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 2.75Ч1016cal/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.