Ohaaki will be a spectacular sight

Tu Tangata, Issue 29, 1 April 1986, Page 40

Ohaaki will be a spectacular sight

Right on schedule for its planned commercial operation in October 1988, the Ohaaki geothermal power station between Rotorua and Taupo won’t come onstream with a hiss and a roar like its counterpart at nearby Wairakei.

But although the most modern methods of steam reinjection will largely dispense with the familiar steam clouds of Wairakei, the station will still be a spectacular sight because of the massive 105 metres of cooling tower that will be an integral part of the operation.

The first stage of the Ohaaki development will add about 100 megawatts to the national grid, with 50 megawatts becoming available after October 1988 and another 50 MW after March 1989.

Should it be considered feasible, plans also allow for a possible second stage development, lifting total output to 150 megawatts. However, there is no commitment to this yet. There is a long history behind Ohaaki. It is the first power station to be built on land with Maori ownership the Ngati Tahu people. Their small Ohaaki marae, consisting of a meeting house and dining hall, stands adjacent to the project.

Steam investigations and other testing started in the Broadlands field 20 years ago and final Government approval to proceed with the construction of the

power station came in 1982, following agreement with the owners on a lease for the land required on the west side of the Waikato River.

The field is now tapped by 45 wells, about half of which will be used to draw off steam for the Ohaaki project. Wells measure between 1000 metres and 1800 metres deep.

One of the problems with New Zealand’s geothermal fields is the toxicity of the separated water, containing poisonous elements including arsenic and boron. Pumped into fresh water they can cause pollution.

At Ohaaki this problem will be overcome by reinjection. It is also expected to reduce ground instability and prolong the life of the field.

Reinjection is used both in Japan and the Philippines but has only previously been tried on an experimental basis in New Zealand.

The plant will consist of two intermediate pressure turbo generators and two smaller high pressure turbo generators which were formerly used in the Wairakei field before failing steam pressure there made their use unecon-

omic. Refurbished, they will go back into duty at Ohaaki as an integral part of the power station, although separate turbine houses are being built for the two sets of generators.

The turbines will be driven by a mixture of steam delivered by pipeline from separation plants, which are themselves each fed by an average of five bores sunk into the ground. The separators will remove the hot water, which will be at about 150 deg C, and reinject it into bores identified for this purpose. About 1800 tonnes per hour will go back into the field. A massive cooling tower, equivalent to a building about 30 storeys high, will be a feature of the station and will be the first time such a structure has been used in New Zealand. A common sight in thermal and nuclear power stations overseas, the tower will be 70 metres across at the base. The tower will be used to cool the steam after it has passed through the turbines, condensing the steam into water at a rate of up to 700 tonnes an hour. The natural draught tower will remove about 420 megawatts an hour of heat from the water, which will be circulated by large pumps at a rate of up to 20,000 tonnes each hour. The gas content of the steam is another problem. Its potentially damaging hydrogen sulphide will be discharged into the cooling tower and then dispersed harmlessly into the atmosphere. Yet another remarkable feature of the station is that it is planned to be an unmanned “satellite” station of Wairakei, to which electricity will actually be sent for onward distribution through the national grid. Fail-safe shut down procedures in the event of any failure will be incorporated into the S2BO million plant. As well as leasing their land, the Ngati Tahu people hope the station will benefit them in other ways, by attracting some of their people back from other areas. They are to build further houses on their land, and there are prospects of a horticultural industry there, using steam-heated greenhouses. Ohaaki may well prove to be the perfect energy source clean, non-pollu-tant, and a development that offers more than just power for far-away places.