Using geothermal fluid for generation


The geothermal reservoir holds a mixture of brine and steam; predominantly brine. Brine is water that is high in naturally occurring dissolved minerals such as: silica, boron, sulphates, chlorides and sodium bicarbonate. The steam carries with it a small proportion of naturally occurring gases such as: carbon dioxide, hydrogen sulphide, argon and methane. Compared to other geothermal fields in New Zealand, the Ngawha geothermal fluid is unique because the amount of steam is low, but it has a high content of carbon dioxide (CO2).

The temperature of the fluid has been measured at up to 301°C in the reservoir’s central area, although the temperature of the brine generally used in generation is 190°C. 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 that is then converted into electricity.