The Stirling engine was invented 1816 by Robert Stirling, Scottish priest. The goal was to have an engine less dangerous than the steam engine.
The operation principle is the work done by the expansion and contraction of a gas (normally helium, hydrogen, nitrogen, or simply air) forced to follow a cooling cycle in a cold source, which is contracted, and heating in a hot source as well, whereby it expands. That is, it requires the presence of a temperature difference between two sources and it is a heat engine.
This engine continues into research because of the versatility of usable energy sources for its operation, and that only need a heat source to the cylinder, you can use a variety of energy sources (solar energy, all kinds of fuels, use of biomass, geothermal, etc.).
here is a list with 26 videos from GreenPowerScience where you where you will be convinced to having one on your backyard.