Waste Heat Engine
A Waste Heat Engine, as the name implies, is an engine that runs on the waste heat from some other source. This heat source could be a heating system, power system or geothermal heat from the earth itself. A prime example fo a waste heat engine is the Stirling Engine.
The Stirling Engine is an external combustion engine, this means that instead of there being an explosion inside the cylinders to create the power (like in most engines), the power can be outside the engine and can take any form at all.
Here is a video of solar powered stirling engine, running on a temperature differential of only a couple of degrees -