A step toward supercritical-carbon-dioxide power generation

By Scott Jenkins |

Construction is complete on a new building to house a fully integrated electricity-generating power plant that will demonstrate operability and performance of a 10-MW power-generation facility using supercritical carbon dioxide (sCO2) as the working fluid in a Brayton thermodynamic cycle. Known as the STEP (supercritical transformational electric power) Demo project, the pilot-plant test facility is located on the property of Southwest Research Institute (San Antonio, Tex.), one of the STEP team members, along with project-leader GTI (Des Plaines, Ill.; www.gti.energy), GE Research (Niskayuna, N.Y.), and the National Energy Technology Laboratory (Morgantown, W.Va.).

The sCO2 cycle (diagram) offers significant potential benefits in efficiency, cost and environmental performance over the steam Rankine cycle, a common method for producing electricity currently. Above its critical point, CO2 has properties of both a gas and liquid. “In a Rankine cycle, you have a liquid-vapor phase change,” explains John Marion, GTI senior program director. “In a Brayton cycle with sCO2, there is no phase change — sCO2 behaves like a gas, but it’s non-compressible like a liquid — and that allows overall cycle efficiencies of over 50% (2 to 5 percentage points higher than steam Rankine), and allows the turbomachinery components to be 10 to 30 times smaller than those for Rankine cycles.” These attributes result in a levelized cost of energy that is 3–4% lower than with steam cycles.

The objective of the STEP Demo is to integrate all the elements required for a grid-connected commercial power-generation plant, which has not been done previously, although sCO2 Brayton cycles have been used in smaller, niche applications. Major equipment components have been fabricated and delivered for assembly.

Recent advances in materials and turbine modeling have allowed the Brayton sCO2 cycle in a practical system for power generation, says Don Stevenson, GTI vice president of energy supply. In order to realize the efficiency gains with the sCO2 Brayton cycle, the system needs to operate at high (500–700°C) temperatures, which necessitates materials that can withstand the temperatures, for example.

Other technological advances developed and built for the STEP Demo project include high-temperature, printed-circuit heat recuperators, a monolithic turbine rotor and the application of dry gas seals for the turbine and compressor, Marion says.

The STEP project leaders anticipate commissioning and testing the pilot facility in summer 2021. It is funded by the U.S. Department of Energy and by industry partners.

supercritical carbon dioxide

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