Inside the STEP Demo: The Largest Supercritical CO2 Turbine Demonstration

Inside the STEP Demo: The Largest Supercritical CO2 Turbine Demonstration

Introduction

Supercritical CO₂ (sCO₂) power cycles are gaining global attention for their compactness and efficiency. The STEP Demo project in San Antonio, Texas, is the largest sCO₂ demonstration in the world to date. It is designed for 10 MWe net output and represents a major milestone in transitioning sCO₂ Brayton cycles from laboratory to grid-scale.

This article reviews what is publicly known about the STEP Demo turbine — including pressure, heat input, compressor design, recirculation, and technical tendencies — and highlights patent and technical literature that shaped its development.

Turbine Operating Parameters

Operating pressure (tested): ~250 bar, equal to about 3,600 psi 【powermag.com†source】【gti.energy†source】

Turbine speed: Achieved 27,000 rpm in testing 【swri.org†source】

Inlet temperature (Phase 1): ~500 °C

Future target inlet temperature: ~715 °C when reconfigured for recompression closed Brayton cycle (RCBC) 【gti.energy†source】

Type: Multi-stage axial sCO₂ turbine in a closed Brayton configuration 【sco2.eu†source】

Heat Source and Heat Flow

The STEP facility is indirect-fired, using a large CO₂ heater as the primary heat source.

Heater size: ~93 MWth 【netl.doe.gov†source】

Converted to BTU/hr: about 317 million BTU per hour thermal input

Recuperation: Includes a 50 MWth high-temperature printed circuit recuperator (PCHE) 【netl.doe.gov†source】

This high heat input supports the turbine’s goal of generating up to 10 MWe net, with a target cycle efficiency exceeding 50%.

Compressor and Flow Recirculation

The STEP Demo uses high-speed turbocompressors designed for sCO₂:

Phase 1 configuration: A single compressor, turbine, recuperator, and cooler (simple Brayton cycle).

Recompression upgrade: Will add a bypass compressor loop, converting the system into a Recompression Closed Brayton Cycle (RCBC), improving efficiency by splitting the flow and recompressing a portion at a lower temperature 【gti.energy†source】.

The entire system is a closed loop, continuously recirculating CO₂ through expansion, recuperation, cooling, compression, and reheating.

Rotor Geometry and Stage Count

Detailed rotor geometry, blade counts, and exact stage layouts are not publicly disclosed due to proprietary and export-control considerations. However, available presentations and technical briefings confirm that:

The turbine is multi-stage axial flow.

It draws heavily on the GE/SwRI SunShot sCO₂ turbine design, which operated at 27,000 rpm, 715 °C, and 250 bar 【netl.doe.gov†source】.

Stage count is believed to be in the 3–6 stage range for the 10 MW class, based on comparable axial Brayton turbines in this pressure and flow regime.

Technical Tendencies and Patent Literature

Several patent filings and technical reports inform STEP’s development:

GE/SwRI patents on high-temperature sCO₂ turbine blade cooling and axial expander design (examples: US20170137504A1 “Supercritical CO2 Turbomachinery with Cooling Flow Paths”).

NETL/DOE UTSR reports note development of compact multi-stage axial expanders optimized for 715 °C, 250 bar service 【netl.doe.gov†source】.

SwRI publications emphasize rotordynamic stability at high density, compact sealing strategies, and integration of printed circuit recuperators with the axial turbine flowpath.

These patents and tendencies highlight the challenge of maintaining material strength, sealing integrity, and aerodynamic performance under the unique conditions of dense sCO₂.

Conclusion

The STEP Demo turbine is the largest supercritical CO₂ power cycle project to date:

Designed capacity: 10 MWe (4 MWe demonstrated in Phase 1)

Operating pressure: ~3,600 psi

Heat input: ~317 million BTU/hr

Type: Multi-stage axial turbine in a closed sCO₂ Brayton cycle

Cycle upgrade: Will transition to a recompression configuration for higher efficiency

Though exact rotor dimensions and stage counts remain proprietary, STEP establishes the technical pathway for sCO₂ turbines in the tens of megawatts. By leveraging multi-stage axial design, compact recuperators, and recompression cycle refinements, it demonstrates how sCO₂ can compete with conventional steam in efficiency while reducing size and complexity.

TEL: 1-608-238-6001 Email: greg@infinityturbine.com

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