Geothermal energy is contained in the heated rocks and fluid that fill the fractures and pores within the earth’s crust. It can be harvested in two ways, direct use of hot water or steam for space heating or industrial use such as aquaculture, thermal baths and hot springs, and to power electricity generation plants. Direct use is confined to low temperatures, usually below 150o C whereas, power generation employs high temperature resources over 150o C. 80 countries have developed direct use of geothermal energy and 20 exploit geothermal energy for power generation. About twice the amount of energy capacity utilized for direct use is used for power generation.
Direct use of geothermal heat has been used for thousands of years. The major direct use applications today are GHP installations for space heating, presently estimated to exceed 500,000 and are the first in terms of global capacity but third in terms of output.
Geothermal power started in 1904 with the Larderello field in Tuscany, which produced the world’s first geothermal electricity. Major production at Larderello began in the 1930s and by 1970; power capacity had reached 350 MW. The Geysers in California started in the 1960s is the largest geothermal plant in the world. Individual geothermal power plants can be as small as 100 kW or as large as 100 MW depending on the energy resource and power demand. The technology is suitable for rural electrification and mini-grid applications, especially in distributed generation systems, in addition to national grid applications.
The three countries with the largest amount of installed direct heat use capacity are USA (5,366 MW), China (2,814 MW) and Iceland (1,469 MW), accounting for 58% of world capacity, which has reached 16,649 MW.
The global installed capacity of geothermal power generation is currently just below 8,200 MW.
Six countries account for 86% of the geothermal generation capacity in the world. The USA is first with 2,228 MW, followed by Philippines (1,909 MW); four countries (Mexico, Italy, Indonesia, Japan) had capacity at the end of 2003 in the range of 550-790 MW each.