The word ‘geothermal’ comes from the Greek and means ‘heat from the earth’. Deep inside the earth heat is released by the decay of radioactive elements such as uranium and thorium. Geothermal systems occur where circulating groundwater is heated and rises as a column of hot water to the surface.
There are two main types of geothermal system:
- Low-temperature systems, which range from 30ºC to 100ºC, are associated with areas of extinct volcanism, or with active faults.
- High-temperature systems are associated with active volcanism. They are heated by shallow reservoirs of molten rock (magma), and temperatures typically reach 200–300ºC.
The surface features of a geothermal system may be an isolated hot spring, mud pool, geyser, or area of steaming ground. A set of these features grouped together is called a geothermal field.
New Zealand’s geothermal features are world famous. In particular, the Taupō Volcanic Zone has one of the greatest concentrations of geothermal activity in the world, and is rivalled only by Yellowstone National Park in the United States.
Before Europeans arrived, Māori used hot springs for heating, cooking and preserving food, and for their medicinal and therapeutic properties. These traditional uses did not affect or modify geothermal features greatly.
European settlers soon discovered the scenic charms and healing benefits of thermal springs, and spa bathing became the basis of a rapidly growing tourism industry. Bathhouses and treatment centres were set up in Rotorua from about 1870. Between 1891 and 1904 the number of spa baths taken each year by visitors increased from 10,000 to 100,000. At first this demand could be met by the natural springs, but eventually shallow wells had to be drilled to increase the hot-water supply.
A geothermal hotel
Rotorua’s Millennium Hotel makes full use of its location. Steam from under the ground is used to heat rooms, tap water, and the swimming and spa pools. It is also used for cooling and air conditioning. Cooling can be produced when geothermal heat evaporates a low-boiling-point liquid.
Geothermal waters have been used for many years in Rotorua, and to a lesser extent in Taupō, to heat homes, businesses and institutions. It would have been efficient to develop municipal heating systems, but this was hindered by a lack of capital and political will. Instead, individuals and organisations drilled their own shallow bores, using small-scale, primitive heating systems that wasted a lot of the heat.
There were severe electricity shortages in the 1950s and restrictions were imposed. This encouraged people in Rotorua to drill wells to heat their homes. By the 1970s it became apparent that drawing off hot water was depleting the Rotorua reservoir and damaging local geysers and springs. Since 1991 geothermal extraction has been managed to protect surface geothermal activity. Recent trends have been towards communal systems, with 10 or more households typically sharing a well.
A major use of geothermal energy in Rotorua is pool heating. Swimming pools can contain clean, fresh water warmed by heat exchangers. Mineral pools use the geothermal waters.