Geothermal heat pumps

In recent years, the term geothermal heating has frequently been used to refer to the heating and cooling that can be achieved through the use of a geothermal heat pump. This technique is generally for residential use. It involves a refrigerant liquid being pumped through pipes in the ground, heating the liquid. This liquid then is brought back into the house, and the heat exchanged. The same technique is used to cool the house.

Geothermal heat pumps take advantage of the natural constant temperature of the earth.Goswami, Yogi D., Kreith, Frank, Johnson, Katherine (2008), p. 9-4. During winter when the ground temperature is warmer than the air above it, geothermal heat pumps use the earth's soil (or groundwater) to recover the earth's heat. In contrast, an air-source heat pump will remove heat from the cold outside air and thus requires more energy.

In the summer months, geothermal heat pumps deliver heat to the same relatively cool soil (or groundwater) rather than delivering it to the hot outside air. As a result, the heat is pumped over a smaller temperature difference with a geothermal heat pump and this leads to higher efficiency and lower energy use.Goswami, Yogi D., Kreith, Frank, Johnson, Katherine (2008), p. 9-7.

Hot dry rock

The conditions necessary for water to circulate to the earth's surface are present in less than 10 percent of Earth's land area.Union of Concerned Scientists - How Geothermal Energy Works, http://www.ucsusa.org/clean_energy/renewable_energy_basics/offmen-how-geothermal-energy-works.html. Therefore, geothermal heating is not always a feasible option. Hot dry rock is a method of capturing the heat embedded in these dry areas. First, high-pressure water is pumped down a borehole and through the rocks to break them apart. Water is then pumped from the surface through the ground and the broken hot rocks. This causes the water temperature to rise. The hot water is returned to the surface through a second well and is used to drive turbines for electricity or to provide heat. The water is then returned to the ground and the process repeated.

Fenton Hill facility

There was an experimental facility built in 1974 in Fenton Hill, New Mexico that studied hot dry rock. A well was drilled 11,500 feet into rock at 430 °F to demonstrate the feasibility of hot dry rock technology. The water returned to the surface at 360 °F after being pumped down the well at 80 °F. The plant produced as much as five megawatts of power, proving that energy from hot dry rock can be extracted for practical applications.