Making use of the natural heat from the earth, geothermal energy is a growing force as a source of renewable energy. It has already been used to provide power to 1.2 million homes in the United States. It has far greater potential and to both heat and provide electricity to homes and is yet to be properly utilized. That’s about to change.
Geothermal energy comes from a variety of sources of heat within the earth: decay of naturally occurring substances within the crust, the earth’s core and movement of continental plates as they slide against and underneath each other. Volcanoes, hot springs and steam vents represent the easily accessible points to this energy but most geothermal energy is trapped under the earths crust and must be accessed by drilling into the resource and harnessing the energy. The thermal energy in the uppermost 6 miles of the earths crust contains 50,000 times the energy of all the worlds gas and oil resources.
What Is Geothermal Energy?
Geothermal energy is the heat stored under the earths surface. In some parts of the world where the earths surface is thin or cracked, steam and molten rock can escape. These are usually locations of high seismic activity such as earthquakes and volcanoes. If water finds its way into these cracks, it becomes heated and may come to the surface as geysers, fumaroles, hot springs and mud pots.
Parts of New Zealand, Japan, USA and Europe have high geothermal activity. High grade geothermal energy such as geysers, hot dry rocks, mud pots and fumaroles is used to generate electricity. Geothermal energy can also be used as a heating source, for example in Iceland hot water is brought to the surface through a bore , then sent through insulated pipes into homes and radiator panels which provide heat. Over 80% of homes in Iceland are heated this way.
Although geothermal energy doesnt pollute the air with greenhouse gases, there are other environmental concerns about its use. Scientists are not sure how the long-term use of this resource could affect our underground water supplies. Some geothermal tourist attractions at Rotorua in New Zealand have already suffered a decline in surface activity due to the draw-off of geothermal fluid from the underground reservoir by domestic and commercial uses.
Geothermal energy can be broken down into 4 main types Geothermal energy can be broken down into 4 main types geopressured, magma, hydrothermal and hot dry rock.
This is the only type of geothermal energy that is currently producing commercial quantities of electricity and is derived from hot water and steam formed in porous or fractured rock at relatively moderate depths from 100 metres to 5 kilometres.
The hot water and steam come from the intrusion of molten magma into the earths crust or the deep circulation and heating of groundwater through faults and fractures.
To generate electricity, hot water at temperatures ranging from 180 250 degrees Celsius is brought from the underground reservoir to the surface through production wells and is flashed to steam in special vessels by release of pressure. The steam is separated from the liquid and fed into a turbine engine which turns a generator. Spent geothermal water is returned back to the reservoir to help maintain reservoir pressure.
Geopressured energy is derived from hot, pressurised waters containing dissolved methane, trapped at depths of three to six kilometres in sedimentary formations. The water temperature ranges from 90C to 200C.
Three forms of energy can be captured from geopressured sources chemical energy from burning the dissolved methane, hydraulic energy from the high pressure, and thermal energy from the hot water.
Hot Dry Rock
Under certain conditions granite at a depth of 3 to 5 kilometers under the ground can get to 250C. Unlike hydrothermal resources, the fractures and faults required to conduct water to the surface are not present, therefore water must be pumped into the rock at high pressure to create an artificial underground reservoir of steam or hot water.
A number of development projects continue into attempting to make use of hot dry rock to create electricity but factors such as cost and questions about resistance of the reservoir to flow, water loss and thermal drawdown remain. Costs are coming down though and geothermal technology in general will improve enough to make hot dry rock economically feasible.
Magma is the molten or partially molten rock that is found at depths between three and 10 kilometres beneath the Earths crust and reaches temperatures up to 1200C. While some magma resources are at accessible depths, a practical means of extracting energy directly from magma has yet to be developed, instead magma is used indirectly by exploiting the water it has heated. Magma is only accessible where volcanic activity or tectonic plate movement occurs.
The Future Prospect of Geothermal Energy in the US
An exciting new development in the future of geothermal energy in the United States has just been announced by the Department of the Interior who have promised to make 190 million acres of federal land available for geothermal power development. This is good news because the proposed land includes no environmentally sensitive areas such as national parks or designated wilderness areas.
By the year 2015 it is estimated that electricity from geothermal power production in the United States can reach 5,500 MW thanks to this new initiative. Its a positive move that will promote a form of renewable energy that has been used for years, but perhaps not to its full potential.