Extinction coefficient of water-based multi-walled carbon nanotube nanofluids for application in direct-absorption solar collectors

In this reported work, the extinction coefficient of water-based nanofluids containing multi-walled carbon nanotubes (MWNCTs) has been experimentally measured. The MWCNTs were dispersed in deionised water with a surfactant, hexadecyltrimethyl-ammonium bromide, and they were homogenised by a bath-type ultrasonicator and a mechanical stirrer. The characteristics of MWCNTs suspended in the nanofluids were examined by transmission electron microscopy and scanning electron microscopy images and their hydrodynamic particle size was measured by a particle size analyser. The extinction coefficient of nanofluids was measured by an in-house developed apparatus at a single wavelength (632.8 nm) based on the Lambert-Beer principle. With the experimentally obtained extinction coefficient, the efficiency of a flat-plate type direct-absorption solar collector (DASC) was theoretically estimated. For this purpose, a modified analytical solution of the DASC efficiency is presented by assuming that the extinction coefficient is not a function of the wavelength. Finally, the efficiency of DASC is demonstrated according to the nanotube volume fractions. The results show that the DASC concept can further improve the efficiency of the conventional flat-plate type solar collectors.