Neither Sir John Houghton's article on climate change ( Physics World February 1998) nor subsequent letters by others (April pp23-24 and May pp21-22) mention the possibility of fuel cells as clean electric power generators. Fuel cells produce DC power, which could be converted easily and efficiently into AC power. Although fuel cells use hydrogen as a fuel, hydrogen could be obtained from fossil fuels by steam reforming or partial oxidation. The initial reaction in each case produces carbon monoxide and hydrogen, which are converted to carbon dioxide and further hydrogen by a steam-shift reaction.

The high efficiency of fuel cells, which can range from 40% to 70%, and the possibility of improving the overall efficiency of such systems using combined heat and power (CHP) designs would significantly reduce emissions. For example, a fuel cell with an efficiency of 60% would emit 35-60% less carbon dioxide at the fossil-fuel stage and 80% less from hydrogen. Other emissions would be significantly reduced (almost to zero in some cases), compared with the emissions from the current mixture of conventional power stations in the UK. Of course, the emissions from fuel cells include those from the fuel-manufacturing stage, and therefore depend on the particular fuel and the method used to provide the hydrogen. However, if pure hydrogen is the fuel, the only by-products are water from the fuel cells and a few emissions produced during the manufacture of hydrogen.

Transport systems currently produce more pollution than power stations, and alternative solutions were mentioned in the letters by Ian Hurley (April) and Cedric Lynch (May). If battery-powered electric vehicles were adopted, the need to recharge them using electricity from conventional power stations would produce about as much carbon dioxide as the vehicles that they replace. Emissions of sulphur dioxide would also rise by up to 85%.

However, if fuel cells were used to recharge the batteries, there would be significant reductions in emissions from the power-generation and transport industries. Fuel-cell power stations could still use fossil fuels, and such stations could be introduced either as part of a central generating system or as small CHP systems dedicated to particular locations and fleets of vehicles, such as urban public transport systems. Electric vehicles could even be introduced alongside conventional transport vehicles.

Many fuel-cell power stations running on natural gas - including one in Japan with a power output of 12 MW - have already been demonstrated, and systems are being considered that can use other fossil fuels, such as diesel. Various vehicles powered by fuel cells have also been shown to work, including buses, cars and a submarine. Although these vehicles mainly carry pure hydrogen as the fuel, mobile systems for fossil fuels are being developed as well.

The fuel cells developed for both power generation and transport could be used alongside the introduction of renewable-energy resources and a run-down in the use of fossil fuels. This combination would be particularly relevant in the UK, where land dedicated to a particular use is at a premium.

Victor Adams

Bath, UK

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