A lternative energy sources currently enjoy good market prospects in Germany: from 1998 to 2000 alone, the number of biogas plants doubled to 1 200. A further boost to this sector is provided by the German government's commitment to reimburse private operators up to E0,1 per kilowatt-hour, as part of its policy on renewable energy. Yet many biogas facilities still do not operate optimally, because cogenerating power stations waste too much energy as heat in the process of electricity generation. "Our experience has been that recovery of this waste heat in most biogas plants is, at best, inadequate," says Stephan Kabasci of the Fraunhofer Institute for Environmental, Safety and Energy Technology, Umsicht, in Germany. Working on a project sponsored by the Land of North Rhine-Westphalia and the European Union, Umsicht researchers tackled this problem by developing an innovative concept, which entails harnessing the waste heat generated by a cogenerating power station for room heating and for the operation of a chiller unit. The system, designed for a farmstead, went into service in June 2001. In addition to using biomass produced at the farm, such as manure, plant clippings, and corn silage, the system also 'digests' vegetable waste from an industrial food-processing plant. The resulting biogas is in turn used as fuel in a cogenerating power station that produces enough electricity to meet the needs of 500 homes. This form of energy is fed into the public power grid by the system's operators. Generating electricity also produces heat. "We use this by-product to heat the stable, the farmhouse with offices and a manufacturing facility," reports Kabasci. The heat is further utilised to operate a thermally-driven chiller unit, which works on the same principle as a gas-operated refrigerator in a recreation van, by using waste heat as the energy supply for the cooling cycle. This allows the stables as well as living rooms and offices to be cooled in summer. "Thanks to this ingenious concept, about three-fourths of the available heat energy is utilised," explains the engineer from Umsicht. Even the effluent from the anaerobic digestion process is a valuable liquid fertiliser for farming. This method of integrated energy supply thus offers economical and ecological advantages: it saves on heating costs, uses waste heat to provide cheap refrigeration, supplies electricity, reduces greenhouse gas emissions, reduces the leaching of nitrates into groundwater, and closes the cycle of organic waste to fertilisers.

Smrcka is an information resources consultant and translator

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