Inventory Management and Control Options for Transient Operation of Supercritical Carbon Dioxide Brayton Cycles

Abstract

Abstract. 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. 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. A linear control system for regulating inventory tank valves is analyzed through step-response testing, revealing rapid response characteristics of inventory rejection, while nonminimum phase behavior of inventory injection. A transfer function-based representation is proposed to aid control system design and analysis. Design of a proportional-integral (PI) controller is carried out using the 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 nonminimum phase behavior. Transient thermal analysis of the recuperator, performed a posteriori step using flownex, quantifies time-varying heat duties and cycle efficiency. During part-load transients, small terminal temperature variations result in nearly constant wall temperature profiles, making the energy stored in the heat exchanger walls orders of magnitude smaller than the convective heat transfer rates. 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.