The invention relates to a process producing useful energy from thermal energy. An overall population of mobile particles confined to a unidirectional flow closed circuit of conducting channels (1-2-3-3′-4-1) is subjected to a conservative or effectively conservative force field. The circuit is thermally insulated with the exception of two non juxtaposed areas a first area (2-3) allowing thermal exchange for heating (Qin) from a warmer environment outside the circuit, a second area (4-1) allowing thermal exchange (Qout) for cooling, as necessary, by a colder environment outside the circuit. The closed circuit is provided with a load (3′-4;) designed to convert the energy it receives from the mobile particles flow to a useful output energy. In two portions of the unidirectional circuit located before (3-3′) and after (1-2;) said load, flow velocity vector is parallel or has a component which is parallel to the conservative or effectively conservative force field one portion with a warm flow and the other portion with a cool flow of mobile particles and in that if the density of the chosen mobile particles decreases when the temperature increases, the direction of the conservative force field is the same as that of the cool flow velocity vector or of a cool flow velocity vector component in the said circuit portion and the inverse if the density of the chosen mobile particles increases when the temperature increases.