The Ocean Thermal Energy Conversion (OTEC) system comprises a condenser 20, eg formed of radial finned-tube heat exchangers, which is located in deep cold waters, to which the vaporised working fluid (eg ammonia) vapour, having driven a surface-mounted turbo-generator 11, is transferred down an insulated conduit 12. The condensed fluid runs into a sump 21 from which it is pumped up by a pump 22 back to the surface, where heat exchangers 15, eg on a platform 10, expose it to surface water heat. The vapour drives the turbo-generator 11 and is returned again to the condenser, to complete the cycle. The condenser may be located at the lower end of a tube containing linking pipework and machinery serving as access and as guide to raise the condenser to the surface for maintenance.; In a modification, fig.4, the condensed working fluid is pumped up via a series of pumps 22 and a series of break tanks linked by an air pipe 40, avoiding the huge pressure of the whole column. The deep location of the condenser obviates the cost and inefficiency of the convention long cold water pipe and causes less environmental disturbance.