Natural gas still holds sway over the power generation profile of the US, but the cracks are beginning to show. Solar and wind resources are more competitive than they were just a few years ago, the cost of energy storage is coming down, and green hydrogen is also showing up in the mix. Now here comes another wild card in the form of solar-friendly supercritical carbon dioxide technology, aimed at shrinking the massive footprint of conventional turbines down to the size of a household appliance.
The Supercritical Carbon Dioxide Difference
Turbines based on supercritical carbon dioxide came across the CleanTechnica radar back in 2020, when we noticed that the US Department of Energy was eyeballing the technology as an energy efficient replacement for steam-driven turbines.
The familiar steam turbines in wide use at power plants today are based on 19th century technology. They typically range in size from less than 100 kilowatts to more than 250 megawatts, depending on the use case. When used to generate electricity in a central power plant they are massive beasts the size of a bus or larger.
Supercritical carbon dioxide turbines are different. They don’t deploy steam as a working fluid. Instead, they use a concentrated form of carbon dioxide — sCO2 for short — that hovers somewhere between a gas and a liquid.
The Energy Department anticipates that new supercritical carbon dioxide turbines can shave energy consumption at power plants by 10%, but that’s just for starters. They have a much smaller footprint than their steam-driven cousins, resulting in manufacturing efficiencies all along the supply chain.
By way of comparison, the Energy Department calculates that a 20-meter steam turbine would shrink down to one meter if replaced with an sCO2 turbine.
“Above the critical point, CO2 does not change phases (that is, change from gas to liquid). Instead, it undergoes a change in density in even small shifts in temperature and pressure,” the Energy Department explains. “This property allows a large amount of energy to be extracted at a high temperature, using equipment that is relatively compact.”
“The sCO2 turbines may be an order of magnitude smaller than today’s utility-scale combustion or steam turbines,” they emphasize.
The Concentrating Solar Power Connection
Like steam turbines, supercritical carbon dioxide turbines are fuel agnostic. The Energy Department anticipates deploying them in coal and gas power plants as energy efficiency upgrades.
The potential for more efficient carbon capture at fossil-fueled power plants is another aspect of the technology that has attracted attention from the Energy Department.
Much more exciting from a planet-saving perspective is the connection between supercritical carbon dioxide turbines and concentrating solar power. Concentrating systems use mirrors to collect and focus heat from the sun (see more concentrating solar coverage here).
The heat factor has attracted the Energy Department, which is looking for ways to bring down the cost of concentrating solar power systems. Despite considerable support from the Energy Department, they have been slow to gain a foothold in the US. An assist from supercritical carbon dioxide technology could flip the script.
“Supercritical carbon dioxide (sCO2) power cycles have the potential to reduce the cost of concentrating solar power (CSP) by far more efficiently converting high-temperature solar heat into electricity,” enthuses the Energy Department.
Putting Tiny Supercritical Carbon Dioxide Turbines To Work
The Energy Department’s showcase for sCO2 technology is taking shape in the form of a new $155 million, 10 megawatt-equivalent demonstration power plant. The power plant is the culmination of the Energy Department’s public-private STEP (Supercritical Transformational Electric Power) program, which launched back in 2016 under the wing of the Energy Department’s National Energy Technology Laboratory.
“Power cycles based on a sCO2 working fluid have the potential for higher thermal efficiencies and a lower capital cost when compared to state-of-the-art steam-based power cycles,” NETL explains.
“These potential benefits, combined with the compounding performance benefits from a more efficient cycle on balance of plant requirements, fuel use, emissions, water use and cost-of-electricity (COE), have created broad interest in sCO2 power cycles,” they add.
The independent R&D organization Southwest Research Institute is a leading partner in the project along with the firms GTI Energy and GE Vernova. Construction of the building shell took place between 2018 and 2020, followed by the startup of a supercritical CO2 compressor earlier this year.
In the latest update from SwRI, the team has just marked the completion of the mechanical work on the system, including new turbines that are about 1/10 the size of a conventional steam turbine.
Though only about the size of an office desk, household refrigerator, pony, credenza, or golf cart, the new turbines are powerful enough to generate the electricity equivalent of 10,000 typical homes.
There Goes Texas, Again
If all goes according to plan the STEP plant will be fully operational early in 2024. When it does, it will put Texas on the map — again — for showcasing powerful new energy technology. The STEP facility is located at the sprawling SwRI campus in San Antonio, Texas.
That’s of a piece with other developments in Texas. Despite its strong foothold in the fossil energy industry, Texas has emerged as a wind and solar power leader in the US. Green hydrogen and electrofuels activities are also stirring in the state.
Public officials in Texas have been on a long, loud rant against renewable energy investing, but SwRI is not shy about tooting the supercritical carbon dioxide horn for itself and its partners.
“The STEP Demo pilot plant is one of the largest demonstration facilities in the world for sCO2 technology to dramatically improve the efficiency, economics, operational flexibility, space requirements and environmental performance of this new technology,” SwRI explained in a press release last March.
No word yet on when or how the Energy Department intends to assess the system’s compatibility with concentrating solar power. However, the connection has been on SwRI’s radar since at least 2012, when the organization received an $8.5 million Energy Department grant to develop a supercritical CO2 turbine for use in concentrating solar systems.
That grant was followed by several others including a $3.5 million award in 2019, aimed at developing the sCO2 connection to concentrating solar power systems.
To put Texas on the renewable energy map even more, last year SwRI earned an R&D 100 award from R&D World Magazine for its work with Hanwha Power Systems on a system to convert thermal energy to electricity. Called an sCO2 compander, the system was designed specifically to convert heat from a 10-megawatt concentrating solar power plant. The work was supported in part by the Office of Energy Efficiency and Renewable Efficiency at the Energy Department.
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Photo (cropped): This building in San Antonio, Texas houses a 10MWe power plant that will demonstrate new supercritical carbon dioxide technology (drone photograph by Bryan Winter, courtesy of SwRI).
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