About Geothermal Energy

"Geothermal energy is a renewable energy source that can provide large scale base load power for homes and industries, 24 hours a day, with zero green house gas emissions."

Geothermal comes from the Greek words "geo" meaning earth and "therme" meaning heat. It is the natural heat of the earth which presents a potentially sustainable and commercial solution to problems of pollution, green house gas emissions, rising prices and long term supply problems associated with fossil fuels.

The principal of geothermal energy is simple. Water pumped below the earth's surface and back again will return hot, very hot. Heat is energy and that energy can be converted into electricity, a renewable form of energy.

Geothermal energy is employed in over 70 countries from direct use applications for space heating, green houses, agriculture through to electricity production.

Sources of geothermal energy

There is a spectrum of geological formations from which geothermal energy can be obtained. At one end of the spectrum there are conventional hydrothermal systems where hot water or steam flows naturally to the surface from underground acquifers or reservoirs; through to hot dry rocks at the other end of the spectrum, where there is no natural permeable reservoir.

Sources of Geothermal Energy

Most of the World's operating geothermal energy are located in hot volcanic regions where hot underground reservoirs have been formed from water seeping from the surface through natural fractures and faults near molten magma or hot volcanic rocks. These systems are often found near active tectonic plate boundaries where volcanic activity has occurred, such as in New Zealand, Iceland and The Philippines.

Hydrothermal systems can also occur outside volcanic regions where the rocks are hotter than normal and water has collected between layers of sedimentary rock. These systems can be found in the Perth Basin in Western Australia and in Hungary where the earth's crust is relatively thin, or below the Cooper Basin in South Australia where rocks generate anomalous quantities of heat.

Geothermal power plants, fed by hydrothermal systems, are therefore mostly located where there is volcanic activity of hot basement rocks. The geothermal energy is recovered by harvesting the hot water, or steam, already in the Earth.

In many hydrothermal systems the permeability of the hot rocks is too low to enable the water to flow at a rate sufficient for electricity generation. The flow rate, and thus productivity, of these hydrothermal systems can often be improved by pumping water, under pressure, down the wells to open additional fractures and pathways in the hot underground reservoirs. This technique, first developed by the petroleum industry, and now being refined by the geothermal industry, is known as fracture stimulation.

A geothermal reservoir, or system, that has been fracture stimulated to improve the water flow rate, is often called an Engineered or Enhanced Geothermal System. Fracture stimulation is used to generate a permeable underground reservoir for hot dry rock systems and is also used to improve the permeability of hydrothermal systems; these fracture stimulated systems are referred to as enhanced or engineered geothermal systems.

This brings us to the other end of the spectrum, Hot Rock systems that do not have a natural system of permeable water filled fractures or faults. They are normally associated with granites that contain anomalously high concentrations of the naturally radioactive elements uranium (U), thorium (Th) and potassium (K). Although enriched in these elements compared to other rocks, element concentrations are still relatively low. The radioactive decay of these elements over millions of years generates heat which is trapped when the granites become buried by insulating sediments.

Hot Rock systems can be either hot wet rocks where there are water filled fractures, through to hot dry rocks where the extraction of heat is achieved by pumping cool water into the rocks at depth and subsequently withdrawing it at a much higher temperature after it has flowed under pressure through fractures in the hot rocks.

Base Load Energy

Unlike nearly all other forms of renewable energy, geothermal energy delivers constant base load power.

Base load energy is the delivery of energy over a full 24 hour period; i.e. the energy is supplied for the full 24 hours a day, 365 days of the year.