The POWER Interview: Pioneering STEP Supercritical Carbon Dioxide Demonstration Readying for 2022 Commissioning

Construction of the 10-MWe Supercritical Transformational Electric Power (STEP) pilot plant, a public-private collaboration to demonstrate and test supercritical carbon dioxide (sCO2) power generating technology, is making marked progress toward a mid-2022 startup date. 

Attendees from Experience POWER, HydrogeNext, and the Distributed Energy Conference, three of POWER’s annual in-person events, which took place last week in San Antonio, Texas, viewed part of the novel 22,000-square-foot facility at the Southwest Research Institute (SwRI) campus that houses the pioneering project. Led by the Gas Technology Institute (GTI), the project’s partners include SwRI, General Electric Global Research, and the U.S. Department of Energy.  

STEP will demonstrate a fully integrated electricity-generating power system that will use sCO2 as the working fluid—in place of the conventionally used steam—to operate an indirect sCO2 recompression closed Brayton cycle. Among the project’s ambitious scope is to verify the performance of first-of-a-kind components—including its turbomachinery, recuperators, compressors, and seals—and demonstrate that they can operate at a turbine inlet temperature of at least 700C. 

If successful, it will dramatically improve the size, efficiency, economics, operational flexibility, space requirements, and environmental performance of the new technology, GTI Senior Program Director John Marion told POWER in an interview. That could open up a wide range of potential power applications, “including fossil-fired systems, waste heat recovery, long-duration energy storage, concentrated solar power, and nuclear power generation,” he said. 

Marion has held various roles in steam technology research and development (R&D), first with ABB and Alstom Power, and then notably serving as director of Technology and Research and Development at power equipment technology giant GE Power for 18 years. Before he joined GTI to head up its projects related to new power conversion technology developments, he also served as director of R&D for Generation at the Electric Power Research Institute. POWER’s interview with Marion here is lightly edited. 

POWER: What are supercritical carbon dioxide power cycles, and why are they noteworthy? 

Supercritical CO2 (sCO2) power cycles are Brayton cycles that utilize supercritical CO2 working fluid to convert heat to power. They offer the potential for higher system efficiencies than other energy conversion technologies such as steam Rankine or Organic Rankine cycles; especially when operating at elevated temperatures. The unique properties of supercritical CO2 offer intrinsic benefits over steam as a working fluid in closed and semi-closed cycles to absorb thermal energy, to be compressed, and to impart momentum to a turbine. The supercritical state of CO2 (nominally above 31C [88F] and 7.4 MPa [1070 psia]) is easily achieved, and above these conditions is a supercritical fluid with compressibility but with higher density compared to steam or air. This results in much smaller turbomachinery (factor 10:1) for a given power level.

sCO2 power cycles can offer several benefits:

  • Higher cycle efficiencies due to the unique fluid and thermodynamic properties of sCO2.
  • Reduced emissions resulting from lower fuel usage.
  • Compact turbomachinery, resulting in lower cost, reduced plant size and footprint, and more rapid response to load transients.
  • Reduced water usage, including water-free capability in dry-cooling applications.
  • Heat source flexibility.

POWER: The STEP demo is especially notable for its joint industry program, which includes “open” participation by original equipment manufacturers (OEMs), engineering companies, utilities and power plant owners and operators, and other energy companies. What is driving interest in the project? What benefit does it offer these companies in the power generating space? 

Enabled by modern design tools and materials, sCO2 power cycles offer the potential for compact and highly efficient power generation when applied to solar, fossil, nuclear, and waste heat recovery energy sources. Depending upon the application, sCO2 power technology offers efficiency and cost improvements over incumbent steam-based or organic Rankine cycle-based approaches. The STEP demo project is a public-private funding partnership currently with $115 million in federal [funding from the Department of Energy’s National Energy Technology Laboratory] and $41 million [from] industry. It has been active in keeping the public informed on project status and achievements. In addition, a Joint Industry Program (JIP) enables interested parties (eight to date) to become project supporters at a small fraction of the project costs (<1%) and obtain detailed project results. Through these actions, the project seeks to be transparent with project results and the exciting potential of sCO2 power systems.

POWER: One objective of the pilot has been to boost the technology readiness level (TRL) of the sCO2 power cycle from TRL 3 to TRL 7. Some of these components have been conceived and engineered from the ground up. Would you describe some of the project’s most challenging undertakings? 

The STEP demo project at 10 MWe and with a turbine inlet temperature of more than 700C is significant in the scale-up and commercialization of the technology. Challenges have been encountered and systematically resolved on low technology readiness equipment and the limited manufacturing supply chain and experience including challenging applications of high-temperature materials of Inconel 740H and Haynes 282 in first-of-a-kind components including primary heater tubes, process piping, and turbine stop and heater protection valves. At this date, most issues have been resolved and the project is progressing with final equipment manufacturing, delivery, and installation. The project team and sponsors’ view is that these issues encountered and being resolved, in addition to the planned testing to confirm performance and operation, are a core purpose for executing this commercial-scale pilot project.

 POWER: During the tour, we viewed a model of the tiny turbine, truly a marvel of engineering. Would you explain how so much power can be packed into that small component? What are the dimensions of the turbine, and what is its function? 

Yes, among the attributes of sCO2 power technology is the compact nature of the sCO2 turbomachinery. The STEP turbine rotor is only about a meter in length including room for seals and bearings. The equivalent steam turbine would be more than 10 times this size. The compact nature results from the high density of carbon dioxide in its supercritical fluid state. Because of its small rotor size in this pilot-scale demonstration, the shaft, blades, and shroud were 5-axis EDM [electrical discharge machining] machined as a single piece from a monolithic block of Nimonic 105 material (a nickel-cobalt-chromium alloy). There are limited shops capable to complete this work. Machining was been time-consuming but has been completed, and final assembly of the turbine system is in progress. The STEP turbine will generate 16 MWe gross power at a [turbine inlet temperature (TiT)] of 715C and 250 bar inlet pressure.

The new STEP facility will be significantly smaller than today’s power plants. For example, a desk-sized sCO2 turbine could power 10,000 homes. Courtesy: GTI

POWER: The pilot, which kicked off in 2018, has evidently made major progress. When are you targeting the completion of construction? Would it be grid-connected? How long do you anticipate the project will operate? 

Mechanical completion is expected in the spring of 2022. Commissioning and testing in a simple recuperated cycle system configuration is scheduled through 2022. The STEP demo system will then be modified to add additional heat recuperation and operate in an RCBC (Recompression Brayton Cycle) configuration to demonstrate the highest efficiency potential of the technology through 2023. This pilot is a fully operational electric generating power plant and testing is planned that will put power generated on a local grid. Extensive testing is planned to fully explore the operating envelope and confirm performance and control strategies.

POWER: STEP evolved from a smaller 1-MW project. Would STEP show the technology is commercially applicable? 

A 1 MW-scale test loop at SwRI was commissioned and operated to STEP-equivalent operating conditions (715C and 250 bar) in December 2018. This test loop was used to perform cost-effective reduced-flow validation testing of the mechanical performance of the first turbine prototype and was later adapted to test other machinery prototypes including the compressor and an integrally geared compressor-expander unit. These design and validation data were used to inform the designs of the turbine and compressors manufactured for full-scale testing at the STEP demo, which will demonstrate both mechanical and aerodynamic performance of the machinery and heat exchangers as well as overall system performance.

POWER: Where are the technology’s nearest-term applications? 

Supercritical CO2 power cycles promise substantial cost, emissions, and operational benefits that apply to a wide range of power applications including coal, natural gas, waste heat, concentrated solar, biomass, geothermal, nuclear, and shipboard propulsion. Most of today’s water-steam power cycle applications can be superseded by sCO2 power cycles. The STEP 10-MWe pilot demo project is demonstrating indirect-fired sCO2 cycles to known available materials limits (T>700C) in a fully integrated 10-MWe electric generating pilot plant. The project will enable the progression of technology readiness level from TRL of 3 to a TRL of 7 and subsequent commercialization.

Early commercial adaptation is expected for waste heat recovery from simple cycle small [gas turbines] such as compressor stations. Supercritical CO2 power attributes for this application include high efficiencies at small scale (1 MW to 20 MW), compact equipment, fast ramp rate capability, avoidance of water cooling and polishing, and the potential for autonomous operation.

Sonal Patel is a POWER senior associate editor (@sonalcpatel@POWERmagazine).

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