Abstract: A power unit, energizable by solar heat, having a thermodynamic system including an evaporator, vapor pressure actuated motor, pump and condenser, the motor being arranged to operate upon evaporation of fluid in the evaporator, which is subsequently condensed and pumped back into the evaporator. The vapor pressure actuated motor is coupled to a hydraulic pump which in turn drives a hydraulic motor. The fluid in the thermodynamic circuit is a low temperature boiling fluid, preferably a fluoro carbon refrigerant, so that use can be made of "low grade" heat.