Tuesday, August 22nd, 2017

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

Available online 3 January 2013
Publication year: 2013
Source:Energy

The dynamics of a direct-heated closed Brayton power conversion system (PCS) with supercritical carbon-dioxide as the working-fluid (sCO 2 PCS) is investigated in this study. Simulations of the dynamic response of the sCO 2 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 sCO 2 PCS dynamic behaviour has been constructed using mathematical models of heat-exchangers and turbomachinery. Changes in solar heat input causes movement of carbon-dioxide (CO 2 ) mass between the hot and cold-sides of the PCS. Movement of mass results in variations in CO 2 mass-flow rate, pressures, temperatures, and net-power output. The sCO 2 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 CO 2 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 CO 2 charge manipulation for sCO 2 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.

Highlights

► A closed supercritical-CO 2 Brayton cycle in a solar thermal power plant is modelled. ► Dynamic response of the cycle on representative summer and winter days is simulated. ► Fluctuations in heat input causes mass movement between hot and cold-sides of cycle. ► Control of compressor inlet temperature is required in winter for increased output. ► CO 2 mass-flow control is required in summer to reduce turbine inlet temperatures.


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