Innovative tech could generate renewable energy and capture carbon

UC Engineering PhD student Karan Titus is researching new ways to combine geothermal with bioenergy, to supercharge renewable electricity and create a carbon sink.

Titus has shown that a combined geothermal and bioenergy plant could remove a million tonnes of CO₂ each year, equivalent to taking 200,000 cars off the road. And the financial incentive to do so could run in the tens to hundreds of millions of dollars.

“Although this work is at an early stage, we think it’s a hugely promising step forward, particularly as the world’s economies continue major plans for decarbonisation,” says Titus. “Aotearoa is a beautiful country with an abundance of natural resources. We have a real opportunity and responsibility to lead the way in the collective global decarbonisation effort.”

His doctoral co-supervisor UC Civil and Natural Resources Engineering Senior Lecturer Dr David Dempsey says: “Basically, we have a way to suck huge amounts of CO₂ out of the atmosphere and put it underground in geothermal fields. It has the potential to be both good for the climate and good for the economy.”

Aotearoa New Zealand is a world-leader in geothermal technology, a renewable source of electricity and heating for homes and industry. In 2020, about 18% of New Zealand’s electricity came from geothermal sources.

However, some of the country’s geothermal fields have been in the spotlight for CO₂ emissions. This is because there is some CO₂ dissolved in the hot water that comes from underground – a result of deep volcanic processes – and this is released into the atmosphere during electricity generation.

Finding new ways to minimise, or even reverse, emissions from geothermal energy sources can help Aotearoa New Zealand decarbonise its electricity sector by 2050 and reach a net-zero emissions economy by 2050, as set out in the 2019 Zero Carbon Act.

“Karan’s PhD project focuses on an exciting method to not only eliminate CO₂ emissions from geothermal, but also take CO₂ out of the atmosphere and lock it up underground,” Dr Dempsey says. Called carbon sequestration, it’s a major component in most international climate agreements.

Unfortunately, traditional carbon sequestration used in the oil and gas industry is still very challenging. The deep injection wells required cost tens of millions of dollars, and the buoyant gas tries to escape upwards.

“Geothermal systems solve both these problems. The wells have already been drilled, and we can dissolve CO₂ into water that has to be reinjected anyway,” Titus says. “This new approach is already being trialled in Iceland, another country with significant geothermal resources.”

Titus has investigated a new technology called Geothermal-BECCS (Bioenergy and Carbon Capture & Sequestration). The BECCS technology sees forestry waste burned to generate electricity, and the CO₂ given off is captured and injected underground. This CO₂ was absorbed from the atmosphere as the forest grew and would return to the atmosphere if the waste was simply left to rot. This concept pairs well with most geothermal plants, which already have systems to put fluids underground (called reinjection wells). Together, bioenergy and geothermal energy can combine to generate more electricity than geothermal alone.

“Karan’s work is targeting some really key issues for Aotearoa’s future energy system,” says Titus’s co-supervisor UC Civil Systems Engineering Lecturer Dr Rebecca Peer. “Providing a reliable pathway for electricity generation that has net-negative emissions is very powerful and sets a great example for the rest of the world.”

Most of New Zealand’s geothermal systems are close to major forestry operations, which means the biomass is available and doesn’t need to be transported long distances. With government incentives and an all-time high CO₂ price above $80/tonne, the academics say there’s an exciting opportunity for cross-industry research in this area.