A closed loop system for generating mechanical energy at high efficiencies from hydrogen, fossil fuels, bio-fuels, solar or other renewable and recoverable energy sources. The system can have a heating source, a superheater, an expander, a receiver, a condenser, vacuum pump, or absorber, a desorber, and regenerator with pumps and controls. The heating source and superheater are used to heat a working fluid (including ammonia, other refrigerants, a combination of refrigerants, or steam). A positive displacement liquid/vapor expander expands the heated working fluid to the near saturated or saturated state utilizing a reduced pressure, low-pressure, or sub-atmospheric exhaust sink. A condenser, vacuum pump, or absorber is used to generate the reduced pressure, low pressure, or sub-atmospheric sink. The desorber is used to reconstitute inlet vapor (for reuse) and the regenerator recovers heat generated by the process. The system can generate mechanical energy (or power) which can be used to drive a wide range of mechanical systems (including pumps, compressors, vehicles, conveyances, or other similar mechanical devices); or used to drive an electrical generator to meet electrical power needs-for residences, businesses or office buildings, or commercial and industrial applications. The system can supply electrical energy to power grids, and can be an alternative to power generation plants.