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Name: Anthony
Status: student
Grade: 9-12
Location: MD 
Country: N/A
Date: 7/25/2005

Is it possible, better yet, feasible, to develop a Stirling engine that would operate off of the excess heat from a home water heater and central heating that otherwise escapes through the exhaust pipes? Could the engine, in turn, be connected to a flywheel or other device to generate an electrical current to be stored in a battery for later home use? I am trying to find a practical everyday use for the engine.

Is it possible? Yes. Is is practical? No.

A Stirling engine is one variant of a "heat engine," and its efficiency depends on the temperature of the input heat. In the case of exhaust pipes, the temperature is only warm, not hot.

Most of the energy of the natural gas or propane has already been used to do what it has been designed to do: keep the house or the water hot. The amount of heat that is not captured by the furnace or water heater is not only small (furnaces and water heaters are pretty efficient nowadays) but the temperature is not very high. Thus it is doubly impracticable to use a heat engine to recapture the energy.

A better route (although still not economical) is to do it backwards: use the natural gas or propane do drive the Stirling engine. Then use the Stirling exhaust to heat the water or the house. The Stirling engine needs really high temperatures, but the furnace or hot water only needs warm temperatures.

An even better route (although still not economical) is to use the natural gas or propane to run an engine. The engine can generate high quality electric power. The engine cooling system and exhaust can be used to heat the house or the water. This is done in industry on a large scale, but not in private houses.

Bob Erck

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