# The Potential For The Exploitation Of Geothermal — страница 2

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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

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