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%. The steam and carbon dioxide it produces can be used to drive a turbine generator (cogeneration) which can raise the total efficiency to 80% - 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 two-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 USA 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 these problems.
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. Most other fuel cells cannot run directly on hydrocarbons which clog the catalyst within minutes.
At the RIBA conference of October 2001 which triggered this book, Professor Tony Marmont offered a scenario whereby the fuel cell in a car would operate in conjunction with a home or office. He estimated that a car spends 96% 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, amounting to about 50-80 kW, 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 to provide the hydrogen. However, if the natural gas cell proves its worth, then it would simply draw its energy from the domestic supply.
Considerable research activity is focusing on fuel cells, particularly on how to reduce the costs of catalysts and improve energy density. One promising research programme is investigating how an enzyme substitute can break hydrogen molecules into their constituent atoms. The goal is a cheap catalyst which could transform the cost-effectiveness of fuel cells. The principle is to mimic the hydrogenase enzyme, adding ruthenium, which performs most of the hydrogen-splitting operation. Ruthenium is up to fifteen times cheaper than the conventional platinum.
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