Graphene oxide/multi‐walled carbon nanotube—Therminol®66 hybrid nanofluids for low‐to‐medium temperature volumetric solar collectors
Jian Qu, Ruomei Zhang, Lu Shang, Zhihao Wang
Abstract
Nano-composites of graphene oxide (GO) and multi-walled carbon nanotube (MWCNT) were prepared by a simple physical method and well dispersed into Therminol®66 to form hybrid nanofluids with mass fractions in the range of 10 to 150 ppm. The suspension stability, optical absorption properties and photo-thermal conversion characteristics of GO/MWCNT nanofluids were tested and evaluated. GO nanosheets can avoid the self-entanglement and agglomeration of MWCNTs and ensure long-time suspension stability. The solar energy absorption capability increased with the composite concentration, and the majority of incoming radiation could be absorbed in a fluid layer of 1.5 cm. There existed an optimum concentration of 100 ppm related to the indoor experiment, and the temperature could be increased up to 94°C with the corresponding receiver efficiency greatly improved from 52% (Therminol®66) to 70% (100 ppm). Besides, a line focusing Fresnel lens was utilized for the outdoor solar concentrating experiment, and the photo-thermal conversion performance of hybrid nanofluids increased with the composite mass fraction from 30 to 100 ppm. A highest temperature of 153°C was achieved at 100 ppm concentration. The thermal oxidation without degradation of Therminol®66 occurred after indoor/outdoor experiments, and slightly enhanced the optical absorption owing to the solar radiation induced heating effect. The GO-MWCNT/Therminol®66 nanofluids exhibit a good prospect in volumetric solar thermal systems from low to medium temperatures.