Feature

Future In Hot Water

Geothermal power is a major energy resource, but cannot be considered an unlimited, non-polluting one.

By Lynette Clelland

New Zealand's major geothermal fields are found in the Taupo Volcanic Zone, a wedge-shaped region extending from Ruapehu to the Bay of Plenty. Only one field -- Ngawha in Northland -- is found outside this region.

Fifteen high-temperature geothermal fields, of greater than 225^oC at depth, have been identified in the TVZ and Northland. There are at least ten other fields in the TVZ, plus many smaller thermal areas.

Geothermal energy is not easily transported over long distances, and must be used at or close to its source. The most practical alternative is to generate electricity, which is easily transmitted. In the future, developments using geothermal heat to produce methane, methanol and other energy sources from biomass may be possible.

Energy Resource

New Zealand's high temperature geothermal fields could generate between 2000 and 5300 MW of electricity. This includes the potential output of a number of fields which, for many reasons, will not be developed. Those fields which could most readily be developed in the near future would produce between 800 and 2300 MW_e -- still a significant resource.

New Zealand's major geothermal power stations at Wairakei and Ohaaki, operating since 1958 and 1989 respectively, generate 240 MW_e, or about 7% of Electricorp's total production. The Bay of Plenty Electric Power Board recently installed a 2.5 MW_e organic rankine cycle (ORMAT) plant at Kawerau, which uses waste hot water separated from the steam supplied to the Tasman Pulp and Paper Mill. The mill has been using geothermal steam for 34 years and is the single largest industrial user of geothermal energy in the world.

There are many other small scale uses throughout the country --  predominantly for residential and commercial heating, swimming and bathing pools, plus some minor horticultural and industrial applications.

Geothermal resources are sometimes grouped with solar, wind and other `renewable' resources. They are also widely believed to be more environmentally benign than other energy sources. These assumptions need to be examined more closely.

Output Not Unlimited

It is not correct to assume that a geothermal field can sustain a given output indefinitely. This may be true at very low levels of exploitation, but in major developments, heat is mined and temperatures will fall over the extraction period.

On the other hand, the rundown period may be longer than the economic life of the plant -- as is the case at Wairakei. There, output has dropped by about 20MW_e over a period of more than 30 years, well beyond the planned economic life of the plant. Because it keeps going reliably, it is now a very economic supplier of electricity.

Effects on the geothermal field and pollution are the main environmental issues related to geothermal developments.

Field-related effects include loss of geysers and flowing springs depositing silica, subsidence, increased hot ground and hydrothermal eruptions. Most development will result in one or more of these effects but the loss of natural activity is particularly important. There has been a cumulative loss of such features over the years, although not all from geothermal development.

Associated Pollution

Large geothermal developments on high temperature fields use steam flashed from the hot fluids produced from deep wells. The water left after steam separation is waste and contains dissolved pollutants. Older developments, like Wairakei and Kawerau, discharge this water to local rivers, where it causes heat and chemical pollution. This practice is now generally unacceptable, and the most recent development at Ohaaki reinjects its waste water.

The steam contains gases -- predominantly carbon dioxide, with small amounts of methane, hydrogen sulphide and mercury -- which are eventually discharged to the atmosphere. In the past, concern about such discharge has largely centred on hydrogen sulphide and mercury, but more recently its contribution to the greenhouse effect has been under scrutiny.

Ngawha is the gassiest field in New Zealand. Most other fields are likely to be similar to either Ohaaki or Wairakei. Mokai, the most promising geothermal prospect at present, is thought to be similar to Wairakei.

Geothermal resources compare particularly favourably with coal, which is the fossil fuel most likely to be used for new electricity generation in New Zealand. Geothermal power may even be able to do better -- a power station in California is reinjecting its waste gas. If this can be achieved economically, the steam-flash system for geothermal power generation (and other uses) would be largely pollution free.

Geothermal fluids are already used in pollution-free ways, by putting the total bore-flow through heat exchangers, and returning the fluid to the ground, or by using downhole heat exchangers. However, these methods are really only practicable for small scale developments.

In general, geothermal developments have a greater potential to be pollution free than fossil fuel developments.

There is no doubt that geothermal energy provides a major energy resource for New Zealand, and can be developed in an acceptable manner. The main problems for the future involve conservation, such as the need to protect the natural features in some fields from development,  and institutional factors, such as the  lack of electricity demand and unresolved issues relating to resource ownership and control.

A number of very promising geothermal fields have been investigated in recent years. Mokai is the best of these. The new ORMAT plant at Kawerau may signal a trend towards small, quickly established developments, utilising a range of fluid temperatures.

Lynette Clelland is a scientist with DSIR Geology & Geophysics