A Startup Wants to Power Data Centers With ‘Supercritical’ Carbon Dioxide

A new company has a plan to make the dirty gas turbines powering data centers more efficient: liquid carbon dioxide.

American Supercritical came out of stealth Wednesday, announcing $8 million in funding. It wants to retrofit inefficient gas turbines that many data centers rely on for power with units that can generate more power, without adding more emissions (though the gas-fired turbines will continue to emit carbon pollution). The technology can also theoretically be used on a wide variety of energy sources at a time when power demand is skyrocketing.

“We want to start with gas turbines but eventually expand beyond that,” says cofounder Simon Shuham.

Most large gas-fired power plants in the United States use an array of heat engines in what’s known as a combined-cycle process: First, turbines generate electricity from burning compressed air and natural gas, then a separate engine uses the hot exhaust to make steam and create additional energy. But for a variety of reasons, data centers across the US have opted to power their operations with what are known as simple-cycle turbines, and exclude the steam component.

These turbines are much less efficient than combined-cycle plants. Usually, only about 35 percent of the energy from simple-cycle turbines is converted to electricity, while the rest escapes as exhaust. (In combined-cycle plants, that figure hovers closer to 60 to 65 percent.) That exhaust includes greenhouse gases, making plants that run on simple-cycle turbines a much worse choice for the environment than combined-cycle plants.

The size of some of these plants combined with their inefficiency is a recipe for climate disaster. A massive data-center power plant in Texas that Amazon is building with just simple-cycle turbines, for instance, is permitted to emit more than 33 million tons of greenhouse gases per year—more than the annual total of some small countries.

But all these small, inefficient turbines could be a great match for supercritical CO₂ technology, American Supercritical’s founders say. Carbon dioxide becomes supercritical when it’s pressurized and held at a certain temperature. In this state, it gets the density of liquid but still behaves like a gas, meaning it can move energy more efficiently through much smaller amounts of equipment.

American Supercritical wants to attach its units to small gas turbines and help generate more energy. While the turbines themselves would still use gas, the supercritical CO₂ unit can use the hot exhaust generated from those turbines to create additional electricity. Instead of using that heat to boil water and create steam, the heat is transferred directly by the pressurized CO₂ to generate additional energy with no additional emissions.

“We’re essentially building miniature combined-cycle plants,” says Shuham.

Using supercritical CO₂ also can eliminate or greatly reduce water use in the power generation process—something that’s drawn intense scrutiny when it comes to data centers. Importantly, the CO₂ involved operates in a closed-loop system, meaning that it doesn’t have to be refilled. Cofounder Matthew Carlson, who researched supercritical CO₂ for more than a decade, likens it to refrigeration systems that circulate CO₂ to facilitate cooling.

The company plans to initially sell a 10-megawatt unit—relatively small by gas plant standards. By American Supercritical’s math, adding supercritical CO₂ units could increase turbines’ efficiency by up to 50 percent without adding any more greenhouse gas emissions or water use.

But retrofitting all of the country’s inefficient gas turbines powering data centers with supercritical CO₂ units won’t reduce emissions to zero. Natural gas production and use, even with efficient turbines, warms the planet. Data centers also represent a massive new source of pollution—right at a time when the world needs to be scaling back on its use.