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The Potential For The Exploitation Of Geothermal

Energy In The United Kingdom Essay, Research Paper


Whether the World Population stabilises at 8 Billion


or 10 Billion, both developing and developed nations will call for increasing


amounts of energy as they strive to achieve ?higher? standards of living.The oil crises twenty years ago gave rise to a


debate about the availability of energy, adequacy of supply and the hunt for


alternatives.? Today, there is no


shortage of energy, the question is how can we generate and deliver more of it


with less environmental impact.? Hence,


the quest for increased use of renewable energy supplies. Wind, wave, solar, hydro, the renewables that are,


increasingly viable, variable in output and much vaunted.? All of these energy sources focus primarily


on the generation and delivery of electricity.?


While electricity is probably the most advanced and flexible form of


energy devised by man, transport and the heating and cooling of buildings are


two equally large consumers of energy.?


Hidden away, beneath our feet, is another, vast, renewable energy


resource.?? At depths of several


kilometres there is a thermal resource available to mankind.? In fact 99% of the Earth?s volume is at


temperatures in excess of 1000°C (Appendix 1).?


This vast resource can be exploited for both electricity production and


direct use applications.? This report


investigates whether there is a potential to exploit geothermal energy


resources in the United Kingdom.HistoryThe exploitation of geothermal resources dates back


to Roman times where hot water was used for mechanical, domestic and leisure


applications.? Roman Spa towns in


Britain sought to exploit natural warm water springs with simple plumbing


technology.? Today, more than 30


countries worldwide are involved with direct uses of warm groundwater


resources. Space heating, bathing, fish farming and greenhouses represent 75%


of the applications, giving a total installed capacity of 10,000 MW thermal


(see Boyle, G10 p359).Geothermal energy was first used for power


generation in 1904, when a 5KWe prototype unit was developed at


Larderello, Italy.? Today the Larderello


power station complex (Appendix 2) has a capacity exceeding 400MW and a


rebuilding programme in progress that will take the capacity to 885MW (see


Batchelor, A5 p39).Another 20 countries now produce power with natural


geothermal steam rising from deep wells drilled into hot permeable aquifers.


The capacity of all the geothermal power plants amounts to 8,000 MW electric


(See IGA2 p3).What is geothermal energy?In order to evaluate the potential in the UK, I have


used a variety of resources to research into the origins, distribution and


geographicalrequirements for the different applications of geothermal


energy.Geothermal energy is derived from the earth?s


natural heat flow, which has been estimated at some 2.75×1016cal/h


(thermally equivalent to 30,000 million KW) (see Laughton8 p61).Heat flows out of the earth because of the massive


temperature difference between the surface and the interior: the temperature at


the centre is around 7000°C.? This heat and therefore the source of geothermal energy exists


for two reasons: first, when the earth formed from particles around 4,600


million years ago the interior heated rapidly, largely because the kinetic


energy of accreting material was converted into heat; second, the earth


contains tiny quantities of radioactive isotopes, principally thorium 232,


uranium 238 and potassium 40, all of which release heat as they decay (See


Boyle, G10 p357).? The distribution of heat flow over the surface of


the globe is related to ?plate tectonics? illustrated in (Appendix 3).? In the zones of active tectonism and


volcanism along the ?plate? boundaries, the heat flow peaks at values of 2-3W/m2


as a result of actively convecting molten rock (magma).? Variations in the vertical thermal gradient are also


considerable, being greatest in the vicinity of active plate boundaries and


least in the continental shields remote from the boundaries, with average


values around 25°C/km (See Laughton8


p61).? The following equation can be used to relate the


heat flow to the temperature at any depth if the thermal conductivity of the


rock is known.This is the heat conduction equationq=KTDT zwhere q is the vertical heat flow in watts per


square metre (Wm-2).? DT is the temperature difference across a


vertical height z.? The constant KT


relating these quantities is the thermal conductivity of the rock (in Wm-1°C-1) and is equal to the heat flow


per second through an area of 1 square metre when the thermal gradient is 1°C per metre along the flow direction (See


Boyle, G10 p368).? If for instance, the temperature is found to be 58°C at a depth of 2km and the surface


temperature is 10°C, the temperature gradient


is(58-10)/2000 = 0.024°Cm-1and if the thermal conductivity of the rock is 2.5Wm-1°C-1, the heat flow rate is2.5 x 0.024 = 0.060 Wm-2Because the heat flow is related to the thermal


conductivity of the rock, it is apparent that the potential for the exploitation


of geothermal energy depends upon the geographical location.? Only in certain areas, is the heat flow


great enough to make geothermal exploitation profitable.? In areas of high heat flow, large quantities of heat


is stored in the rocks at shallow depth, and it is this resource that is mined


by geothermal exploitation and commonly used for electricity generation.? Current U.S. geothermal electric power generation


totals approximately 2200MW or equivalent to four large nuclear power plants


(see reference17).Away from these zones, heat is transferred in the


crust by conduction through the rocks, and locally, by convection in moving


ground water, to give heat flows on the continents averaging no more than


60mW/m2) (see Laughton8 p61).? The fact that the UK is not near a crustal plate


boundary makes the possibility of finding the high temperature sources very


remote.? However, low enthalpy resources


do occur in the UK (see Batchelor, A9 p34).In areas of lower heat flow, where convection of


molten rock or water is reduced or absent, temperatures in the shallow rocks


remain much lower, and the resources are suitable only for direct use


applications (Appendix 4).? Uses for low and moderate temperature resources can


be divided into two categories: direct use and ground-source heat pumps:? Direct use, involves using the heat in the water


directly for heating buildings, industrial processes, greenhouses, aquaculture


and resorts.? Direct use projects


utilise temperatures between 38°C to 149°C.?


Current U.S. installed capacity of direct use systems totals 470MW or


enough to heat 40,000 average sized houses.Ground-source heat pumps use the earth or


groundwater as a heat source in winter and a heat sink in summer.? Using temperatures of 4°C to 38°C, the heat pump, a device


that moves heat from one place to another, transfers heat from the soil to the


building in winter and from the building to the soil in summer.? Accurate data is not available on the


current number of these systems; however the rate of installation is between


10,000 and 40,000 per year (see reference17).Over 150 years ago, Lord Kelvin theoretically


demonstrated the concept of the heat pump, a thermodynamic engine capable of


taking large quantities of low-grade heat and upgrading it to smaller


quantities of high-grade heat using a pump or compressor.? Today, the best known manifestation of this


technology is the domestic refrigerator ? a heat pump collecting low grade


energy from the inside of the fridge and rejecting to the outside at a higher


temperature.? There are now many air


source heat pumps that?provide


heating and, in some cases, reversible heat pumps that deliver both heating and


cooling.? The IEA Heat Pump Centre makes


the case that heat pumps could be one of the most significant technologies


currently available for utilising renewable energy to deliver substantial


reductions in CO2 emissions worldwide.? The figures suggest that in 1997, heat pumps in general saved


only 0.5% of the total annual CO2 emissions of 22 billion tonnes.? It is now advocated that heat pumps could save


between 6% and 16% of total annual CO2 emissions (see Curtis, R3


p2).? I asked Dr Curtis (Technical


Manager, GeoScience Limited), of the potential for the use of heat pumps in the


United Kingdom.? He stated that: ?there


is enormous potential for ground coupled heat pumps to provide heating and


cooling for buildings ? anywhere in the UK?.?


This means that geothermal resources for direct use


applications such as those listed in (Appendix 4) would be possible in the


United Kingdom.? Therefore the future


for the development of geothermal resources in the UK using heat pumps looks


very promising.AquifersDue to the geographical position of the United


Kingdom in relation to plate tectonics and the distribution of high heat flows,


only sedimentary basin aquifers and Hot Dry Rock Technology (assisted by heat


pumps) may be used.? In the mid-1970?s, the Department of Energy in


association with the EEC initiated a programme of research aimed principally at


assessing the UK?s geothermal resources by the mid-1980?s.? By 1984, new maps of heat flow (Appendix 5a) and of


promising geothermal sites (Appendix 5b) had been produced.? Three radio-thermal granite zones stand out


with the highest heat flow values, but heat flow anomalies also occur over the


five sedimentary basins identified, partly because these are regions of natural


hot water upflow.? Many shallow heat


flow boreholes were drilled during this period, together with the four deep


exploration well sites of (Appendix 5b) and (Appendix 6) (see Boyle, G10


p386).The Southampton borehole has led to the development


of the first geothermal energy and combined heat and power (CHP) district


heating and chilling scheme in the UK.Following successful trials, Southampton City


Council formed a partnership with Utilicom, a French-owned energy management


company to form the Southampton Geothermal Heating Company (see Smith, M4


p1).This partnership exploits the hot brine (76°C) proved in the exploratory well previously


drilled by the Department and the EEC at the Western Esplanade in central


Southampton (see Allen, D12 and Downing, R13).A single geothermal well, was drilled in 1981, to a


depth of just over 1,800m beneath a City Centre site in Southampton (Appendix


7).? Near the bottom of the hole, 200


million year old Sherwood Sandstone containing water at 70°C was encountered.? This is both porous and permeable allowing it to hold and


transmit considerable volumes of water.?


The fluid itself contains dissolved salts and, as in most geothermal


areas, is more accurately described as brine.?


Within the aquifer the brine is pressurised and so it rises unaided to


within 100m of the surface.? A turbine


pump, located at 650m in the well, brings the hot brine to the surface where


its heat energy is exploited.The brine passes through coils in a heat exchanger


where its heat energy is transferred to clean water in a separate district


heating circuit.? Heat exchangers


operate on a similar principle to many domestic hot water tanks in which a


working fluid (also usually water passing through a coil of pipes in the tank)


is used to heat water for washing.In this case, the cooled geothermal working fluid


(brine) is discharged via drains into the Southampton marine estuary.? The heated ?clean? water is then pumped


around a network of underground pipes to provide central heating to radiators,


together with hot water services (see Boyle, G10 p354).A scaled-down district-heating network was installed


in 1989, and initially served the Civic Centre, Central Baths and several other


buildings within a 2km radius.? Today


with improved heat extraction from the geothermal brine, using heat pump


technology, the scheme also includes the BBC South headquarters, Novotel and


Ibis Hotels, ASDA, Southampton Institute, Royal South Hants Hospital, West Quay


Shopping Centre and many other buildings (see Smith, M4 p1-6).The geothermal heat supply, originally 1 mega watt


thermal (1MWt), has now been increased to 2MWt using heat


pumps, and this is capable of satisfying the base load demand.? However, during periods of higher demand,


fossil fuel boilers boost the plant?s heat output to a maximum of 12MWt.? The Southampton Geothermal Heating Company, which


now runs the operation, charges the modest sum of about 1 penny per KWh of heat


energy consumed, but it must be emphasised that neither the drilling nor the


testing costs were met by the company, and the scheme was partly financed as an


EC demonstration project.? Moreover,


most similar geothermal district heating schemes require the drilling and


operation of a waste brine re-injection well.?


Nevertheless, the scheme is seen as environmentally acceptable, and is


saving over a million cubic metres of gas (of 1000 tonnes of oil) a year (see


Boyle, G10 p355).The Southampton City Geothermal and CHP scheme


provides a useful case study within the UK of a small-scale geothermal scheme


that actually works.? So why are


geothermal aquifers not being exploited much more widely?? The problem is not just one of marginal


economics and geological uncertainty, but is to do with the mismatch between


resource availability and heat load, itself a function of population


density.? Over half the resources are


located in east Yorkshire and Lincolnshire, essentially rural areas lacking


concentrated populations.? The other UK


areas are little better, though several large conurbations in the midlands and


North West could benefit form geothermal schemes such as that in Southampton.? For example, there has been discussion about


reopening and exploiting the Cleethorpes well if high flow rates could be


maintained at around 50°C (see Boyle, G10


p388).? Should fossil fuel prices ever


escalate again, no doubt geothermal aquifers in the UK will receive much more


attention than at present.Hot Dry Rock Technology (HDR)When asked whether there is potential in the UK for


geothermal electricity production Dr Robin Curtis of GeoScience Limited stated


?there is no potential for electricity power generation in the UK other than by


Hot Dry Rock Technology which is still being developed in a few other countries


but is currently on hold in the UK?.Hot Dry Rock technology is often referred to as


?heat mining? and aims to exploit volumes of hot rock that contain neither


enough permeability nor enough ?in situ? fluid in their natural state for


commercial exploitation.? The


permeability is created by stimulation techniques and the fluid is placed and


circulated artificially (see Ledingham, P1 p4).Research on hot dry rock technology began in the


1970?s to develop reservoir creation and exploitation techniques that would


allow access to an almost limitless resource base virtually independent of


location. The original dream behind HDR concept was that if a method could be


found to induce permeability into basement rocks that would not otherwise


support significant flows of water, then this would give access to the huge


amount of thermal energy stored within the accessible layers of the Earth?s


crust.Such a resource would be available virtually


everywhere, would reduce dependence on imported fuels, provide temperatures


adequate for electricity generation even in tectonically stable regions, and


would discharge very little waste and almost no greenhouse gases (see


Ledingham, P11 p296).Of the three principal granite zones in the Eastern


Highlands, Northern England and Southwest England, the latter is characterised


by the highest heat flow, as shown in (Appendix 5a).? However, large areas of the more northerly granite masses are


covered by low thermal conductivity sedimentary rocks and so, from The Heat


Conduction Equation, temperatures will be higher at depth than if the granite


bodies came to the surface.By the mid-1980s, detailed evaluation of the


radio-thermal and heat conduction properties of all the granite areas still


demonstrated, as shown in (Appendix 8a), that the South-West England granite


mass is the best HDR prospect.?


Substantial areas of Cornwall and Devon are projected in (Appendix 8b)


as having temperatures above 200°C at 6km depth and it has


been estimated that the HDR resource base in South?West England alone might


match the energy content of current UK coal reserves.? One estimate suggested that 300-500MW (about1016Ja-1)


could be developed in Cornwall over the next 20-30 years with much more to


follow later (see Boyle, G10 p388).?


However, for technological and economic reasons, the pace of progress is


unlikely to be that fast.The principle of HDR technology is to circulate a


fluid between an injection well and a production well, along pathways formed by


fractures in hot rocks. A deep heat exchanger is then created, and the fluid


transfers heat to the surface, where it can be converted to electricity. This


process is contained in a closed-loop and no gas or fluid escapes in the atmosphere.


The hot fluid produced under pressure at the wellhead flows through a heat


exchanger, vaporizing a secondary low-boiling working fluid This fluid, usually


isobutane, is then passed through a turbine driving an electric generator


(Appendix 10) (see reference16).Since the early days of HDR research, the main


question has been whether HDR technology can be made to work, i.e. whether a


sufficiently large heat exchanger with acceptable hydraulic properties can be


created in rock of low natural permeability so that economic quantities of heat


can be extracted. The only method of testing the concept and of developing the


techniques for engineering the reservoir is via large-scale field experiments.


The UK-project in Rosemanowes, Cornwall was the second such project to be


initiated and has produced a great deal of new information about deep


crystalline rock masses and techniques to investigate them (see reference15).


The Experiments with HDR carried out at Rosemanowes


in Cornwall served to demonstrate some of the outstanding uncertainties in HDR


projects, and hence the risk factor that may be inadequately covered by the


drilling contingency in the cost breakdown shown in (Appendix 8).? Phase 1 of this project (1977-80) saw the


drilling of four 300m deep boreholes to demonstrate that controlled explosions


within the boreholes could improve permeability and initiate new fractures


which might then be stimulated hydraulically.?


This was highly successful and target impedances of 0.1Mpa1-1


were achieved.? (Incidentally, 22°C water from a measurement borehole now


supplies a small-scale, commercial horticultural scheme at nearby Penryn ? a


second, albeit minor, UK use of geothermal resources) (see Boyle, G10


p388).If and when drilling and hydro-fracturing technology


is improved, large areas of the UK are potentially available for HDR


development.? One estimate by the


British Geological Survey is that 360 x 1018J could ultimately be


available from this source, enough to provide UK electrical energy for 200


years!? However, major technological


breakthroughs, coupled to a significant increase in the market price of


conventional energy resources, would be needed to make HDR a viable source of


power for the UK.? The Renewable Energy Advisory Group concluded in


1992 that, within the UK, market penetration by geothermal aquifer-based energy


systems will be difficult and that hot dry rock systems would not be


economically viable in the foreseeable future (see Boyle, G10


p391).? However, when I recently asked John Garnish Director


General of Research and Development of the European Commission in Brussels


about electricity production from HDR technology in the UK.? He stated that ? the development of Hot Dry


Rock continues, on a collaborate European basis, and is looking very promising.? A pilot plant generating a few MW should be


built in the next five years.? If that


is successful, then it is realistic to foresee this energy source being able to


provide 10-15% or more of the UK?s electricity needs.Environmental ImplicationsAlthough there are many advantages to using


geothermal energy, there are some environmental issues that need to be


considered before the exploitation of geothermal resources can take place.Environmental concerns associated with geothermal


energy include as noise pollution during the drilling of wells, and the


disposal of drilling fluids, which requires large sediment-lagoons.? Longer-term effects of geothermal production


include ground subsidence, induced seismicity and, most importantly, gaseous


pollution. Geothermal ?pollutants? are mainly confined to


carbon dioxide, with lesser amounts of hydrogen sulphide, sulphur dioxide,


hydrogen, methane and nitrogen.? In the


condensed water there is also dissolved silica, heavy metals, sodium and


potassium chlorides and sometimes carbonates.?


Today these are almost always re-injected which also removes the problem


of dealing with waste water (see Boyle G10 p380). Atmospheric emissions are minor compared to fossil


fuel plants. It has been estimated that a typical geothermal power plant


emits 1% of the sulphur dioxide, <1% of the nitrous oxides and 5% of the


carbon dioxide emitted by a coal-fired plant of equal size (Appendix 9) (see


reference14). A geothermal plant requires very little land, taking up


just a few acres for plant sizes of 100MW or more.? Geothermal drilling, with no risk of fire, is safer than oil or


gas drilling, and although there have been a few steam ?blow out? events, there


is far less potential for environmental damage from drilling accidents.? In direct use applications geothermal units


are operated in a closed cycle, mainly to minimise corrosion and scaling


problems, and there are no emissions.?


So while the acidic briny fluids are corrosive to machinery such as


pumps and turbines, these represent technological challenges rather than


environmental hazards.The ideal geothermal development site is either in a


remote location or well screened like the quarry at Rosemanowes in Cornwall;


unfortunately, not all commercially viable sites have this advantage.An HDR plant in Cornwall would produce no


?greenhouse? gas emissions, no acid rain and no long-term wastes (see


Batchelor, A5 p47).? However,


there will be a significant fresh-water consumption and the generation of


micoearthquakes at depths well below those used in the experimental


programme.? The mechanism of


micro-earthquake generation is understood and the risk of triggering a damaging


event is considered to be insignificant (see Engelhard, L6 p47).ConclusionGeothermal energy is not merely a hope for the


future.? High temperature geothermal


resources are found in many places on the earth and approximately 8,000MW of


generating capacity is installed in 20 countries, producing 45 billion kilowatt-hours


of electricity per year from geothermal energy.? The growth of geothermal utilisation for power generation has


averaged 9% per year over the last 20 years, probably the highest growth rate


for a single energy source over so long a period of time.As a result of geothermal production, consumption of


exhaustible fossil fuels is offset, along with the release of greenhouse gases


and acid rain that are caused by fossil fuel use.? Today?s geothermal energy utilisation worldwide is equivalent to


the burning of 150 million barrels of oil per year.? In Europe alone, every year geothermal production displaces


emissions to the atmosphere of 5 million tons of carbon dioxide, 46000 tons of


sulphur dioxide, 18000 tons of nitrogen oxides and 25000 tons of particulate


matter compared to the same production from a typical coal-fired plant (see IGA2


p3).The environmental and political factors suggesting


future limitations to the availability of fossil fuels has promoted research


into alternative and renewable resources of energy, particularly for electricity


generation in the UK.? Aquifers are not


able to provide the high entropy energy required for this purpose but interest


has been stimulated in the expectation of high temperature heat from Hot Dry


Rocks at depths of 6km or more in some areas of the UK. ???The occurrence of high heat flows in the


radio-thermal Cornish granites led to a major research programme and much of


this research is ahead of comparable work elsewhere in the world.? The prospects for a successful conclusion to


this research and development are encouraging.?


Economic analysis indicates that both electrical power generation and


CHP systems could be deployed economically in the early part of the 21st


Century to provide some 2-3% of the UK?s present energy demands for some 200


years, although CHP is seen at the present time as a less likely commercial


proposition (see Laughton8 p72).?


Economic analysis also suggests that district


heating schemes fed from HDR well be economical in given circumstances at the


present time and some areas warrant site-specific studies, particularly those


where high heat loads are underlain by radio-thermal granites.? The application of low enthalpy geothermal


resources to district heating from aquifers has proved commercially


advantageous in many parts of the world and is expected to continue


supplementing such energy demands well into the future.? In the UK, however, the geographical


distribution of the aquifers and the difficulty of forecasting their yields at


given sites, coupled with the abundant availability of low-cost fossil fuels


and various institutional barriers, have inhibited development of such local


energy supplements.? The commercially


led applications at Southampton and Penryn may lead to a change in this


situation.

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