A practical guide to exploiting low temperature geothermal resources

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Thain, I.; Reyes, A.G.; Hunt, T.M. 2006 A practical guide to exploiting low temperature geothermal resources. Lower Hutt, N.Z.: GNS Science. GNS Science report 2006/09 72 p.

 Abstract: New Zealand has 435 MWe installed capacity for geothermal power production, the 7th largest in the world. However at 310 MWt it is only 13th in the world for direct heat utilization despite its vast resources. At present the main uses for direct heat in the country includes industrial process heat (67%), agricultural drying (9%), bathing, swimming and balneology (9%), space heating (8%), fish and animal farming (6%) and greenhouse heating (1%). More than 90% of the annual energy from direct heat uses in New Zealand is derived from wastewater from geothermal power stations (cascade); and little is made use of other sources such as hot spring systems. There are more than 100 warm to hot springs found all over New Zealand that could be harnessed directly or by drilling wells. However, less than 20% of these hot springs are being used at present. With the increased availability and efficiency of ground source or geothermal heat pumps, for space heating and cooling and domestic use, a number of low grade geothermal sources in New Zealand can be exploited including the natural conductive heat flux in the ground, below a few metres from the surface, where temperatures remain constant at 12-15 degrees C throughout the year. This is the most pervasive source of low temperature heat in the country. Other sources are warm waters from flooded abandoned mines and abandoned oil, gas and water wells. The cost of installing a domestic geothermal heat pump is twice that of a natural gasfired system, at about $12,000.00, although operating costs are lower for a heat pump with an annual cost saving of nearly $800.00. Kiln-drying of timber is a direct heat application cascaded from geothermal power production in Kawerau. In the kiln high temperature water of 80-140 degrees C, circulated in heating tubes, heats the air to the required drying schedule temperature. As the circulating air picks up moisture from the timber, part of the air is vented and replaced by dry air from outside. It takes about 570 kW of thermal energy to dry one cubic metre of Radiata pine lumber. The rate of applying this energy to the timber dictates the drying schedule, type of kiln used and the quality of lumber product. Another direct heat use, cascaded from the production of geothermal power in Mokai and Poihipi, is greenhouse cultivation of vegetables and orchids. The greenhouse heating system is dictated by the type of crops being grown. To heat a greenhouse the temperature of the geothermal water should be between 60-80 degrees C. The quantity of hot water required depends on the optimum growing temperature for the crop, size of greenhouse and the lowest outside temperature in the area. In installing a direct heat use system the following are required: borehole with casing, downhole and circulation pumps, transmission and distribution pipelines, peaking and backup plants and various heat exchange mechanisms. (auth)