Deep heat

Mighty River Power’s latest geothermal venture is the company’s biggest scheme yet and a vital part of its drive to produce 500MW of geothermal power by 2012.    BY NEIL RITCHIE

The Nga Awa Purua power station being built at Rotokawa north of Taupo is still in its early stages, with the foundation works started.

At a cost of $450 million, the project will be the second power station at Rotokawa and expected to provide 132MW of new capacity (an average of 1100GW annually). It will be the second largest geothermal station in New Zealand.

Though the construction of Nga Awa Purua is only about 10 percent complete, the design has already been finished and the main steam turbine is already being manufactured by Fuji Electric in Japan, says company geothermal operations manager, Spence McClintok.

“And we recently had a team of six go to Japan to review the design and meet with the main contractors there, Sumitomo Corporation and Fuji Electric. These Japanese people are great engineers and they produce top notch equipment.”

Sumitomo and Fuji were also involved in the recently commissioned Mighty River Power Kawerau plant further northeast. As well as Fuji Electric, New Zealand’s Hawkins Construction is assisting Sumitomo, which will be letting the required subcontracts over the coming months.

There’s an almost two-year construction timeframe, with the project scheduled for completion by May 2010.

In Taranaki, you sometimes find oil and gas wells and production stations, in unlikely places – in the middle of a farm or in rugged hill country. And with the North Island’s volcanic plateau, the geothermal fields are often surrounded by farms or forests, with power stations sited in seemingly incongruous places. So it is with Nga Awa Purua and Rotokawa – the smell of sulphur, hot steam and an industrial development.

With foundations out of the way, works begins on the above ground flat area for the cooling tower and the construction of the building that will house the turbine. These are not insignificant tasks, says McClintok, with the cooling tower being about 150 metres long, by 30 metres wide and nine metres high. It will use about five million litres of recycled river water, cooling the geothermal steam that can be at temperatures of up to 300˚C when it comes out of the ground and goes through the condenser.

The turbine will be a “triple flash unit”, with three separator units and three steam flashes at different pressures and temperatures. “Rotokawa is a very hot field, similar to Mokai,” adds McClintok.

Rotokawa’s geothermal resource comes from between two and three kilometres below the surface (compared to around two kilometres at Kawerau). The geothermal resource is a two-part fluid, which is separated into steam and geothermal water (brine) at the surface.

Energy for electricity is also extracted from the hot brine enhancing the efficiency of the power station. All post-production geothermal fluid (condensed steam and brine) is then re-injected into the ground through purpose-built wells.

McClintok adds that there are many similarities between Nga Awa Purua and the new 90MW plant at Kawerau. The design is essentially the same.

Interest in Rotokawa as a commercial geothermal resource dates back to 1953, when studies of the steam field showed that it offered significant potential for geothermal development. Mighty River Power already runs a smaller (35MW) geothermal plant on the same field that was commissioned in 1997. It was bought by MRP in 2000 and recently upgraded. The new plant is being built close to the existing one and will connect into existing 220kV transmission lines directly over the field.

The new Nga Awa Purua power station is a joint venture between Mighty River Power and Tauhara North No.2 Trust. Its chairperson Makere Rangitoheriri says the joint venture gives local people a significant stake in the long-term economic benefits of the development and enables the trust to extend its programme of health and education initiatives. In other Maori trust partnerships, Mighty River Power is also a 25 percent shareholder in the Tuaropaki Power Company’s 112MW geothermal station at Mokai and planning initial exploratory work on the Nga Tamariki field from late this year.

Long term plans

Mighty River Power chief executive Doug Heffernan says geothermal energy is now a core business activity for his company and will be a high priority for future development.

“Geothermal is the only renewable fuel not subject to climate variations and has a key part to play in ensuring security of supply for the future. It also perfectly complements our hydro generation portfolio and the wind-power initiatives we are exploring,” he says.

“Its high capacity factor also makes it a more efficient use of transmission capacity.”

The generator’s diverse generation assets – hydro projects, geothermal, wind, cogeneration and bio energy (utilising landfill gas) – can produce up to 22 percent of New Zealand’s peak energy demand. Geothermal currently accounts for about 18 percent of the company’s generation portfolio, but that is expected to almost double, to over 30 percent by early next decade.