A process and system of extracting useful work or electricity from a thermal source, wherein heat energy from the thermal source is used in the form of a heated collection fluid; a first side of a heat exchanger is filled with a liquid or supercritical working fluid; fluid flow out of the first side of the heat exchanger is closed such that a fixed volume of the working fluid is maintained in the first side; the heated collection fluid flowed through a second side of the heat exchanger that is adjacent to the first side to affect a transfer of heat from the heated collection fluid to the fixed volume of the working fluid to raise its temperature and pressure; the pressurized working fluid is released from the first side of the heat exchanger upon the working fluid reaching a threshold state; a flow of the pressurized working fluid is directed to an expander capable of converting the kinetic energy of the pressurized working fluid into useful work or electricity; and the foregoing steps are repeated. A plurality of such operably coupled heat exchangers may be used in a manner such that the timing of the pressurized working fluid from each heat exchanger to the expander is offset.