Experimental investigation on optical and thermal properties of propylene glycol–water based nanofluids for direct absorption solar collectors

Abstract

Direct absorption solar collectors (DASCs), which absorb solar radiation energy by the working medium, attract considerable attention. Because the addition of nanoparticles can improve the efficiency of solar collectors, nanofluids can be used as the working fluid. The stability, optical properties and thermal conductivity of propylene glycol–water based nanofluids containing TiO2, SiC and multiwalled carbon nanotubes (MWCNTs) are investigated in this paper. The results confirmed that nanofluids with surfactant exhibited good stability. By combining comprehensive transmittance, extinction coefficient and the solar-weighted absorption coefficient (Am) measurements, the transmittance of TiO2 and SiC nanofluids decreased from 65% at 0.025 vol% to about 5% at 0.2 vol%, and the light can hardly pass the MWCNT nanofluids over 0.05 vol%. MWCNT nanofluids had the highest Am, followed by SiC nanofluids and TiO2 nanofluids, which exhibited optimum properties even at low concentration. In addition, thermal conductivity of nanofluids increased with the growing volume fractions and temperature. MWCNT nanofluids had the maximum thermal conductivity (0.508 W/m k at 0.2 vol%) at 20 °C. This work provides valuable insights into the optical and thermal property enhancement of nanofluids for increasing the efficiency of DASCs.