A Stirling engine is a heat engine that is operated by the cyclic compression and expansion of air or other gas (the working fluid) at different temperatures, resulting in a net conversion of heat energy to mechanical work. More specifically, the Stirling engine is a closed-cycle regenerative heat engine with a permanent gaseous working fluid. Closed-cycle, in this context, means a thermodynamic system in which the working fluid is permanently contained within the system, and regenerative describes the use of a specific type of internal heat exchanger and thermal store, known as the regenerator. Strictly speaking, the inclusion of the regenerator is what differentiates a Stirling engine from other closed-cycle hot air engines.Originally conceived in 1816 by Robert Stirling as an industrial prime mover to rival the steam engine, its practical use was largely confined to low-power domestic applications for over a century. However, contemporary investment in renewable energy, especially solar energy, has increased the efficiency of concentrated solar power.