Thermofluid design and dynamic simulation of an sCO2 power cycle for a 50 MWe concentrated solar thermal tower plant in Southern Africa

Du Sart, Colin F

Thermofluid design and dynamic simulation of an sCO2 power cycle for a 50 MWe concentrated solar thermal tower plant in Southern Africa

Thesis / Dissertation

2026

Publisher

University of Cape Town

Department

Department of Mechanical Engineering

Faculty

Faculty of Engineering and the Built Environment

Abstract

Supercritical carbon dioxide (sCO2) power cycles have been identified as an attractive technology to reduce the cost, complexity and footprint, and to increase the thermal efficiency of thermal power plants. In this work, two promising sCO2 power cycles are investigated for use in a proposed dry-cooled 50 MWe concentrated solar power (CSP) plant in Southern Africa. These include the partial cooling with reheating (PCRH) cycle and the recompression with intercooling and reheating (RCICRH) cycle. Initially, a techno-economic design point study is performed to determine near optimal operating conditions and component requirements in which the recuperators are optimally sized, by volume, as printed circuit heat exchangers (PCHEs). This contrasts with other comparative studies where simplified models are used that do not consider the effect of actual recuperator geometry on heat transfer and pressure drop. The results suggest that the RCICRH cycle requires larger recuperators and turbomachinery, resulting in a marginally higher capital outlay for the power cycle, but offers superior thermal efficiencies, which suggests that a smaller solar field is required. However, the PCRH cycle requires a smaller solar receiver system and a smaller thermal energy storage (TES) system. Thereafter, conceptual designs for the cycle components (compressors, turbines, heaters and coolers) are developed. Additionally, auxiliary equipment (piping, valves, fans, TES tank and inventory tank) is sized and selected. Given the limited data available on sCO2 component modelling, unique sizing methodologies that employ fundamental one-dimensional (1D) thermofluid network modelling approaches are developed and verified. Compared to models used by others, which often contain significant simplifying assumptions, the models used in this work are more suited for off-design and transient power cycle studies, and for costing. Furthermore, by developing component designs for both cycles, the similarities and differences of the PCRH and RCICRH cycles are further clarified. A dynamic model of the RCICRH cycle is then developed. Using the model, a bypass and fan on-off control strategy for a multi-cell mechanical forced draft air-cooled heat exchanger (ACHE) system is proposed and demonstrated for the first time during changing ambient conditions. The model is also used to characterise the off-design performance of the cycle and the changing component boundary conditions for various methods of load control. Finally, the transient performance and operating requirements for the cycle are investigated. Accurate and stable load-following can be achieved using a combination of inventory and upper cycle bypass control. Furthermore, by using both upper cycle bypass control and compressor outlet throttling, the cycle can continue to operate during a load-rejection event. This is the first sCO2 study where all major cycle components are designed, fully integrated, and used to perform a comprehensive off-design and dynamic study for a cycle of this output capacity for a CSP application. While the selected boundary conditions and technological limitations considered in this study are unique to Southern Africa, many of the methods presented in this work may be used to develop conceptual designs for sCO2 power cycles of a similar scale.