NETL researchers are working to overcome key challenges associated with supercritical carbon dioxide (sCO₂) power cycles, which will help advance next-generation power systems, such as the NETL-supported Supercritical Transformational Electric Power (STEP) pilot plant, that could help achieve a decarbonized energy future.

“Using CO₂ as a working fluid for power cycles has a number of potential advantages compared to steam, including higher efficiency, less maintenance, more compact equipment and lower cost,” said NETL’s Eric Liese. “Because a section of the cycle operates near the fluid critical state where properties change dramatically, control strategies are needed to prevent flow instability. This is especially true as power ramp rates are increased. The faster a power cycle can follow a desired power setpoint, the more value it has since it can better help meet variable grid demand. Our studies show that the NETL control strategies could solve this challenge, providing sCO₂ plants and test facilities such as STEP with viable solutions to real-world problems.”

Liese and his colleagues looked at operational control scenarios for increasingly fast power ramping of the sCO₂ cycle using a dynamic model based on the STEP project. The team found that as ramp rates are increased, variations in flow conditions intensify. This phenomenon has been noted by other works in literature, both modeling and experimental. Excessive variance in the flow of the sCO₂ will lead to unacceptable power delivery and possible system shutdown.

Liese and his colleagues proposed two control options to mitigate these variations in flow, and their performance was compared for power ramping operations. Their results were published in Applied Energy in an article titled, “Control methods for mitigating flow oscillations in a supercritical CO₂ recompression closed Brayton cycle.

“Two control approaches were developed,” Liese said. “Both options mitigated the flow variance problem and achieved close power setpoint following at a ramp rate of 7.5% of full power per minute.”

This achievement is another example of NETL’s important research under the Strategic Systems Analysis and Engineering directorate that is helping to optimize advanced energy systems and advance technology validation and deployment.

The mission of the STEP program is to design, build, and operate a 10 MWe pilot scale facility using an indirect-fired sCO₂ power cycle, demonstrate component performance and cycle operability over a range of operating conditions and show progress towards a lower cost of electricity. The STEP facility will serve as an opportunity for industry and government to work together to develop and mature the technology at the pilot scale to facilitate commercialization.  

NETL is a U.S. Department of Energy national laboratory that drives innovation and delivers technological solutions for an environmentally sustainable and prosperous energy future. By leveraging its world-class talent and research facilities, NETL is ensuring affordable, abundant and reliable energy that drives a robust economy and national security, while developing technologies to manage carbon across the full life cycle, enabling environmental sustainability for all Americans.