--Work informs “roadmap” for a superhot future By Elizabeth A. Thomson, correspondent for Quaise Energy

HOUSTON, Texas--Clean, renewable geothermal energy has the potential to become a major player in the energy transition, according to recent reports from organizations including the International Energy Agency. According to the Clean Air Task Force and others, the size of the prize could increase by an order of magnitude if we can tap into superhot rock over 375 degrees Celsius (about 700oF).

But the geothermal plants of today work at considerably lower temperatures, most ranging from about 100 to 250oC, and relatively little is known about designing plants for much higher temperatures.

Now, Daniel W. Dichter of Quaise Energy reports insights to that end in two papers. One was published online recently by Geothermal Rising (the work was presented at the 2024 Geothermal Rising Conference); the other was presented at the 50th Stanford Geothermal Workshop last month and is now also available online.

"We have a good understanding of how to design geothermal power plants in the conventional temperature domain, but we don't have much experience with geothermal source temperatures any higher than that. These papers apply conventional geothermal design principles to a higher-temperature range beginning at 300oC," says Dichter, a senior mechanical engineer at Quaise.

Dichter hopes the work will inform a roadmap toward a superhot future.

In geothermal plants, water exposed to hot rock picks up that heat and is then pumped to the surface where the carried energy is converted into electrical power. Among Dichter's conclusions: for systems tapping into superhot rock, it may not be necessary to maintain water at temperatures above 375oC all the way to the surface. He also found that plants working with geothermal fluids at temperatures higher than 300oC at the surface can use common turbines to turn the resulting steam into electricity. Most geothermal plants operating today at lower, conventional geothermal temperatures must employ a more expensive turbine system that is not as commercially available.

Toward the Superhot

Water pumped into rock at temperatures above 375oC will become supercritical, a steam-like phase that most people aren't familiar with. (Familiar phases are liquid water, ice, and the vapor that makes clouds.) Supercritical water, in turn, can carry some 5-10 times more energy than hot water at conventional geothermal temperatures, making it a power-dense energy source if it could be pumped above ground to turbines that could convert it into electricity.

Today, however, rock at those temperatures can only be accessed at a few locations around the globe, such as Iceland, where they are relatively close to the surface. The number one problem: we can't drill down far enough.

The mother lode of superhot geothermal energy is some two to 12 miles beneath the surface. Drills used by the oil and gas industries can't withstand the formidable temperatures and pressures that are found that far down. As a result, drilling becomes exponentially more expensive with depth. Quaise aims to solve the problem with a completely new way to drill using millimeter wave energy (cousins to the microwaves we cook with) that can literally melt and vaporize rock.

New Insights

Among other conclusions, Dichter found that if we can tap into superhot rock miles down, we don't have to maintain the water at supercritical temperatures (above 375oC) all the way to the surface. In other words, "supercriticality is not necessary for maximum performance at the surface," he says. And that could make superhot systems more economical.

It turns out that there are diminishing returns in keeping the water that hot all the way to the surface. That's due to the physics involved in transporting it over long distances through the slender pipes associated with geothermal, which are only a little over eight inches in diameter.

"While the heat content of supercritical water is higher, the mass flow rate through the pipes decreases, and they basically cancel each other out," Dichter says. Nevertheless, Dichter found that water at a production, or surface, temperature as low as 350oC can still lead to power outputs "up to an order of magnitude higher than those of conventional geothermal systems." He emphasized, however, that "you may still need supercritical conditions in the reservoir [the superhot rock deep below] to achieve sub-critical production because of losses between the surface and the reservoir."

Another of Dichter's conclusions involves the turbine system used to convert superhot water into power. Many of the geothermal plants in operation today involve a binary cycle with two working fluids. That's because heat cannot be converted to electricity efficiently by only using water at lower temperatures, and the geothermal water pumped to the surface contains impurities that can damage the above-ground equipment associated with power generation.

The solution is to transfer heat from the geothermal water to a second fluid flowing in separate but adjoining pipes. Hydrocarbons like isobutane are the preferred secondary fluid in most binary cycles, but Dichter found that they could be replaced with water (a clean source separate from that pumped to the surface). He specifically found that at higher temperatures, water works better than most hydrocarbons as the secondary fluid.

Replacing hydrocarbons with water has several advantages. Water is less expensive, and steam turbines using it are quite common, as opposed to those based on hydrocarbons (organic Rankine cycles, or ORCs). "Steam turbines have a much more mature supply chain than ORC turbines," says Dichter, who presented more about water-based geothermal binary cycles and how they compare to ORCs at the 50th Stanford Geothermal Workshop in February.

Dichter is excited about our geothermal future. "The applications are diverse, from power plants to regional heating to domestic ground-source heat pumps, and there are a lot of fresh new eyes on the field," he says. "There's a renaissance happening in geothermal right now."