http://www.youtube.com/watch?v=CMSuVbpLkfU
Not OU, just for fun.
Stirling engine (not mine) which draws energy from the evaporation of only a few grams of pure water. The top plate is covered with a cloth kept moist by a wick dipped in a cup of water. The temperature difference between the two plates is sufficient for the engine to run for several hours at about 130 tr/min (ambient temperature is 34C, humidity 27%).
I think all heat engine are based on the same principle. Stirling use gas instead of NiTi. Both undergoes expansion/retraction when heat apply. The amount of work for these types are Work = Qin - Qout roughly. This is an excellent machine convert heat into work. The low temperature difference hold promises to OU. Because heat pump COP increase with temperature difference decrease, it is desire to have heat engine work in low delta T. Lenr would appreciates this engine.