Geothermal system ready and able to help us fight climate change

A test-case geothermal system is happy to store not just its own but the emissions already heating our world, say researchers with New Zealand’s Carbon Removal Project. Now, they’re out to discover how real systems would react.

The geothermal reservoirs currently powering our homes could also be harnessed in the fight against climate change, preliminary research suggests.

“Geothermal systems already provide us low-carbon energy. But they could be even better for the planet, as they have the potential to take some of the carbon created from historic emissions,” said Ryan Tonkin of the Carbon Removal Project.

A novel approach could send this greenhouse gas back to the Earth through current geothermal wells.

Geothermal power stations pipe up hot steam to rotate turbines, creating electricity. Stations then send the cooled liquid back underground, creating a renewable loop.

Along with the sulphuric substance that gives that distinctive geothermal smell, carbon dioxide was typically vented to the air at power stations.

Deep below the power plant, the geothermal fluid contains dissolved carbon dioxide. Akin to opening any soda bottle, this turns into a gas when it reaches the surface.

Although some fields are “gassier” than others, Tonkin said geothermal power stations only produce a fraction of the emissions compared to a gas or coal-burning power plant.

Across New Zealand, these stations are shifting towards net-zero. Some have started to capture this gas and mix it back into the fluid before it is piped back down to the Earth.

Tonkin and senior lecturer John O’Sullivan – both with the University of Auckland’s Geothermal Institute – used numerical reservoir simulations to assess carbon reinjection.

John O'Sullivan (left) and Ryan Tonkin at a geothermal power station.

The team’s early research indicates that a representative reservoir is “happy” to accept not just its own carbon dioxide, but additional emissions. In that way, its power station would become “carbon negative”, Tonkin said, fighting climate change.

“We wanted to know if the reservoir would behave any differently, because that could negatively impact power generation. In this case, we saw a negligible difference – a very encouraging start, because geothermal is so critical to New Zealand.”

The University of Auckland research team used a computer model to simulate how a hypothetical system would react after being drilled to supply a new power plant.

For the first three years, the power station vented carbon dioxide to the atmosphere. Then, researchers ran three scenarios.

Under the first, the plant continued to vent all its emissions. In the second, the plant captured 100% of all the carbon dioxide arriving at the surface and dissolved it into the fluid sent down the reinjection wells.

For the third, 110% of the arriving carbon dioxide – including 10% taken from the air – was mixed into the fluid.

“For our system, the early years were its gassiest,” Tonkin said. “For that reason, new geothermal power stations should consider introducing reinjection right away.”

Tonkin, who is presenting the non-peer-reviewed research at the European Geothermal Congress conference this week, tracked the carbon dioxide above and below ground for a 25-year period while the power plant was running – and then for another 25 years after it closed.

Many geothermal reservoirs naturally connect to the Earth’s surface: producing the stunning hot pools that our country is known for. As well as venting warmth, these pools can release natural emissions to the atmosphere.

Spectacular hot pools at the Earth's surface can be a natural source of emissions from a geothermal reservoir.

Often, the background levels of natural emissions fall once we drill a reservoir.

The team’s hypothetical reservoir did not exceed its pre-drilling levels of natural emissions, even after 110% of emissions were dissolved and reinjected.

“We want carbon put underground to stay underground, so this finding is a positive start,” Tonkin said. “Even so, more work is needed to find whether this is true of real geothermal systems and how long the carbon dioxide is stored for.”

Accounting for the change to natural emissions, the researchers found that capturing emissions could prevent 4 million tonnes of emissions from entering the atmosphere over a 25-year period of electricity generation. That’s the equivalent of nearly 750,000 return tickets from Auckland to London.

Add in carbon removal, and the reservoir could take another million tonnes of carbon dioxide – or 185,000 across-the-world tickets – out of the air, halting its damage to the planet.

“I hope more energy companies across the globe begin to reinject their carbon emissions, and seriously consider the prospect of carbon removal,” Tonkin said.

Because each reservoir is different, energy companies should repeat the carbon modelling for their particular system as part of their homework before introducing carbon reinjection and removal, Tonkin said.

Reinjection may not make sense for systems that are comparatively less “gassy”, he added.

Following this encouraging early work, the team will investigate how different types of reservoirs would react to carbon storage, and how long the emissions would be stored for once power generation ceases.

The study is part of a wider Government-funded project to make New Zealand a world leader in carbon removal by developing a suite of evaluated technologies.