The word geothermal comes from two Greek words – ‘geo’ meaning of the Earth, and ‘thermal’ meaning heat. Therefore, geothermal energy is energy produced from the heat of the Earth. If we took a slice through the Earth, we would find that it was made up of a number of layers with an extremely hot core at the centre.
The deeper we go, the hotter it gets. When water seeps into the ground, it percolates until it gets to a layer of rock that is impermeable, meaning it will not let any water seep through. If the rock that the water is surrounded by is hot, the water gets heated, and if there is a big enough reservoir of hot water, this resource can be used to generate electricity.
A geothermal power plant drills wells into a reservoir and uses either the steam or hot water from within the Earth to generate energy. In New Zealand, we have a number of geothermal power plants, most of them in the central North Island, like the Wairakei geothermal power plant near Taupo, which has wells sunk to an average depth of 600 metres and uses hot water with temperatures of 200–250°C. This hot water is used to generate dry steam, which can drive large turbines used to generate electricity.
Steam makes the large blades of the turbine spin, transforming heat energy into kinetic energy. The turbine is attached to a generator, which uses electromagnets to transform the kinetic energy into electrical energy that we use. At present, geothermal energy supplies around seven percent of New Zealand’s energy, although there is potential to supply much more.
Processing waste water
One of the problems faced with using hot water in electricity production is what to do with the water once it has been though the plant. Two methods are used – discharge and reinjection.
Discharge is when the spent water is released into a surface water source. For example, Wairakei discharges water directly into the Waikato River. Originally, this was the only method used to dispose of the hot water, but problems arose.
These include a change in water chemistry as dissolved solids from the geothermal water entered the river (an increase in elements such as arsenic) and changes in pH level and water temperature, as well as ground subsidence and reduced geothermal activity from the removal of too much water. Changed water temperatures also have an effect on the nearby flora and fauna. To counter these problems, the amount of water discharged was reduced. Currently, around 60,000 tonnes of water are discharged into the Waikato River daily.
Reinjection returns the water to the Earth, either by injecting it straight back into the geothermal system itself or into deep ground water. Wairakei reinjects around 46,000 tonnes a day to reduce the problems associated with discharge
A fact sheet on geothermal energy from the NZ Energy Efficiency and Conservation Authority