Abstract: A heat pump with a closed cooling medium circuit for transport of heat from one air flow to another, comprises an evaporator (27) provided in one air flow for evaporation of a cooling medium, a compressor for compression of the vaporiform cooling medium, a condenser (28) provided in the other air flow for condensation of the cooling medium, and a return system for condensed cooling medium from the condenser (28) to the evaporator (27). The evaporator (27), the compressor and the condenser (28) are located in a fan casing (32) and arranged to rotate about a common shaft (1), with the compressor in the middle. The compressor works according to the liquid ring principle and comprises a rotating compressor housing (17), an intermediate shaft (2) mounted eccentrically on the outside of the shaft and one or more free-running impellers (3A, 3B), thus causing the compressor housing (17) to transfer rotary energy to the impellers via the liquid ring during operation.