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.