Enhancing the solar energy conversion and harvesting characteristics of multiwalled carbon nanotubes‐modified 1‐hexyl‐3‐methyl‐imidazolium cation ionic liquids
Tsogtbilegt Boldoo, Minjung Lee, Honghyun Cho
Abstract
The concept of solar energy absorption or photothermal energy conversion of nanofluids as a working fluid is a promising idea utilized in various applications. However, the lack of diversity of the base agents restricts the usage of solar thermal conversion systems in extreme conditions such as extreme cold or hot environments. The ionic liquids were used as a base agent to broaden the applicable conditions of the solar thermal conversion system considering the remarkable favorable characteristics of ionic liquids. In this study, key factors of solar energy conversion characteristics and performance of the various 1-hexyl-3-methylimidazolium cation-based ionic liquids and MWCNT nanoparticle-enhanced ionic liquids (NEILs) were experimentally investigated. This study dives deep into the optical properties of the ionic liquids compared to the previous studies in the open literature. In the manufacturing process, MWCNT nanoparticles are dispersed in various ionic liquids at 20 kHz and coated with the Gum-Arabic. It was found that 1-hexyl-3-methylimidazolium cation-based pure room temperature ionic liquids were almost transparent in the visible light spectrum with nearly 0% absorbance; however, adding a small amount of additional MWCNT nanoparticles sharply increased the absorbance rate up to nearly 100% (F ≈ 1). Besides, the transmittance of the MWCNT NEILs decreased with the increasing concentration. Under the simulated solar light, the final temperature of the samples increased with the increasing concentration of MWCNT NEIL. Moreover, the maximum photothermal energy conversion efficiency ( η PEC ) of each MWCNT NEILs was 38.0%, 40.9%, 40.2%, and 39.8% in the case of [HMIM][Tf2N], [HMIM][TfO], [HMIM][Pf6], and [C12MIM][Tf2N], respectively, at a concentration and operating time of 1 wt% and 7200 seconds. In addition, SER value sharply increased with the increasing nanoparticle concentration, and it almost doubled compared to the base agents. Furthermore, the nanoparticle concentration affects the solar energy absorption capability of ionic liquids more than anions in their chemical structure. This study can deliver the necessary data to design a direct absorption solar thermal collector using ionic liquids, which are new green designer working fluids.