Application of large-scale prepared MWCNTs nanofluids in solar energy system as volumetric solar absorber

Abstract

Multi-walled carbon nanotubes (MWCNTs) nanofluids suffer from low dispersion and short shelf life which hinder their large-scale application in solar energy system. To respond this, in this study a novel method of MWCNTs nanofluids preparation has been designed. It is a one-pot method by stirred media mill technique achieving the objective of low cost and scalable for nanofluids production, in which nearly 1 L MWCNTs nanofluids of 3 wt% could be gotten in a laboratory-scale production process. Apart from with concentration adjustability and long-lasting stability, obtained MWCNTs nanofluids also displayed superior optical properties. It showed that the average extinction coefficient of 0.01 wt% milling-treated MWCNTs nanofluid was 7.49 cm-1, which was obviously higher than that of 0.01 wt% acid-treated sample with 6.55 cm-1. Especially, the spectral analysis revealed that milling-treated MWCNTs nanofluids had better energy absorption capacity in visible and near-infrared regions. Moreover, the evaluation of solar energy absorption fraction showed that 0.01 wt% MWCNTs nanofluid could absorb nearly 100% incident energy, when fluid thickness was more than 0.5 cm. It means that MWCNTs nanofluids could be tailored easily for volumetric solar receiver to achieve optimal design. Furthermore, the solar-thermal conversion tests were conducted under natural solar radiation and a high conversion efficiency (95%) was obtained at first 10 min in 0.02 wt% MWCNTs nanofluids. It is worth to note that the performance degradation of high concentration MWCNTs nanofluids did not happen during conversion tests. Such results revealed that MWCNTs nanofluids were prepared successfully in large-scale and the high performance of them showed their application potential in solar energy system.