sources of energy for geothermal direct heat use - GNS Science

Estimated temperatures in the coal mines range from 18 o to 24 o C, with a mean of 18 o C.Temperatures are higher in gold mines at 19 o to 35 o C with a mean of 23 o C mainly becausethe mines could be as deep as 700 to 850m and are also located in high heat flux areas inthe Coromandel and the South Island where the thermal gradient is about 35-40 o C/km(Table 6).The volume of the underground coal mines are calculated from the total mass extracteddivided by the mean density of coal at about 1260 kg/m 3 . For the gold and copper mines thearea covered by the underground mines and the depth of the deepest stopes are included inthe calculation. The estimated potential heat energy from the warm waters is about 0.64 PJfor the 22 coal mines and 13.6 PJ for the gold and copper mines.5.0 PROJECTED USE OF UNCONVENTIONAL GEOTHERMAL RESOURCES5.1 SHALLOW CONDUCTIVE HEAT FLOW AND GROUND SOURCE HEAT PUMPSIn calculating the attainable use of heat for ground source heat pumps in New Zealand forthe next 10 years, it is assumed that about 5% of the buildings in New Zealand will have atleast 6 kW installed.According to New Zealand Statistics, there are 1,961,290 buildings in New Zealand of which309,750 are business buildings and 1,651,540 dwellings. About 18.6 PJ will be harnessed if5% of the buildings (98,065) will have a 6 kW ground source heat pump installed within thenext 10 years (Table 7).5.2 ABANDONED HYDROCARBON WELLSIn the next 10 years about 20 of the 140 abandoned wells in Taranaki could be exploited forgeothermal direct heat uses, approximately amounting to 25 MWt or 0.5 PJ on an annualbasis. Another 20 could probably be harnessed for use with 6 kW ground source heat pumpsamounting to an additional 0.004 PJ. Hence for the next 10 years, at least 0.504 PJ may beexploited from these wells.5.3 ABANDONED UNDERGROUND MINESWithin the next 10 years two mines could probably be installed with 6 kW ground source heatpumps yielding an energy of 0.0004 PJ. Access to abandoned underground mines may,however, be hampered by several factors such as high concentrations of combustible gas insome coal mines, cave-ins, highly polluted environment as in the Tui mine and distance frompopulated areas. In some cases the abandoned mines may be filled-in and secured.6.0 SUMMARY AND CONCLUSIONSPotential sources of geothermal energy in New Zealand are widespread and are not onlyconfined to active volcanic regions such as the TVZ and Ngawha in Northland (Figure 14).The sources of geothermal energy include (1) more than 100 hot spring systems occurring allover New Zealand, ranging in temperatures from 17 o C to 100 o C, (2) heated waters inabandoned hydrocarbon wells and flooded underground coal and mineral mines and (3)conductive heat in rock below 15m that can be harnessed by ground source heat pumps andin the future, by using the wellbore heat exchangers.GNS Science Report 2007/16 20