Photo-thermal conversion characteristics of carbon black-ethylene glycol nanofluids for applications in direct absorption solar collectors

Highlights: • Carbon black-ethylene glycol (CB-EG) nanofluids were investigated. • Extinction coefficient increased linearly with CB concentration within 450–820 nm. • An enhancement of overall photo-thermal efficiency of 27.43% was obtained. • Local photo-thermal efficiency improves with concentration and depth of fluid. -- Abstract: Direct absorption solar collector (DASC) is a promising method of harvesting solar energy. Present work considers carbon black-ethylene glycol (CB-EG) based nanofluids as the working fluids for DASC applications. Nanofluids were synthesized by the two-step method. Various studies carried out in this work include long time homogeneity, energy absorption characteristic and transient temperature profiles as functions of fluid thickness, light exposure time and concentration of the nanoparticles. Improved absorption characteristics, compared with those of the base fluid, towards incident irradiance were observed in all cases. About 27.90% increment in overall photo-thermal conversion efficiency over that of the ethylene glycol (EG) alone is observed for the case of 15 ppm carbon black (CB) concentration with an exposure time of 1200 s. Measured data show increasing trends in local photo-thermal efficiency with the thickness of the liquid layer as well as with the concentration of the suspended nanoparticles. These studies confirm that CB-EG based nanofluids can be used as potential working fluids for DASCs.