Ngawha’s geothermal field, the only high temperature geothermal resource in New Zealand outside the Taupo Volcanic Zone, is thought to be between 20 and 40 square kilometres in area. It has few natural surface features, apart from the springs adjacent to Ngawha Village which, although modest in flow, are highly valued for their cultural and therapeutic properties.
The resource is characterised by very porous layers of rock beginning at about 600 metres below the surface which are saturated with water and heated from below. This ‘geothermal reservoir’ is sealed above by layers of solid caprock. The caprock allows considerable pressure and temperature to develop as the water is heated. Up to 230 degrees Celsius has been measured in the reservoir’s central area. Although this is well above boiling point, steam is not present in the reservoir because of the very high pressure.
Steam is produced only when the water is brought to the surface via a well bore, which allows the pressure of the rising fluid to reduce. There is ample pressure within the reservoir to drive the water up the well and in this way a mixture of steam and hot water is produced, both of which contain considerable energy which is then converted into electricity.
© Top Energy Limited 2010