The plate tectonic theory provides a unified basis for explanation of many geophysical and geological phenomena. This theory, which gained widespread credence as evidence for it was gathered during the nineteen sixties, postulates the existence of moving crustal plates 100km thick on the earth’s surface. These plates have their origins in the middle of deep oceans and are destroyed where they converge at active continental margins; at the convergence, one plate is usually thrust beneath the other forming deep ocean trenches, causing deep earthquakes and, as the descending plate melts, volcanoes. Many of New Zealand’s physical characteristics can be ascribed to its position at the meeting of the Indian and Pacific plates.
Seismic events (earthquakes) and volcanoes both constitute hazards to man and his creations. In New Zealand, scientists have invented base isolation devices for preventing damage to buildings and bridges and earthquake engineering has become an important knowledge cluster.
Not all technological effort devoted to these phenomena has been to protect against them though; positive advantage has been taken of the surface heat flows associated with volcanism by the harnessing of the geothermal activity in the North Island volcanic zone as an energy source. This has led to development of expertise among New Zealand technologists, which has been widely employed in other countries with a similar geophysical setting.
In November 1918, Volume 1 of the New Zealand Journal of Science and Technology carried an abstract of an article from the journal, ‘Engineering’, describing the Lardarello natural-steam power plant. Near Voltera in Tuscany, Italy, an industry had been established based on the extraction of chemicals from the naturally occurring steam. A demand for power existed because the region is an industrial one and experiments had been carried out from 1897 using the steam in engines – initially in piston engines and then in 1912 in a 330 horsepower (250 kilowatt) turbo generator. In 1916 a turbine power station of 10,000 horsepower (7.5 megawatts) was built.
Steam for the generating plant had been obtained by sinking 16 inch (400 mm) bores 200-400 feet (60-120 metres) into the ground and lining them with iron tubes. The natural steam was not used directly but in heat exchangers because of the high concentration of non-condensable gases. Experiments in direct use of the steam were being carried out at another locality in the same region where the quantity of gases was lower.
The New Zealand Journal (S H Jenkinson) commented, "These experiments are interesting to new Zealanders because of the much more extensive and active thermal regions of Rotorua. The question of adequate power may at once be granted, but that of cost compared with water power or a large coal-power plant at Huntly would need close examination. If natural steam superheated to 400-500`F could be obtained around Rotorua, directly suitable for turbines, it would probably prove the cheapest source of power supply for the North Island; but experiments and investigation are needed. Transmission advantages presumably favour Huntly."
The active volcanic region of New Zealand extends from Ruapehu through Taupo and Rotorua to White Island in the Bay of Plenty, an area about 250km long and 50km wide. Within the region there are numerous localities with hot springs and in some cases fumaroles (steam discharges), geysers, and boiling mud pools. Volcanic eruptions have occurred on a number of occasions in historic times.
The agricultural scientists, Bruce and Shorland, had advocated use of the heat resources of the volcanic region in the early 1930’s. They were supported by the geologist, Grange, in a bulletin published in 1937 in which he stated that, without doubt, drilling could yield steam for power production. These scientists had in mind the Lardarello development where, by 1944, before retreating German forces wrecked the installations, the Italians had installed an electricity generation capacity of 135 megawatts. Captain F. Tuck of the second NZEF, a Ministry of Works engineer, visited and reported on the Lardarello installation in 1944, shortly after Italy capitulated and, in 1948, the Commissioner of Works and the Director of the New Zealand Geological Survey visited the area.
By 1949 an investigation of the New Zealand resource by DSIR scientists was underway, with measurement being made of temperatures, composition, acidity and flows of both water and steam from bores and natural vents. A plan was proposed for a 5-year scientific investigation to acquire basic geological, geophysical and geochemical data relating to the whole geothermal region in order to gain an understanding of the resource. Geophysical equipment was acquired.
But then came a change of pace. The scientists were not to be allowed to carry out a completely thorough and systematic investigation. The need for power was regarded as too urgent. Risks would have to be taken in order to assess the feasibility of large-scale power production. In what some have regarded as a lucky choice, it was decided to concentrate investigations in the Wairakei area, where the DSIR had, by drilling to 500 ft, been successful in obtaining steam for the Wairakei Tourist Hotel. An area of 13 miles square was chosen as the focus of the studies.
The most wide-ranging intensive technological investigation in New Zealand’s history began in January 1950. A great number of unknowns existed at this point. Not only did the availability of steam in sufficient quantities to justify a power station have to be proven, but its use had to be shown to be technically and economically feasible.
The Ministry of Works was brought on to the scene along with three rigs for prospecting drilling. As is still the case today, the only sure way of ascertaining that steam can be obtained from a geothermal field is to drill and show that steam is discharged. Scientific work can indicate where drilling might be successful and its ability to do this has improved over time but it cannot substitute for drilling. Drilling however is a very expensive and time consuming activity (a typical well drilled to 1300 metres costs about $750,000 (1980) ) and thus detailed scientific prospecting work to delineate potential steam fields can be justified on economic grounds. Geological and geophysical work is supplemented by that of geochemists, radio chemists and mathematical modelers.
When work started at Wairakei, expertise had still to be developed in the techniques for drilling for steam. Essentially the method used is the same as that for oil drilling but particular emphasis is needed on coping with high temperatures and the wells are completed in different ways, with geothermal wells not having casing to the bottom of the hole.
The prospecting drills could only drill four-inch diameter holes to a depth of 500-700 feet. Most of the holes were drilled along a prospect line known as the D line running from the Waikato river through the Waiora valley for about three miles. High temperature steam (greater than 180`C) was encountered in most holes. By March 1952 there were sixteen holes and, when a larger drilling rig came available from the Mines Department, the holes were drilled to 1000 ft. Spectacular results were obtained when the first of the development wells was blown, with a column of wet steam shooting 300-400 feet with occasional rock fragments going even higher. It became evident to the geologists that unlike Lardarello there was no major structural trap for the steam, which came from the bores in company with water.
The differences between the Wairakei and the Italian fields meant that the New Zealand technologists were on their own. They would have to do original work to cope with the particular conditions at Wairakei. Because the steam discharged at Wairakei was wet, the engineers carried out experiments and devised a steam/water separator. The chemists made encouraging measurements, which showed that the steam had low gas content and was not greatly corrosive, and staff of the Dominion Physical Laboratory designed a geothermograph for measuring temperatures at depth.
When a successful bore had been drilled it was allowed to discharge to atmosphere between measurements. An important reason for this practice was to establish that the bore field was capable of sustained discharge. No indications occurred to suggest that the heat supply was in any way being diminished, but the noise level from even a small discharging bore was great enough to cause deafness, headaches, and dizziness in people working nearby. With a large number of bores discharging, the noise became a general nuisance. Measurements were made of the nature of the noise and a silencer was developed which was effective in reducing it, particularly in the more dangerous upper frequency range.
As investigations proceeded, the nature of the resource was becoming better understood. Wairakei is a hot water field in contrast to the dry steam or vapour dominated system that occurs at Lardarello. The temperatures at depth tend to the value corresponding to the boiling point for the hydrostatic pressure at that depth (ie the pressure due to the head of water). Thus at a depth of 500 metres temperatures of up to 250`C can occur. When a hole is drilled into a permeable location and the head of water removed the bore will "blow" and the hot water flashes to steam.
The promising results led to the ordering of heavier drilling equipment specially designed for the higher temperatures and pressures expected at greater depths.
The target became to obtain steam sufficient to produce 20,000 kilowatts (20 MW) of electricity.
By late 1952 the heavier drilling rigs were at work and one hole had reached a depth greater than 3000 feet. Overseas expertise in the form of the British consulting firm of Merz and McLennon was engaged to look at the design of a power station to use the geothermal steam.
At this point another embryo technology intruded – nuclear power. In a "thermal" nuclear reactor (the most common type) the chain reaction, which involves the splitting by neutrons of atoms of uranium or platinum, is sustained because further neutrons are produced in the fission process. These further neutrons must be slowed down, or moderated to "thermal" energies before they induce further fissions. It had been found that a very efficient agent for producing this slowing down (acting as a moderator) was heavy water.
In heavy water the atoms of hydrogen have been replaced by its isotope deuterium. Heavy water occurs naturally as a very small fraction of ordinary water (one part in 5,400 by weight) and at the expense of the application of large quantities of energy it can be separated out. One way to do this is by distillation. Geothermal steam appeared to offer a very cheap source of heat to carry out this distillation. Arrangements were made with the United Kingdom Atomic Energy Authority to investigate the production of heavy water for its nuclear power programme using the geothermal steam as the energy source in a plant that would also produce electricity. This use of geothermal steam as a heat source for heavy water distillation was not a new idea. S.H. Wilson, DSIR scientist, had mooted it in 1946.
The investigations entered a new phase – greater depths were being achieved in drilling and greater volumes of hot water and steam were being produced successfully; it was evident that sufficient was available. The emphasis shifted to the problems associated with the utilization of the steam. Foundation conditions were examined as part of the exercise to determine the site for a powerhouse and test rigs were established to measure corrosion on various metals and alloys to establish the most suitable for construction of the heavy water plant and the steam turbines.
It was found that mild steel shows good resistance to geothermal fluids as long as no oxygen is present.
A joint company "Geothermal Developments Limited" was established between the British and New Zealand Governments to direct the scheme. Preparation of designs and specifications for the heavy water plant in conjunction with a 45MW electricity station proceeded.
Cost estimates for the heavy-water plant were prepared, based on the more detailed design, and these proved higher than expected. The Atomic Energy Authority took fright, decided that they could get the heavy water they wanted cheaper from an alternative source in the USA, and withdrew from the project. This left all the steam available for electricity production. Rather than redesign the project, and in the face of what appeared to be an urgent demand for more generating capacity, it was decided to proceed with the existing design and to substitute further turbines in the scheme where the heavy water distillation plant was to have been. This brought the power station capacity up to 69MW.
Because the winning of the steam was proceeding successfully the consultants were asked to report upon the feasibility of extending the project. They recommended in November 1956 that further plant should be installed to raise the total capacity to 192MW.
The Government accepted this recommendation in September 1957.
In the design of the power station a number of technical decisions had to be made. Essentially conventional steam station technology was adopted with allowances being made for the relatively poor quality (low temperature and pressure) of the steam. The site chosen for the power station is adjacent to the Waikato river; steam separated at the well heads in the bore or produced by flashing hot water is piped at three different pressures down to the power station. Jet condensers in which cooling water taken from the Waikato is mixed directly with the outlet steam were chosen since there is no need to recycle water to the boiler as would be done in a conventional steam station. The cooling water is discharged directly into the Waikato River.
In the original design for the first stage of the power station, steam turbines operating over two different pressure ranges were chosen. The exhaust steam from the two 6MW topping sets was to be fed to the distillation plant, which in turn was to exhaust to four 11.2MW condensing sets which had an inlet pressure of 1lb per square inch. When the heavy water plant was abandoned, two intermediate pressure sets, also of 11.2 MW, were substituted so that the total stage 1 output of 69MW was produced in a somewhat complicated arrangement from 8 turbo-generater sets.
In stage II, two topping sets of 11.2MW were added together with three 30MW mixed pressure condensing sets which operated over the same pressure range as the medium and low-pressure sets of stage I. The first machine, one of the 6MW back pressure sets, was commissioned in November 1958 and both stages of the station were in operation by October 1963.
From the 1963-64 year, Wairakei has operated as a base-load station consistently contributing over 1000 GWh to the New Zealand power system. Modifications to the steam collection system have been made over the years to increase the efficiency of steam use, counteracting the reduction in availability of energy from the field as pressures in the bores have dropped.
Steamlines at Mokai
When the initial scientific investigation of the volcanic region had been narrowed down to the Wairakei area, the scientists had still been hopeful that the choice of a thirteen-mile by thirteen-mile section for study would allow general conclusions to be reached about prospecting methods and the nature of geothermal fields. The geophysicists carried out seismic magnetic, gravity and resistivity surveys but as the emphasis came to be placed upon obtaining a return for the money expended on drilling, efforts became concentrated on the potential production area and the broader question of establishing the geothermal potential of the whole region was lost sight of.
None of the geophysicists’ methods proved particularly enlightening in the Wairakei region though one in particular, that of ground resistivity measurement, might have been expected to prove effective in a geothermal zone where the presence of high temperatures and dissolved salts in the ground water should have lowered resistivity appreciably.
In the early 1960’s ground resistivity measurement was again tried in hydrothermal areas. To improve the measurements, electrodes were put down to the water table to increase current input to the ground. This improved sensitivity but was costly. The breakthrough came when a newly available method of measuring a small voltage (the vacuum-tube voltmeter) was found to simplify the measurement of ground resistivity. By 1965 a survey had been made covering from Taupo through to Waiotapu. (The latter location because of its high natural heat flows had been chosen as the next potential power station site after Wairakei but the output of the test bores was poor.) As a result of this survey the value of the prospecting method became apparent.
At Broadlands, about 20km north-east of Wairakei, there was little surface expression of thermal activity but the 5-ohm-metre resistivity contour gave a definite indication of the presence of a geothermal field and the geophysicists were able to tell the Ministry of Works where they should drill to find heat at depth. Since then the technique of resistivity traversing has been used extensively in New Zealand and elsewhere.
The long story of the development of the field at Broadlands can be regarded as starting in October 1965 when drilling of the first well started. The well failed to sustain a discharge but a temperature of 280`C was recorded at depth. More success was obtained with the next well which was bored the following year; a permeable zone was encountered and discharge with energy content equivalent to water at 245`C obtained. The potential steam output of the well was gauged to be 60 tons/hour at 7 bars gauge pressure. A further 32 wells were drilled in the period up until 1976, of which about half were "producers".
Investigations did not proceed smoothly over this period however. Although by 1970 sufficient steam was available to justify a power station – a stop was called when Maui gas was discovered and it seemed probable that it would be used for power generation. The promoters of geothermal power were thwarted by the Electricity Department’s belief that since it had only limited design resources it should concentrate on big power stations which could make a significant contribution to meeting the steadily increasing electricity demand. Geothermal stations were too small.
A limited drilling programme was then reinstated since an active drilling crew was necessary to carry out maintenance at Wairakei, and studies were made of other possible uses of the steam. Once again the production of heavy water was considered – this time using a different process involving exchange with hydrogen sulphide. The initiative fro this study came from New Zealand Electricity engineers who had just returned from a secondment to the United Kingdom Atomic Energy Authority where a design study was carried out on a heavy water moderated reactor of potential interest to Australia and New Zealand. Investigations into nuclear power, like those into geothermal power, had been halted because of the Maui gas discovery. Although the heavy water production investigation came to naught, pressure from the Ministry of Works and the oil price hike of 1973 led to a geothermal station at Broadlands being recommended for 1987 commissioning in the 1974 Power Plan.
Environmental concern had been manifested in the Manapouri issue, in protest about the damming of the Clutha, in the establishment of the Commission for the Environment, and in the first stirrings of protest about nuclear power when in 1974 the whistle was blown on geothermal power. Officials within the Government were reluctant to acknowledge that there was anything significant in a report produced within DSIR but, given the spirit of the times, the emergence of the story was inevitable, although without this particular report it might well have appeared at a later date, and in a less eloquent form.
A visiting American scientist, Robert C. Axtmann, at the DSIR’s Physics and Engineering Laboratory, produced “An Environmental Study of the Wairakei Power Plant” after several monthsÕ study and discussions with a wide range of people with experience of the Wairakei project. Axtmann was able to draw on the extensive knowledge developed within the DSIR on geothermal phenomena and the effects of the Wairakei plants effluents on different systems.
He noted that geothermal power had been spawned in an era when environmental sensibilities were less acute and that it had won a public reputation for "cleanliness". He did not say, though it clearly emerges from his report, that, in comparison with other power production technologies, the environmental impact of the Wairakei plant when taken in relation to the amount of power produced is considerable.
Chemical and thermal effluents from the bore field and power station were the main source of impacts. Of particular significance was thermal pollution of the Waikato River. For a power production of 143MW(e), 805MW of heat were discharged into the river. Axtmann attributed the deterioration of a trout fishing between Huka Falls and Lake Aratiatia to the discharge of this heat together with hydrogen sulphide in the turbine condensate. Other effects on the river resulting from mercury and arsenic concentrations in the river could exceed World Health Organisation levels for potable water. Axtmann also considered that dissolved carbon dioxide in the condensate could accelerate weed growth in Lake Aratiatia.
Gaseous discharges he drew attention to were those in the powerhouse, which gave use to unpleasant levels of hydrogen sulphide and water vapour from the silencers at the time of station shutdown, which increased local fogging.
Axtmann, a technologist himself, had produced a penetrating commentary from the viewpoint of the environment-conscious seventies on the impacts of the Wairakei geothermal power plant. Clearly though he was fascinated by the technology and in sympathy with it as the following extract from his section on aesthetic considerations shows.
"If a tramper on State Highway 1 pauses at dusk 5 miles north of Taupo on a moist day with a stiff breeze, he is treated to an eerie sight of haunting beauty. Scores of fleecy plumes arc skyward only to be seized and devoured by green demons that haunt the boughs of imperial confers; bundles of silvery bull whips, cracked by an invisible giant who lurks behind the western hill, are caught in stop-action as they rise and fall in unison. It is an odd amalgam of technology and nature, of the Tin Woodsman of Oz and the Sorcerer’s Apprentice, gently underscored by the whispering, slightly syncopated "whuff-whuff … whuff-whuff" of the well-head silencers.
The Broadlands field, if it is developed, could look like an oil refinery and would be the worse for it. Improvements in geothermal technology since Wairakei was built, though promising greater efficiency, will probably be less visually appealing.
The Wairakei power station is not usually visible from the Highway, but hikers in the scenic reserve across the river have an excellent view. Fossil-fuelled stations are visual abominations; nuclear plants, those that are imaginatively designed, can evoke planetaria; the Wairakei power station is somewhere in between. It is "all right." The fierce noise levels (up to 90 dB) that permeate its interior, the reek of H2S that seeps off the roof and down into the offices of the station’s supervisory force, and the ugly pile of insulators, conductors, and transformers that constitute a switchyard, are all tucked away from public exposure. Even the transmission lines make a modest exit through a stand of stately Pinus radiata."
By the time that a decision was made to proceed with a power station at Broadlands, the field had been thoroughly studied from many angles. When the environmental impact report appeared the authors had clearly taken on board the lessons preached by Axtmann and the report is apologetic for the technology’s shortcomings. Extensive measures, including re-injection of the waste fluid were proposed to alleviate the impacts, making the proposed plant considerably different from that at Wairakei and somewhat more costly.
Since the construction of Broadlands, now known as Ohaaki, the deregulation of the electricity sector has led to further geothermal plant being committed on a number of fields.
The Mokai geothermal station uses a binary cycle to maximise the energy recovered from the steam.
A J Ellis, The Development of Geothermal Power in New Zealand is Different Chemical Milestones in New Zealand History Clerestory Press, 1999
Location Map Credit: Stagpoole,V. M. and Bibby, H. M., 1998. Electrical resistivity map of the Taupo Volcanic Zone, New Zealand; nominal array spacing 500 rn, 1:250 000, version 1.0. Institute of Geological & Nuclear Sciences geophysical map 11. Institute of Geological & Nuclear Sciences Limited, Lower Hutt, New Zealand. (note: the low resisitivity at the coast is due to seawater)
Resistivity Map Credit: Risk G.F. 1983: Delineation of Geothermal Fields in N.Z. using Electrical Resistivity Prospecting. Proceedings 3rd ASEG Conference, Brisbane, November 1983, p 147-149. (Note the point data gives drillhole temperatures measured between 0.7 and 1.0 km depth)