Steaming up

Is geothermal the key to New Zealand’s energy future? (11 minute read)

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One of my favourite holidays as a child and teenager was a trip to Rotorua. As we drove into town, the distinctive sulphurous smell would creep into the car and I would be thinking about pools of boiling mud and cracks in the earth which emitted scalding steam. Even as an adult, I remain fascinated by the region’s wonderful geothermal features.

Rotorua is part of the Taupō Volcanic Zone, one of the largest volcanically active areas in the world, stretching from Whakaari/ White Island in the north-east to Mount Ruapehu in the south-west. The whole area is filled with reminders that our earth is not solid and stable but an eggshell-thin crust over layers of super-heated rock. When cool water from the surface soaks down to reach some of that hot rock, the water heats up and then rises back to the earth’s surface – just as hot air rises, so too does hot water.

Early Māori had an intimate knowledge of the Taupō Volcanic Zone, building a rich lore which recognised that the volcanoes were connected to the other geothermal features. They used the geothermal waters for bathing, cooking and washing, but these were not the only uses they made of the geothermal features. For example, in the early spring, kūmara tubers were sprouted in houses built over heated ground, giving the crop a head start when it would otherwise be too cold for this largely tropical species.

Māori weren’t alone in recognising the value of geothermal energy. There are records of people using hot springs in North America around 10,000 years ago and at least 3000 years ago in Japan. Geothermal heat has been used to warm buildings for hundreds of years. Then, in the early part of last century, a new idea emerged – using geothermal energy to create electricity. The first experimental power plant was built in Italy in 1904, followed by the first commercial plant in 1913.

It wasn’t until the 1950s that the next geothermal plant was built – this one at Wairākei, near Taupō. New Zealand became interested in geothermal energy after drought affected our hydroelectric dams in the 1940s. The plant was opened in 1958.

I’m interested in geothermal energy, because it doesn’t get much attention but I’ve occasionally heard it promoted as part of the solution to climate change. However, I also know that it’s not without environmental impacts – an area of land just to the west of State Highway 1 has been subsiding since the 1960s, and beautiful features like the Wairākei geyser are gone, thanks to the Wairakei power plant. So, I’ve been wondering what part geothermal energy will play in New Zealand’s future? And what will it cost us?

While I’m researching the topic, I come across a page about a research programme called Geothermal: the next generation. It’s part of GNS Science, the research organisation which looks at the earth sciences in New Zealand. For once, I’m actually organised and researching my article well in advance of when I plan to publish it, so I send off an email to the programme leader, Isabelle Chambefort. Would she be willing to do an interview with me to talk about her research?

She is willing, but she’s also on the organising committee for a conference that’s coming up, so we schedule a Zoom call that’s a couple of weeks away and I spend some more time reading.

By the time we speak, I’ve read up on the basics of geothermal electricity generation. Like many fossil fuel-powered thermal plants, a geothermal power plant uses steam to drive turbines and generate electricity. In a geothermal plant, though, the steam is not created by heating water. In a plant such as the Wairakei geothermal power station, the steam comes from hot geothermal fluids – a mixture of water and dissolved chemicals heated by magma (molten rock trapped underground). Wairakei is what’s known as a flash steam power plant, the most common type around the world. A hole is drilled and the fluid, which is at temperatures greater than 182^oC, is drawn to the surface under high pressure. Because of the high pressure, the fluid stays liquid even though it is well above the boiling point of water. When the fluid is then released into a low pressure tank, it “flashes” into steam and drives the turbine.

I’ve also started to get some understanding of the drawbacks of geothermal electricity generation – aside from the obvious point, that you need to build it near a geothermal field. I’ve learned that geothermal plants emit carbon dioxide – a major drawback when we desperately need to cut our emissions. I’m hoping that my conversation with Chambefort will help me better understand both the advantages and disadvantages.

I’m not disappointed. Within a few minutes of starting the Zoom call, I can see her enthusiasm for her subject, and I love hearing enthusiastic scientists talk about their work.

Chambefort’s research is focused on a particular type of geothermal fluid, one which is much hotter than the fluids used at the Wairākei power station. It’s called a supercritical fluid, something that was new to me until I started looking at geothermal energy.

Most of us know about substances such as water having three different states – solid, liquid and gas. What we don’t hear about so much is that there are other states too, states which can exist under extreme conditions like combinations of high temperatures and pressure, several kilometres down in the earth’s crust. “What we know,” Chambefort tells me, “is that if you drill deeper, you can reach super hot or supercritical fluid – it’s not steam, not a gas but it’s not liquid water either. It’s a phase that is extra energetic. So, if you can tap the fluid, you can have a lot more energy per well. It’s really great on paper, but comes with a lot of engineering complexity.

“From a geological point of view there’s still a lot that we do not understand. We know the Taupō volcanic zone is where most of the magma is in New Zealand, we know there is magma at shallow depths but is it a little droplet or big? And the connection between the magmas and what we are seeing on the surface, in Waiotapu, Waimangu, all of those beautiful features – we still have so many questions about it.

Boiling mud, Kuirau Park, Rotorua (image credit: Melanie Newfield)

“One of the key aspects that is missing is because those fluids are not common at the surface, the thermal chemistry of it, how this will react with the rock, is poorly understood. So we were lucky enough to be able to invest into a new experimental laboratory here in Wairākei, and so some of my colleagues are leading some research reacting some of this rock at high temperature and high pressure. When I say high temperature, it’s beyond 372 degrees C. We have experimented at 500 degrees, pulsing some fluid at 500 degrees at high pressure. It’s really very new science, to beyond what we know, and it is quite exciting I must admit.”

I’m starting to realise, from what Chambefort is telling me, that it isn’t quite as simple as drilling a hole in the ground and catching what comes out. Each geothermal field is different.

“Overall, if you are talking worldwide, they are kind of similar, but when you are starting to operate, you need to know the specificity of every little detail of how you are going to manage and design a power station. It’s all dependent on all these little intricacies. It’s actually an absolutely amazing engineering process, geothermal. It’s mind blowing.”

Despite the difficulties, though, there are some real advantages with using geothermal energy. One important difference between geothermal electricity generation and wind or solar is that the output from a geothermal plant is constant, not dependent on the weather. “From an energy point of view, we need to increase the electricity generation in New Zealand. You can put wind and solar but they are not baseload, not a secure source of energy that can always be put through the grid. That’s why we still have Huntly, still some gas power stations, because you kind of need to balance the grid all the time and it’s a bit challenging.”

But, Chambefort tells me, we shouldn’t just be thinking about using geothermal energy for electricity. “The other thing as well is that it’s opening the door for any high temperature processes – we can utilise the heat directly. The best way to heat something is not to take the geothermal heat to generate electricity and then use it to generate heat. When you think about it – the heat is right there.”

In particular, geothermal heat could be used for industrial processes, for example wood drying or milk drying. “There’s a company drying wood for pellet fires that’s now fully geothermal. It’s a company that’s started here and now they are selling worldwide. There is also some research that’s happening right now in utilising really high temperature in carbon dioxide capture or hydrogen generation. I’m pretty sure that we’ll see some transformation in the next 20 years. But if we don’t invest in this now it’s not going to happen.”

Geothermal energy does have considerable potential, but I ask Chambefort about the drawbacks too. Despite her obvious passion for the subject, she gives me a realistic picture of the challenges. “One of the problems of geothermal is the upfront cost, it’s so expensive. There’s is a lot of work going on trying to see how we can reduce the cost of geothermal so it’s competitive. But it’s really hard to make a business case for something that does not exist with the market model that we have. Investment in high end technology that no one has – it’s really challenging. New Zealand doesn’t have an Elon Musk.

“I have to go back sometimes to trying to understand how magma works. That makes me happier than thinking about markets.”

The other challenge with geothermal energy is managing the environmental impacts. The impacts can be local, like reducing the flow of water to geysers and hot springs, or global because geothermal electricity plants emit carbon dioxide – the very problem that we are trying to solve with our energy transition.

The problem with carbon dioxide comes because volcanoes and other geothermal features naturally emit carbon dioxide into the atmosphere. Unless there’s a major eruption, the amount is at a rate many times less than burning fossil fuels, and we can’t do much about it. But it does mean that geothermal plants, which use geothermal fluids, are classed as emitters. “I have a PhD student and other people looking at that, and the key aspect is that if you put a power station or don’t put a power station, the CO₂ will come at the surface regardless, it’s a natural process. So you can’t really blame the geothermal company for putting CO₂ out in the atmosphere because it’s happening anyway.

“This said, they still have to pay carbon tax and so they are looking at designing engineering and capturing some of the CO₂ and putting it back to the reservoir. The whole industry came together and shared their knowledge on how to do reinjection and they are testing different things in different fields. I’m pretty sure that we’re going to see a lot of reinjections with CO₂ very quickly which is great.”

Chambefort’s point about reinjection is an important one, and not just in relation to carbon dioxide, so I’ll take a moment to explain in more detail. Earlier geothermal plants, such as Wairakei, simply discharged the geothermal fluids that they used into the environment. Since the 1980s, geothermal plants have been reinjecting some of the fluids back into the ground. Partly this is done to reduce discharges of geothermal fluid but mostly it’s to maintain pressure in the reservoirs of geothermal fluids. The best way to do reinjection, though, depends on the geothermal field. Get it wrong, and it can do more harm than good.

It is clear, then, that geothermal energy is not an easy solution to our problems. There are scientific and engineering challenges to be overcome, environmental impacts to be managed and big upfront costs. But, Chambefort says, “coming from a mining world and still doing some research in mining, when you are looking at what is needed for wind turbine, for example, it’s quite impressive what we are going to need to mine.” That is an important point. In 2018, well over 1000 tonnes of rare earth metals were required for constructing wind turbines, much of it mined in China. She makes the point, too, that even hydro-electric dams flood a whole lot of valleys, and she’s quite right. Every source of energy we tap has some form of impact, some might be worse than others but none are perfectly clean and green.

New Zealand, then, has some big challenges, but also a unique opportunity. As Chambefort says, “we are living here in a country that has so much heat at shallow level, it’s such a gift that a lot of countries are probably envying.” I agree with her. We have a real opportunity here, and I hope that we can make the best of it.

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