Heat pump apparatus comprising: - an evaporator, such as an evaporation tank, configured to evaporate by direct evaporation a liquid received from an external liquid source through a liquid inlet line and an evaporator inlet to an evaporation chamber, the pressure in the evaporation chamber being lower than the pressure in the liquid inlet line and sufficient low for evaporating the liquid entering the evaporation chamber, i.e. the pressure in the evaporation chamber is so that the evaporation temperature of the liquid associated with the pressure is lower than the temperature of the liquid entering the evaporation chamber through the evaporator inlet and allowing for generating a pressurized vapor leaving the evaporator through an evaporator vapor outlet, the evaporator additionally comprising an evaporator liquid outlet, - an expander having an expander inlet and an expander outlet, the expander inlet having a fluid connection to the evaporator vapor outlet for receiving pressurized vapor from the evaporation to drive the expander, - a compressor having a compressor inlet and a compressor outlet, the compressor being operatively driven by the expander for compressing a gas from a low pressure, low temperature inlet gas at the compressor inlet to a high pressure, high temperature outlet gas at the compressor outlet, - a first condenser (13a, 51) having a first condenser inlet and a first condenser outlet, the first condenser inlet having a fluid connection to the expander outlet and being configured for condensing the fluid received from the expander and the first condenser outlet being connected to a first liquid outlet line (16a, 33). - an evaporator, such as an evaporation tank, configured to evaporate by direct evaporation a liquid received from an external liquid source through a liquid inlet line and an evaporator inlet to an evaporation chamber, the pressure in the evaporation chamber being lower than the pressure in the liquid inlet line and sufficient low for evaporating the liquid entering the evaporation chamber, i.e. the pressure in the evaporation chamber is so that the evaporation temperature of the liquid associated with the pressure is lower than the temperature of the liquid entering the evaporation chamber through the evaporator inlet and allowing for generating a pressurized vapor leaving the evaporator through an evaporator vapor outlet, the evaporator additionally comprising an evaporator liquid outlet, - an expander having an expander inlet and an expander outlet, the expander inlet having a fluid connection to the evaporator vapor outlet for receiving pressurized vapor from the evaporation to drive the expander, - a compressor having a compressor inlet and a compressor outlet, the compressor being operatively driven by the expander for compressing a gas from a low pressure, low temperature inlet gas at the compressor inlet to a high pressure, high temperature outlet gas at the compressor outlet, - a first condenser (13a, 51) having a first condenser inlet and a first condenser outlet, the first condenser inlet having a fluid connection to the expander outlet and being configured for condensing the fluid received from the expander and the first condenser outlet being connected to a first liquid outlet line (16a, 33).