In a big breakthrough for the event of supercritical carbon dioxide (sCO2) energy cycles, the 10-MWe Supercritical Transformational Electrical Energy (STEP) Demo pilot plant in San Antonio, Texas, has wrapped up Section 1 testing, demonstrating the business readiness of its next-generation oblique sCO2 Brayton cycle.
The $169 million undertaking—the biggest of its sort on the earth—demonstrated a easy cycle configuration of a sCO2 Brayton energy cycle at a 10-MW scale on the totally built-in energy plant constructed on the Southwest Analysis Institute (SwRI) in Texas. The preliminary system configuration includes a single compressor, turbine, recuperator, and cooler, with warmth provided by a pure gasoline–fired heater (that carefully resembles a duct-fired warmth restoration steam generator).
Throughout Section 1 testing, the undertaking achieved full operational pace of its turbine at 27,000 RPM, working at 500C and 250 bar. It generated 4 MWe of grid-synchronized energy. “The info collected from the pilot plant will optimize the design, efficiency, and operability of future sCO2 energy methods, laying the inspiration for widespread business deployment,” stated expertise improvement agency GTI Vitality, which is spearheading the undertaking.
“The ten-MW scale and 500C temperature are properly fitted to waste warmth functions, offering emission-free energy from in any other case wasted warmth,” defined William Follett, GTI Vitality program director of sCO2 Applied sciences. “The following part of the undertaking, anticipated to begin in 2025, will reconfigure the plant to a Recompression Brayton Cycle (RCBC) configuration and improve the turbine inlet temperature to 715C to supply a big increase in effectivity. RCBC shall be relevant to energy era for a lot of zero/low emission warmth sources, similar to photo voltaic thermal, nuclear (fission and fusion), and biomass and fossil gas with carbon seize,” he stated.
When the second—and ultimate—part is accomplished, the STEP Demo is predicted to generate 10 MWe hourly. However its greater contributions shall be to spice up expertise development for the promising energy cycle. “The success of this undertaking underscores the potential of sCO2 expertise to remodel the way forward for energy era, bringing us one step nearer to commercializing improvements that can drive extra sustainable and resilient power methods,” famous Don Stevenson, vp of carbon administration and conversion at GTI Vitality on Monday.
sCO2: A Energy Cycle With Massive Potential
As POWER has reported, sCO2 energy cycles are primarily Brayton cycles that make the most of supercritical CO2 working fluid to transform warmth to energy, providing the potential for greater system efficiencies than different power conversion applied sciences (similar to steam Rankine or Natural Rankine cycles), particularly when working at elevated temperatures.
The Brayton cycle circulates heated working fluid and compresses it, which spins a generator that creates electrical energy. The distinctive properties of supercritical sCO2 provide important benefits over steam as a working fluid in closed and semi-closed cycles. In its supercritical state—achieved at temperatures above 31C (88F) and pressures above 7.4 MPa (1,070 psia)—sCO2 acts as a dense, compressible fluid, GTI has famous. This permits it to effectively take up thermal power, be compressed, and drive generators, leading to turbomachinery that’s as much as 10 instances smaller than comparable steam or air methods for a similar energy output. Together with greater efficiencies, the cycle might scale back water utilization and warmth supply flexibility, it says.
STEP has got down to show an oblique sCO2 recompression closed Brayton cycle in two distinct phases. In Section 1, testing was based mostly on a easy cycle configuration comprising a single compressor, turbine, recuperator, and cooler.
RCBC, wherein STEP shall be reconfigured as a part of Section 2, entails a high-efficiency cycle able to reaching this system’s thermodynamic effectivity targets of greater than 50%. In Section 2, the second (lower-temperature) recuperator and a bypass compressor shall be put in. Whereas Section 1 sought to show the system’s controls and operability and validate the efficiency of key elements, Section 2 will deal with technical dangers whereas minimizing the added complexity at every part.
Culminating a Decade Rigorous R&D
Completion of Section 1 culminates a serious effort by the STEP Demo group, which started in 2013 to determine a working sCO2 energy cycle with backing from the DOE’s Workplace of Fossil Vitality and Carbon Administration. The ability notably achieved mechanical completion in October 2023, which paved the way in which for full system commissioning checks to kick off in December 2023. Efficiency testing started in March 2024.
“This has been an 8-year-long undertaking (to date) with quite a few challenges to reaching this breakthrough expertise,” Follett famous on Monday.
Amongst important hurdles was the event of a number of world-first elements, together with the biggest printed circuit warmth exchanger, the biggest turbine cease valve and management valve, and the biggest set up of Inconel 725 piping ever constructed. These improvements required main advances in supplies science and manufacturing. For instance, engineers needed to discover ways to solid large-scale elements utilizing high-temperature alloys like Haynes 282 for the turbine cease valve and 740H for heater tubes and piping.
Specialised methods for inspecting and repairing turbine blades had been additionally developed to deal with the acute operational situations. Lastly, the group needed to develop intensive commissioning procedures that may be accepted by regulators to make sure protected operation of the novel system.
Improvement of the turbine, designed to advance from Know-how Readiness Degree (TRL) 6 (an engineering prototype) to TRL 7 (a full-scale prototype) has marked a very notable expertise triumph. The three-stage turbine encased in a fabricated barrel-style casing has a gross energy output of 16 MW and an influence density of 200 kW/kg (120 hp/lb)—boasting the very best terrestrial energy density ever achieved on the earth. Reaching this degree of energy density required a single-piece rotor and blades, which resulted in lengthy lead instances, and cautious thermal administration, Follett instructed POWER final 12 months.
The profitable commissioning of the compressor loop marked one other essential milestone. The compressor, which was examined at each easy cycle and RCBC speeds, features a cooling tower, an sCO2 stock administration system, and a major course of cooler. Commissioning the compressor, notably, recognized a number of gaps in information and efficiency for business sCO2 compressors. The compressor’s map efficiency differed considerably from predictions, decreasing its turndown functionality, and liquid operation emerged as an important requirement to assist chilly begins, Follett famous final 12 months. Moreover, precisely calculating compressor effectivity proved tough because of the low-temperature differentials throughout the compressor. That required the set up of torque and density meters to enhance measurement accuracy.
The STEP Demo group has additionally utilized superior dynamic and steady-state modeling to refine system efficiency. Changes, similar to decreasing compressor pace improved stability, whereas simulations of startup and shutdown processes helped optimize compressor operations and system reliability for future sCO2 functions.
sCO2 Cycles Discovering Traction in Evolving Energy Area
Together with GTI and SwRI, the pilot undertaking’s companions embrace GE Vernova’s Superior Analysis and the DOE’s Nationwide Vitality Know-how Laboratory (NETL). The undertaking additionally consists of contributions from 10 U.S. and worldwide companions: American Electrical Energy, Southern Co., Companies, Australia’s Commonwealth Scientific and Industrial Analysis Group, Thailand’s EGAT, France’s Engie, the Korean Electrical Energy Firm, Pure Sources Canada, Brazil’s Petrobras, Indian agency Spectrum Affect, and the Texas Fee on Environmental High quality (TCEQ).
“Industrial companions present steering to the operation, have entry to undertaking knowledge, and have preferential rights to undertaking [intellectual property,” STEP has noted. However, additional partners are welcome, it says.
For now, the STEP demo, which has $125 million in funding from the DOE, is set to remain an ongoing testbed for future sCO2 cycle-based power development, the team said. “The plant design is flexible and can be reconfigured to accommodate future testing to continue technology optimization and component development,” it said.
The joint-industry partnership is proving pivotal as interest in sCO2 power cycles ramps up. At least one company, NET Power, has set out to build a utility-scale 300-MW gas-fired power plant based on the Allam-Fetvedt sCO2 power cycle at an Occidental-hosted site near Odessa, Texas. While originally anticipated to begin operations in 2026, NET Power CEO Danny Rice said during an earnings call in August that the plant is now on schedule for startup “between the back half of 2027 and the first half of 2028.” NET Power has partnered with Baker Hughes to commence the first phase of a rigorous turboexpander equipment validation program at its La Porte demonstration facility in Texas.
Other pilot programs are also advancing sCO2 technology, including the CARBOSOLA project in Germany and several U.S. DOE-funded projects exploring sCO2 use in concentrated solar power (CSP) and waste heat recovery. Sandia National Laboratories, for example, continues to refine components and system performance for broader applications in both CSP and nuclear energy. Companies like Echogen Power Systems are developing sCO2-based systems for industrial applications.
“Achievement of the simple cycle max milestone [at the STEP Demo] is extremely encouraging each for the undertaking and commercialization of sCO2 energy cycle expertise,” famous John Crane, supervisor of NETL’s Superior Generators Know-how on Monday. “That is the biggest scale demonstration of the expertise so far and, as a result of the plant makes use of turbomachinery and course of gear which can be consultant of economic implementation, it additionally demonstrates a scalable expertise pathway to bigger functions within the 10s-100s MW vary.”
—Sonal Patel is a POWER senior editor (@sonalcpatel, @POWERmagazine).
