Device for conversion of a thermal energy into a mechanical energy, comprising: - a first fluid supply line, - a coolant fluid supply line, - a vapour generator having: ∘ a first inlet connected to the first fluid supply line, the first fluid following a first path between the first inlet and a first outlet, ∘ a second inlet receiving the coolant fluid, the coolant fluid following a second path between the second inlet and a second outlet, the second path being different from the first path, the first path being thermically coupled to the second path so as to form vapour from the first fluid, said vapour leaving the generator through the first outlet, - a chamber provided with: ∘ a first inlet connected to the vapour generator's first outlet, the first fluid following a first path within the chamber between the first inlet and a first outlet, the chamber being configured to realize the isothermal decompression of the first fluid in the chamber through a decompression split by a plurality of elementary isothermal decompressions, ∘ a second inlet connected to the coolant fluid supply line, the coolant fluid following a second path different from the first path between the second inlet and a second outlet, the second outlet of the chamber being connected to the second inlet of the vapour generator, the first path being thermically coupled to the second path so as to heat the first fluid between each decompression, - a mixing device connected to the chamber's first outlet and to the vapour generator's second outlet and configured so as to mix the first vapourous fluid with a coolant fluid to obtain a double phase mixture, characterized in that - the chamber is an isothermal spillway configured to maintain the vapour generator's outlet pressure at a constant pressure, - the mixing device is configured with a spray nozzle and the first outlet of the isothermal spillway so as to create a double phase mixture. - a first fluid supply line, - a coolant fluid supply line, - a vapour generator having: ∘ a first inlet connected to the first fluid supply line, the first fluid following a first path between the first inlet and a first outlet, ∘ a second inlet receiving the coolant fluid, the coolant fluid following a second path between the second inlet and a second outlet, the second path being different from the first path, the first path being thermically coupled to the second path so as to form vapour from the first fluid, said vapour leaving the generator through the first outlet, ∘ a first inlet connected to the first fluid supply line, the first fluid following a first path between the first inlet and a first outlet, ∘ a second inlet receiving the coolant fluid, the coolant fluid following a second path between the second inlet and a second outlet, the second path being different from the first path, the first path being thermically coupled to the second path so as to form vapour from the first fluid, said vapour leaving the generator through the first outlet, - a chamber provided with: ∘ a first inlet connected to the vapour generator's first outlet, the first fluid following a first path within the chamber between the first inlet and a first outlet, the chamber being configured to realize the isothermal decompression of the first fluid in the chamber through a decompression split by a plurality of elementary isothermal decompressions, ∘ a second inlet connected to the coolant fluid supply line, the coolant fluid following a second path different from the first path between the second inlet and a second outlet, the second outlet of the chamber being connected to the second inlet of the vapour generator, the first path being thermically coupled to the second path so as to heat the first fluid between each decompression, ∘ a first inlet connected to the vapour generator's first outlet, the first fluid following a first path within the chamber between the first inlet and a first outlet, the chamber being configured to realize the isothermal decompression of the first fluid in the chamber through a decompression split by a plurality of elementary isothermal decompressions, ∘ a second inlet connected to the coolant fluid supply line, the coolant fluid following a second path different from the first path between the second inlet and a second outlet, the second outlet of the chamber being connected to the second inlet of the vapour generator, the first path being thermically coupled to the second path so as to heat the first fluid between each decompression, - a mixing device connected to the chamber's first outlet and to the vapour generator's second outlet and configured so as to mix the first vapourous fluid with a coolant fluid to obtain a double phase mixture, - the chamber is an isothermal spillway configured to maintain the vapour generator's outlet pressure at a constant pressure, - the mixing device is configured with a spray nozzle and the first outlet of the isothermal spillway so as to create a double phase mixture.