Comparative energy, exergy and CO2 emission evaluations of a LS-2 parabolic trough solar collector using Al2O3/SiO2-Syltherm 800 hybrid nanofluid

Abstract

The use of hybrid nanofluids in parabolic trough solar collectors (PTCs) is a promising way to improve their performance. There have been some studies on utilization of hybrid nanofluids in PTCs, however there has been no study on utilization of Al2O3/SiO2-Syltherm 800 hybrid nanofluid. In the present study, after showing the superiority of utilization of this hybrid nanofluid in a LS-2 PTC over four previously studied hybrid nanofluids, thermal and hydrodynamic performance evaluation studies together with exergy and environmental analyses have been performed. Also for the sake of comparison Syltherm 800 as the base fluid and the corresponding mono nanofluids of Al2O3 and SiO2 have been considered and for all of the cases the flow regime has been considered turbulent. In some previous studies related to hybrid nanofluids the acceptable range for Reynolds and Prandtl numbers have not been observed, thus the results such as energy and exergy efficiencies and other output parameters stated before are not trustworthy. In this study, proper ranges of 10000–22000 for Reynolds number and 550–600 K for inlet temperature have been considered. The results showed that the energy and exergy efficiencies of the Al2O3/SiO2-Syltherm 800 hybrid nanofluid are always the highest, such that the maximum enhancements compared to those of the base fluid are 5.22 % and 5.49 %, respectively; whereas for mono nanofluid of SiO2 they are 1.71 % and 1.75 % and for mono nanofluid of Al2O3 they are 1.19 % and 1.36 %, respectively. Moreover, the results showed that the hybrid nanofluid is the most suitable with the highest PEC of 1.76. Environmental analysis based on exergoenvironmental method showed that if the hybrid nanofluid is utilized the maximum carbon dioxide emission in the life cycle of PTC is 1.657 kg/day; whereas for Syltherm 800 and mono nanofluids of SiO2 and Al2O3, it is 2.089, 1.925 and 1.961 kg/day, respectively. Energy, exergy and environmental analyses showed that the best performance of PTC is attained when hybrid nanofluid is utilized and Tin and Re are 600 K and 104, respectively.