Thermal performance evaluation of various nanofluids with non-uniform heating for parabolic trough collectors

Nanoparticles when used even in relatively low concentrations, can significantly alter the thermal properties of a base working fluid, thereby substantially enhancing the thermal performance of a power generation system. In the present study, numerical simulations were performed for a solar collector to test the effectiveness of six non-metallic nanoparticles, namely aluminum oxide (Al2O3), cerium oxide (CeO2), copper oxide (CuO), ferric oxide (Fe2O3), titanium dioxide (TiO2) and Silicon dioxide (SiO2). These nanoparticles were dispersed individually in three different base working fluids; therminol VP-1 (at 400 K), water (at 400 K) and molten salt (at 600 K) to form different nanofluids. Each of these nanofluids were then examined with three different volume fractions (2%, 4% and 6%) in addition to the pure base working fluid case for a range of Reynolds number (Re=104-105). For the simulation the Monte Carlo Ray Tracing (MCRT) model was used to represent the non-uniform heat flux around the absorber tube of the Parabolic Trough Collector (PTC). The results show that the enhancement of the thermal and the hydraulic performances depended upon the combination of the nanoparticles and the base working fluid. Silicon dioxide, SiO2, however, was found to be the most efficient nanoparticle regardless of the choice of the base working fluid for all the tested volume fractions. For example, for typical operating conditions for SiO2 with a volume fraction (VF) of 6%, the average Nusselt number of the water-based mixture was enhanced by 32.4%, with a thermal efficiency improvement of 5.11% and performance evaluation criterion (PEC) of 1.313. On the other hand, for a molten salt-based SiO2 mixture, the average Nusselt number was improved by 21.36% and thermal efficiency by 9.92% with a PEC of 1.155. Finally, the corresponding improvements with therminol VP-1-base fluid were 15.6%, 9.18% and 1.21 for the average Nusselt number, thermal efficiency and PEC respectively.