Inventory Management and Control Options for Transient Operation of sCO2 Brayton Cycles

Supercritical Carbon Dioxide (sCO2) power cycles have emerged as a promising technology for a variety of applications involving transient operations. These include load-following power plants and systems operating with intermittent heat inputs, such as waste heat recovery, CO2 batteries, and solar thermal power generation. In this context, this paper explores dynamic response and control system design of a 5 MW simple recuperated sCO2 Brayton cycle, focusing on inventory and turbine bypass control strategies. The study examines a linear control system for regulating inventory tank valves, analyzing system’s step response during inventory rejection and injection. It is found that inventory rejection yields rapid responses, while inventory injection exhibits non-minimum phase behavior. Transfer function-based representation is proposed to aid control system design and analysis. Design of a Proportional-Integral (PI) controller is carried out using classical Bode plots technique. Although the PI controller improves tracking accuracy, its performance degrades at high ramp rates, and due to plant gain reduction and non-minimum phase behavior. Turbine bypass control is demonstrated to serve as an alternative, offering first-order dynamics and faster ramp rates. Overall, the study demonstrates the efficacy of simple PI controllers for load regulation in sCO2 Brayton cycles.