Mighty River Power has built a new binary geothermal power plant to add to its list of energy assets

Ngatamariki is a new binary geothermal power plant about 17km north east of Taupo. The plant runs four OEC (Ormat Energy Converter) turbine units, generating 84MW of power – enough electricity to power 80,000 homes.

In 2013, Mighty River Power was awarded the geothermal project of the year at The World GeoPower Markets Award Ceremony, part of the annual 2013 World GeoPower Markets conference in Amsterdam.

WSP Opus played a significant part in this project, fulfilling key roles including Lead Civil Engineer, Construction Programme Manager and Lead Landscape Architect. We were responsible for design and construction supervision of the haul and access road network, as well as geotechnical engineering, structural and seismic assessment and design, project management, construction supervision, surveying and quality control.

There are currently three production wells between 3,400m and 3,800m deep, and the geothermal fluid at the bottom of the wells is 290 degrees Celsius. This fluid is used to super-heat pentane and the pentane drives the turbines. The pentane is then cooled and recirculated for constant reuse.

The current cooling system comprises four 15m high structures, covers a combined area of over 2.5 rugby fields, and has a combined length of 300m. The generated electricity is fed to the National Grid via 8.5km of transmission line at an output of 220,000 volts. Ngatamariki has been designed to allow for future expansion to six OEC units.

There are four reinjection wells of similar depth to the production wells, to return the geothermal fluid back into the geothermal reservoir to replenish it. There are also 21 sentinel and monitoring wells, of depths up to 1,500m. The total combined depth of all the wells is about 40km.

From initial earthworks to handover, the Ngatamariki project was completed in 21 months, with more than 1 million person-hours at the site. And with the commissioning of Ngatamariki, Mighty River Power’s total geothermal generation was boosted to 40% of its total generating capacity.