In a way most heat pumps recover or reclaim heat energy for space heating and cooling, water heating or process heating. In the building sector, the natural heat sources which heat pumps transfer to useful heat, such as outside air, the ground, ground water and sea/lake/river water are in fact all heat sources that consist of solar heat and cannot be directly used for heating due to their temperature. Hence, one can argue that heat pumps which use these sources are (solar) heat-recovery devices. What is more, these heat pumps use renewable energy sources which are replenished continuously by the sun. However, the most obvious source for recovering heat is the building itself, locally and on a larger scale, e.g. districts, cities etc.