sources of energy for geothermal direct heat use - GNS Science

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4.0 UNCONVENTIONAL SOURCES OF GEOTHERMAL ENERGY4.1 INTRODUCTIONThe heat in place for unconventional sources of energy in New Zealand is estimated usingEquation 2.Total heat in place = Heat in Rock + Heat in Circulating Fluid i.e.,Total heat in place =[ V(1-Φ)C rock ρ rock (T f -T o )] + [VΦC water ρ water (T f -T o )] Equation 2Where:V = volume in m 3Φ = porosityC rock = heat capacity of rock in kJ/kg)ρ rock = density of rock in kg/m 3 )C water = heat capacity of water (4.18 kJ/kg)ρ water = density of water in kg/m 3 (1000 kg/m 3 )T o = base temperature (15 o C used in report)= source temperature for extraction of geothermal energyT fWhere the rock is dry and low in permeability and circulating fluids, a porosity of 0.01 isassumed. In flooded underground mines, where heated flood waters fill mine caverns, aporosity of 1.0 is assumed.Heat capacity and density values for andesite and greywacke, rock formations generallyintersected at depth in New Zealand, are shown in Table 2.The calculations in this report are rough estimates and need to be updated in the future by:1. Refining the surface heat flow map into smaller regions that may be accessed later on bypeople who may wish to install ground source heat pumps, for example,2. Correcting or rechecking certain regions of the present heat flow map of New Zealand,3. Estimating the heat flow and heat reserves at various depths in New Zealand,4. Defining the heat capacity, density and porosity of the various lithologies intersected atvarious depths and temperatures,5. Refining the base temperatures used in the different regions and,6. Establishing factors that affect the efficiency of energy extraction from various sources ofunconventional geothermal energy and define this parameter for heat reservecalculations.One of the most basic data used in calculating heat at depth is heat flow. Heat flow trendswithin a large area of the TVZ is fairly well-defined by the temperature measurements innumerous wells drilled in the geothermal areas and several heat flow studies (e.g. Studt andThompson, 1969; Alllis, 1979; Bibby et al, 1984; Bromley and Hochstein, 2000). The mainheat flow data for the rest of the country, used in this report, is based on Pandey (1981) andthe heat flow isoline map published by Allis et al (1998; Figure 7).GNS Science Report 2007/16 8
Table 2. Specific heat capacity and bulk density values of andesite, greywacke and water. Greywacke densitydata is from Malengreau et al (2000), upper value for andesite from Johnson and Olhoeft (1984). The heatcapacity of greywacke is that of sandstone (www.edumine.com) and the heat capacity of granite(www.EngineeringToolbox.com) at 27 o C and 100 kPa is used as a proxy for andesite. Water constants are fromWeast et al (1989).Material Heat Capacity (kJ/kg) Density (kg/m 3 )Andesite 0.79 2800Greywacke 0.92 2670Water 4.18 10004.2 CONDUCTIVE HEAT FLOWThe calculation of available heat reserves from conductive heat flow is based on a total areaof ~184,415 km 2 , nearly 70% of the New Zealand landmass that excludes national parks,forest parks, reserves, archaeological sites and protected private land under the jurisdictionof the Department of Conservation (Figure 8) and large lakes outside these areas e.g., LakeTaupo.In the following calculations the country is divided into two general heat flow regions: lowheat flow at 70mW/m 2 ) where the thermal gradient is >33 o C/km. In regions of low heat flow, a thermalgradient of 28 o C/km is used in the calculations. Within the high heat flow region a thermalgradient of 33 o C/km is used outside the TVZ, Coromandel-Thames and Ngawha-Northlandsubdivisions.On land, the TVZ has a total area of 9000 km 2 with about 18% (1620 km 2 ) occupied by partsof the Tongariro National Park, and other protected forests, and parks in the region. LakeTaupo has an area of 616 km 2 (www.doc.govt.nz). The area encompassed by thegeothermal systems is 291 km 2 (Figure 9; Appendix 2; Bibby et al, 1995). Therefore the areain the TVZ used in the evaluation of low-grade heat is about 6,475km 2 . The regional thermalgradient used in calculating heat reserves in this region is 50 o C/km (Table 3).The area of Coromandel is about 2300 km 2 with 30% of the land governed by theDepartment of Conservation (www.doc.govt.nz) leaving an area of 1,610 km 2 for possibleheat exploitation. The area is within the 80 mW/m 2 heat flow isoline and the thermal gradientis about 40 o C.The whole of Northland has an area of 6670 km 2 with 1680 km 2 under the jurisdiction of theDepartment of Conservation (www.doc.govt.nz). About 65% of this area is encompassed bythe 80 mW/m 2 heat flow isoline (Figure 7). The total area used in the calculations is 3245 km 2and the thermal gradient set at 40 o C/km, similar to Coromandel-Thames.GNS Science Report 2007/16 9
Page 3 and 4: BIBLIOGRAPHIC REFERENCEReyes, A. G.
Page 5: Figure 11 Location of onshore aband
Page 11 and 12: the South Island, however,
Page 13: Table 1. Conservative projected ene
Page 17 and 18: eyes 07Figure 8. Map showing areas
Page 19 and 20: eyes 07Figure 10. Estimated constan
Page 21 and 22: Table 5. Estimate of heat reserves
Page 23 and 24: Figure 12. Potential uses of heat f
Page 25 and 26: Table 6. Extractable heat energy fr
Page 27 and 28: Table 7. Present and potential extr
Page 29 and 30: eyes 07Figure 15. Potential energy
Page 31 and 32: 7.0 REFERENCESAllis R G (1979) Heat
Page 33 and 34: APPENDIX 1. WASTE WATER AND STEAM F
Page 35 and 36: APPENDIX 2. WARM GROUNDS AND WATERS
Page 37 and 38: Well NameTotal Depth(m)BHT(C o )MWt

sources of energy for geothermal direct heat use - GNS Science

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