Solar energy and the Stirling engine.

The Guardian reports that a combination of mirrors to focus the sun's heat and a Stirling engine to drive a generator could be the game changer for solar energy. The Stirling engine works by moving gas between a heated cylinder and a cooled one, or by moving the gas between a heated and a cooled end of a single cylinder. Any source of heat will work. Wikipedia has an article about it here. Stirling Builder is dedicated to the engine, and has some free instructions on how to build one. The main advantage of  Stirling's invention is that it can exploit much smaller temperature differences than the steam engine.