matrix. The electrodes are both nickel based. The operation of the MCFC differs from that of other fuel cells in that it involves carbonate ion transfer across the electrolyte. This makes it tolerate both carbon monoxide and carbon dioxide. The cell can consume hydrocarbon fuels that are reformed into hydrogen within the cell.

The MCFC can achieve an efficiency of 55 per cent. The steam and carbon dioxide it produces can be used to drive a turbine generator (cogeneration) which can raise the total efficiency to 80 per cent - up to twice that of a typical oil or gas fired plant. Consequently this technology could be ideal for urban power stations producing combined heat and power. The Energy Research Corporation (ERC) of Danbury, Connecticut, USA, has built a 2 megawatt unit for the municipality of Santa Clara, California, and that company is currently developing a 2.85 megawatt plant.

Development programmes in Japan and the US have produced small prototype units in the 5-20 kW range, which, if successful, will make them attractive for domestic combined heat and power.

The main disadvantage of the MCFC is that it uses as electrolytes highly corrosive molten salts that create both design and maintenance problems. Research is concentrating on solutions to this problem.

In March 2000 it was announced that researchers in the University of Pennsylvania in Philadelphia had developed a cell that could run directly off natural gas or methane. It did not have to be reformed to produce hydrogen. Other fuel cells cannot run directly on hydrocarbons which clog the catalyst within minutes. This innovative cell uses a copper and cerium oxide catalyst instead of nickel. The researchers consider that cars will be the main beneficiaries of the technology. However, Kevin Kendall, a chemist from the University of Keele, thinks differently. According to him 'Millions of homeowners replace their gas-fired central heating systems in Europe every year. Within five years they could be installing a fuel cell that would run on natural gas . . . Every home could have a combined heat and power plant running off mains gas' (New Scientist, 18 March 2000). That prediction should perhaps be raised to 2010.

International Fuel Cells (US) is testing a cell producing 5 kW to 10 kW of electricity and hot water at 120-160°C for heating. This is a residential system which the US company Plug Power, which is linked to General Electric, is marketing as the 'GE HomeGen 7000' domestic fuel cell.

Professor Tony Marmont, the initiator of the fuel cell in his West Beacon farm, considers a scenario whereby the fuel cell in a car would operate in conjunction with a home or office. He estimates that a car spends 96 per cent of its time stationary so it would make sense to couple the car to a building to provide space and domestic hot water heat. The electricity generated would be sold to the grid. The car would be fuelled by a hydrogen grid. Until that is available a catalyser within the car would reform methanol or even natural gas from the mains to provide the hydrogen.

The reason for the intensification of research activity is the belief that the fuel cell is the energy technology of the future in that it meets a cluster of needs, not least the fact that it can be a genuine zero carbon dioxide energy source. It could also relieve us of reliance on a national grid which, in many countries, is unreliable. Perhaps the greatest beneficiaries will initially be rural communities in developing countries who could never hope to get access to a grid supply. Access to energy is the main factor which divides the rich from the poor throughout the world. A cheap fuel cell powered by hydrogen electrolysed from PV, solar-electric or small-scale hydroelectricity could be the ultimate answer to this unacceptable inequality.

There is little doubt that we are approaching the threshold of the hydrogen-based economy. Ultimately hydrogen should be available 'on tap' through a piped network. In the meantime reforming natural gas, petrol, propane and other hydrocarbons to produce hydrogen would still result in massive reductions in carbon dioxide emissions and pollutants like oxides of sulphur and nitrogen. The domestic-scale fuel cells will have built-in processing units to reform hydrocarbon fuels and the whole system will occupy about the same space as a central heating boiler.

The fuel cell will really come into its own when it is fuelled by hydrogen produced from renewable sources like solar cells, wind- and marine-based renewables. If tidal energy is exploited to its full potential there will be peak surpluses of electricity which could serve to create hydrogen via electrolysis.

The first domestic scale fuel call was installed in the experimental Self-Sufficient Solar House created by Fraunhofer Institute for Solar Energy Systems in Freiburg in 1994. Its hydrogen was electrolysed from PVs on its roof and stored in an outside tank (Figure 5.6).

In the US there are growing problems in some areas over the reliability of the power supply and this is increasing the attractiveness of fuel cells. In Portland, Oregon hydrogen extracted from methane from a sewage works generates power sufficient to light 100 homes. The same happens in California where sewage from the Virgenes Municipal Water District in Calabasas reforms methane into hydrogen to supply a fuel cell that provides 90 per cent of the power needed to run the plant. If it were available to the grid it would power 300 homes.

The US Department of Energy plans to power two to four million households with hydrogen and fuel cells by 2010 and ten million by 2030. If the hydrogen is obtained from sewage, livestock waste, underground methane or water split by PV/wind electrolysis then this programme will certainly be one to be emulated by all industrialised countries.

Fuel cells reliant on renewable energy will be heavily dependent on an efficient electricity storage system. At present this is one of the main stumbling blocks to a pollution-free future.

The main barrier to the widespread adoption of fuel cells is the cost. The US Department of Energy estimates that the current cost of a fuel cell is ~$3000 per kilowatt. A UK firm ITM Power of Cambridge is claiming that it should be able to reduce this to ~$100/kW by developing a simplified fuel cell architecture based on a patented unique family of ionically conducting polymers which are cheap to produce. Production costs will be considerably reduced due to its patented one-stop manufacturing process. A complete fuel cell stack would be made in a single process. It plans to have domestic-scale fuel cells on the market in 2005.

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