Combined Heat and Power, Biomass, Generation

A new version of the device, the Stirling engine, is set to turn ordinary domestic gas boilers into miniature power stations, generating electricity whenever you switch on the central heating or hot water.

It won't make electricity meters run backwards. But for an estimated £500 extra on the price of a new boiler, the machine will generate electricity for the home for nothing, using excess heat that would otherwise escape out the flue.

In Britain, a confidential report prepared for electricity companies by energy consultants EA Technology estimates that by 2025, 13m of the country's 23m households could have their own little power station humming away in the boiler cupboard.

The Stirling engine was patented in 1816 by a Scottish clergyman, Robert Stirling. It consists of two cylinders, filled with gas, each containing a piston. The cylinders are heated and cooled in turn, making the pistons move up and down to turn a drive shaft.

In existing domestic gas boilers, about a third of the heat is wasted. With the latest make of Stirling engine fitted, that spare heat is used to drive a small generator.

The idea of turning homes into power stations is known as "micro chp" (combined heat and power).

EA Technology is championing a Stirling engine made by WhisperTech, a New Zealand company, which can generate a kilowatt of electricity - enough to power three fridges.

British Gas is working on a different design with a similar power output, based on an engine from the US firm SunPower.

"We have now got to the stage where we've overcome the technical barriers, and it's simply a matter of product development, things like designing the right colour for the casing," said EA Technology's Jeremy Harrison.

He said the first units would be on sale in 2002. Depending on the size of the home, he said, they would save householders between £150 and £300 a year on their electricity bills.

The beauty of micro chp is that it cuts in when demand on the grid from households is greatest. On a summer's afternoon, with the fridge on and the TV and VCR on standby, a household might be using only a few hundred watts of electricity.

On a winter's evening, with kettles, electric cookers, irons, washing machines and tumble dryers working, that might rise to 20 kilowatts. But with the central heating and hot water on, the home would be generating its own electricity, lightening the load on the grid, the family budget, and the environment.

EA Technology estimates that at peak times, a network of 13m households would generate 15,000 to 23,000 megawatts of electricity, compared with 15,000 megawatts produced by the country's nuclear power stations.

However, if the government wanted to promote micro chp specifically as a way of cutting down on the carbon dioxide emissions held responsible for global warming, it would need to close high-emitting coal-fired power stations, rather than nuclear ones.

The government has been keen to promote chp as a means of implementing the 1997 Kyoto protocols, which called for reductions in carbon emissions.

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ENERGY SAVING	PASSIVE	ENERGY MAKING	PROACTIVE
Heating		Wind Power
Hot Water		Photovoltaics
Roofs		Solar Hot Water
Walls		Heat Pumps
Windows/Doors		Bio-Mass
Lighting		C.H.P.


	Combined heat and power (CHP), or co-generation, is the simultaneous generation of electricity and heat. Tri-generation is a further extension to include a refrigeration process for air conditioning as well.


	Conventional electricity generation Many electricity generation processes make use of a heat engine. The efficiency of the heat engine depends on the ratio of the input heat of the working fluid to the output heat. This typically requires a form of heat exchanger to transfer heat into a cooling fluid i.e. water or air. Due to the relatively low efficiency of most heat engines, the energy available in this cooling fluid may be two or more times as much as in the electricity generated. This heat energy in most conventional power stations is simply dissipated to air and/or river or sea water, in cooling towers.

	Applications of CHP CHP is most suitable when there is year round demand for heat to balance the demand for electricity, but is useful: When there is a requirement for space heating or process heat close to the generator To provide low temperature (up to 90ºC) hot water heating for local district schemes For applications that require (low grade) process heat, especially those that can supply their own fuel (i.e. sawmills and wood process industry which use heat for timber drying and steaming) At sites such as hospitals, leisure centres, greenhouses, and retirement complexes which have a year round heat demand To provide steam for other industrial applications Where there is a requirement for environmentally reponsible disposal of waste (i.e. sewage sludge, clinical waste or agricultural residue) and where transport costs for disposal are high To power an absorption refrigerator to provide cooling in summer, giving tri-generation.