Solar energy utilization is very important in the background of global warming and reduction of carbon dioxide emission. Solar air/water collectors are convenient to convert solar energy into heat. Compared with regular solar collectors, direct absorption solar absorbers are not common. However, nanofluids can be used in direct solar absorbers to improve their performances. The objective of this paper is to evaluate the potential of application of nanofluids in direct solar absorbers. Spectral transmittances of nanofluids within the solar spectrum have been measured by a customized spectrophotometer. A direct solar absorber has been built to use nanofluids as the working fluid and its performance has been experimentally investigated. It is found that the radiative properties of nanofluids deviate significantly from that of the base fluid. A remarkable amount of visible light can pass through the SiO2 nanofluid while solar radiation can hardly transmit through the TiO2 nanofluid and ZrC nanofluid. The ZrC nanofluid shows the highest solar absorbance among the studied nanofluids. The temperature increase rate of the nanofluid is faster than that of water when the liquid stagnant in the absorber is illuminated by solar radiation. Furthermore, the temperature differences between the inlet and outlet of the TiO2 nanofluid and ZrC nanofluid are higher than that of the SiO2 nanofluid at the same flow rate. This work is beneficial for the exploration of nanofluids in direct solar absorbers.

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