Geothermal Heating and Cooling
Low enthalpy (low temperature) Geothermal Energy is to be found everywhere in the ground, it is basically stored heat energy from the sun. This heat can be used as a source of heating if it is combined with a heat pump. It can also be used as a sink for heat when cooling is needed, either directly, free cooling, or using a reverse heat pump that lowers the temperature from the ground. The technology is often referred to as Ground Source Heat Pumps, GSHP.
The heat pump needs electricity and you typically get 3 to 5 times as much heating and/or cooling energy as the amount of electric energy you put in. It is most effective when the (same) ground can be used both as a source of heat, in the winter, and as a sink of heat, in the summer.
There are two common types of storages of Geothermal energy. One is called Aquifer Thermal Energy Storage, ATES, in which case the subterranean water is used directly as a carrier of energy. The water is pumped from the ground and then returned, when heating (or cooling) has been extracted. That technology can only be used when you have plenty of water, an aquifer. The other is called Borehole Thermal Energy Storage, BTES, and in that case heating and cooling are collected by using heat exchanger (plastic tubes with a fluid) in a number of bore holes. In that case the subterranean water is not affected at all except for the actual extraction or addition of heat. A BTES can be put anywhere, it does not rely on subterranean water or aquifers.
The technologies of ATES and BTES are mature technologies, especially in Northern Europe where it has been around for 30 years. It is also well established in USA and it is now spreading to large areas in the rest of the World, not least in China.
There is also a third type called CTES, Cave Thermal Energy Storage, where you use underground caves, usually abandoned mine that are filled with water, as heat source and sink.