The Stirling engine is a heat engine. Like all heat engines it works on the principle that heat is allowed to flow (from the hot side to the cold side) and in this process some heat is converted to work. The principle is something like the water mill.
How engines work
Theoretically, a heat engine can operate because of the heat flow between two cups of ice cream one of which is just a little bit colder than the other. An internal combustion engine burns fuel inside itself to produce the heat, and hence has to be very selective about the chemical effects of the fuel to be burnt.
Stirling engine: heat, not fuel
The Stirling engine is different in that it will accept heat (not fuel) from any source outside it. Thus it can run on heat provided by sunlight which is focused on to the ‘hot’ side of the engine by a solar dish reflector. Not being an internal combustion engine, the Stirling engine runs much more quietly. The engine contains a gaseous ‘running fluid’ which could be air, hydrogen or helium. This gas remains confined within the engine and hence, the Stirling engine is a ‘closed cycle’ heat engine.
The Stirling engine has another special characteristic which distinguishes it from other heat engines. A Stirling engine has some form of a mechanism for heat re-utilization or ‘regeneration’. This mechanism is called a regenerator. This gives the engine a higher upper limit of efficiency approaching 40 percent.The Stirling engine is of two types by way of construction, called type alpha having two cylinders, and beta having only one cylinder, although a third variant named gamma type is also described in the literature. In each case the rotary mechanical motion is available for utilization as mechanical work or driving an electrical generator. The engine and generator are normally mounted at the focus of the large reflector dish.
Advantages of the Stirling engine
It will run on any heat source (outside itself), rather it runs due to a heat differential.