Cold water, for example from the ocean or a lake or a river, is used as heat input medium of a heat-pump system of large capacity, and the water is brought to freezing in order to release its freezing heat. Freezing is brought about by injecting the water into a vacuum chamber, from which the ice produced is sluiced out, while the great flow of steam produced is slightly compressed and fed into a condenser which forms the evaporation part of the heat-pump system. A simple single-stage centrifugal steam compressor is sufficient to raise the steam pressure to such a level that the steam is subsequently condensable at a temperature of 0 DEG C or slightly above, as a result of which ice formation problems in the heat-pump system are avoided. The freezing system forming the heat pump can be a simple and highly effective system as the evaporation temperature can lie as high as approximately 0 DEG C. The invention allows the use of cold lake water at 1 DEG -3 DEG C as heat-supplying medium in a heating system of large capacity with a high degree of efficiency and good economy. IMAGE
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