Humans have used geothermal energy for thousands of years, using hot springs initially for cooking and building reservoirs around springs to create shrines and bathing complexes such as those built at Bath by the Romans. 1

Geothermal energy has been harvested commercially since the early part of the 20th century. The use of geothermal energy has increased rapidly since 1970 and now occurs in more than 45 countries. 2

Geothermal energy uses heat energy from beneath the surface of the earth. Some of this heat finds its way to the surface in the form of hot springs or geysers. Other schemes tap the heat energy by pumping water through hot dry rocks several kilometres beneath the earth's surface. Geothermal energy is used for the generation of electricity and for space and water heating in a small number of countries. 3

Commercial exploration and development of geothermal energy to date have focused on natural geothermal reservoirs—volumes of rock at high temperatures (up to 662°F or 350°C) and with both high porosity (pore space, usually filled with water) and high permeability (ability to transmit fluid). The thermal energy is tapped by drilling wells into the reservoirs. The thermal energy in the rock is transferred by conduction to the fluid, which subsequently flows to the well and then to the Earth's surface.

Although geothermal energy is present everywhere beneath the Earth's surface, its use is possible only when certain conditions are met: (1) The energy must be accessible to drilling, usually at depths of less than 2 mi (3 km) but possibly at depths of 4mi (6–7km) in particularly favorable environments (such as in the northern Gulf of Mexico Basin of the United States). (2) Pending demonstration of the technology and economics for fracturing and producing energy from rock of low permeability, the reservoir porosity and permeability must be sufficiently high to allow production of large quantities of thermal water. (3) Since a major cost in geothermal development is drilling and since costs per meter increase with increasing depth, the shallower the concentration of geothermal energy the better. (4) Geothermal fluids can be transported economically by pipeline on the Earth's surface only a few tens of kilometers, and thus any generating or direct-use facility must be located at or near the geothermal anomaly.

Equally important worldwide is the direct use of geothermal energy, often at reservoir temperatures less than 212°F (100°C). Geothermal energy is used directly in a number of ways: to heat buildings (individual houses, apartment complexes, and even whole communities); to cool buildings (using lithium bromide absorption units); to heat greenhouses and soil; and to provide hot or warm water for domestic use, for product processing (for example, the production of paper), for the culture of shellfish and fish, for swimming pools, and for therapeutic (healing) purposes.

The use of geothermal energy for electric power generation has become widespread because of several factors. Countries where geothermal resources are prevalent have desired to develop their own resources in contrast to importing fuel for power generation. In countries where many resource alternatives are available for power generation, including geothermal, geothermal has been a preferred resource because it cannot be transported for sale, and the use of geothermal energy enables fossil fuels to be used for higher and better purposes than power generation. Also, geothermal steam has become an attractive power generation alternative because of environmental benefits and because the unit sizes are small (normally less than 100 MW). Moreover, geothermal plants can be built much more rapidly than plants using fossil fuel and nuclear resources, which, for economic purposes, have to be very large in size. Electrical utility systems are also more reliable if their power sources are not concentrated in a small number of large units. 4

Geothermal technology utilises the natural heat of the earth to provide energy. In the Philippines geothermal energy represents 27 per cent of the country’s total electricity production, and is second only to the United States in global geothermal energy production. Development started as early as 1977 on the island of Leyte and continues today.5

When a heat pump is used to provide domestic heating, the savings on electricity can outweigh the cost of installing and running the system. Where geothermal energy is used in agriculture (such as to heat greenhouses) heating costs can be cut by up to 80%. The cost of electricity from geothermal power plants is slowly becoming competitive with that from traditional power plants.

An environmental impact assessment is usually required to explore and extract geothermal energy. Extracting geothermal energy can release noxious waste gases such as sulphur dioxide and carbon monoxide. These risks can often be addressed with technology that re-injects waste gases back into the geothermal well. Other adverse impacts may include land subsidence and increased seismic activity.

The next step in using heat trapped inside the earth, is to drill deeper and circulate water through the hot dry rock found 3 - 5 miles under the surface, or to venture even deeper and try to utilise the hottest source of all - the molten and semi-molten magma that makes up the Earth's core. 6


Notes

  1. Geothermal Energy, http://www.bbc.co.uk/climate/adaptation/geothermail_energy.shtml
  2. UNEP Renewable Energy Technology Fact sheet, Geothermal, http://www.uneptie.org/energy/act/re/fact_sheet/docs/geothermal.PDF
  3. Power for a Sustainable Future, Geothermal Energy, http://www.sustainableenergy.qld.edu.au/sources/geo.html
  4. http://www.answers.com/geothermal%20power
  5. P. Brown, ‘Global Warning: The last chance for change,’ Dakini Books NP (2006) p.309
  6. Geothermal Energy, http://www.bbc.co.uk/climate/adaptation/geothermail_energy.shtml