Design and Operability Challenges for Supercritical CO2 Plants: The sCO2-Flex Centrifugal Compressor Test Experience

Interest in supercritical carbon dioxide (sCO2) cycles has grown significantly in recent years. The peculiar properties of the fluid open unique opportunities but also poses several challenges. This article aims at narrowing the knowledge gap in operating such plants, leveraging the test experience of a full-scale 5 MW centrifugal compressor (CC) prototype designed for the sCO2-flex project (H2020 funded program, grant agreement No. 764690). The test rig replicated a complete recycle loop for the compressor. In addition to validating the compressor's performances in its design condition, a larger envelope of operation and transients was explored. The chapter describes engineering groundwork and design choices regarding: (i) transient operation such as pressurized startups and emergency shutdowns (ESDs); (ii) safety relief valves and discharge lines, with possible CO2 solidification; and (iii) vibrations due to extremely high density. For all these topics, specific challenges brought by the sCO2 are described. The most meaningful results, in excellent agreement with predictions, are shown; key learnings from the test experience are discussed. The importance of anticipating risks during the design phase is highlighted and recommendations for similar installations are given. The article concludes with hints on testing severe off-design conditions, with two-phase flow at compressor suction.