(547f) Optimal Design and Dynamic Modeling of Microtube Recuperators in an Indirect Supercritical Carbon Dioxide Recompression Brayton Power Cycle

Supercritical carbon dioxide (sCO₂) has strong potential as the working fluid for power generation cycles because of the potential for high efficiency, compact equipment, and lower costs. For instance, the highly-recuperated recompression Brayton cycle will use compact recuperators, rather than conventional shell-and-tube heat exchangers, resulting in a lower capital cost and footprint for the sCO₂ application. In the sCO₂ Brayton cycle, the overall thermal efficiency strongly depends on the design and performance of the recuperators because of the smaller cycle pressure ratio and the higher temperature of the turbine outlet stream exchanging heat with the cooler, compressed CO₂ entering the turbine inlet. Thus, the compact recuperator design is important. Furthermore, modeling the dynamic behavior is particularly important with regards to its impact within the cycle model for analyzing systems control and operation. It is also noted that the transport and thermodynamic properties of CO₂ vary considerably near its critical and pseudo-critical points, which makes most of the design approach and conventional thermal-hydraulic correlations based on the constant property assumption not suitable. Hence, instead of using conventional heat exchanger models available in commercial process modeling software, customized models must be developed.

With this motivation, a rigorous design model has been developed using Aspen Custom Modeler for high-temperature and low-temperature microtube recuperators in an indirect sCO₂ Brayton recompression cycle with improved steady-state economic characteristics and preferred dynamic performance characteristics. Additionally, a dynamic model has been developed to simulate the operating conditions spanning normal design point, to startup, shut down and other off-design conditions. This model is imported to a system-level dynamic model of the entire sCO₂ Brayton cycle to improve simulation accuracy when investigating the system dynamic behavior and developing advanced control strategies. In both models, the recuperators are discretized along the axes to handle the changing properties, and simulated with rigorous thermal-hydraulic correlations and commercially available microtube sizes. In this presentation, we will focus on the following aspects: (1) one-dimensional steady-state design approach of the recuperators in sCO₂ Brayton cycle, (2) design optimization for better economics along with superior dynamic characteristics by considering various objectives such as maximization of the heat transfer area, maximization of the compactness, minimization of the metal mass, minimization of the thermal residence time, by using successive quadratic programming algorithm, (3) sensitivity studies of different values of the design parameters, such as minimum temperature approach and maximum allowable pressure drop, which highly impact the thermal efficiency and the economic performance of the entire sCO₂ Brayton cycle, (4) two-dimensional dynamic model of the recuperators spanning both design and off-design conditions, and (5) re-design to obtain superior transient response of the recuperators.