Stirling engines are powered by the expansion (heating) and contraction (cooling) of gas. The fixed amount of gas inside a Stirling engine is transferred back and forth between a hot end and a cold end, which cyclically expands and contracts the gas.
I recently built a Stirling Engine from a kit. This clip shows the engine running off of the waste heat emanating from a cup of coffee. In the video you can see how the heat from the cup is used to expand air in the base of the engine, causing a blue floater to push air out of the engine and into a small cylinder tube. This in turn causes the cylinder to be displaced which results in the flywheel being turned (the spinning wheel on top of the engine) at a speed of up to 3000 RPM.
All Stirling Engines are basically powered by a difference in temperature (either hot or cold). Stirling engines are unique among heat engines because they have a very high theoretical Carnot efficiency, in fact it is almost equal to their theoretical maximum Carnot efficiency. The term “Stirling Engine” applies to all types of closed cycle regenerative gas engines.
My little Stirling engine can also run on cold as well as waste heat. The following clip shows the same engine running on top of a tray containing 4 ice cubes. Although it is difficult to see in this video, the flywheel is turning in the opposite direction from the video above. The heat difference is not as great compared to the waste heat from my morning coffee, so the flywheel is not turning quite as fast.
The first Stirling engine was designed in 1816 by a Church of Scotland minister Robert Stirling in Edinburgh, Scotland. The device was in the form of an inverted beam engine, and incorporated the characteristic phase shift between the displacer and piston that we see in all Stirling Engines today. The engine also featured the cyclic heating and cooling of the internal gas by means of an external heat source.
Stirling engines were just becoming popular in the late 1800s when the electic and internal combustion gas engines were invented. The gas engine had higher power from the exploding fuel and made Stirling engines obsolete until recently. Now with growing concern over carbon emissions coupled with diminishing oil supplies, Stirling engines are poised to make a comeback. Stirling engines can work off of waste heat, combustion, solar, geothermal, nuclear, or even biological heat sources as well as off of cryogenic fluids such as ice water or freon. For example using solar energy, sunlight can be focused through a lens to heat air in the Stirling engine. This is considerably more efficient than using solar panels to create electricity. At least 3 different Stirling car engines have been designed, and there are at least 8 different submarines running on Stirling engines.
These two pictures show the engine normally and in a cut-away view. You can see the heat chamber on the bottom that drives the piston directly. The smaller piston at the top captures the return stroke and pushes cool air back into the heat chamber. This cool air is heated again and the cycle repeats.