Geothermal energy is the natural heat within the earth
produced from fluids that absorb the heat within crustal rocks. Geothermal energy is the natural heat within the earth
produced from fluids that absorb the heat within crustal rocks.
The amount of heat that is theoretically available within a depth of 5 kilometres is in the order of 140 x 1024 joules.
However, exploitation of only a tiny fraction, 5 x 1021 joules, can be regarded as economically viable within the next five decades, and only an estimated 500 x 1018 joules is likely to be exploited by the year 2020.
Although this energy is non-uniform its distribution is globally widespread.
Exploitation of geothermal energy has concentrated on hydrothermal resources from aquifers.
Geothermal aquifers exploit heat from the earth's crust through naturally occurring ground waters in deep porous rocks. The exploitation of these aquifers as a source of energy requires a production borehole to extract the water and an injection hole to dispose of the cooled water.
An alternative single hole configuration can be used where, instead of using an injection well the used water is simply discharged to the sea or some other convenient sink.
Because of the poor thermal conductivity of rock and low-fluid recharge rates, heat is usually extracted at a greater rate than it is replenished from the surrounding rock mass.
Geothermal aquifers are, therefore, not 'renewable' resources in the strict sense of the word, but are usually grouped along with renewables.
During the 1970's the Hot Dry Rock (HDR) concept was developed, which extracts heat from artificial injection of cold water into crystalline plutonic rocks or metamorphic 'basement' complexes. Water is pumped down one well to induce hydraulic fracturing to create a reservoir.
Water is then circulated under pressure through these fractures, absorbing heat before returning to the surface via one or more production wells.
The concept was initially pioneered in the UK and the USA. The focus has more recently switched to large scale experimental work at Soultz, under a EU co-ordinated program, and in Japan. Other concepts such as direct exploitation of heat from magma bodies and energy extraction from geopressured formations remain longer term options.
Direct use of geothermal energy is only likely to be economically viable where there is a large base load energy demand.
Detailed geological conditions, which ultimately govern the quality of the resource (formation fluid temperature and flow rate) are difficult to predict in advance without capital investment in drilling and well tests.
Consequently, geothermal energy is regarded as a high risk investment relative to other forms of energy production. However, both the French and Icelandic Governments have shown that with imaginative assistance schemes, the resource can be successfully developed and sustained.
Moreover, new energy markets suited to large base load demand could be developed either from existing systems or newly developed resources. The high risk associated with development and the continued influence of low World fossil-fuel prices will probably lead to only modest increases of 12% in the direct use of geothermal energy by the year 2010.
For deployment, the following non technical market barriers play an important role:
Information
Risk
Financing
Environmental considerations
The key factors for successful Geothermal development are:
Identification of good quality resource.
Large coincident heat demand such as district heating, horticulture, fish farming etc.
Good scheme design to optimise heat demand pattern with reservoir management.
Long term repayment provided by a guaranteed income stream over the life of the project to offset initial high capital cost.
Insurance system for both the development stage, including drilling, and the production stage.
Information dissemination to potential developers via technical associations (e.g. European Branch of the International Geothermal Association (IGA) or a new dedicated European orientated geothermal trade association. Strong links with district heating and utility companies would also be desirable.
Compilation of case studies orientated towards potential developers which exemplify key features of successful development and solutions to technical problems that can be implemented in new schemes.
Compliance with environmental regulations or requirements, particularly with regard to the discharge of emissions of noxious gases or formation waters with a high saline content.
