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Last edited 11 Sep 2018
 What is geothermal energy?
Geothermal energy is the second most abundant source of heat on earth, after solar energy. It is the natural heat energy stored in the earth. This energy is contained in about 260 billion cubic metres of rocks and metallic alloys, just below the outer surface of the earth, that are at or near their melting points (Lanterman & Lee, 2007).
Calculations show that the Earth, originating from a completely molten state, would have cooled and become completely solid if the only energy input was that of the Sun, so it is believed that the ultimate source of geothermal energy is the decay of naturally radioactive isotopes (Dincer et al., 2007).
The thermal energy from the earth continuously flows outwards. This heat transfer from the core to the surrounding mantle is principally via conduction. When the temperature and pressure of the system becomes high enough, some of the rocks that make up the mantle melt and form magma. As the liquid magma is less dense than surrounding rocks, it slowly rises, convecting thermal heat towards the earth’s crust (Lanterman & Lee, 2007).
Geothermal temperature increases with depth in the earth’s crust. Using the technology available at present, it has been found that the average geothermal gradient is about 3°C per 100m (Dincer et al., 2007). According to Lund (2009) the approximate total thermal energy above surface temperature to a depth of 10 kilometres is 1.3x10² Joules, equivalent to using 3x10¹ barrels of oil.
As global energy consumption is equivalent to about 100 million barrels of oil per day, the thermal energy to a depth of 10km would supply all of mankind’s energy needs for six million years. However, based on current technology, only a fraction of this energy is available as a recoverable source. The rest of the energy is too widely spread over the surface of the earth or is too deep for it to be practical to reach.
Geothermal water from natural pools and hot springs has been used by humans for tens of thousands of years for cooking, bathing and heating. The Romans used geothermal energy for space heating, and direct heating has been used universally for agricultural purposes for many years, for example for greenhouse heating (Lanterman & Lee, 2007).
The world’s first geothermal district heating system was developed in the 1300s at Chaudes-Aigues in France and it is still operational. The oldest and still functional geothermal district heating system in the United States of America is in Boise, Idaho. It became operational in 1892, is powered directly by a deep geothermal well and provides space heating for up to 450 homes. In Iceland, municipal heating was provided using hot geothermal sources in the 1930s and they are still a major source of heating today. Early industrial applications of geothermal energy included chemical extraction in the Larderello region of Italy, with geyser steam being used to extract boric acid for commercial use in the 1800s (Lund, 2009).
Geothermal energy was first used for electrical power generation with experimental work by Prince Gionori Conti in the Larderello field in Tuscany, Italy in 1904. This was followed by the first commercial power plant being commissioned in Larderello in 1913. In Japan, an experimental site for geothermal work in Beppu in 1919 led to a pilot plant in 1924 (Lanterman & Lee, 2007). These developments were followed by a plant in Wairakei, New Zealand in 1958 and an experimental plant at Pathe, Mexico in 1959.
The first plant in the USA was then set up at The Geysers in 1960 and is currently the largest geothermal power producer in the USA (Collie, 1978). Due to the corrosive properties of most groundwater and steam, there were complications with the utilisation of geothermal energy for power generation until 1950, as metallurgy was not advanced enough to enable the manufacture of corrosion-resistant steam turbine blades (Dincer et al., 2007).
 Current usage
Geothermal energy has been used on a commercial scale for over 100 years and more than 70 countries now exploit geothermal resources (Batchelor, 2005). At present, the USA remains the biggest producer of electricity from geothermal energy (Lanterman & Lee, 2007). Due to Iceland’s geological location, it has abundant geothermal resources and leads in the use of geothermal energy for space heating, hot water supply and agricultural uses, with over half its population living in houses heated by geothermal energy (Collie, 1978).
The main benefits of geothermal energy are:
- The resources are continuous, reliable, sustainable and clean.
- The cost of geothermal energy is not prone to fluctuation.
- It provides a large resource, readily available in one form or another in every country, leading to a reduction of energy imports, therefore lowering dependency on external economical or political situations.
- It helps reduce dependence on fossil or nuclear fuels.
- It can be cost-competitive in providing base-load electricity, heating, cooling and hot water.
- There is diversity of use: electricity generation and direct use of heat.
- It can be used simultaneously for both power generation and direct-use applications.
- It has low operating and maintenance costs.
- There is a low land area requirement for geothermal power plants.
- Geothermal systems can be installed at remote locations without requiring other infrastructure; the region can prosper without pollution.
- Geothermal energy can easily be combined with other energy systems.
(Lanterman & Lee, 2007)
 High-temperature applications
High temperatures are primarily used for power stations and require temperatures of greater than 150°C. Typically the geothermal fluids used are at 200-280°C and are from wells 1500-2500 metres deep (Batchelor, 2005). High-temperature reservoirs are only found in regions with active volcanism and tectonic events on major plate or fault boundaries (Batchelor, 2005).
The characteristics of the hydrothermal resources (resources containing water and/or steam) determine the power cycle of the geothermal power plant. In rare and geographically limited locations, dry steam is produced and this can be used directly to turn the turbines. However, in most cases the hot water resources need to be flashed, by reducing their pressure, in order to produce the steam required (Lund, 2009). Although geothermal power generation only accounts for a fraction of the world total, it is very important locally in many countries.
 Medium-temperature applications
Temperatures between 40°C and 150°C are used for large-scale heating and process applications, and some limited power generation. Some medium-temperature thermal energy reservoirs are found in the same regions as the high-temperature reservoirs (plate and fault boundaries), where the heat-source is more diffused in reaching the surface or less completely trapped (Collie, 1978). Another type of medium-temperature reservoir exists where poorly conducting rock strata in the crust accumulates regional heat flows (Collie, 1978).
These resources are extensively used in countries such as Hungary and Iceland, for space and district heating and agriculture. The difference between space and district heating is that space heating systems only supply heat to one structure, whereas district heating systems serve many structures from a common set of wells (Lanterman & Lee, 2007). The countries that have the highest usage of medium-temperature geothermal resources for direct use are China, the USA, Iceland and Turkey, accounting for 68% of the geothermal energy used directly as heat (Batchelor, 2007).
 Low-temperature applications
Low temperatures are used with heat pumps on ground source systems to provide heating, cooling and hot water, using temperatures of less than 40°C. As the ground temperatures required for this application reduce, the area where geothermal utilisation is possible expands rapidly, making them suitable for small-scale and even domestic use in almost any location (Batchelor, 2007).
The systems can be either open-loop, using ground water directly through an evaporator heat exchanger, or closed- loop, using a water-based antifreeze mixture circulating through sealed pipes (Batchelor, 2007). Although open-loop systems provide the highest energy yield, they require the highest financial input and pose the highest technical risks (Boennec, 2008).
 Environmental and economic considerations
Geothermal energy can be considered a clean and environmentally friendly energy source as it generates no (or minimal) greenhouse gases (such as carbon-dioxide and nitrous oxide) because the conversion and utilisation processes do not involve any chemical reactions, in particular, combustion (Lanterman & Lee, 2007). It is also classified as both renewable and sustainable (Lund, 2009). Furthermore, geothermal energy is available continuously, regardless of weather conditions, a stark contrast to solar and wind power.
The current economic benefits of geothermal energy are that it is flexible and can be used centrally (power plants) or locally (district heating), aiding economic development of small, isolated communities (Dincer et al., 2007). The deeper the boreholes required, the more capital-intensive the systems are, so as technology develops, allowing us to use lower temperatures for electricity generation and heating, the more economically attractive geothermal energy becomes. This will further aid the development of remote communities that do not have ties to main electricity distribution systems.
Most of the emissions of geothermal power plants are associated with the cooling towers that produce water vapour and possibly carbon dioxide, sulphur dioxide, nitrous oxides and hydrogen sulphide, but in amounts that are a fraction of those produced by fossil-fuel-fired power plants (Lund, 2009). Geothermal power plants are also generally not visually intrusive as ca be made to blend in with the landscape and use minimum land (Lund, 2009).
Heating and hot water provision do not pose many environmental issues. The extraction of heat leads to a temporary drop in ground temperatures but in times of low heat demand the ground partly recovers due to underground water flows and geothermal flow (Boennec, 2008). The extraction of water is also managed as in most cases once the water is used it is returned to the aquifer, albeit at a different temperature.
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