Control strategy for actual constraints during the start–stop process of a supercritical CO2 Brayton cycle

Abstract

Supercritical carbon dioxide (sCO2) Brayton cycles have been actively researched for potential use in a wide range of energy-conversion applications. The start–stop process, as an important transition stage, has a significant impact on the stability and security of system operation. Previous studies have ignored practical constraints which are important for the safe operation. This study developed a dynamic model of the recompression Brayton cycle and validated its components carefully. The effect of the operating variables on different constraints is analysed, and corresponding measures to practical constraints are proposed. A complete start–stop scheme considering multiple constraints is developed, and a simulation verification is performed. The results indicate that the rotation speeds of compressors should be changed simultaneously and the rate of temperature change must be constantly controlled for successful start–stop operation. Compared to the start-up process, overloading is more likely to occur during the shutdown process. Timely lowering of the turbomachinery rotation speed after the system load drops can effectively prevent overloading. There are various practical constraints and strong coupling relationships between the countermeasures and constraints. The coordination between the heat source and rotation speeds of the two compressors is the key factor that affects each constraint condition.