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Micro CHP, major issues: from next year, British domestic consumers will be able to generate their own electricity from microCHP equipment. What is more, they will have a choice of technology, as two new domestic CHP products, both based on the Stirling cycle, will reach the market almost simultaneously.

Article Excerpt
Stirling engines are powered by the expansion of a gas when heated, followed by the compression of the gas when cooled. The Stirling engine contains a fixed amount of gas, which is transferred back and forth between a cold end (at room temperature) and a hot end. Heat is supplied to the hot cylinder by a simple continuous flame, provided in domestic units by a gas burner. Heat is removed from the cold cylinder by cooling water, which can be used for domestic space and water heating.

The two Stirling cycle based products about to hit the UK market are based on very different applications of the cycle.

Powergen has jointly developed its 'Whispergen' product with New Zealand company Whisper Tech (www.whispertech.co.nz), which began work on Stirling engines in 1989 and released its first commercial DC units in 1998. Whisper Tech says that it has produced a small engine with very low vibration, smooth torque and simple construction by combining four piston-cylinder sets in an axial arrangement, with the hot end of one cylinder attached to the cold end of the adjacent cylinder.

The WhisperGen's pistons have a series of guides and seals to retain the working gas within the cylinders. The piston rings and seals operate in a dry environment with no lubrication inside the cylinders. The company says that if the power from the pistons was transferred to a rotary motion by a traditional crank type of mechanism it would put considerable side loading on the pistons and cause rapid guide and seal wear--traditionally a life-limiting factor in Stirling engines. Instead, it has developed a "wobble-yoke" system to convert the linear motion of the engine's four pistons into the rotary motion necessary to drive a generator, while putting very little side load on the piston seals and guides. The wobble-yoke mechanism connects the...