Geothermal energy is one of the most important energy source in zero energy home. Heat can be extracted from any source, even from the cold one, but naturally a warmer source provides higher efficiency.
Ona a small scale, for example for a family house, this can be performed using a geothermal heat pump. While an air source heat pump requires more energy, because it draws heat from the cold air from the outside, a geothermal heat pump uses ground as a source heat, so it takes advantage of surface ground temperatures which starts at 10 °C, 50 °F. Alternatively a ground water heat source can by drawned.
Ground based heat pumps do not cread any pollution, do not require any fuel and they work regardless of weather, bacause of the constant temperature of surface ground. However, once we´ve mentioned the advantages we have to mention the disadvantages too.
Operation of an air source heat pump is recommended only in mild temperate. As we mentioned, geothermal source heat pumps are more efficient, but compared to air sourced ones are far more expensive. Also, mainly in cooler climates a secondary heat source is needed.