It has been calculated that 99.9% of the earth’s mass is over 100 degrees centigrade with this temperature being maintained by natural radioactive decay. Geothermal technology harnesses this energy to provide surface heating (and cooling) and steam-generated power. Geothermal has been exploited since Roman times, by tapping into naturally occurring hot springs and aquifers and the UK provides examples today in Bath and Southampton.
Developments in drilling technology and geophysical techniques mean that systems can be engineered at depths of up to 5km. In such systems, water is injected into the hot rock via one well, it is then superheated by the rock before being abstracted via another well. Despite high initial exploration risk and early capital requirements, such systems are expected to provide energy for up to 50 years making them a very reliable and cost effective source of on-demand power, as well as heat and cooling.
Steam power plants
These plants use very hot steam and water resources. The steam is used to turn turbines which drive generators to produce electricity. Steam is the only significant emission from these plants. Whilst very small amounts of carbon dioxide, nitric oxide and sulphur are also emitted this is 50 times less than what is emitted by conventional fossil-fuel power plants.
Binary plants
These plants use lower-temperature hot water resources. Hot water is passed through a heat exchanger as the same time as a secondary fluid with a lower boiling point. This secondary fluid vaporises, turning turbines which drive generators. The geothermal water is returned to the reservoir. Binary plants do not emit anything as their cycle is self-contained.
District & Commercial Heating Plants
Depending on the temperature achieved in the geothermal reservoir, the water extracted can be used to provide heat through a district heating network or connected to a other large heat load. This type of plant is growing in popularity with cities such as Paris deploying the technology to provide heat to many of its residents.
UK Applications and Potential
The UK currently has a deep geothermal district heating plant in Southampton and recently Geothermal Engineering Ltd has successfully produced deep geothermal energy for the first time in Cornwall for 25 years.
According to DECC, Cornwall alone has the potential to supply 3GW(e) of electricity from deep geothermal sources. Deep geothermal power plants are normally within the 3 to 50MW(e) range and are well suited to urban areas or specific locations where there is a Combined Heat and Power load. Drilling rigs have now been specifically designed and built for urban locations, and the plants themselves are often contained within a standard industrial unit.
DECC’s 2050 Pathways predicts deep geothermal could supply 35TWh, or 10% of current annual electricity demand. This would translate to a theoretic potential to deliver a third of UK heat demand, however in practice this will be constrained by the need for heat networks. Further reports such as SKM’s Geothermal Potential: Great Britain and Northern Ireland are also optimistic regarding the UK’s potential.
The REA has been successful in securing a dedicated Renewable Heat Incentive tariff for deep geothermal heating which will make projects in well suited areas viable. Changes to planning policy are also underway that will simplify the development of deep geothermal wells.
There is also the potential to greatly expand the drilling, geophysical, manufacturing and support sectors in the UK.
For more detailed information and to find out more about this technology please see the REA’s Deep Geothermal Group.
