Geothermal energy originates from a variety of sources of heat within the earth: the planet core, decay of naturally occurring substances within the crust 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 earth’s crust and must be accessed by drilling into the resource and harnessing the energy. The thermal energy in the uppermost 6 miles of the earth’s crust contains 50,000 times the energy of all the world’s gas and oil resources.

Geothermal Power Projects In Progress Capacity

Expected Completion Date
Heber South, California 10MW 2008
Thermo Power Plant, Utah 10MW 2008
SDG&E Adds 40 MW of Geothermal Energy 40MW
North Brawley Geothermal Power Plant 50MW End 2008
Kawerau Geothermal Power Station 90MW End 2008
Blue Mountain Gaulkner 1 Power Plant 49.5MW End 2009
Lightning Dock Power Plant, New Mexico 10MW 2009
Sarulla Geothermal Project 350MW End 2010
Rotokawa Power Station 132MW End 2010
Carson Lake Power Plant, Nevada 30MW TBC

Blue = proposed
Green = construction started
Red = proposal has been abandoned
Grey = project complete

What Is Geothermal Energy? 

Geothermal energy is stored in the form of heat below the earth’s surface. In some parts of the world where the earth’s surface is cracked or thin, molten rock and steam can escape. These are usually locations of high seismic activity such as volcanoes and earthquakes. 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, fumaroles, mud pots and hot, dry rocks 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 doesn’t 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 – hydrothermal, geopressured, hot dry rock and magma.

Hydrothermal

Hydrothermal is the only source used to generate commercially viable energy 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 are formed from the intrusion of molten magma into the earth’s 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

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 90°C to 200°C.

Three forms of energy can be captured from geopressured sources – thermal energy from the hot water, hydraulic energy from the high pressure, and chemical energy from burning the dissolved methane.

Hot Dry Rock

Hot dry rock is a heated geological formation consisting of dry, impermeable rock. 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.

Obtaining energy from hot dry rock has not been implemented commercially since well costs increase dramatically with depth, 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

Magma is the molten or partially molten rock that is found at depths between three and 10 kilometres below the Earth’s crust and reaches temperatures up to 1200°C. While some magma resources are at accessible depths, a practical means of extracting magma energy has yet to be developed. Magma is only accessible where volcanic activity or tectonic plate movement occurs.