Direct Use And Ground Source Heat Pumps example essay topic

Whether the World Population stabilizes 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 Earths volume is at temperatures in excess of 1000^0 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. History The 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, G 10 p 359). Geothermal energy was first used for power generation in 1904, when a 5 Kde prototype unit was developed at Larderello, Italy. Today the Larderello power station complex (Appendix 2) has a capacity exceeding 400 MW and a rebuilding programme in progress that will take the capacity to 885 MW (see Batchelor, A 5 p 39). 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 IGA 2 p 3). 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 geographical requirements for the different applications of geothermal energy.

Geothermal energy is derived from the earths natural heat flow, which has been estimated at some 2.751016 cal / h (thermally equivalent to 30,000 million KW) (see Laughton 8 p 61). 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 7000^0 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, G 10 p 357). 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 teutonism and volcanism along the plate boundaries, the heat flow peaks at values of 2-3 W / m 2 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 25^0 C / km (See Laughton 8 p 61). 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 equation = K TDT where 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-1^0 C-1) and is equal to the heat flow per second through an area of 1 square metre when the thermal gradient is 1^0 C per metre along the flow direction (See Boyle, G 10 p 368). If for instance, the temperature is found to be 58^0 C at a depth of 2 km and the surface temperature is 10^0 C, the temperature gradient is (58-10) /2000 = 0.024^0 Cm-1 and if the thermal conductivity of the rock is 2.5 Wm-1^0 C-1, the heat flow rate is 2.5 x 0.024 = 0.060 Wm-2 Because 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 2200 MW or equivalent to four large nuclear power plants (see reference 17). 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 60 mW / m 2) (see Laughton 8 p 61).

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, A 9 p 34). 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 38^0 C to 149^0 C. Current U.S. installed capacity of direct use systems totals 470 MW 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 4^0 C to 38^0 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 reference 17). 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 CO 2 emissions worldwide. The figures suggest that in 1997, heat pumps in general saved only 0.5% of the total annual CO 2 emissions of 22 billion tonnes. It is now advocated that heat pumps could save between 6% and 16% of total annual CO 2 emissions (see Curtis, R 3 p 2). 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. Aquifers Due 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 UKs geothermal resources by the mid-1980's. By 1984, new maps of heat flow (Appendix 5 a) and of promising geothermal sites (Appendix 5 b) 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 up flow. Many shallow heat flow boreholes were drilled during this period, together with the four deep exploration well sites of (Appendix 5 b) and (Appendix 6) (see Boyle, G 10 p 386).

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 Utili com, a French-owned energy management company to form the Southampton Geothermal Heating Company (see Smith, M 4 p 1). This partnership exploits the hot brine (76^0 C) proved in the exploratory well previously drilled by the Department and the EEC at the Western Esplanade in central Southampton (see Allen, D 12 and Downing, R 13). A single geothermal well, was drilled in 1981, to a depth of just over 1,800 m beneath a City Centre site in Southampton (Appendix 7). Near the bottom of the hole, 200 million year old Sherwood Sandstone containing water at 70^0 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 100 m of the surface. A turbine pump, located at 650 m 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, G 10 p 354). A scaled-down district-heating network was installed in 1989, and initially served the Civic Centre, Central Baths and several other buildings within a 2 km 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, M 4 p 1-6). The geothermal heat supply, originally 1 mega watt thermal (1 MWt), has now been increased to 2 MWt using heat pumps, and this is capable of satisfying the base load demand.

However, during periods of higher demand, fossil fuel boilers boost the plants heat output to a maximum of 12 MWt. 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, G 10 p 355). 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 50^0 C (see Boyle, G 10 p 388). 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, P 1 p 4). 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 Earths 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, P 11 p 296). 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 5 a). 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-1980's, detailed evaluation of the radio-thermal and heat conduction properties of all the granite areas still demonstrated, as shown in (Appendix 8 a), that the South-West England granite mass is the best HDR prospect. Substantial areas of Cornwall and Devon are projected in (Appendix 8 b) as having temperatures above 200^0 C at 6 km depth and it has been estimated that the HDR resource base in SouthWest England alone might match the energy content of current UK coal reserves. One estimate suggested that 300-500 MW (about 1016 Ja-1) could be developed in Cornwall over the next 20-30 years with much more to follow later (see Boyle, G 10 p 388).

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 iso butane, is then passed through a turbine driving an electric generator (Appendix 10) (see reference 16). 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 reference 15). 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 300 m 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 impedance's of 0.1 Mpa 1-1 were achieved. (Incidentally, 22^0 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, G 10 p 388). 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 1018 J 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, G 10 p 391).

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 UKs electricity needs. Environmental Implications Although 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 G 10 p 380).

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,.