Here is another example, of a fairly common Stirling engine representative. It became widespread for the reason that having a large area of heating and cooling it can work even been put on a mug of coffee or a piece of ice. The demo of its functioning at small temperature differences is very impressive.
The principle of operation is based on the movement of hot air from the heating sphere to the cooling sphere and vice versa by displacing (large) piston. This causes an increase or decrease of pressure inside the system. The change of pressure, in turn, moves the working (smaller) piston.