Running hot - Mighty River's geothermal gems

Geothermal is the ultimate renewable energy source - and one reason why Mighty River Power is kicking off the government's asset sales. Energy reporter Grant Bradley visits the company's brand-new plant

Site Manager Greg Thompson on top of Ngatamariki Power Station with some of the 240 cooling fans. Photo / NZH
Site Manager Greg Thompson on top of Ngatamariki Power Station with some of the 240 cooling fans. Photo / NZH

In a dusty pine forest clearing near Taupo, super-heated fluid from deep below the surface has started pushing its way towards Mighty River Power's new geothermal station.

Yesterday contractors opened a massive valve at the head of a 3km deep well, and pumped down liquid nitrogen to stimulate the 280C salt-laden brine and steam that will soon be used to generate power at Ngatamariki, a project that will cost close to half a billion dollars.

Water flow at the site is a significant milestone for the station, coming in the same week that the partial sale of the state-owned company took a major leap with the start of pre-registration by potential shareholders.

When the station is commissioned around the middle of the year, 40 per cent of Mighty River's power will be generated from five geothermal stations, giving it a diverse generation mix that was big part of the thinking behind the government's decision to put Mighty River at the front of the sale queue of state-owned enterprises.

There are risks. These include costly exploration, building costs - as with all new power stations - are high, and poorly managed fields can literally run out of steam. But modern geothermal energy is widely seen as the ultimate renewable. Unlike wind and hydro, it is available around the clock every day of the week no matter what the weather, and geothermal fields are replenished by the brine that has gone through power stations being reinjected.

The desperately dry farmland on the 17km drive from Taupo to Ngatamariki represents something of a mixed story for Mighty River Power.

The good news is that its early conversion to the geothermal energy renaissance is paying off, plugging a gap now that the company's hydro generation is curbed because of low water levels on the Waikato River.

Lake Taupo is near historic lows, there is no significant rain on the horizon and the company last month said its full-year result could be affected by reduced output from its nine hydro stations, which still generate the bulk of its power.

Mighty River said yesterday it was "using water prudently" and was also running its gas-fired plant at Southdown, in Auckland, hard to meet demand and respond to wholesale market conditions.

Across New Zealand the use of geothermal energy for power generation has grown three-fold in the past decade, to about 15 per cent of capacity, although the energy source has been used in the North Island for centuries, first by Maori for cooking and warmth and then for tourism.

Mighty River was formed out of electricity sector corporatisation in 1998 and inherited the Waikato River hydro system - where there was no opportunity for further dams - and no other generation.

In order to grow, the Auckland-based company needed to diversify, so with Contact Energy it became a leader in the geothermal resurgence.

Ngatamariki will be the company's fifth geothermal station and on the eve of tapping the well, site manager Greg Thompson says there is excitement among the workers.

"For us its a pretty big deal - it's massive."

Icelandic firm Iceland Drilling drilled the bore holes - a choice which annoyed the Engineering, Printing and Manufacturing Union - Israeli firm Ormat is installing much of the power station's generation gear, New Zealand firm Hawkins Construction laid 8.5km of carbon steel pipes and did the civil works, and Boots & Coots, part of the US company Halliburton, is responsible for the well heads.

Most of the equipment comes with a one-year guarantee and there are agreements covering station performance targets.

Thompson says many of the peak work force of 350 were locals. When the plant is commissioned and feeding the grid around the middle of the year there will be 14 staff, including operations room and maintenance workers and managers.

Dave McLernon has been the project manager at Ngatamariki since early last year, and says the oval-shaped geothermal field, first explored by the government in the mid-1980s with four wells, was picked up by Mighty River early this decade.

The station itself is a 300m long supersize Meccano-style structure that houses a maze of equipment including four generators and turbines, vaporisers, heat exchangers, super heaters and cooling fans. Next to it is a plant to separate water and steam, and ponds for fluid overflow.

Rather than being used to drive the turbines directly, the hot water from underground is used to heat pentane, a liquefiable gas with a boiling point of 36C. The gas is heated to around 180C and used to drive the turbines before being recycled though a closed loop system within the plant. The cooled water is returned to the outer perimeter of the field and reinjected deep into the ground.

Power from the turbines is stepped up to 220Kv in the neighbouring switching yard, then transmitted to Mighty River's geothermal plant at Nga Awa Purua, where it joins the national grid, which has been fortified in the Taupo region to cope with the geothermal boom.

Mighty River's chief executive Doug Heffernan, credited with leading the geothermal charge, once described mastering the energy source as a "dark art".

Power companies investing billions of dollars in stations rely on geo-scientists to get the dynamics of the field right - but there are no guarantees.

Ohaaki, now owned by Contact Energy, operates at less than half of its capacity after being run too hard in its early years by its government owners. Unrestricted and unplanned tapping of Rotorua's geothermal field led to emergency restrictions.

At Mighty River's Kawerau station, fluid flowing through the plant turned out to be too rich in silica, so pipes clogged up and a chemical wash was required.

The development risks have been highlighted by Mighty River's overseas ventures, where in the first half of this financial year it was forced to write off $89 million. Hard drilling in Chile and environmental court challenges in Germany have slowed joint venture projects, although a partnership in California's geothermal renaissance added $57 million to its after-tax interim profit. The company has so far spent US$250 million on its overseas geothermal projects, which it says gives it growth prospects now that the New Zealand electricity market is saturated, and exploits its growing, and internationally recognised, expertise.

Greenpeace has given geothermal energy a big tick of approval.

Energy campaigner Simon Boxer says the organisation regards it as completely green.

Closed cycle systems using heat exchange technology ensure that nearly all the carbon dioxide produced during the process is returned to the ground during water reinjection.

New technology has rectified the problem of chemical leaching, and geothermal sites are well known, so drilling can be precisely targeted, rather than causing wider damage to the landscape.

In its Energy Future document released last month, Greenpeace calculates that the potential New Zealand share of the global geothermal energy market up to 2050 could amount to more than US$85 to US$114 billion ($121 to $163 billion at current exchange rates). "We are exporting our expertise now. New Zealand is well placed to capture a sizeable share of this part of the geothermal market," Greenpeace says.

"The world is on the cusp of a sustained boom in the development of geothermal electricity."

Greenpeace wants more government support for research and development, and industry to train more New Zealanders in geothermal science and engineering.

It calculates that more than 70 businesses based in this country have extensive expertise in all areas of geothermal development, from resource mapping and exploration through to design, project management, operation and even manufacturing heavy industrial components for geothermal power stations.

Contact Energy's geothermal resources manager James Kilty says geothermal works for business and the environment.

"The great thing about geothermal is the commercial imperatives are very aligned with the care of reservoir; there's no money to be made in overbuilding a resource. It's a fast track to a bad outcome."

Back at Ngatamariki, McLernon talks of quiet satisfaction at the geothermal fluid's flowing, but the big celebration is on hold.

"We'll wait until we've got power to the grid before we do that."

Full steam

280C: Temperature of the brine in the Ngatamariki reservoir
90C: Temperature of fluid reinjected into the field
36C: Boiling point of pentane gas used to drive turbines
180C: Temperature of pentane after heating by geothermal brine


Mighty River Power assisted with the Herald's travel to Ngatamariki

- NZ Herald

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