Uses of geothermal resources fall into three general categories, heat pumps, direct use, and electricity generation. The use depends primarily on the temperature of the resource, but also to some extent on the need of the user.

Direct use is any application of heat from the Earth at, or below, the temperature that it comes from the ground (excluding heat pumps). Historically, direct-use applications have used hot water from natural hot springs, artesian (naturally flowing) or pumped wells to transfer the heat to the surface. Since the 1970’s, however, especially in Germany, natural hot water in the subsurface has been more aggressively exploited for direct use by drilling pairs of wells into aquifers with hot water. Water is pumped from the well and returned to the aquifer in the second well [1]. The distance between the wells is sufficient that the reinjected water is reheated before reaching the extraction well. At the surface the water passes through a heat exchanger. Surface water heated in the heat exchanger is used for district heating systems and other direct uses. In Colorado, at present, direct-use applications are restricted to sites where hot water flows naturally to the surface, and a few hot well sites. As demonstrated in Germany, there is much more hot water available for direct use that that at the surface. In Germany this subsurface hot water has been exploited as a pollution-free, sustainable resource.

For electricity generation, water must be hot enough to produce a vapor phase to turn a turbine to drive a generator. Prior to about 1980, the vapor was either natural steam or steam produced by flashing hot water to steam at the surface. A hot water temperature of about 175°C (350°F) or higher was required for efficient electricity generation. In 1981, the first commercially successful binary power plant was introduced into the US, which allowed power production with lower temperature resources. Binary technology uses a secondary working fluid with a boiling point lower than that of water. Currently the lowest temperature at which a binary power plant operates is about 75°C (165°F) and is in Chena, Alaska [2]. Older power plants running on natural or flashed steam may have a small component of pollution, especially if the resource is associated with young volcanic activity. Volcanic gasses, such as carbon dioxide, sulfur dioxide, and hydrogen sulfide may be brought to the surface with the geothermal fluid and released to the atmosphere. With newer binary power plants, however, the geothermal fluid is passed through a heat exchanger and reinjected with no exposure to the atmosphere, and no pollution. Binary power plants produce clean, renewable, baseload, sustainable electrical energy.

Heat pumps are typically the lowest temperature use and do not formally require a geothermal resource. The ground is used as a storage reservoir for heat. In summer, heat is pumped out of a building to cool the structure and the ground is used to absorb the heat. In winter, heat is pumped out of the ground and into the building to heat the structure. Electricity is used to operate the heat pumps, but in terms of heating or cooling a building, a heat pump transferring heat into and out of the ground is a much more efficient use of the electricity than using the electricity directly to produce heat. Heat pumps can contribute significantly to a clean energy economy by saving a very substantial fraction of total energy use, although they are not usually included in discussions of alternative energy, unfortunately.

[1] Schellschmidt, R.,Sanner, B.,Pester, S. and Schulz, R., 2010, Geothermal Energy Use in Germany, Proceedings World Geothermal Congress 2010, Bali, Indonesia, 25-29 April, 2010, pp. 1-19.

[2] Chena Hot Springs Resort, Geothermal Power: