Supercritical geothermal power holds the promise of meeting humanity’s energy needs for millions of years, but how practical is it? A new analysis by Lux Research’s Karthik Subramanian suggests it could be somewhere between unlikely and impossible.
At first glance, geothermal energy seems like a promising source of energy. It's clean, there's enough heat in the Earth to power civilisation for the foreseeable future, and all you have to do is drill down to harness it.
Even better is what’s called supercritical geothermal. Traditional geothermal systems work by drilling holes into areas marked by volcanoes or hot springs and either pumping water into the ground and then extracting the steam, or by attaching a heat exchanger to heat the water inside a closed loop of pipes.
This works, but such plants are very expensive to build and there are only a few places in the world where they can be built. Their output is also limited because they only reach temperatures of about 200 °C (400 °F), which equates to an energy output of 5 MW for a single generating plant.
In practice, this means that geothermal energy accounts for only about 0.5% of world electricity production and never grows by more than 3.5% a year.
Supercritical geothermal takes the concept to the next level by finding magma pockets as deep as 2 km (1.2 mi) near the surface, or by searching for the hot interior of the Earth as deep as 20 km (12.4 mi). Here, the temperature and pressure are so high that water is heated to over 373 °C (703 °F) and a pressure of over 220 bar (3,190.83 lb/in², 217 ATM). In this state, the water is superheated, but it cannot turn into steam. It can also hold four to 10 times the energy of normal water or steam.
In other words, a supercritical geothermal power plant could have a capacity of 50 MW, and three wells could have the power output of 42 conventional geothermal wells. In addition, finding supercritical heat is simply a matter of finding a magma pocket or digging deep enough somewhere.
This sounds great, but it’s a huge engineering challenge that pushes drilling techniques and materials to the extreme. Not only do they have to dig incredibly deep boreholes, they also have to withstand pressures, gases, and erosive forces that would quickly destroy any conventional drilling rig. But there’s a bigger problem.
To get to that beautiful supercritical layer, or bubble, the drill must pass through a region called the Brittle-Ductile Transition Zone (BDTZ). Simply put, the rock above the supercritical zone undergoes a change when exposed to the temperatures and pressures at such depth. Instead of becoming brittle, the rock becomes flexible and plastic. Imagine granite that you can roll into doughy snakes and you get the idea.
The BDTZ is not uniform. The top is still brittle, the bottom is plastic, and the center is a hybrid of the two. This means that drilling is a terrible thing, requiring too much effort for too little progress, and is too hard on drill bits and other equipment.
According to Karthik Subramanian, getting through the BDTZ requires a thorough understanding of the drilling area to find the fragile points along the entire path, if any. It also means dealing with direct heat and corrosive volcanic gases from below, which can include hydrogen sulfide and sulfur dioxide.
In fact, these problems are not only difficult, but may also be difficult to overcome with current technology, meaning that the dream of supercritical geothermal energy may remain just that: a dream.
“Supercritical geothermal is far from commercialization and will depend on advances in drilling methods, digital resource modeling and materials development to access these resources for power generation,” Subramanian said. “Despite its potential, supercritical geothermal will not play a significant role in energy conversion due to its inherent technical hurdles. If the technical hurdles are addressed, supercritical geothermal will only be economical in regions with volcanic or tectonic activity at lower depths unless new deep geothermal drilling methods prove otherwise.”
Deep drilling issues could mean supercritical geothermal could be limited to volcanic regions like Iceland or the Pacific Ring of Fire, but crossing the BDTZ could be overcome with technology developed by the MIT and University of Cambridge-backed startup Quaise, which plans to use a particle beam produced by a “Gyroscope” that can break up, melt and vaporise the offending layer of plastic rock.
How well he can achieve this will be determined by time and trial.
Source: Lux Research