What is geothermal energy?

Geothermal energy is heat energy available from the Earth. Some of this heat energy finds its way to the surface in the form of hot springs or geysers. Geothermal energy has been used for thousands of years in the form of hot springs for bathing, heating, cooking and medicinal reasons.

Today geothermal energy is used around the world for heating and electrical generation. To produce electricity, the geothermal resource must be concentrated and easy to access.
Geothermal energy may be used in a number of applications such as:

• Hydrothermal Power Stations - Steam or water is released from geysers at a high pressure and used in a turbine to generate power.
• Hot Dry Rock - Another technology being developed to harness geothermal energy uses the heat in found in zones of hot dry rock (HDR). Water is pumped into the hot dry rock, heated, and then returned to the surfaceas steam for direct use or in power production.
• Heat Pump Systems - These use the relatively low but stable temperatures just below the Earth's surface to improve the efficiency of heating and cooling systems.

Unlike natural springs and geysers which bring geothermal energy to the surface, many sources are buried deep in the Earth's surface. This means, we often drill wells up to five kilometres deep to reach useful high temperature sources of geothermal energy.

Where does geothermal energy originate from?

The heat in the Earth's crust comes from three main sources:

1. Solar radiation - Ground temperatures down to about 10 metres show seasonal variations and so most of that heat is solar energy from the sun.
2. Heat conducted from the underlying layers - Heat from the layers below the crust (the mantle and core) conduct heat outwards towards the surface. On average, the temperature increases 25^oC every kilometre below the surface. As well as this, heat may be brought up to the surface through cracks in the Earth in the form of molten rock or lava during volcanic eruptions.
3. Radioactive decay - The major source of geothermal heat in the crust, from 50 to 75%, depending on the location, is the radioactive decay of elements such as uranium, potassium, and thorium. The decay of the elements may occur slowly over very long periods of time e.g. Potasium-40 has a half life of 1.3 billion years or in Stages e.g. Uranium-238 decays through 14 stages before becoming the stable element lead.

Using geothermal energy

Geothermal energy is currently used for the generation of electricity and for space and water heating. There are four main types of resource: hydrothermal reservoirs, hot dry rock, geopressured brines, and magma. In Australia, the potential sources of geothermal energy are hydrothermal reservoirs and hot dry rock.

Hydrothermal Energy Use

Hydrothermal reservoirs are bodies of hot water or steam which can be tapped for electricity generation or district heating applications.

Most hydrothermal reservoirs consist mainly of hot water and are referred to as water-dominated. If the water is trapped by the rock, there can be a build-up of pressure. This means the boiling point of the water can reach temperatures of 150^oC or more (at high pressures, the boiling point of a liquid will rise). Once this water is released from the rock, to the atmosphere, it will boil or 'flash off' into steam. Steam-dominated reservoirs are less common but certainly more spectacular. Hydrothermal resources can be used directly or to produce electricity:

• Direct use: It can be used to provide space heating, hot water, heat for industrial drying applications and to greenhouses. The technology is well established and works in a similar way to heat pumps. Naturally occurring hot water has been used for centuries throughout the world. However, because of heat losses and the cost of piping, the direct use of hot water is usually only economical if the application is close to the reservoir.
• Electricity: The production of electricity is a convenient way of converting geothermal energy into an easily transported form. Temperatures greater than 180^oC are usually required to produce steam to drive conventional turbines, however, lower temperature resources can be harnessed to drive engines with working fluids other than steam e.g. Freon-12 (a refrigerant) has a boiling point of -30^oC and can be vaporised easily at relatively low temperatures. The vapour may be used to drive a generator. Freon, however, is a chlorofluorocarbon compound which is now recognised as being an ozone depleting agent. Research is therefore being carried out to develop heat engines which run on hydrocarbons such as butane or isopentane.

Hot Dry Rock (HDR)

In Hot Dry Rock (HDR) installations, water is pumped to deposits of high temperature rock and the heated water is then used for generation.

HDR geothermal energy is a potentially huge resource. They consist of areas of high temperature granites buried below the Earth's surface. Hot dry rock resources are harnessed via injection and production wells. These are holes drilled to depths of 4 to 5 kilometres. Water is pumped into the injection well, at high pressure, forcing open existing weaknesses in the rock. These weaknesses consist of minute fractures, which crack and shear when the water is introduced. They are in turn affected by the local natural stresses making it important to know about the local geology before attempting to tap the hot dry rock resource.

Water is injected over a period of time through the injection well. It is then heated as it passes through a zone of hot cracked rock. It returns through production well as steam under pressure which is used to drive a turbine and generate electricity. Drilling is the most costly part of the system, the most promising sites are therefore those where hot rocks are found close to the surface.

Geopressured Brines

Consist of hot brine saturated with methane and found in large deep aquifers under high pressure. Heat energy can be obtained from these sources at temperatures between 90-200^oC.

Magma

Molten rock is the largest global geothermal resource and is found at depths below 3-10km. Its great depth and high temperature (between 700^oC and 1200^oC) make the resource difficult to access and harness.

Geothermal heat pumps

Geothermal or ground source heat pumps are another means of using the Earth as a source of heat. In this case, its heat constantly absorbed from the sun. Heat, of course, is also lost and the first few metres below the ground show a seasonal variation in temperature which lags the average air temperature by about a month.

Air temperatures can vary by 40^oC or more over a year, but the temperature below the ground might only change by half this amount. The variation reduces with depth and at about 10 metres the Earth's temperature is constant and close to the annual average air temperature.
The ground source heat pump draws heat from the Earth and transfers it to the space or load it is heating. Heat pumps tend to be thought of as air conditioners for buildings. However, they are equally as effective in drying materials or producing hot water. In winter, when the air temperature can get close to 0^oC, the temperature a few metres underground could be 15^oC, making it a better source of heat than the air.

The key feature of the geothermal system is the heat exchanger which consists of polythene pipes called ground loops buried in trenches or bore holes. A heat exchange fluid (usually just water) circulates through the ground loops to pick up or carry away heat from the ground.

The relatively constant temperature of the Earth enables a geothermal heat pump system to operate with a much greater efficiency than an equivalent air-to-air system. The Coefficient of Performance (CoP) is a measure of the energy efficiency of a heat pump. It is equal to the heat energy delivered (or removed) by the heat pump, divided by the energy consumed. In summer, the ground temperature may only have risen a few degrees so it is a better heat sink than the air. Geothermal heat pumps therefore have even bigger CoP than air-to-air systems: typical CoPs range from 4.5 to 6.0, compared to 2.5 - 3.0 for conventional air-to-air systems.

Links

PIRSA Geothermal Energy Projects - The department of Primary Industries and Resources of South Australia is involved in Geothermal projects in South Australia. Further information regarding geothermal exploration in our State can be found at this site.