Supercritical Geothermal: The future of clean energy?

This is the first in a series of Harbour’s research on the energy transition. Our Navigators will explore New Zealand’s transition to a clean and reliable energy system by 2050.

We felt it best to start with a combination of controversy and innovation. The recent $60 million investment by the Government to explore the potential of supercritical geothermal highlights the long-term thinking needed in the energy transition. We are not picking winners; instead, we will explore a large range of new and existing energy technologies.

Let’s jump in: Supercritical Geothermal: Harbour’s view – this is worth ten minutes of your time!

Geothermal energy is an existing vital renewable resource for New Zealand with significant potential to provide clean, sustainable power at the right price and an environmental footprint that meets our current goals.

Traditional geothermal energy systems, commonly found in areas like the Taupō region of New Zealand, Japan or Iceland, rely on hot water and steam rising naturally from underground reservoirs heated by magma and surrounding rock. However, there is another more powerful form of geothermal energy that could revolutionise the industry: supercritical geothermal.

In recent developments, the New Zealand Government has allocated up to $60 million from the Regional Infrastructure Fund to explore the potential of supercritical geothermal technology. The initial funding of $5m is to investigate the possibilities of drilling deeper and tapping into more powerful New Zealand geothermal energy sources, potentially securing New Zealand’s future energy needs.

Understanding conventional geothermal energy

To fully appreciate the potential of supercritical geothermal, it’s important to first understand how conventional geothermal energy works. Over decades rainwater has filtered down through the ground into the field reservoir where it is heated by rock and magma that has also moved up the towards the surface over many years.

In New Zealand geothermal fields this extraction of fluid currently occurs at depths ranging from 1.5 - 3 km. Think Wairakei and Contact Energy’s new Tauhara geothermal development.

The natural heat convection causes the steam and fluid to rise towards the surface. This rising fluid further stimulates the geothermal system, with the fluid reinjection at the outer edges, and deeper than the extraction point, maintaining pressure in the field and allowing for continued generation.

Some key considerations for a geothermal field to function effectively include the rock having good permeability (allowing water to flow easily). The geothermal field also requires a hard, impervious cap rock to trap the heat below, much like a lid on a pot.

The ideal temperature range for efficient geothermal power generation is typically 260 - 320°C, with a sweet spot around 270 - 300°C in most New Zealand fields. That sounds hot but wait….

What is supercritical geothermal?

Supercritical geothermal is a more extreme form of geothermal energy, involving fluids that exist at higher temperatures and pressures than conventional geothermal systems.

A supercritical fluid occurs when the temperature exceeds 374°C and atmospheric pressures are above 218 bar, indicating that water cannot exist as a liquid above this. In this state, the fluid becomes highly efficient at transferring heat, with the potential to generate much more energy than the equivalent volume of conventional geothermal fluid, making it ideal for power generation.

Source: Lumenlearning.com

In supercritical conditions, less fluid is required to generate a megawatt of electricity, making it potentially far more efficient than conventional geothermal systems. The fluid’s ability to carry more heat also means that supercritical geothermal systems could provide a more compact and powerful solution for large-scale power generation. This efficiency is similar to the high temperatures reached in coal power plants, but with far less environmental impact.

The challenges of supercritical geothermal

Despite its promising benefits, supercritical geothermal presents significant challenges, primarily due to the extreme conditions involved. At temperatures above 300°C, the increased presence of silica in the water can lead to scaling problems in the power station. When the fluid cools, this silica can clog turbines and other equipment, making maintenance costly and frequent. In addition to silica scaling, the higher temperatures cause rocks to become more soluble, leading to mineral-rich, often salty water that can further damage equipment.

Extracting fluids at greater depths creates further challenges especially when close to volcanic activity where gases such as chlorine and fluoride can be present. These gases can be highly corrosive, causing rapid deterioration of drilling equipment and power plant infrastructure. This makes it critical to develop technology that can withstand the extreme conditions found in supercritical geothermal reservoirs.

Drilling for supercritical geothermal: deeper, riskier, and potentially costlier

One of the biggest hurdles to unlocking the potential of supercritical geothermal energy is the challenge of drilling to the necessary depths. Traditional geothermal systems operate at depths of 1 - 3 km, but supercritical systems require drilling potentially much deeper—often thought to be beyond 4km in New Zealand, where the hotter fluids reside. The fact is that we do not know the exact depths, and, in part, this is where the New Zealand Government initial funding might prove useful.

The increased depth brings new challenges, including the need for specialised drill bits and frequent changes in drilling equipment, as well as the risk of encountering unstable rock formations. This variability in drilling times and costs makes it difficult to estimate the true cost of supercritical geothermal exploration.

While conventional geothermal drilling can involve the use of many wells with varying costs, supercritical geothermal wells are likely to be more difficult to manage. This is one reason why supercritical geothermal projects in places like Japan and Iceland have struggled to deliver long-term results. These projects were located near active volcanic zones and drilled to depths of just 2.5 km. Despite their proximity to hotter fluids, these projects experienced rapid casing deterioration and were ultimately shut down, largely due to the challenges of managing the extreme conditions.  Perhaps in New Zealand this may be different with fields likely to be further away from volcanic activity?

Can supercritical geothermal be harnessed?

The future of supercritical geothermal depends on overcoming several technical barriers. The ideal scenario would involve drilling into zones where the temperature is high, but the magma and its associated gases don’t cause equipment damage. By locating these systems at a distance from active volcanoes, it may be possible to avoid the worst impacts of volcanic gases while still benefiting from the high temperatures necessary for supercritical conditions.

Technological advancements in power plant design are also needed to handle more aggressive fluids. Binary power plants, which separate the geothermal fluid from the turbine, may be better suited for supercritical conditions, as they can protect the turbine from the corrosive and abrasive effects of hot geothermal fluids.

However, the success of supercritical geothermal also hinges on the ability to control the chemistry of the water in the system, something that coal and nuclear plants can already do with their cooling systems. For geothermal energy, this remains an unknown challenge, with research still ongoing to understand how best to handle the fluid chemistry at these extreme conditions.

Is supercritical geothermal the future?

Supercritical geothermal represents one of several strong long-term possibilities for the future of renewable energy. The ability to generate more power with less fluid and higher efficiency is a major advantage. However, technical challenges and costs related to extreme temperatures, corrosive fluids, and the depths required for successful extraction remain significant barriers. Further research into drilling technologies, depth to drill, power plant materials, and fluid chemistry should go hand-in-hand with drilling. These factors will be crucial in determining whether supercritical geothermal can move beyond experimental projects and become a reliable source of large-scale, clean energy.

There will be many challenges around regulation and consenting, consideration of Māori cultural values and technology risk; meaning having commercial investment with acceptable returns may still be many years or even decades away. 

Despite these hurdles, countries like New Zealand, Japan, and Iceland have shown their willingness to innovate by being at the forefront of exploring supercritical geothermal, hoping to unlock a new, powerful energy source that could one day power the world.

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