The original settlers of New Zealand, the Maoris, who arrived around 800 years ago from the central Pacific, used the natural geothermal springs for bathing and the very hot springs and geysers for cooking. Now the country uses this natural energy to supply around three per cent of its power needs, and is seeking to further develop its potential.
New Zealand is on the south-west corner of the Pacific "ring of fire", the chain of volcanic activity which extends up through the Pacific Islands, Indonesia, the Philippines, Japan, Alaska, the west coast of the USA, and down to the tip of South America. This volcanic activity arises from weaknesses in the earth’s crust where the Pacific tectonic plate meets neighbouring plates.
There have been a number of major volcanic eruptions in the region, the most recent being the Tarawera eruption of 1856 and the largest being the eruption which formed Lake Taupo around 400 AD. The ancient Chinese recorded the impact of the ash emitted by the latter on their weather. Where there is volcanic activity there are often geothermal fields capable of commercial development for power generation.
New Zealand was one of the early developers of geothermal power with the Wairakei project being the first large scale development of a water dominated geothermal field in the mid-1950s. There has been a second phase of geothermal development since the mid-1980s, stimulated largely by deregulation of the electricity industry which allowed the distribution companies to be more active in generation development.
There is geothermal activity spread over both islands of New Zealand with the main high temperature fields of volcanic origin in the Rotorua-Taupo area in central North Island. Most of the other geothermal activity in the country is of tectonic origin and the heat flows and temperatures are not sufficient for commercial power generation.
Early developments
The first geothermal fields to be developed were Kawerau and Wairakei in the Rotorua-Taupo area. The Kawerau field is 20 km inland from the town of Whakatane and is the site of New Zealand’s first pulp and paper mill. Here, geothermal steam is used directly in the mill for paper drying and for boiler feedwater heating as well as for electricity generation. The wells produce two phase geothermal fluid which is flashed to produce steam leaving the brine to be discharged to the ground. More recently some reinjection wells have been drilled and there is a programme to introduce full reinjection. Despite 40 years of abstraction, the field has maintained a good pressure and should have many years of further life.
In 1989 the local power company, Bay of Plenty Electricity, installed an Ormat binary plant of two units with an output of 2.2 MW (known as TG1) to utilise the energy in the 172°C brine from one of the wells. This was followed in 1993 by a second Ormat unit of 3.6 MW (known as TG2) on a separate brine flow. Both units utilise isopentane as the working fluid and are air cooled.
Wairakei, 9 km to the north of Taupo and first commissioned in 1959, was the first major geothermal power station in New Zealand. The Wairakei geothermal field consists of a pumiceous pyroclastic reservoir underlain by ignimbrites and capped by lacustrine mudstone. The upflow temperature is 270°C with typical production temperatures of 230°C. The wells generally produce two phase flow, with dry steam in some areas. This was the first wet geothermal field in the world to be utilised for commercial power production.
Their are two stations at Wairakei utilising steam at intermediate pressure (3.5 bar) and low pressure (0.1 bar). The current net output is 153 MW and the annual energy production 1300 GWh. Originally there were high pressure (12.6 bar) turbines as well, but these were moved to Ohaaki when the reservoir pressure declined. The station design originally incorporated a heavy water plant for the British nuclear programme, but this was dropped before construction.
However, the Wairakei power station is a major source of contaminants in the Waikato River. Around 1100 t/h of steam is condensed in the turbines’ spray condensers which use the river water directly, and around 3500 t/h of brine is discharged to streams which flow to the river. A programme is therefore underway to reinject the brine and the feasibility of further generation from the brine energy is also under review.
In response to the oil crises of the 1970s, the government investigated and drilled most of the known geothermal fields in the Taupo-Rotorua area. Many of these showed commercial potential and the 13 km2 Broadlands field was selected for the development of the Ohaaki geothermal station. This station is owned and operated by Contact Energy, one of the two state-owned generating companies, which operates the station remotely from its other geothermal station, Wairakei.
A total of 51 wells with an average depth of 1000 m were drilled, and a 103 MW station was completed in 1989. The 24 production wells produce two phase fluid which is separated to a steam flow for the power station and a brine flow which is pumped to eight reinjection wells. The 12.5 bar steam is fed to two 11.5 MW back pressure turbines which were originally installed at Wairakei. Steam from these turbines at 3.5 bar, supplemented by low pressure steam from the wells, supplies two 47 MW Mitsubishi condensing turbines.
Ohaaki was the first large scale reinjection of brine in New Zealand and the first use of a concrete natural draught cooling tower. Due to field pressure decline the station now operates at 80 MW giving an annual energy production of around 750 GWh. Despite the reinjection of brine and condensate, there has been considerable subsidence in parts of the steam field and areas adjacent to the Waikato River.
New capacity
The McLachlan power station, located about 5 km from Taupo, is the first privately owned power station in New Zealand and has been developed by a joint venture of a Taupo businessman and the Auckland electricity distribution company Mercury Energy. These partners have taken an unconventional approach in order to reduce the capital costs – it can be hard to reach financial closure for a geothermal project in New Zealand’s wholesale market.
The power plant is sited on one edge of the Wairakei geothermal field, and is drawing from the same resource as the Wairakei geothermal station. In that area of the field there is a relatively shallow steam cap and the station takes dry steam from four wells of around 750 m depth. The total project cost was $57 million (NZ$81 million) and full commercial production started in June 1997.
Because the station draws from a resource utilised by an existing station there were considerable difficulties in obtaining the necessary planning consents. The consents to draw steam were granted under the now repealed Geothermal Energy Act following protracted negotiations with the government-owned generating company and public hearings. The consents to draw steam are not sufficient to operate the station at the full output of 53 MW, and the station operates at minimum load during the night when electricity market prices are at their lowest levels and at full output during the day. There were also problems with obtaining the air discharge consents when a local group appealed the consents granted, resulting in a six month delay in project commissioning.
The McLachlan plant has a single 55 MW gross output Fuji condensing turbine of conventional design including an underslung shell and tube condenser and a hydrogen cooled generator. This gives a turbine hall configuration very similar to any other steam unit, with the operating floor high above ground. The turbine and generator were originally destined for a station on the Geysers geothermal field in California, but were never installed due to the over exploitation of the field and consequent pressure decline becoming apparent before the installation commenced.
The unit was stored under controlled conditions for a number of years and refurbished for 50 Hz operation and the lower steam pressure before shipment to New Zealand. This mainly involved changes to the turbine blades and the installation of a much larger non-condensable gas extraction system. Cooling takes place by conventional mechanical draught cooling towers, with the condensate used as tower make-up water. The blow down from the cooling towers and any excess condensate is pumped 2 km to a shallow well.
As part of the de-rating from 3600 r/min to 3000 r/min for 50 Hz operation, the generator voltage was reduced to 11 kV allowing use of standard New Zealand switchgear. The generator output is stepped up to 220 kV and fed into the Wairakei-Whakamaru 220 kV line, which is part of the National Grid, by a short simple Tee connection.
The turbine and generator came with only the control systems closely associated with their operation, and a new computer based overall station control system employing sophisticated graphic interfaces has been installed. Considerable work was necessary to uprate and adapt the electrical auxiliary systems to meet the current design codes, particularly as relates to the hazardous areas around the hydrogen cooling systems. The station is manned at all times, and all the maintenance work will be carried out by contract.
By utilising a second hand refurbished plant and new auxiliary and control systems, the joint venture has been able to complete the station with full manufacturers warranties at a considerably lower capital cost than if new plant had been utilised. The full engineering of the station and the detailed design of the changes to pipe work and the electrical systems was undertaken by the New Zealand consultant HPM Power.
The Rotokawa geothermal field is a deep high-temperature field covering 25 km2 and located 12 km north of Taupo. The surface manifestations of the field include small hot springs and an acidic lake which contains large quantities of colloidal sulphur from the oxidation of hydrogen sulphide as it bubbles to the surface. Eight wells were drilled between 1966 and 1986 as part of the government’s programme to assess the region’s geothermal resources. Most of these have since been capped and abandoned, either because they were not commercial producers or because of casing corrosion. The field has an estimated capacity of 100 to 200 MWe, and is being developed by a 24 MW station in order to allow careful monitoring of the resource before full exploitation.
The project is unique as it involves the participation of the indigenous Maori people in the development as a joint venture partner with Auckland electricity distributor Power New Zealand. The Tauhara North No. 2 Trust owns the land over the middle of the geothermal field, including the land around the well RK5 which is the best of the wells drilled by the government.
The project has two production wells of around 2000 m depth producing two phase fluid which is piped to a separator at the station. Steam is separated from the brine at 23 bar and both streams are used for electricity generation. The condensed steam is pumped up to the brine pressure, combined with the high pressure brine, and reinjected with no further pumping. There are three reinjection wells of 500 m depth, one of which is one of the original field exploratory wells.
In the process of selecting a turnkey contractor for the power station, the alternatives of condensing steam turbines and binary plant were compared. The plant finally selected is configured to use steam turbine and binary plant, and obtain the best of both technologies. The contractor for the 24 MW plant is Ormat Industries Ltd. of Israel.
To maximize the benefits of the high steam pressure, a back pressure turbine of 12 MW output is utilised to drop the steam pressure to 1.5 bar. This low pressure steam is condensed in two binary units of 4 MW output each. This configuration, called a "geothermal combined cycle unit" by Ormat, has the advantage of the low capital cost of a simple back pressure turbine and of condensing the steam in a heat exchanger where steam wetness is not a problem. There is a third binary unit also of 4 MW output utilising the hot brine flow, cooling it from 219°C to 150°C. The motive fluid in the binary units is isopentane and cooling is by air radiators.
Future development
There are a number of geothermal fields at various stages of development and some projects may proceed despite the low wholesale electricity prices, which are averaging around 3.0 c/kWh. In the far north the Ngawha field is being developed with an initial 8 MW Ormat binary plant due for commissioning in early 1998. The Mokai field is probably the best geothermal field in New Zealand with an output of 200 to 400 MWe. A contract has also been awarded to Ormat for a 50 MW plant of similar configuration to Rotokawa, and finance is now being finalized.
Resource use and planning consents have been granted for drilling the Rotoma and Taheke fields near Rotorua and have been filed for the Tauhara field near Taupo. There are also proposals to further develop the Kawerau field. Geothermal energy has met around three per cent of New Zealand’s electricity needs for many years, and has the potential to supply up to ten per cent. However, major impediments exist, including an excess of generating capacity in the short term leading to low wholesale electricity prices, and difficulties in obtaining the necessary planning and resource consents.
TablesTable 1. New Zealand’s geothermal generating plants Table 2. The McLachlan power station Table 3. The Rotokawa power station
