Why is not possible to use this simple cycle for recover energy from unique temperature ? 1/ (first drawing) For limited friction I think it's possible to put outside pressure of 10 nPa. The speed velocity of the cylinder is constant, I think it's possible to use small speed like 20 m/s but bigger is the speed bigger is the efficiency. The cylinder has a gas inside. If velocity is constant, this need rotational speed is changing in time but like that it's possible to prevent problem for lost kinetics energy when radius change and problem with frictions. The goal when cylinder rotating is to keep constant the kinetics energy of gas, velocity is constant not rotational speed. 2/ (Second drawing) Or for not change kinetics energy of gas, put inside cylinder an object when pressure is low at bottom. When cylinder move in translation, move out red object and recover energy.