Tuesday, 23 February

Nga Awa Purua to give a 132-MW boost to the NZ grid

Mighty River Power announced that its $430 million 132MW geothermal plant at Rotokawa northeast of Taupo will start producing power by June 2010. The plant will be producing about 1000 gigawatt hours of power a year. That is more than an average year's growth in New Zealand's electricity demand and it signifies the largest geothermal development in New Zealand after the initial development of Wairakei 50 years ago. Might River, as the name implies, is a hydroelectric company but it decided to diversify due to increasing variability of the hydroelectric power stations. For example, the company generated 15 per cent less power in the latest half-year, thanks to smaller inflows into the Waikato River.

The figure shows the Nga Awa Purua construction site in October 2009.

The rectangular building on the right is obviously the cooling tower. The company brochure states that the cooling tower sits on a 165x20 m concrete foundation and houses 10 fans 10-m in diameter and spinning at 99 RPM. It is a wet cooling tower that relies on the evaporative cooling of water as well as forced-convection of air.

The geothermal fluid at 300 oC is provided is extracted from a depth of 2500 metres using eight production wells. The average geothermal production is 65 kg/second/well. The plant is a triple flash plant with a single-shaft turbine using three expansion stages. After the last stage, the steam is condensed and the majority of the condensate is reinjected (the rest is probably used in the cooling tower to provide evaporative cooling). There are five injection wells injecting the cold water to a depth of 3000m. The choice of a flash plant technology is interesting because the fluid in the reservoir contains elevated Au, As, Sb, W, Tl, Hg, Ag, Ge and Ga concentrations. Carbon dioxide (CO2) is the dominant gas, while hydrogen sulfide (H2S) accounts for 0.5% by weight of the total gas discharge [Wilson, Webster-Brown, and Brown, "Controls on stibnite precipitation at two New Zealand geothermal power stations", Geothermics, 36(2007),330-347]. An earlier combined-cycle binary plant operating in the same field has been reported to have significant scaling and corrosion problems (Reyes, "Mineral Deposits in the Rotokawa geothermal pipelines:, 2002). I could not find information on the technical details for the new plant and how the scaling and corrosion issues are being addressed in the three flashing processes.

The plant was constructed by Sumitomo of Japan with the turbine and the generator provided by the Fuji Electric Systems. This is reportedly the largest single casing geothermal steam turbine in the world, capable of providing 139MW (gross)(D Hoyer and T Gray, "Implementation of Efficient Plant Designs in a Time Constrained market", SPE Paper 121360-MS, 2009).

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