Geothermal Energy is the heat store of the earth. It relies on the existence of a very high temperature source (of the order 7000oC) within the earths core. This creates a flow of heat from the centre to the earths surface. This heat is then lost to the atmosphere (at a rate, typically of 60mW/m2).
It is not strictly a renewable energy as the hot core was created by the hot rocks which made up the earth when it was created and is maintained by the decay of long lived radioactive isotopes. At a global level, the magnitude of this heat source has reduced since the earths creation and will continue to decrease over a timescale of billions of years.
The high temperature core creates temperature gradients from the centre to the earths surface. The bulk of the earths mass consists of molten solids which transmit heat much more readily than the solidified rock making up that part close to the earth's surface. This creates a fairly uniform temperature within all but approximately 100km of the surface. Thus, change in temperature with depth is much greater close to the earth surface than it is within the bulk of the earth and temperatures above 100oC are common at depths of a few kilometres. Though the temperature gradients throughout the earth's crust are relatively high across the surface of the earth some locations have greater potential than others. The following three characteristics help to classify any geothermal source
Some medium is necessary to transport the heat to the surface. Where this medium (generally water/brine) is contained within the rock, it already acts as a heat store which can be transported to the surface. It may be pressurised such that it requires little or no effort to bring it to the surface. Such a store is referred to as an aquifer. All water containing geothermal stores are referred to as Hydrothermal. Fluid transport acts in only one direction (upwards). When no suitable fluid is present then this must be injected into the well and sufficient heat transferred before it is returned to the surface. This is not considered to be economically feasible unless the source is at a very high temperature (say, over 300oC).
The geological formation is the main determinant of the quality of the source. Three main classifications are made
Volcanic Plate. The rigid outer layer of the earth is divided into plates which move slowly across the surface under the control of the molten layers beneath. At the boundaries of the plates, heat flow is a maximum, it is along these boundaries that volcanoes form.
Sedimentary Basins. Certain sedimentary rocks are highly porous and permeable. Their porosity allows them to store water within their structure and their permeability allows the transport of water through the rock. When such rocks are trapped beneath two layers of impermeable rock but are exposed at one or more margins (and therefore able to absorb rain or fallen water) they may be a good source of stored geothermal energy.
Hot Dry Rock. HDR refers to when a large amount of heat is stored in rock containing no water. Rocks such as granite created from Volcanic Magma offer the best potential. With HDR sources a fluid (water) must be supplied to a borehole and withdrawn after the heat of the hot rock has been transferred to it.
Enthalpy represents the energy value of the geothermal fluids for which the temperature is an indicator. High Enthalpy sources (above 160oC) exist largely as steam (at atmospheric pressure) and are generally most suited to electricity generation. Medium Enthalpy sources (100 - 160oC) may be suitable for electricity generation but may require 'special' technology to enable this. Low Enthalpy sources (below 100oC) are only suitable for direct heating applications.
Utilising the energy
Electric power generating geothermal plant is based on conventional steam turbine technology. Differences depend on how the geothermal fluid is treated before it enters the turbine and once it leaves. Greatest efficiencies are achieved when the supply to the turbine is dry steam.
Dry steam requires no treatment before entering the turbine, these plants are referred to as Dry Steam Power Plants. Wet steam has lower enthalpy and therefore less energy to 'lose' within the turbine and will require higher flow rates to achieve the same power.
Water stored in aquifers where the temperature is above its boiling point at atmospheric pressure, may exist as liquids at the pressures experienced a few kilometres into the earth's crust. When they rise up the borehole the reduction in pressure can induce boiling and steam generation (called 'flashing') within the bore. This is not desirable and under these circumstances, flashing is suppressed by pressurising the bore. Upon reaching the surface, steam is allowed to form which can be fed into the turbine. Such plant are referred to as Single Flash.
Typically, 80% of the energy remains stored as fluid which is effectively waste if no suitable direct heating application exists locally. Though adding a significant cost to the installation, the waste heat is best fed down a second borehole back to the reserve. This helps extend the life of the resource. Double Flash installations can be 20 - 25% more efficient. Here, the waste hot water is depressurised to create more steam which is then mixed with exhausted steam from the first turbine. The mixture is fed into a second turbine.
Where temperatures are too low to induce significant flashing (around 100oC), a fluid with lower boiling point (e.g. Pentane or Butane) may be used to power the turbine. The heat of the geothermal waters is transferred to such fluids via a heat exchanger. Such plants are referred to as Binary Cycle. This method has the advantage that relatively impure sources may be exploited. Such waters may contain dissolved gases or have a high salt concentration which may be detrimental to either the turbine or the environment. The fluid may be pumped straight back down a return borehole upon leaving the heat exchanger, forming a closed system.
Direct Use. Low Enthalpy sources (below 100oC) are unsuitable for electricity generation but may be used as a means to provide space (and, perhaps water) heating either to homes and places of work or for industrial and farming applications. Here, the final application helps determine the technology necessary. One problem with such installations is that the pressure within the borehole may be inadequate to bring the water to the surface and pumping may be necessary. Where the temperature is inadequate for the application, heat pumps may be used to upgrade the heat content.
Hot Dry Rock. The enthalpy and therefore the application of Hot Dry Rock sources will depend on the depth of drilling. To extract the heat from this source it is necessary to drill two wells and set up a pathway between them at their base which will act as a heat exchanger. The pathways are best achieved by enlarging the natural fracturing of the rock.
Global and Local potential
Installed geothermal capacity world-wide amounted to 7GW installed electrical power (GWe)and 11.3GW thermal power (GWth) . This compares with figures of 6GWe and 4GWth in 1992 . Of the electrical installations, half is installed within the Geyser Field in Northern California, other significant capacity exists in Mexico, The Philippines, Italy and Indonesia. Japan heads those nations exploiting geothermal energy for direct heating use, followed by China and Iceland. Many other countries also have plants.
The total capacity represents approximately 0.2% of Primary Energy Use in the World. Whilst geothermal energy has been exploited commercially for a century now, it remains a marginal energy industry. However, there remains a significant resource yet to be exploited. The amount of heat available within a depth of 5km (the accessible resource base) is 1.4 x 1026Joules  which equates to more than one quarter million years at current energy consumption. However this heat is distributed much too broadly around the globe and Palmerini states that only 5 x 1020Joules will be economically available over the next forty years. This represents a small growth in capacity. Historically, installed geothermal capacity has grown almost linearly since the first plants of 1913 in Italy.
The annual geothermal heat flow is about 1021Joules. The magnitude of solar energy reaching the earth is some 5 000 times this value. However, the variation in solar energy contribution across the globe, is much less than geothermal energy: resulting in a number of high energy geothermal sources. Many of these exist in the developing world. There are six regions where concentration of geothermal energy is highest:
The prominent role of Western USA in power generation is as much to do with technological and social aspects as its geothermal potential. Many of the resources in the Developing World are yet to be exploited. Some areas have high reserves in fossil fuels (e.g. Eastern Africa/Arabian Peninsula) and therefore it is improbable that these regions will exploit Geothermal Resources before the middle of the next century.
The economics of geothermal exploitation vary enormously with the circumstances of the resource. The most favourable sites producing reasonably pure, dry steam for electricity generation tend to incur the lowest capital costs and produce the greatest power. The Geysers Site in California produces electricity which is very competitive with the fossil-fuel derived alternatives and a recently installed plant in Costa Rica provides electricity at one quarter the cost of its oil derived rivals. Lower temperature sources incur additional expense both in capital and maintenance as equipment is necessary to prepare the steam or maintain pressure within the borehole adding to the running energy costs. The cost of district heating systems in the UK, based on the Southampton plant is approximately 5p/KWhth.
The prospects for the UK may depend on whether Hot Dry Rock technology becomes economically and technically feasible. Low-enthalpy sources exist in a number of areas in the UK.
Over half the UK potential lies in the Yorks-Lincs basin away from major centres of population. As low-enthalpy geothermal energy is only suited to heating applications, it is not transportable over large distances and therefore its adoption is linked to identifying applications for the heat.
One of the best established applications is in agriculture where geothermal heat may be useful for producing crops out of season or high value crops which are usually imported and which therefore are usually associated with high energy costs due to this transportation.
Table 14 Potential Resources, UK, EJ (1018J) 
Many of the activities on which the geothermal Industry depends already have a strong base in other energy industries. Although the circumstances are different, some oil, gas and coal industry operators have adapted expertises in Geothermal Energy Engineering. One such is Unocal. Unocal claim to be the worlds leading producer of geothermal energy, with thirty years experience in the field. They operate the Geyser Field in the U.S.A (the worlds largest geothermal electric installation) as well as significant plant in The Philippines and Indonesia. Many of the most successful companies have grown up through a presence at the Geyser.
Main international projects
The U.S. Department of Energy (DOE) Geothermal Technologies Program . The U.S. Government has sponsored research in Geothermal Energy since 1971. The current program has two specific objectives concentrating on short term and long term needs of the Geothermal Industry.
Four main technical areas are identified, which are summarised below:
Exploration Technology: Objective is to develop techniques which assist in identifying currently hidden resources.
Drilling Technology: Objective is to develop borehole technology which is more appropriate to the geothermal environment rather than the relatively more benign environment of oil and gas drilling, from which it has developed. Drilling is the most significant expense of most geothermal energy plants, accounting for between 30 and 60% of costs.
Reservoir Technology: Objective is to develop technologies which are appropriate to ensure the sustainability of geothermal energy extraction. Efficient utilisation of a source of geothermal energy, particularly with respect to sites where power is generated, is directly linked to the maintenance of a constant supply of 'material' of the same quality. Over extraction will result in a lower enthalpy supply which may no longer be appropriate to the technology set up to make use of it. Knowledge of reservoir potential is essential for plant sizing and efficient conversion.
Conversion Technology: Objective is specifically to reduce the costs and increase the efficiency of binary power technology. The majority of existing Geothermal Power plants utilise high enthalpy sources. Binary Power plants have been developed to tap the more abundant moderate enthalpy sites. The efficiency of such operations is marginal as the quality of the resource is lower and other inefficiencies, such as heat exchange are introduced. Improving the efficiency of this technology will increase the range of sites considered to be economically feasible.
Europe. The EU has recently adopted a White Paper  which sets out a 'Strategy and Action Plan' to double renewables contribution to primary energy needs by the year 2010. It assumes a 1995 base figure of 6% which is to rise to 12%. The specific targets for Geothermal Energy are not very ambitious in comparison with other renewables. Targets of 1GWe installed electrical power and 2.5GWth heat are specified. The targets for Wind and Hydro are 40GWe and 105GWe respectively.
The main vehicle for meeting these targets will be the ALTENER II programme which sets out ways to overcome obstacles to renewables. Research and Demonstration programmes are set within the framework of the Joule-THERMIE programme.
Hot Dry Rock (HDR) Research. One of the greatest challenges in the field of Geothermal Energy is the establishment of a working model of Hot Dry Rock technology. A number of nations are involved with research programmes in this field including Japan, U.S.A. and Australia as well as a cross-European group. The nations involved in the European group are France, Germany, Italy, Switzerland, Sweden and the UK. A European programme  has been in existence in one form or another for over ten years. The centre of present activity is Soultz in Alsace. A three stage plan has evolved:
Work is currently centred on the first of these stages and some significant achievements have been made. A closed loop, forced circulation system has been set up between two boreholes of 4km depth where temperatures of 165oC are experienced. Circulation of the water is achieved without loss at a temperature of 140oC. The power required for pumping to maintain the flow is minimal compared with the energy value of the heated water. This represents a successful demonstration that the fundamental principles work. Further trials are to be implemented prior to moving onto stage 2.
Main barriers and how to overcome them
Geothermal Energy is a mature renewable energy technology. However, Geothermal Energy Technologies are sophisticated technologies involving a high level of risk. The main problems to the expansion of the industry lie in the economics of existing technologies and the uncertainties of future technologies.
Power Generation. Typical production costs are distributed as shown in Table 15. It can be seen that the main costs are incurred prior to operation of the plant. Time is also an important factor and it may take a number of years to reach full operation whilst absorbing a high investment capital. Expenses are not dissimilar to those incurred by the oil industry. However the scale of operation within the oil industry is several magnitudes higher than that of the geothermal industry and the oil industry is able to afford the long term investment and risks which the geothermal industry is unable to. The economic problems can be partly tackled by addressing those factors that give rise to them. The expense of drilling exploration wells in order to identify a resources potential may be reduced by improving the methods employed in surface exploration and development of more reliable data interpretation methods. Drilling technology needs to target the specific problems encountered at geothermal sites to increase the rate at which wells are produced and thus reduce the timescale from project inception to plant operation.
Table 15 Typical divisions of expenditure for the different phases of Geothermal Power projects 
Direct Uses. The points raised above also apply to direct operation. The incurred costs of drilling tend to be lower as low-enthalpy reservoirs tend not to be as deep as the hotter sources. However the lower enthalpy store has a lower commercial value and produces a product which is not transportable. Thus the economics of exploitation of low-enthalpy sources rely on the identification of a local market for the product. Where geothermal resources are located below centres of high population density a ready market exists, though installation of distribution plant may not be straight forward. Generally though this is not the case and thus if exploitation of these resources is to be expanded it will need to be achieved by the integrated sponsorship of the energy and those industries which can make use of it.
Advanced Techniques. These have a number of problems to overcome to prove that they are commercially viable. If successful they will open up a vastly greater resource than that which is currently seen to be viable. Successful commercial operation of these technologies will in themselves create the opportunity for a considerable expansion of geothermal energies and the establishment of significant industries which are able to take on further challenges themselves.