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Nga Awa Purua, the second power station on the Rotokawa geothermal resource north of Taupo, is almost completed. Alan Titchall reviews its unique design and construction.
Mighty River Power’s decision to run with a single turbine set the pace for the design and construction of the country’s latest geothermal generator. In comparison, Contact Energy’s neighbouring Wairakei geothermal plant, commissioned in 1958 and the second oldest in the world, operates on 10 turbines, and its proposed Te Mihi project, closer to Taupo, will likely feature three generators pumping out 75MW each. The massive turbine, generator and related condenser at Nga Aawa Purua arrived from Yokohama, Japan at the Port of Tauranga a year ago in March 2009. The 177 tonne generator made dramatic news as it was cautiously transported via state highways and through the Kaingaroa Forestry roads to the Rotokawa site on specialised transporters. The painstaking journey took three days and careful planning to avoid a number of the bridges on the state highways that were not capable of supporting a load of 177 tonnes – a typical logging truck and trailer has a maximum road weight of 45 tonnes. The journey was one of many for the freight/transport industry, as over the construction of the power house, more than 65 shipments of special equipment was imported from 12 countries during construction. Construction allianceAt a cost of $430 million, and a joint venture between Mighty River Power and the Tauhara North No.2 trust, Nga Awa Purua was built by Beca under the auspices of the Japanese companies Sumitomo Corporation and Fuji Electric Systems, in a similar alliance to MRP’s Kawerau geothermal project that was finished in 2008.
During peak construction the workforce numbered 400 – many had already worked on the Kawarau Geothermal Power Station project and were relocated to Taupo. Taking on the full ‘liquidated damages’ risks of up to $190,000 a day, the design-build contractor Hawkins Construction (Hawkins) set a target programme of 98 weeks – the contractual programme was 104 weeks – and construction kept four to six weeks in front of that. The project was finished six weeks early. Harsh environmentBeca was engaged by Hawkins to provide the engineering design for the civil and structural components of the plant, which is made up of a vast array of pipe and plant support structures, a series of cooling towers, and the main turbine hall with a whopping, reinforced, 17 metre concrete pedestal supporting the turbine, generator and condenser with their combined mass in excess of 400 tonnes. The design of this turbine-generator pedestal involves a broad range of engineering disciplines to tackle both the massive size of the turbine machinery and the harsh geothermal environment. A very seismic area, structures had to be designed to withstand large earthquakes—not only for stability under a very large quake, but the smaller earthquakes the plant will experience during its 35-year expected lifetime.
Concrete designs involved dense, heavily enforced, high-quality concrete mixed with fly-ash; a very fine pozzolanic material that is a by-product from the burning of pulverised coal. It’s a nice irony that the durability one of the country’s renewable energy sources is dependant on material produced by thermal generation. When added to concrete mixes, the fly-ash particles react with the cement and fill void spaces in the concrete, reducing the permeability of the cured concrete to water and aggressive chemicals. The concrete also had to contend with high temperatures generated by hydration reactions in the cement from within the thick concrete structures that cause cracking in the young concrete and impair its water-tightness. A special mix design along with the fly-ash slowed down this reaction heat, and insulation was used during the curing to reduce differential temperatures. A lot of concreteThe largest and most technically challenging structure at Nga Awa Purua Nga was the turbine hall building and its huge turbine pedestal that contains 1500 cubic metres of concrete, or enough to fill 300 standard concrete trucks.
A grillage foundation supports the top of the turbine structure that is 10 metres above ground level. This grillage is supported on six reinforced concrete columns that sit on a 1.6 metre deep (and 25 metre square) reinforced concrete foundation slab at seven metres below ground. The slab is supported by bored piles 0.9 metres in diameter that are founded at around 25 metres below the ground surface. The pedestal has been designed with the 50-year design life and is tough enough to withstand a ‘1000-year’ earthquake. Triple flash technologyThe stream drawn from 10 wells in the local Rotokawa geothermal field is of very high pressure. In fact, Nga Awa Purua works with the highest steam pressure in the world.
The geothermal fluid is ‘flashed’ at three different stages and pressures to drive the mixed pressure steam turbine at maximise generation. The first high-pressure flash is at 24-26 bar (80 percent of the turbine’s generation is created at pressure). The remaining brine has enough energy left to be re-flashed at an intermediate pressure of 12-13 bar. The brine is then flashed a third time at a low pressure of two bar. Steam at Mighty River’s Kawerau power station, by comparison, is only double flashed at the highest pressure of 12/13 bar. The generation parts are stacked vertically, with the turbine and generator sharing the top level (about 21 metres high) and the condenser sitting beneath the low-pressure end of the turbine. The condenser rapidly condenses the exhaust steam leaving the turbine, creating a vacuum that effectively ‘sucks ‘ the low-pressure steam through the turbine that is being ‘pushed’ from the input side. Condensing the exhaust steam converts it into water (referred to as steam condensate), which releases a lot of heat energy as it changes from gas to liquid. Two large 1.4MW hotwell sumps/pumps on the bottom of the condenser collect the liquid condensate and pump it (at between 10 and 15 bar) to the top of a forced-air evaporative cooling tower in a circuit where the condensate is reused in the steam generator, or the excess pumped to the re-injection wells. The triple system produces 26,000 cubic metres per hour of condensate. The four rejection wells were drilled a couple of kilometres south and east from the plant and are between two and three kilometres deep. It is always a big challenge for engineers to determine the depth of re-injection point – too far and it wont work its way back into production and if too close, it cools down the production field. From extraction to re-injection, the geothermal reservoir fluid has traveled some nine kilometres of pipeline. Into the gridThe gross generation of the turbine is 140MW, with the plant using five to eight percent of this energy for generation. The net result is feed directly into the national grid that swings past the area on its way north across the central Waikato. The turbine revs at a constant speed (3000 rpm) at 50 hertz so it is set into the frequency of the grid.
Nga Awa Purua has been adding electricity to the grid since January 2010, gradually stepping up productions in stages. After hand-over in April, some six weeks ahead of target, the country’s newest geothermal power station will pump 132MW into the grid, or sufficient electricity to power 130,000 homes, or the equivalent of every home in Hamilton, Tauranga, Rotorua and Taupo. Investing in geothermalThe Nga Awa Purua power station follows the commissioning of the 100MW Kawerau Geothermal Power Station two years ago (2008) and is the forerunner to yet another major geothermal power station. Mighty River Power, again in partnership with Tauhara North No. 2 Trust, lodged consents earlier this year for another $400 million geothermal power station at Ngatamariki, 17 kilometres northeast of Taupo. The Ngatamariki project involves taking up to 60,000 tonnes per day of geothermal fluid from the Ngatamariki geothermal system. The project will generate between 800 and 900 gigawatt hours per year of electricity.
“At $430 million for Nga Awa Purua; $300 million for Kawerau; and at least $400 million for Ngatamariki, its easy to see how we are nearing the end of our initial $1.2 billion geothermal investment programme,” says the chief executive of Mighty River Power, Doug Heffernan. “These are great projects that are providing a secure source of renewable electricity to help power the economy. However, it’s a huge challenge to fund this level of investment and to provide a reasonable return to the company’s shareholders while at the same time providing additional sources of energy.” A steaming partnershipThe Rotokawa Joint Venture is a limited liability company formed between two partners – the Tauhara North No.2 Trust and Mighty River Power.
The Tauhara North No. 2 Trust will receive substantial income from a combination ofroyalties, land rentals and equity returns over the lifetime of the Awa Purua project (35 years at least) and future joint projects. The Trust has around 550 listed owners, consisting of a mixture of individuals and family trusts, many of whom reside in and around the Taupo region. Several thousand people will gain access to the downstream benefits associated with the Rotokawa II project.
Energy NZ Vol.4 No.2 March-April 2010 |
No two geothermal power stations are the same design and Nga Awa Purua near Taupo generates its 140 gross megawatts on just one, single-shaft, condensing steam turbine driven by three pressures of steam from a triple ‘flash’ system – the largest such operation in the world.
The alliance arrangement has proved to be very efficient. The construction contact was signed in April 2008 and key elements of the power station built in 10 months. From start to finish, including two months of test running the turbine, it took less than two years to finish the project.
In a geothermal environment the groundwater, soil and atmosphere are chemically aggressive – with humidity, heat, hydrogen sulphide, carbon dioxide, sulphates and chlorides in abundance. At Rotokawa, the main challenge for the builders was the heat – up to 80 degrees C at bottom of the deepest foundation piles.
To eliminate vibration in the turbine hall building the pedestal and the surrounding building is structurally separated and, in order to reduce the overall height of the turbine hall, about half of the pedestal is set below ground.
When in full production, over 45,000 tonnes of geothermal fluid will be extracted each day from seven production wells that are between 2000 and 2500 metres deep. Compare this with neighbouring Wairakei, the country’s first geothermal power station, where the wells (55 in all) are just 660 metres deep.
The technology to be used is yet to be confirmed however, it will be based on a conventional flash plant as employed at Kawerau, or a binary plant as employed at the Turoapaki power station at Mokai, or some combination of both (combined cycle). The final station cooling system to be used is also yet to be confirmed – whether it will include conventional wet cooling towers, air-cooled condensers (dry cooling) or some combination of both (hybrid cooling).
The company holds resource consents over the geothermal development on the Rotokawa Geothermal System – the sites of the 33MW Rotokawa I Power Station and the Nga Awa Purua Power Station (140MW). The pair has agreed to jointly develop a new power station at Ngatamariki, another field about five kilometres north of Rotokawa.