Applications of the Stirling engine range from mechanical propulsion to heating and cooling to electrical generation systems. A Stirling engine is a heat engine operating by cyclic compression and expansion of air or other gas, the "working fluid", at different temperature levels such that there is a net conversion of heat energy to mechanical work. The Stirling cycle heat engine can also be driven in reverse, using a mechanical energy input to drive heat transfer in a reversed direction (i.e. a heat pump, or refrigerator).
There are several design configurations for Stirling engines that can be built, many of which require rotary or sliding seals, which can introduce difficult tradeoffs between frictional losses and refrigerant leakage. A free-piston variant of the Stirling engine can be built, which can be completely hermetically sealed, reducing friction losses and completely eliminating refrigerant leakage. For example, a Free Piston Stirling Cooler (FPSC) can convert an electrical energy input into a practical heat pump effect, used for high-efficiency portable refrigerators and freezers. Conversely, a free-piston electrical generator could be built, converting a heat flow into mechanical energy, and then into electricity. In both cases, energy is usually converted from/to electrical energy using magnetic fields in a way that avoids compromising the hermetic seal.