Abstract: This is a method and an apparatus for performing said method, for the conversion of thermal energy into mechanical or electrical energy by means of an exchange of temperature between two water sources having a temperature differential, and utilizing a compressible fluid to be alternately compressed and expanded by the use of the thermal differential, with the flow imparted to the compressible fluid utilized through an improved positive displacement rotary valve and a motor wherein the motor is operated by hydraulic cylinders alternately pressurized and depressurized and connected between a pair of canted discs.