Geothermal heating

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United States geothermal reservoir temperatures

Geothermal heating is best defined as the use of the Earth's thermal energy for space and water heating.[1] This energy can be heat that was stored in the ground's thermal mass during the summer, or it can come from the radioactive decay of minerals deep within the Earth.[2]. The latter form has been used since the time of the Roman Empire as a way of heating buildings and spas by utilizing sources of hot water and steam that exist near the Earth's surface.[3] Some areas, including substantial portions of many western states (U.S.), are underlain by relatively shallow geothermal resources.[4] Similar conditions exist in Iceland, parts of Japan, and other geothermal hot spots around the world. These resources can be classified as low temperature (less than 90°C or 194°F), moderate temperature (90°C - 150°C or 194 - 302°F), and high temperature (greater than 150°C or 302°F). Where such geological geothermal resources are available, it is possible to distribute hot water or steam to multiple buildings. This technique, long practiced throughout the world in locations such as Reykjavik, Iceland,[5] Boise, Idaho,[6] and Klamath Falls, Oregon [7] is known as geothermal district heating. Heat is extracted from the ground with earth tubes or downhole heat exchangers.

Even in regions without large geothermal resources it is possible to benefit from the Earth's thermal mass by using geothermal heat pumps or enhanced geothermal systems (EGS). Heat pumps extract useful energy from even relatively cool sources of heat below the surface (typically at 10-12°C, 50-54°F) while EGS uses high pressure water to extract energy from broken rock. These technologies make geothermal heating an attractive proposition in many geographical locations. One geothermal district heating system at Drake Landing enhances storage of solar energy in the ground to such an extent that no heat pump is needed.

Contents

[edit] Geothermal heat pumps

Main article: Geothermal heat pump

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. For heating, it involves pumping cool water (often containing an antifreeze) through pipes in the ground. As the water circulates underground it absorbs heat from the ground and, on its return, the now warmer water passes through a heat exchanger which uses electricity to extract the heat from the water. The re-chilled water is sent back through the ground thus continuing the cycle. The heat extracted and that generated by the heat exchanger unit as a byproduct is used to heat the house. The addition of the ground heating loop in the energy equation means that more heat is generated than if electricity alone had been used directly for heating. Switching the direction of flow, the same system can be used to circulate the cooled water through the house for cooling in the summer months with the warm water being circulated through the ground loop for cooling.

Geothermal heat pumps take advantage of the natural constant temperature of the earth.[8] 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 an air source pump does. As a result, the heat is pumped over a greater temperature difference with a geothermal heat pump and this leads to higher efficiency and lower energy use.[8]

[edit] Hot dry rock

Main article: Hot dry rock geothermal energy

In some parts of the world, water circulates to the earth's surface naturally and can be used for heating purposes. Unfortunately, such conditions are present in less than 10 percent of the Earth’s land area.[9] In areas without adequate conditions, EGS can be used to extract both energy and district wide heating from hot dry rock. 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.[9] The water is then returned to the ground and the process repeated.


[edit] Benefits

Geothermal energy is a type of renewable energy that encourages conservation of natural resources. According to the U.S. Environmental Protection Agency, geo-exchange systems save homeowners 30-70 percent in heating costs, and 20-50 percent in cooling costs, compared to conventional systems.[10] Geo-exchange systems also save money because they require much less maintenance. In addition to being highly reliable they are built to last for decades and can add considerably the resale value of a structure.

Some utilities, such as Kansas City Power and Light, offer special, lower winter rates for geothermal customers, offering even more savings.[8]

[edit] Problems

In geothermal heating projects the underground is penetrated by trenches or drillholes. Large projects may cause problems if the geology of the area is poorly understood as with all underground work. In connection with a geothermal heating project for the historical city hall of Staufen im Breisgau, Germany, subsidence of the ground up to eight millimeters has occurred while other areas have been uplifted by a few millimeters. A relation to the geothermal wells is suspected. The subsidence has caused considerable damage to buildings in the city center.[11]

[edit] Future of geothermal heating

Geothermal Lake Technology

Geothermal energy is one of the few renewable energy technologies that—like fossil fuels—can supply continuous, base load power.[9] The future for the direct use of geothermal resources is becoming more attractive as prices continue to drop. It is predicted that geothermal heating will continue to become a more popular option for homes and businesses seeking long term cost and energy savings.

[edit] References

  1. ^ Sci-Tech Encyclopedia, http://www.answers.com/topic/geothermal-power
  2. ^ Heat Pumps, Energy Management and Conservation Handbook, 2008. pp. 9–3. 
  3. ^ Climate.Org - Renewable Energy: Geothermal, http://www.climate.org/topics/green/geo.shtml.
  4. ^ What is Geothermal?
  5. ^ University of Rochester - History of the utilization of geothermal sources of energy in Iceland, http://www.energy.rochester.edu/is/reyk/history.htm.
  6. ^ District Heating Systems in Idaho, http://www.idwr.state.id.us/energy/alternative_fuels/geothermal/detailed_district.htm.
  7. ^ Klamath Falls Geothermal District Heating Systems
  8. ^ a b c Goswami, Yogi D., Kreith, Frank, Johnson, Katherine (2008), p. 9-4.
  9. ^ a b c Union of Concerned Scientists - How Geothermal Energy Works, http://www.ucsusa.org/clean_energy/renewable_energy_basics/offmen-how-geothermal-energy-works.html.
  10. ^ "Geothermal Heat Pump Consortium, Inc.". http://geoexchange.us/about/how.htm. Retrieved on 2008-04-27. 
  11. ^ Waffel, Mark (March 19, 2008). "Buildings Crack Up as Black Forest Town Subsides". Spiegel Online International (Der Spiegel). http://www.spiegel.de/international/zeitgeist/0,1518,541296,00.html. Retrieved on 2009-02-24. 

[edit] See also

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[edit] External links


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