Supercritical CO₂ Could Make Data Center Gas Turbines More Efficient

American Supercritical has a plan to make the gas turbines powering data centers more efficient: use pressurized, supercritical carbon dioxide.

The company emerged from stealth on Wednesday and announced an $8 million funding round. American Supercritical wants to retrofit the relatively inefficient gas turbines many data centers rely on with systems that can generate more electricity without increasing emissions. However, because the turbines still burn natural gas, they will continue to produce carbon pollution.

The technology could eventually be used with other energy sources when electricity demand spikes.

“We want to start with gas turbines and eventually expand beyond that,” says co-founder Simon Schuham.

Why data centers are using inefficient gas turbines

Most large gas-fired power plants in the United States use a process known as combined-cycle generation. First, a turbine burns compressed air and natural gas to produce electricity. The system then captures the turbine’s hot exhaust and uses it to create steam, which powers another engine and generates additional electricity.

For a variety of reasons, data centers across the United States are choosing to operate so-called simple-cycle turbines instead. These systems eliminate the steam component, allowing them to be deployed more simply but making them considerably less efficient than combined-cycle plants.

Typically, only about 35% of the energy in a simple-cycle turbine is converted into electricity. The rest leaves the system as hot exhaust. In combined-cycle plants, the share converted into electricity is closer to 60% to 65%.

Because their exhaust contains greenhouse gases, simple-cycle plants are also a significantly worse environmental option than combined-cycle facilities.

The size and inefficiency of some of these plants can contribute to substantial climate pollution. For example, a large data center power plant in Texas that Amazon is building with only simple-cycle turbines will be permitted to emit more than 33 million tons of greenhouse gases per year—more than some small countries emit annually.

How supercritical carbon dioxide works

American Supercritical’s founders say these small, inefficient gas turbines could be well suited to supercritical CO₂ technology.

Carbon dioxide becomes supercritical when it is pressurized and maintained at a constant temperature. In this state, it has the density of a liquid but continues to behave like a gas. That allows it to transfer energy efficiently while requiring less equipment than conventional systems.

American Supercritical wants to attach its units to small gas turbines to generate more electricity. The turbine would continue to burn natural gas, but the supercritical CO₂ system would use the turbine’s hot exhaust to produce additional power.

Instead of using the exhaust heat to boil water and produce steam, the system transfers heat directly through pressurized carbon dioxide. This could generate more electricity without creating additional emissions from the retrofit itself.

“We are essentially building a small combined-cycle plant,” says Schuham.

Supercritical CO₂ could reduce water use

The technology could also eliminate or significantly reduce the amount of water used in the power-generation process, an important consideration as data center water consumption faces increasing scrutiny.

The CO₂ system operates in a closed loop, so it does not need to be continually refilled. Co-founder Matthew Carlson, who has researched supercritical carbon dioxide for more than a decade, compares the system to a refrigeration unit that continuously circulates CO₂ to facilitate cooling.

American Supercritical targets 10-megawatt units

The company initially plans to sell 10-megawatt units, which are relatively small by power-plant standards. According to American Supercritical’s calculations, adding a supercritical CO₂ unit could increase a turbine’s efficiency by up to 50% without increasing greenhouse gas emissions or water use.

The retrofit would not eliminate emissions. Producing and burning natural gas still warms the planet, even when the turbines using it become more efficient. As a result, data centers could remain a major source of pollution at a time when the world needs to reduce its use of fossil fuels.