A highly stable hydroxylated graphene/ethylene glycol-water nanofluid with excellent extinction property at a low loading for direct absorption solar collectors

In order to explore a graphene-containing nanofluid with good dispersion stability as the working fluid for a low-temperature direct absorption solar collector (DASC), firstly, two kinds of functionalized graphene, hydroxylated graphene (GOH) and carboxylated graphene (GCOOH), were dispersed into a mixture of ethylene glycol (EG) and water (1:1 mass ratio), at the same loadings. It is found that, although the obtained GOH nanofluid exhibits a slight lower zeta potential than the GCOOH one, its optical absorption property is obviously higher. Consequently, GOH was employed to prepare the EG-water based nanofluids at different loadings, followed by the investigations on their optical absorption and thermophysical properties, photo-thermal conversion performance, and dispersion stability. It is shown that, at a loading as low as 0.007 wt%, the transmittance of the corresponding GOH/water-EG nanofluid is reduced to zero, and its extinction coefficient is 100 times higher than that of the base fluid. Under the illumination of 1000 W/m(2), the 0.007 wt% GOH/EG-water nanofluid can reach a high final temperature of 55 degrees C. More significantly, the zeta potential of the 0.007 wt% GOH/EG-water nanofluid increases with temperature and reaches 47.63 my at 60 degrees C, suggesting its good dispersion stability at the elevated temperatures. These good characteristics of the GOH/EG-water nanofluid make it a very promising working fluid for a low temperature DASC.