Geothermal energy has a reputation for being green – but though it’s much cleaner than fossil fuels, it does produce some greenhouse gas emissions. With more than $6 million in newly announced funding from the Ministry of Business, Innovation and Employment, researchers at Waipapa Taumata Rau, University of Auckland, are working to change that.

Associate Professor Sadiq Zarrouk, co-director of the Geothermal Institute at the Department of Engineering Science, is leading a team aiming to turn the greenhouses gases that geothermal plants currently emit into rock, trapping it permanently underground. 

“New Zealand is a world leader in geothermal energy technology and many countries implement our research,” says Research Fellow Eylem Kaya, a member of the team. “This could transform the industry, bringing it from low to no emissions.”

And while geothermal plants are the first target, the process has the potential to be applied to other industries, including fossil fuel power plants. If widely applied around the world, it could take a significant chunk out of global emissions, says Zarrouk.

“We don’t claim we’ll be able to eliminate all global emissions but we’re going to help reduce them, starting with geothermal energy and going beyond.”

Geothermal energy: A primer

The exact amount of emissions varies, mostly due to the natural conditions under the surface, but in New Zealand, some plants emit enough that the carbon price they pay is significant, with the price expected to go up as the country moves towards the goal of net-zero emissions by 2050. This is part of what’s motivating the industry to cut emissions.

That’s not all. The Climate Change Commission has recommended closing several high-emitting geothermal fields by 2030 despite the billions the infrastructure cost to build. For the industry to survive, it’s going to have to lower emissions quickly. 

Geothermal power currently supplies 18.1 percent of New Zealand’s electricity. However, if the country’s full geothermal potential were realised, it could supply significantly more of the electricity demand. If geothermal energy were made carbon neutral and scaled up, it would be a big step towards net zero.

Turning gas into rock

Scientists are already working on ways to re-dissolve greenhouse gases in water before the liquid is injected back underground. From there, however, the gases can again escape, whether by naturally bubbling up or through geothermal energy production. That’s why Zarrouk and his team are working on a second step: turning the dissolved gases into rock. 

It may sound like science fiction – but the process of turning carbon into rock exists in nature. It’s why fossils exist. Of course, geothermal engineers aren’t interested in waiting millennia. They’re working on ways to mineralise CO2 much faster. 

In Iceland, scientists are capturing carbon by injecting it underground into the local basaltic rock, which has the right structure and chemical composition to trap CO2 and petrify it into calcite. However, New Zealand rock isn’t as naturally suited for this as Iceland’s. 

Zarrouk’s team’s big idea is to introduce the ions that can trap CO2 and H2S and inject them underground along with the dissolved gases. The result will be that even in areas with non-basaltic rock, greenhouse gases can be solidified underground. The CO2 will petrify into calcite and the H2S into pyrite (fool’s gold) – both non-toxic, common minerals.

The team plans to take its technology from the lab to the field within a few years. After the technology is proven, the team plans to commercialise it, whether through licensing, consultancy or by starting a company. In addition to helping advance the team’s research, UniServices is providing early advice on moving from research to commercialisation.

If every existing New Zealand geothermal plant adopted the technology, emissions would be reduced by 560 kilotonnes a year – worth $42.3 million under current carbon pricing. If the technology were applied to other industries, such as dairy – milk plants often use fossil fuels for purposes such as milk drying – emissions would decline still further.

If fossil fuel plants internationally were to use the team’s process to capture emissions and pump them underground to mineralise, the technology would go from carbon neutral to carbon negative, say the researchers. While there would be a cost to bring in the technology, this would be more than offset by the savings in carbon pricing.

“If this technology is taken up around the world, it will make a real dent in emissions,” says Zarrouk. “Others are also doing their bit, working on the electrification of cars, switching to hydrogen, etc. – this work is part of the big jigsaw to help solve the problem of climate change.”