Dynamic modelling and simulation of a supercritical CO2 power system for waste heat recovery

The growing focus on energy efficiency and environmental sustainability has increased interest in waste heat recovery power systems, among which supercritical CO2 Brayton cycles are a promising option. Compared to conventional Rankine cycles, supercritical CO2 Brayton cycle-based systems offer the advantages of high efficiency and compactness. This work focuses on the development of a dynamic off-design model of a simple recuperated supercritical CO2 system for power generation from the waste heat recovery of an industrial process. The system consists of a compressor, an internal heat exchanger (recuperator) that recovers heat from the turbine outlet to preheat the working fluid after compression, a heater, the heat exchanger that recovers heat from the hot flue gases, a turbine, and a cooler, all arranged in a closed-loop configuration. The dynamic model is developed starting from a given design of the system, which has been obtained in previous works studying realistic waste heat recovery applications. The model of each component of the system is first developed individually, highlighting all the equations and assumptions adopted. After verifying that the dynamic simulation of each component shows a coherent dynamic response, the complete system is assembled by connecting all components together. All the models are implemented in MATLAB-Simulink environment. Finally, several simulations are carried out to analyse the transient behaviour of the system under different operating conditions. These variations include typical situations that may occur, such as changes in the mass flow rate or temperature of the hot source. The simulations show that the system is able to reach a new equilibrium point when variations are applied to the boundary conditions. By analysing the transients, it is possible to observe how the dynamic response of the system is mainly influenced by the cycle components with higher thermal inertia. From the simulation results and the issues encountered during the model development it is found how the compressor inlet is the most critical point of the system. The reason is that the thermodynamic conditions of CO2 in that point are very close to the CO2 critical point, where CO2 properties vary abruptly, as result of small changes in operating conditions. This highly influences the operation of the compressor, which may face instabilities if appropriate control strategies are not implemented. The developed model provides information to better understand the behaviour of supercritical CO2 systems and offers valuable insights for future studies and the development of adequate control strategies.