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] It 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.[2] The ultimate source of geothermal energy is believed to be from the radioactive decay that occurs deep within the Earth’s crust.[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 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.

Even in regions without large geothermal resources it is possible to benefit from the Earth's heat 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.

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[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 anti-freeze) 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 extracts 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 by-product 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

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

Geothermal heating projects may cause problems if the geology of the area is poorly understood. An example is a 2007 geothermal heating project for the historical city hall of Staufen im Breisgau, Germany. After the 140m borehole caused groundwater to come into contact with a hitherto isolated gypsum layer, the ground underneath the city underwent a chemical reaction, causing the city centre to rise after an initial drop. This has been causing considerable damages to buildings in the city centre.

[edit] Future of geothermal heating

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. ^ Climate.Org - Renewable Energy: Geothermal, http://www.climate.org/topics/green/geo.shtml.
  3. ^ Heat Pumps, Energy Management and Conservation Handbook, 2008. pp. 9–3. 
  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.". Retrieved on 2008-04-27.

[edit] See also

[edit] External links


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