Geothermal energy

Krafla Geothermal Station in northeast Iceland

Geothermal power is the use of geothermal heat to generate electricity. Geothermal comes from the Greek words geo, meaning earth, and therme, meaning heat. Giovanni Conti first discovered geothermal-generated electricity at Larderello, Italy, in 1904.

Capacity

If heat recovered by ground source heat pumps is included, the non-electric generating capacity of geothermal energy is estimated at more than 100 GW (gigawatts of thermal power) and is used commercially in over 70 countries.

During 2005, contracts were placed for an additional 0.5 GW of capacity in the United States, while there were also plants under construction in 11 other countries [2].

Resources

Estimates of exploitable worldwide geothermal energy resources vary considerably. According to a 1999 study, it was thought that this might amount to between 65 and 138 GW of electrical generation capacity 'using enhanced technology'.

A 2006 report by MIT that took into account the use of Enhanced Geothermal Systems (EGS) concluded that it would be affordable to generate 100 GWe (gigawatts of electricity) or more by 2050 in the United States alone, for a maximum investment of 1 billion US dollars in research and development over 15 years.

The MIT report calculated the world's total EGS resources to be over 13,000 ZJ, of which over 200 ZJ would be extractable, with the potential to increase this to over 2,000 ZJ with technology improvements - sufficient to provide all the world's energy needs for several millennia.

The key characteristic of an EGS is that it reaches at least 10 km down into hard rock. The MIT report estimated that there was enough energy in hard rocks 10 km below the United States to supply all the world's current needs for 30,000 years. At a typical site two holes would be bored and the deep rock between them fractured. Water would be pumped down one and steam would come up the other. There seems no reason why the steam should not feed an existing coal, oil or nuclear fired generating plant.

Drilling at this depth is now routine for the oil industry (Exxon announced an 11 km hole at the Chayvo field, Sakhalin. Lloyds List 1/5/07 p 6). The technological challenges are to drill wider bores and to break rock over larger volumes. Apart from the energy used to make the bores, the process releases no greenhouse gases. Compared to the difficulties of developing other forms of energy supply (nuclear, wind, wave, solar etc)EGS seems to be well worth encouragement.

Although geothermal sites are capable of providing heat for many decades, eventually specific locations may cool down. It is likely that in these locations, the system was designed too large for the site, since there is only so much energy that can be stored and replenished in a given volume of earth. Some interpret this as meaning a specific geothermal location can undergo depletion, and question whether geothermal energy is truly renewable, but if left alone, these places will recover some of their lost heat, as the mantle has vast heat reserves. The government of Iceland states It should be stressed that the geothermal resource is not strictly renewable in the same sense as the hydro resource. It estimates that Iceland's geothermal energy could provide 1700 MW for over 100 years, compared to the current production of 140 MW.

Power plants

Geothermal power plant in the Philippines

Three different types of power plants - dry steam, flash, and binary - are used to generate power from geothermal energy, depending on temperature, depth, and quality of the water and steam in the area. In all cases the condensed steam and remaining geothermal fluid is injected back into the ground to pick up more heat.

Dry steam

A dry stream power plant uses hot steam, typically above 455°F, to directly power its turbines. This is the oldest type of power plant and is still in use today.

Flash steam

Flash steam power plants use hot water above 360°F from geothermal reservoirs. As the water is pumped from the reservoir to the power plant, the drop in pressure causes the water to vaporize into steam to power the turbine. Any water not flashed into steam is injected back into the reservoir for reuse.

Binary-cycle

The water used in binary-cycle power plants is cooler than that of flash steam plants. The hot fluid from geothermal reservoirs is passed through a heat exchanger which transfers heat to a separate pipe containing fluids with a much lower boiling point. These fluids, usually Iso-butane or Iso-pentane, are vaporized to power the turbine.. The advantage to binary-cycle power plants is their lower cost and increased efficiency. These plants also do not emit any excess gas and are able to utilize lower temperature reservoirs, which are much more common. Most geothermal power plants planned for construction are binary-cycle.

Distribution

The largest dry steam field in the world is The Geysers, 72 miles (116 km) north of San Francisco. The Geysers began in 1960, has 1360 MW of installed capacity and produces over 750 MW net. Calpine Corporation now owns 19 of the 21 plants in The Geysers and is currently the United States' largest producer of renewable geothermal energy. The other two plants are owned jointly by the Northern California Power Agency and the City of Santa Clara's municipal Electric Utility (now called Silicon Valey Power). Since the activities of one geothermal plant affects those nearby, the consolidation plant ownership at The Geysers has been beneficial because the plants operate cooperatively instead of in their own short-term interest. The Geysers is now recharged by injecting treated sewage effluent from the City of Santa Rosa and the Lake County sewage treatment plant. This sewage effluent used to be dumped into rivers and streams and is now piped to the geothermal field where it replenishes the steam produced for power generation.

Another major geothermal area is located in south central California, on the southeast side of the Salton Sea, near the cities of Niland and Calipatria, California. As of 2001, there were 15 geothermal plants producing electricity in the area. CalEnergy owns about half of them and the rest are owned by various companies. Combined the plants have a capacity of about 570 megawatts.

The Basin and Range geologic province in Nevada, southeastern Oregon, southwestern Idaho, Arizona and western Utah is now an area of rapid geothermal development. Several small power plants were built during the late 1980s during times of high power prices. Rising energy costs have spurred new development. Plants in Nevada at Steamboat near Reno, Brady/Desert Peak, Dixie Valley, Soda Lake, Stillwater and Beowawe now produce about 235 MW.

Geothermal power is very cost-effective in the Rift area of Africa. Kenya's KenGen has built two plants, Olkaria I (45 MW) and Olkaria II (65 MW), with a third private plant Olkaria III (48 MW) run by geothermal specialist Ormat. Plans are to increase production capacity by another 576 MW by 2017, covering 25% of Kenya's electricity needs, and correspondingly reducing dependency on imported oil.

Geothermal power is generated in over 20 countries around the world including Iceland (producing over 50% of its electricity from geothermal sources in 2006), the United States, Italy, France, New Zealand, Mexico, Nicaragua, Costa Rica, Russia, the Philippines (production capacity of 1931 MW (2nd to US, 27% of electricity), Indonesia, the People's Republic of China and Japan. Canada's government (which officially notes some 30,000 earth-heat installations for providing space heating to Canadian residential and commercial buildings) reports a test geothermal-electrical site in the Meager Mountain–Pebble Creek area of British Columbia, where a 100 MW facility could be developed.

Water injection

In some locations, the natural supply of water producing steam from the hot underground magma deposits has been exhausted and processed waste water is injected to replenish the supply. Most geothermal fields have more fluid recharge than heat, so re-injection can cool the resource, unless it is carefully managed.

Benefits

Geothermal energy a number of advantages over traditional fossil fuel based sources. From an environmental standpoint, the energy harnessed is clean and safe for the surrounding environment. It is also sustainable because the hot water used in the geothermal process can be re injected into the ground to produce more steam. In addition, geothermal power plants are unaffected by changing weather conditions. From an economic view, geothermal energy reduces reliance on foreign fossil fuel imports. Given enough excess capacity, geothermal energy can also be sold to outside sources such as neighboring countries or private businesses that require energy. It also offers a degree of scalability: a large geothermal plant can power entire cities while smaller power plants can supply more remote sites such as rural villages.

Environmental concerns

There are several environmental concerns behind geothermal energy. Construction of the power plants can adversely affect land stability in the surrounding region. For example, increased seismic activity can occur because of well drilling and land subsidence can become a problem as older wells begin to cool down.. Dry steam and flash steam power plant also emit low levels of carbon dioxide, nitric oxide, and sulfur, although at roughly 5% of the levels emitted by fossil fuel power plants.

Notes

• The United States is the country with the greatest geothermal energy production.
• Iceland gets over 50% of its energy from geothermal sources; it heats many buildings using geothermal heat.
• Chevron Corporation is the world's largest producer of geothermal energy.
• Geothermal energy is a renewable resource along with hydroelectric power, wind power, and solar energy.

See also

• Hot-dry-rock geothermal power
• Geothermal power in Iceland
• Geo-exchange
• Geothermal exchange heat pump
• Geothermal desalination
• The Geysers
• Category:Geothermal power and heating plants
• World energy resources and consumption

External links

• Australian National University - Hot Rock Energy
• Bassfeld Technology Transfer - Introduction to Geothermal Power Generation (3.6 MB PDF file)
• Energy and Geoscience Institute at the University of Utah
• Energy Efficiency and Renewable Energy - Geothermal Technologies Program
• Enhanced Geothermal Resources
• Geothermal Education Office
• Geothermal Energy Association
• Geothermal Resources Council
• Idaho National Laboratory - Geothermal Program
• International Geothermal Association
• Los Alamos National Laboratory - Hot Dry Rock Geothermal Energy Technology
• MIT - The Future of Geothermal Energy (14 MB PDF file)
• National Renewable Energy Laboratory - Geothermal Technologies Program
• Oregon Institute of Technology - Geo-Heat Center
• Southern Methodist University - Geothermal Lab
• RES -The School for Renewable Energy.