The word ‘Geothermal’ comes from the Greek ‘Geo’ meaning Earth, and ‘Thermos’ meaning heat. Within the Earth’s crust, thermal activity is maintained by radioactive decay. This is why it is defined as a source of Renewable Energy.
There are two forms of Geothermal Energy: Hot Dry Rock and Conventional ‘Hydrothermal’ Geothermal.
Hot Dry Rock is a relatively new technology and is yet to become economically viable; an example of this is at Rosemanowes Quarry1, Cornwall. The Hot Dry Rock project at Rosemanowes Quarry found that if they pushed water into the ground, they couldn’t get it to return to the surface. This was down to water percolating into the rock.
Hydrothermal has been used for decades, with Iceland being the pioneering country in using geothermal energy on a large scale. Today, the USA, closely followed by the Philippines, are the largest generators of Geothermal Energy within the world.
Within hydrothermal energy plants, there are four ways of generating energy.
The first is an ‘open system’. It is known as an ‘open’ system because fluid from a geothermal resource is used directly within the generating process. The system is made up of two wells. Hot steam is extracted from one well and passed through a turbine which in turn is attached to a generator, generating electricity. The steam is then passed through a condenser thus cooling into a hot water form which is then injected into the other well, back into the ground.
The second is a ‘closed system’. Instead of using the geothermal fluid directly from the wells into a turbine, the fluid transfers its heat to another fluid through a heat exchanger (much like a refrigerator). This heated fluid then drives the turbine and generator much like before.
The last two are a mix of a closed and open system. The geothermal fluid being extracted from the well will produce steam, but as itself, will not completely turn into steam. This is known as a ‘flash evaporation’. This steam is then used through a turbine as before. The difference between these two is that the first system uses one turbine and one flash evaporation, whereas the second system uses two flashes and two turbines. With the second system, the geothermal fluid unused within the first flashing process is used in the second flashing process.
Advantages and Disadvantages of Geothermal Energy
The process of generating energy from geothermal sources should produce no by-products. Heat or waste water is often recycled by pumping it back into the earth or used to heat other industrial processes. However, fluids drawn up from the earth will contain a mixture of gases, such as CO2, Methane, Sulphides and Ammonia. Yet, in comparison to fossil fuels, geothermal fluid extraction will emit a lot less pollutants and therefore could be used as a mitigation tool in reducing CO2 emissions.
Conventional ‘Hydrothermal’ Geothermal plants are site specific. To harness heat from the earth the plant will need to be built close to a heat hot spot close to the earth’s surface, i.e. close to a contact between tectonic plates. However, the ratio of a geothermal plant to its energy output is much better than most plants. For example, for a geothermal plant to generate a GW of energy it will require 3.5km2, compared with a wind farm which will need 12km2, or coal at 32km2.
Freshwater consumption is also minimal and means that geothermal plants will require 50 times less freshwater than that of nuclear or fossil fuel power plants per MW generated.
It has been known that some plants may over extract heat from the earth. Thus leaving the area redundant of heat for many years -much the same way as over extracting water from aquifers).
Geothermal energy requires no other fuel. It is therefore unaffected by rises in other types of fuel cost. Incidentally in some pioneering countries, geothermal energy was developed as a response to the oil crisis of 1973.
Geothermal Energy in the UK
As a resource, geothermal energy is not as prolific in the UK as other countries such as Iceland and the USA. This is due to the UK not being situated near areas of volcanic activity or tectonic plate boundaries. The potential in the UK for geothermal projects must therefore come from Hot Dry Rock schemes, which as a technology is still within infancy, or underground reservoirs – aquifers.
Geothermal feasibility was first looked at within the UK as a response to the 1973 oil crisis but was not continued as oil prices fell after. The first geothermal ‘scheme’ in the UK was built in Southampton2 in 1987. The scheme extracts heat from an aquifer which feeds a local district heating system.
Eastgate in County Durham has a hot dry rocks project in planning which will be built on an old cement works.
A commercial sized geothermal project is also in its planning stage which will be built near Redruth in Cornwall on the United Downs industrial estate.
There is potential for geothermal projects in the North Sea3 using spent oil reservoirs.
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1. Rosemanowes Quarry. http://en.academic.ru/dic.nsf/enwiki/4979917
2. Southampton District Heating Scheme, Southampton County Council. http://www.southampton.gov.uk/s-environment/energy/Geothermal/default.aspx
3. North Sea Geothermal Energy, Healer George. Accessed [Online] 22 Oct 2010. Available at: http://www.healergeorge.com/geothermal/index.html
