Geothermal Energy is defined as the energy available as heat contained in, or discharged from the Earth s crust. The amount of heat available is largely determined by local geology, and thus exploitable energy is distributed very unevenly, both globally and locally. Extractable heat can be used to produce electricity directly or indirectly, and/or provide heating / cooling (using, for instance, absorption chillers) for industrial processes or buildings.
The term “Geothermal Energy” covers a wide spectrum of geological settings from volcanic sources, through “hot rocks”, hot sedimentary aquifers (HSA), oil-geothermal co-production, and the exploitation of warm mine waters using heat pumps. Although normally described as geothermal, Ground Source Heat Pumps (GSHP) exploit stored solar radiation and virtually none of the heat is derived from the Earth s interior. Thus GSHP technically do not utilise geothermal energy but the International Energy Agency (IEA) nonetheless includes GSHP as a form of Geothermal Energy. GSHP therefore will be covered in the “Other Renewables” UKERC Research Landscape rather than in this document.
Geothermal Energy is usually regarded as a renewable resource, although this depends on the rate of recharge of the resources being exploited. In geothermally active areas of the world associated with volcanoes, such as Iceland and New Zealand, the reservoirs of heat are large compared with demand and resources may also be regularly replenished, so such geothermal energy can be regarded as renewable. In the UK, where active volcanism is not present, the rate of recharge of crustal heat tends to be slow. When the rate of exploitation exceeds the rate of replenishment then heat is essentially being mined. In general, a practical level of replenishment (e.g. 95 ) will occur on time scales of the same order as the lifetime of the geothermal production systems (Rybach and Mongillo 2006). It is likely that the main contribution of geothermal energy in the UK will be to local area heating, although the potential for power generation exists especially in hot rocks such as the granites of Cornwall, northern England and eastern Scotland.
Some of the attractive characteristics of Geothermal Energy are that it can provide a steady base power load, with no seasonal, weather or climate-change related variation, and that both of its outputs (electricity and heat) are easily exploited. On the other hand, the short history of deployment means that there is a lack of experience and awareness of the technology, safety issues and environmental impacts, which coupled with the high capital cost of installation makes investing in the deepest hot rock geothermal resources a high risk strategy in the UK, although shallower resources, especially mine waters, represent a lesser risk. A detailed review of hot rock and aquifer resources in the UK was published by Downing and Gray (1986) whereas a more recent perspective can be found in Younger et al. (2012).
Each type of Geothermal Energy and associated exploitation has its own specific characteristics, risks and benefits.
Hot Rocks: Temperature increases with depth at about 26°C per kilometre on average in the UK but in certain areas where granite rocks have concentrated radiothermal elements (notably U, Th and K) much higher gradients may befound. Thus at depths of 4-5 kilometres rock temperatures in favourable locations may be well in excess of 150°C. Extracting this heat usually requires drilling boreholes into the rock and creating an artificial reservoir by hydraulic fracturing, a process known as stimulation leading to the creation of an Engineered (or Enhanced) Geothermal System (EGS). An important exception is the Weardale granite in Co.Durham where very high natural permeability obviates the need for stimulation (Younger and Manning 2010). Extracting the heat typically involves a doublet closed loop system for pumping cold water down into the reservoir and recovering steam or hot water under pressure. Steam may be used to drive turbines for power generation usually via a heat exchange stage as geothermal waters are usually corrosive to turbine components. Cooler waste water is then returned to the reservoir. The process of stimulation can lead to small amounts of seismic activity locally, and ahot rock development at Basel (Switzerland) in an ill-judged tectonically-active location has been halted. It should be noted that the stimulation process is not the same as the “fracking” process used in shale gas exploitation. An international protocol has been developed to address these concerns, but the technology is still being developed and there remain critics of this form of exploitation. In the past, subsidence has also occurred causing concern at a few high temperature developments; but with modern closed-loop systems, where fluid pumped up is balanced by fluid pumped down, this is now much less of a problem. The UK has some good hot rock prospects among the granites of Cornwall, the North of England, and in the Eastern Grampian Highlands of Scotland, all in relatively stable tectonic settings. Readers should be aware that there is also some confusion of terminology: Hot rock sources are sometimes referred to as Hot Dry Rock (HDR) or Hot FracturedRock (HFR) whereas Enhanced (or Engineered) Geothermal System (EGS) has been defined as “engineered reservoirs that have been created to extract economical amounts of heat from low permeability and/or porosity geothermal resources” (MIT 2006). EGS technology may be applied to any deep geothermal system including hot sedimentary aquifers (see below) but it should be noted that all the above terms are often used interchangeably.
Volcanic Geothermal:This is “classical” Geothermal Energy exploitation as practiced in the volcanically active areas of the world. Here magma lies sufficiently close to the surface to heat natural groundwater and convert it into superheated steam, which can then be exploited to drive turbines, with residual heat being used for district heating. This utilises the same phenomenon as geysers, and it is commercially exploited in the same volcanic areas. There are few risks with this technology although a borehole into an active magma chamber in Iceland resulted in a short-lived ash eruption near a geothermal power plant. There is no active volcanic activity in the UK, ruling out this source of geothermal energy for in situ exploitation, although certain UK Overseas Territories, currently most notably in Montserrat, have potential to exploit volcanic geothermal sources (Younger 2010)
Hot Sedimentary Aquifers (HSA): An aquifer is a body of saturated rock which stores and transmits significant quantities of water. At depth, trapped water may reach useful temperatures and providing the host rocks are sufficiently porous and permeable adequate flow rates may be achieved to efficiently extract the thermal resource. As with some hot rock sources the water is usually not sufficiently hot to flash to steam and drive turbines directly but may be exploited for both heat and electricity by utilising binary power units. These employ working fluids (usually organic) that have lower boiling temperatures than water. The rate of hot water replenishment is an important factor, and for the resource to be sustainable either a balance must be achieved between extraction rate and replenishment rate, or a system of rotation between active wells and recovering wells must be implemented. The UK has some good potential HSA resources, most notably in Permian and Triassic rocks of England and Northern Ireland, and Lower Carboniferous and Upper Devonian rocks of the Midland Valley of Scotland and northern England. Warm waters in the Sherwood Sandstone aquifer of the Wessex Basin have been exploited at about 1800 metres depth in a geothermal development that has provided heat and power to the city centre in Southampton for over 20 years. The Science Central borehole in Newcastle reaches a similar depth and also aims to support city centre heat demands.
Oil-Geothermal co-production: Each barrel of oil extracted from an oilfield may be accompanied by some 10-20 barrels of hot water, especially towards the end of the oilfield s life. As this water has to be separated from the oil and reinjected or disposed into the environment, it requires little additional technology to extract its available heat. In the USA, research is underway with small retrofitted binary power modules to convert this heat to power for the purposes of driving the rig itself and thus offsetting some of therunning costs. In addition, there is some interest in drilling regions such as the Gulf States of the USA in pumping in and recovering fluid cyclically to exploit the increased temperatures at oil reservoir depths a way to make continued use of oil rigs and extraction systems after oil reservoirs are exhausted. A significant factor is that the major costs of rig construction, installation and drilling the oil well have been offset by the hydrocarbon activities. Most of the UK s oil isrecovered from the North Sea, far distant from power grids and centres of population, and any co-produced geothermal energy will need to be used in situ. This is useful, as many platforms ironically become energy-scarce late in the productive life of a field, and, furthermore, if current plans for carbon capture and storage are to come to fruition, a local energy source above the undersea injection zones would be extremely useful.
Warm Mine Waters: Indeep mines, including those used for coal production, temperatures can be sufficiently elevated to heat naturally occurring groundwater. Once pumping ceases, mines tend to fill with this warm water that can then be extracted and used for heating, usually in combination with a heat pump. This type of resource is generally used to support district heating schemes, especially in new build projects close to disused collieries. A few small schemes exist in the UK. On a larger scale warm mine waters may serve as the rationale for urban regeneration projects, such as at Heerlen in the Netherlands (Veld et al., 2010).
The exploitation of geothermal energy is largely dependent on geology. The UK has an enormously varied geology for such a small land area, although its lack of active volcanism means that geothermal energy still has a low profile among so-called renewables. Intense research during the 1980s culminated in a focus on the geothermal potential of some Cornish granite masses, but until recently there had been little subsequent research in the UK. New models of geothermal resources have recently been developed and there is now a need to reassess the UK s geothermal resources. With the development of more efficient methods of utilising lower grade geothermal resources so it is likely that larger scale and commercially viable geothermal resources will soon be identified and developed in the UK, although it is difficult to predict the rate of uptake. Research that would help bring this about includes:
Geothermal energy is plentiful beneath the UK, although it is not readily accessible currently except in specific locations. Unlike many geological resources, anomalous heat is not easily detected at the surface, indeed the best resources will be insulated by rocks of low thermal conductivity. Furthermore heat resources at depth are not easily detected remotely using geophysics, although associated features such as aquifer fluids might be discernible. The subsurface of the North Sea is much better characterised than the subsurface of mainland UK, and a new focus on the few kilometres beneath the onshore surface veneer might result in the discovery of significant amounts of exploitable geothermal energy.
Useful Resources:
References
Downing RA and Gray DA (1986b) Geothermal Energy The Potential in the United Kingdom. British Geological Survey, Nottingham, UK/Her Majesty’s Stationery Office (HMSO), London, UK. MIT (2006) The future of geothermal energy: Impact of Enhanced Geothermal Systems(EGS) on the United States in the 21st century. Massachusetts Institute of Technology (ISBN 0615134386) Rybach L, Mongillo M. (2006) Geothermal Sustainability - A Review with Identified Research Needs. GRC (Geothermal Resources Council) Transactions 30: 1083-90. Veld, P.O, Malolepszy, Z., Bojadgieva, K., Vertsek, J. (2010). Geothermal Development of Low Temperature Resources in European Coal Mining Fields in practice: the EC REMINING-lowex project. Proceedings World Geothermal Congress 2010 Bali, Indonesia. Younger, P.L., & Manning, D.A.C., (2010), Hyper-permeable granite: lessons from test-pumping in the Eastgate Geothermal Borehole, Weardale, UK. Quarterly Journal of Engineering Geology and Hydrogeology 43: 5 10. (doi:10.1144/1470-9236/08-085) Younger, P.L., (2010) Reconnaissance assessment of the prospects for development of high-enthalpy geothermal energy resources, Montserrat. Quarterly Journal of Engineering Geology and Hydrogeology 43: 11 22. (doi: 1144/1470-9236/08-083) Younger, P.L., Stephens, W.E. and Gluyas, J.G. (2012). Development of deep geothermal energy resources in the UK. Energy (Proc. I.C.E.) 164: 1 14.
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