Performance of a solar thermal power plant with direct air-cooled supercritical carbon dioxide Brayton cycle under off-design conditions

The use of an efficient and compact supercritical carbon dioxide (sCO2) Brayton cycle in concentrated solar thermal power plants has the potential to reduce costs of electricity generation. Heat rejection in the hot-arid climate is of great concern to the power cycle, especially by natural draft dry cooling technologies. For this purpose, a comprehensive design and rating analyses of the sCO2-air cooling process was conducted based on short natural draft dry cooling towers. This approach is featured with the capture of non-linear characteristics in physical properties of CO2 and geometry of fin-tube air-cooled heat exchangers. It is found that the proposed methodology successfully predicted the experimentally observed outlet temperatures of an existing cooling tower. By utilizing off-design models of heat exchanger and turbomachinery, a direct air-cooled recompression sCO2 cycle was investigated for a parabolic trough solar plant with thermal energy storage (TES). The impacts of pressure ratio, recompression fraction, shaft speed and boundary conditions, i.e., ambient air temperature and solar intensity, were investigated on the power output and key parameters of the power plant under quasi steady state conditions. The results show that the recompression fraction significantly affects the pitch point in the recuperators, the optimum value of which decreases with an increase in compressor inlet pressure and in shaft speed. In addition, the direct air-cooled power system depends strongly on ambient environments, and is able to handle lower solar intensities without deterioration in electricity generation by the buffering of TES. The cooling tower approach decreases non-linearly as the ambient temperature increased, indicating that a fixed approach of typical 15 °C results in a conservative electricity production at hot climatic conditions.