Reyes, A.G. 2007 A preliminary evaluation of sources of geothermal energy for direct heat use. Lower Hutt, N.Z.: GNS Science. GNS Science report 2007/16 37 p.
Abstract: New Zealand is one big geothermal system with pockets of subsurface high temperatures where heat can be mined economically, from fluids and/or rock. Exploitable temperatures range from as low as <10oC to as high as >350oC. Depths of heat extraction vary from about 15m to >5000m. Conventional sources of geothermal energy with high grade heat reserves include high enthalpy geothermal systems in the Taupo Volcanic Zone (TVZ) and Ngawha and hot spring systems outside the TVZ. About 265 PJ of energy can be extracted from conventional sources of geothermal energy such as hot springs and hot waters discharged from power plants or from wells used expressly for direct heat use from high enthalpy geothermal systems in the TVZ and Ngawha. Only 5% of energy is being used at present at about 14.2 PJ. There are, however, a wide range of unconventional sources of geothermal energy from low to high grade heat reserves that have hardly been exploited including edges of high-enthalpy geothermal systems in the TVZ and Ngawha, warm to hot water in abandoned hydrocarbon wells and underground flooded coal and mineral mines, and the natural conductive heat in sedimentary basins and igneous terrain. At relatively shallow depths from 15m to about 250m, where temperatures remain constant at about 15o to 27oC throughout the year, heat can be extracted from the ground using ground source heat pumps. However harnessing heat at deeper levels from dry rock is not a viable option at this time due to technological and economic restraints. Only about 0.002 PJ of unconventional geothermal heat is being used at present. Projections show that, at most, <20% of conventional and <0.1% of unconventional sources of geothermal heat will be used in the next 10 years due to accessibility and a number of economic, environmental, social, political and technological barriers. (auth)