The ambient energy used most is thermal energy from the air. The temperature of the outside air is so low that the heat cannot be used directly. With the aid of seasonal underground storage, or via a heat pump, however, this thermal energy can be used.
A heat pump is a machine which converts heat (e.g. from the surrounding air or groundwater) to heat that can be used to heat a home, building or industrial process. With the aid of auxiliary energy (e.g. electricity) it moves heat from a low temperature heat source to a high temperature heat sink. The process is the same as that in a refrigerator. There, too, heat from a low temperature heat source (inside the refrigerator) is moved to a high temperature heat sink (outside the refrigerator).