Comparison of direct and indirect natural draft dry cooling tower cooling of the sCO2 Brayton cycle for concentrated solar power plants

This study investigates the performance of direct and indirect cooling in conjunction with natural draft dry cooling towers (NDDCT) for the heat rejection of a supercritical carbon dioxide (sCO2) recompression Brayton cycle. One dimensional models were developed for direct and indirect cooling NDDCTs. The inlet conditions to the heat rejection system that correspond to the peak cycle efficiency for the scenario considered were identified and used to determine the minimum NDDCT size for direct and indirect cooling. The direct cooling configuration requires a smaller tower than indirect cooling, due to the much higher temperature difference between the two fluid streams. When analysed in isolation the direct cooling NDDCT is found to over cool at both high and low inlet temperatures. The direct and indirect dry cooled recompression cycles were analysed at a range of ambient temperatures, using a coupled NDDCT and cycle model approach, in order to represent daily and annual variances. In both direct and indirect configurations, the cycle is overcooled at low ambient temperature, reducing cycle efficiency. A novel bypass arrangement was proposed that allows control of the compressor inlet temperature by reducing heat rejection from the tower, and allows both cycles to maintain peak efficiency at low ambient temperatures. At the design point ambient temperature of 20 °C the cycle efficiency is 49.6%. This is maintained down to 0 °C for both configurations, and drops to 44.7% and 43.8% at 50 °C ambient for direct and indirect respectively.