An energy conversion system for deriving useful power from the thermal gradients in the ocean, or from solar, geothermal, or other sources of low level heat, by using warm water to heat a confined working gas such as air whereby a pressure increase results due to warming the gas, arranging so that the expansion moves a piston or other device to extract power, and then cooling the gas and compressing it back to initial conditions while directly or indirectly contacting it with cooler water to thereby decrease the work needed for recompression. Net useful work results from the difference between the work of expansion at higher temperature and the work of recompression at lower temperature.