Mercury Energy project engineer Lexi Richards, Aimee McGregor, Mercury Energy Programme Manager - Major Geothermal Projects – Generation and Mercury Head of Strategy – Generation Mat Kedian stand on top of the new section of the Ngā Tamariki geothermal power plant. Photo: Dan Hutchinson

Mercury Energy is close to completing a $220 million project at its Ngā Tamariki geothermal power station just north of Taupō.

By Dan Hutchinson

The expansion will increase the capacity of the station from 86MW to 132MW, by adding a fifth electricity generation unit.

Once completed the station will generate enough electricity to power the equivalent of 158,000 average homes – or a city the size of Christchurch.

The company has also invested $175m to drill eight geothermal wells over the past couple of years.

One of those wells will supply geothermal fluid for the new generation unit at Ngā Tamariki and another will be used for reinjection of the cooled liquids back into the geothermal reservoirs, to reduce the station’s carbon emissions.

The other six wells drilled into different fields are being used to maintain supply for geothermal stations in Rotokawa and Kawerau.

About 300 people have spent more than 200,000 hours working on the project construction phase, since April 2024, including Mercury staff and contractors from Taupō, Rotorua and Hamilton.

The largest sub-contractor is Taupō company Steiner and Moses.

It is the mechanical contractor which has had the largest presence, peaking at more than 60 workers on site at one time.

The company is responsible for installing and connecting the equipment and piping.

The power station is a binary plant, meaning it uses a second fluid in a closed loop – Pentane – to heat and cool the geothermal liquid and drive the turbine.

Mercury Head of Strategy – Generation Mat Kedian says the heat of the fluids coming out of the Ngā Tamariki reservoir is about 290 DegC, so “still pretty hot”.

“So here we're mostly hot fluid brine, salty water. And so you can use that without flashing off the steam.

“And the good thing about that is that all of the energy, all of the volume you take out of the rock, you can put back into the ground. So, you can maintain a really good pressure.

“So, it's pretty efficient from a renewable point of view but people still use the flash turbines because they're just a little bit more efficient.”

The second advantage of reinjecting fluids back into the same reservoir it was taken from is that it maintains pressure, avoiding any subsidence in the geothermal field.

Mat says the important feature of a successful geothermal field for generation is that you need a geothermal system heating the water beneath a natural, impermeable layer of clay.

Generation wells are drilled as close to the middle of the field as possible and then reinjection wells are drilled into the edges, so the fluid has enough time to re-heat before it gets back to the generation wells.

The majority of a modern geothermal binary power station is taken up by cooling fans, with the fifth generator at Ngā Tamariki only taking up the space of a bus.

Aimee McGregor, Mercury Energy Programme Manager - Major Geothermal Projects – Generation says they did not build the power station all in one go because it was the first plant they had built on that field.

“We wanted to understand … I guess could it sustain this kind of take, and it has. It's been monitored, it's really healthy, and so then the decision was made, okay, cool, it can sustain more.”

Mat says they have stations on five fields and all of them have the capacity for more generation.

The power station should be generating electricity by the end of the year and running reliably at full capacity early next year.

It is mostly operated remotely and just half a dozen staff will be required during the day for maintenance once it is operational.