Abstract: A geothermal energy conversion system extracts heat from a heat source below the Earth's surface, which has a temperature above that of a fluid circulating through a closed system. A geothermal well and an enclosed well casing define a space that extends from ground level to the heat source. A heat exchanger is in thermal communication with the heat source, and a system of valves and at least one pump is in fluid communication, through the space within the well, with the heat exchanger. The system of pumps and valves regulates at least one of the flow rate or the pressure of the fluid so that the fluid vaporizes within the heat exchanger, the rate of vapor production in the heat exchanger is set at a predetermined value, and build-up of liquid within the heat exchanger is controlled.