Abstract: A closed Rankine-cycle power unit 10 for powering a turbine 16. The unit comprises a single closed loop in which a working fluid is circulated. The fluid, in liquid phase, is brought to gaseous phase by passing it through a heat exchanger 14 and the gas is used to drive a turbine 16. The gas is then passed through supersonic condenser 20 of supersonic condenser-radiator 18. Supersonic condenser 20 comprises a supersonic DeLaval nozzle 30 connected to a long barrel 38 which converts the gas stream into liquid droplets. The droplets are formed into a droplet beam 22 by a spray nozzle 40, the droplets radiating away a large amount of their heat energy. The beam 22 is collected as a liquid by a collector 24 and pumped through a compressor 26 which raises its pressure. The liquid from the compressor 26 is then circulated through the heat exchanger 14 again.