Abstract: Ocean thermal energy conversion (OTEC) uses a fluid, such as ammonia, hea by high-temperature surface water to provide a turbine-driving working gas. To condense the gas for re-use, a slurry of phase-transformation particles and cold ambient sea water is mixed in a deeply-submerged tank and delivered to a surface tank essentially at the cold sub-surface temperature. Condensing of the working gas is performed at the ocean surface level by exposure to the cold slurry temperature. Particle phase-transformation, which occurs at a temperature between that of the cold sub-surface water and the reject temperature of the heat-exchanger, maintains a surface tank temperature at about that of the sub-surface water.