Thermal energy
Thermal energy storage can also be achieved by circulating a fluid through a buried heat exchanger, that usually consists of a horizontal or vertical pipeline. As these systems do not extract or inject groundwater, they are called closed systems and are known as borehole thermal energy storage or ground source heat pumps. Another thermal application that uses the subsurface to provide thermal energy is geothermal energy production, which commonly uses the deeper subsurface where temperature is higher.