Geothermal energy is heat generated and stored in the ground. New Zealand has an abundant supply of geothermal energy because we’re located on the boundary between two tectonic plates. This means that the Earth’s crust is thinner and the hot mantle below is much closer to the surface. Centuries before we began turning geothermal energy into electricity, Māori were cooking, washing and warming themselves with heat directly from the earth’s core. These days geothermal energy provides 17% of our electricity and about 8 PJ directly as heat.
How geothermal energy works
For producing electricity geothermal fluid (a mixture of high pressure water and steam) from wells several kilometres deep is piped to a central generation plant where it is turned into steam or increasingly commonly used to vaporise another fluid in organic Rankine cycle (ORC) systems. The steam or vapour then drives the turbine generators poducing electricity. Used geothermal fluid is re-injected through wells back into the geothermal field to maintain its pressure and structure although some fluid is still discharged into rivers by older plants. New Zealand uses hot water and steam directly for industrial processes including pulp and paper-making, wood processing, dairy manufacturing and heating greenhouses.
Pros and cons of geothermal energy
Geothermal energy supply isn't dependent on weather conditions, making it consistent and reliable. It’s a renewable resource, but it needs careful management and monitoring to control reservoir water and pressure levels and prevent land subsidence and depletion.
Geothermal fluids also contain gases and minerals, though amounts vary from field to field, and the amount released depends on the design of power station. While electricity generated from geothermal energy produces some greenhouse gas emissions the overall emissions intensity of geothermal electricity is about one-quarter of that of the cleanest natural gas-fuelled power station.
The future of geothermal energy
Total geothermal electricity capacity in New Zealand stands at over 900 MW. It has been estimated that there is sufficient geothermal resource for about another 1,000 MW of electricity generation.
We could also use more geothermal energy directly, for example as industrial process heat, or by finding uses for waste heat from geothermal power stations.
Energy in New Zealand 2016 – Ministry of Business, Innovation and Employment website
Using geothermal energy in your business - EECA BUSINESS website
New Zealand Geothermal Association website
GNS Science website