Dynamic characteristics of a direct-heated supercritical carbon-dioxide Brayton cycle in a solar thermal power plant

The dynamics of a direct-heated closed Brayton power conversion system (PCS) with supercritical carbon-dioxide as the working-fluid (sCO2 PCS) is investigated in this study. Simulations of the dynamic response of the sCO2 PCS to changes in ambient air temperatures and solar energy input from parabolic trough collectors on representative days for summer and winter are presented. A control-oriented model describing sCO2 PCS dynamic behaviour has been constructed using mathematical models of heat-exchangers and turbomachinery. Changes in solar heat input causes movement of carbon-dioxide (CO2) mass between the hot and cold-sides of the PCS. Movement of mass results in variations in CO2 mass-flow rate, pressures, temperatures, and net-power output. The sCO2 PCS maintains a relatively stable net-power output when operating under conditions representative of an average day in summer with capped heat input. Turbine inlet temperatures rise well above nominal values due to reductions in CO2 mass-flow rates. A significant power output penalty is incurred on a winter day due to conditions at the compressor inlet becoming subcritical. The simulations highlight the potential for utilising CO2 charge manipulation for sCO2 PCS mass-flow rate control in summer, and the need for control of compressor inlet conditions in winter, for sustained fully supercritical operation of the PCS within allowable limits.

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