A little about stirling engines

The Stirling engine is a heat engine that uses a piston engine whose heat-exchange process allows for near-ideal efficiency in conversion of heat into mechanical movement by following the Carnot cycle as closely as is practically possible with given materials. Its invention is credited to the Scottish clergyman Rev. Robert Stirling in 1816 who made significant improvements to earlier designs and took out the first patent. He was later assisted in its development by his engineer brother James Stirling.

Mr Stirling sought to create a safer alternative to the steam engines of the time, whose boilers often exploded due to the high pressure of the steam and the inadequate materials. Stirling engines will convert any temperature difference directly into movement. This is a clean power producer with no emmissions.

The Stirling engine works by the repeated heating and cooling of a usually sealed amount of working gas, usually air or other gases such as hydrogen or helium. This is accomplished by moving the gas between hot and cold heat exchangers, the hot heat exchanger being a chamber in thermal contact with an external heat source, e.g. a fuel burner, and the cold heat exchanger being a chamber in thermal contact with an external heat sink, e.g. air fins or water jacket.

Engineers classify Stirling engines into three distinct types:

Alpha-contains two separate power pistons in separate cylinders, one "hot" piston and one "cold" piston. The hot piston cylinder is situated inside the higher temperature heat exchanger and the cold piston cylinder is situated inside the low temperature heat exchanger. This type of engine has a very high power-to-volume ratio but has technical problems due to the usually high temperature of the "hot" piston and its seals.

Beta-has a single power piston arranged within the same cylinder on the same shaft as a displacer piston. The displacer piston is a loose fit and does not extract any power from the expanding gas but only serves to shuttle the working gas from the hot heat exchanger to the cold heat exchanger. When the working gas is pushed to the hot end of the cylinder it expands and pushes the power piston. When it is pushed to the cold end of the cylinder it contracts and the momentum of the machine, usually enhanced by a flywheel, pushes the power piston the other way to compress the gas. Unlike the alpha type, the beta type avoids the technical problems of hot moving seals.

Gamma-is simply a beta Stirling in which the power piston is mounted in a separate cylinder alongside the displacer piston cylinder, but is still connected to the same flywheel. The gas in the two cylinders can flow freely between them and remains a single body. This configuration produces a lower compression ratio but is mechanically simpler and often used in multi-cylinder Stirling engines.

Heat sources

Any temperature difference will power a Stirling engine. A heat source may be the result of combustion but can also be solar, geothermal, or nuclear or even biological. Likewise a "cold source" below the ambient temperature can be used as the temperature difference. (see here). A cold source may be the result of a cryogenic fluid or iced water. Since small differential temperatures require large mass flows, parasitic losses in pumping the heating or cooling fluids rise and tend to reduce the efficiency of the cycle.

There is a great potential for nuclear powered Stirling engines in electric power generation plants. Replacing the steam turbines of nuclear power plants with Stirling engines would greatly simplify the plant, yield greater efficiency, and provide above all, a much greater margin of safety, while reducing radioactive by-products.

Stirling engines could be the wave of things to come.