Extended Duration Operation of a Pilot-Scale Supercritical CO2 Test Loop

Abstract Supercritical carbon dioxide (sCO2) power cycles offer significant advantages over steam and organic Rankine cycles particularly for high-temperature thermal input, but remain a relatively new technology with limited extended duration operations and testing experience. As part of a test of a solid-fueled heater system, a simple recuperated sCO2 test system was designed, built and installed at the Utah San Rafael Energy Laboratory for extended test operations. The system design parameters included a maximum pressure of 20 MPa, and a maximum primary heater discharge temperature of 600 °C, and a design CO2 flow rate of 5 kg/s. The design point thermal input from the fired heater to the CO2 working fluid was 1.2 MW. Residual heat rejection to the environment was via evaporative water cooling using a conventional cooling tower, and the CO2 to water heat exchanger was of the diffusion-bonded heat exchanger type. The recuperator was of similar design, while the primary heat exchanger was similar to a tangent-tube fired boiler design, while the heat source was alternately coal, biomass and natural gas. The primary purpose of the test campaign was the demonstration of the fired heater — thus a work-extraction turbine was not included in the system for simplicity. A variable area throttle valve was used to control the system flow and pressure, allowing for more flexible operation than possible with a fixed geometry turbine. A robust closed-loop control system was designed for the test system permitting unattended fully automated operations. During the test campaign, extended-duration fired testing in excess of 200 hours of continuous operation were successfully achieved including several switches between gas and solid fuels, and variation in firing rate.