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.