Thermal efficiency enhancement of nanofluid-based parabolic trough collectors

The use of nanofluids in parabolic trough collectors is one of the most promising techniques for enhancing their performance. The objective of this work is to investigate the use of various nanoparticles (Cu, CuO, Fe2O3, TiO2, Al2O3 and SiO2) dispersed in thermal oil (Syltherm 800). A detailed parametric analysis is performed for flow rates from 50 to 300 L min−1, for inlet temperatures from 300 K to 650 K and for nanoparticle concentrations up to 6%, while the impact of the solar irradiation level on the thermal efficiency enhancement is also investigated. Moreover, a new index for the working fluid evaluation in solar collectors is introduced. The analysis is conducted with a developed thermal model in Engineering Equation Solver. According to the final results, the most efficient nanoparticle is the Cu, with CuO, Fe2O3, TiO2, Al2O3 and SiO2 to follow, respectively. It is found that the higher enhancement is observed for lower flow rates, higher inlet temperatures and higher nanoparticle concentrations, while it is approximately constant for the different solar irradiation levels. For the typical operating conditions with 150 L min−1 flow rate and 600 K inlet temperature, the thermal efficiency enhancement is found 0.31, 0.54 and 0.74% for Cu concentrations 2, 4 and 6%, respectively.