Stirling engines can generate power using only the temperature difference between two surfaces

Patented by Robert Stirling in 1816, the Stirling engine is a closed-cycle heat engine that operates without internal combustion or steam. It works by moving a fixed amount of air between a hot end and a cold end; as the air heats up, it expands to push a piston, and as it cools, it contracts to pull it back. The key to its high efficiency—often reaching 30%—is the 'regenerator,' a wire mesh that captures heat from the air as it moves to the cold side and gives it back as it returns.