PERFORMANCE ENHANCEMENT OF PARABOLIC TROUGH SOLAR COLLECTORS AND THERMODYNAMIC ANALYSIS OF SOLAR-DRIVEN ABSORPTION SYSTEMS

Abstract

In this research, a V-shape ribbed tube is utilized to improve the thermal performance of a parabolic trough collector (PTC). Six different rib arrangements are employed, and a detailed analysis is presented. In addition, the effect of adopting a secondary reflector (SR) on the temperature distribution around both a smooth and a ribbed parabolic trough receiver (PTR) tube is conducted. A computational fluid dynamics model is employed to study the heat transfer and fluid flow characteristics inside the tube. Results show that V-shape ribs are an effective tool to stir up the flow and increase the velocity gradient of the fluid near the inner surface of the tube. This helps increase the convective heat transfer rate and reduce the tube’s maximum circumferential temperature. Moreover, results from the study show that the secondary reflector contributes to a further decrease in the tube surface temperature and the circumferential temperature difference. The combination of a V-shape ribbed PTR tube and a secondary reflector is thus shown to be beneficial for the PTC system, especially at high Reynolds numbers. A parametric investigation of the thermal enhancement of a double-reflector parabolic trough collector when employing an in-line mixed V-shape (IMVS) ribbed absorber tube was also conducted. Three heat transfer fluids (HTFs) are investigated, and a wide range of fluid inlet temperatures are studied. Various geometric parameters of the V-shape rib are analyzed to determine the optimum design of such a modification to the wall of the absorber tube. Results show that the heat transfer fluid (HTF) thermal oil Syltherm 800 is superior to the other HTFs that were studied. Results also show that a lower inlet temperature of the HTF leads to better thermo-hydraulic performance. The study provides a set of values for designing a V-shape ribbed absorber tube that produces optimum thermo-hydraulic performance. The optimum ribbed tube design shows a performance enhancement of about 64% compared to a smooth tube. Finally, this research proposes an innovative approach to improve the performance of solar cooling systems by utilizing a cascaded absorption cooling (CAC) system. The dissertation also examines the viability of coupling an NH3-H2O absorption system with an H2O-LiBr absorption system to simultaneously satisfy both a refrigeration load and an air-conditioning load. Results of this analysis shows that the CAC system uses 7.1% less thermal energy than the sum of the energies used by the ammonia absorption system and the LiBr absorption system if they were to operate separately to meet the same cooling load. In addition, this research investigates the impact of a performance-enhanced parabolic trough collector (PEPTC) on the thermal and exergetic efficiencies of the solar cooling system. By employing a PEPTC, the area required for the solar field in a given solar cooling system will be reduced by 14% compared to the area required by a conventional PTC. Combining the CAC system with the PEPTC results in a 22% increase in the overall efficiency of a cooling plant compared to a conventional PTC coupled with an ammonia system and a LiBr system in the same plant. In summary, it is suggested that the simultaneous utilization of the proposed CAC system and the PEPTC can considerably improve the efficiency of solar cooling systems. Doing so, will lead to sustainable cooling alternatives.