The invention is an ocean thermal energy conversion method and a system in which a motive fluid having predetermined characteristics is circulated in a closed loop between a cold source in cold deep ocean water and heat sources in warm surface water. The motive fluid is compressed between the cold source and a first primary warm water heat source resulting in the motive fluid being substantially totally vaporized at an outlet of the warm water heat source. The motive fluid is heated downstream from the primary heat source by a secondary heat source. The thermal energy of the heated motive fluid is recovered from a turbine and the motive fluid is condensed in the cold source.