Thermal and thermodynamic optimization of the performance of a large aperture width parabolic trough solar collector using gaseous and supercritical CO2 as heat transfer fluids

The degradation of thermal-oil-based working fluids in parabolic trough solar collector (PTSC) systems at temperatures above 400 degrees C has accelerated the search for cost-effective and efficient heat transfer fluids (HTFs) for high-temperature applications. Carbon dioxide is abundant and has the potential to be used as a working fluid in PTSCs. In this work, the performance of a larger aperture width PTSC with CO2 as the working fluid (either in gaseous or supercritical phase) is investigated. A PTSC system with an aperture width of 9 m (geometric con-centration ratio of 113) was considered. The operating pressure is in the range of 40-100 bar, the HTF inlet temperatures are between 650-1000 K, and the flow rate varies from 32.6 to 653 m3/h. A thoroughly verified and validated computational model using a combination of Monte-Carlo ray tracing and computational fluid dynamics was used for the numerical investigations. In addition, the Realizable k-epsilon model was used for turbu-lence modeling. The results show that CO2 at 80 bar has comparable thermal performance relative to current thermal oil-based HTFs. Results further show that supercritical CO2 (sCO2) has the best thermal performance compared to gaseous CO2. Moreover, the entropy generation minimization method was used to determine the Reynolds number that minimizes the irreversibilities at a given operating pressure. At low pressures, much higher flow rates are required to achieve high heat transfer rates and reduce temperature gradients in the absorber tube. Furthermore, correlations have been developed for the optimal Reynolds numbers at which the entropy generation rate is minimum and the collector efficiency is maximum.