Most of New Zealand’s geothermal energy goes to produce electricity, but it can be used for any processes where heat is required. The main non-electrical user is the Tasman Pulp and Paper Mill at Kawerau, which was built in 1957 and deliberately sited to take advantage of the underlying geothermal field. The heat is used for digesting wood pulp, drying timber and paper, and generating electricity.
Geothermal prawn farming
The world’s only geothermally heated prawn farm was established in 1987 on the banks of the Waikato River, next to the Wairākei power station. The first prawns were imported from Malaysia in 1988, and by 2005 the 5.8-hectare farm was producing about 20 tonnes per year. The farm heats its own water with heat exchangers, which draw heat from the power station’s waste water before it flows back into the Waikato River.
This is a good example of what is known as ‘cascade use’, where geothermal heat has a function past its primary purpose. Cascading improves the overall efficiency of a resource by using its waste products. In the case of the prawn farm, cascading also reduces the discharge of hot water into the river, where it can harm aquatic life.
Geothermal waters are used for heating greenhouses on a small scale (covering 10 hectares in total), especially for the commercial, out-of-season production of vegetables, flowers and fruit. This includes a large greenhouse (0.8 hectares) for growing orchids for export, and another set up to grow capsicums with heat from the Kawerau geothermal field.
Crop and timber drying
Drying lucerne (alfalfa) using geothermal energy was pioneered in Ōhākī in the 1970s. Geothermal heat from the Ōhākī power station has been used to make high-protein pellets to feed stock and to process dried juice into a protein concentrate. A timber-drying operation on site produces fence posts and poles, mainly for the local farming industry.
The Tasman Pulp and Paper Mill uses geothermal steam in heat exchangers to heat kiln air to 140ºC for timber drying.