Stable and environmentally benign nanofluids for direct absorption solar collectors based on natural deep eutectic solvents

Direct absorption solar collectors (DASCs) are a new generation of collectors that using nanofluids for directly converting solar radiation into thermal energy, which exist inevitable drawbacks such as unstable working fluid, complex preparation process, high cost etc. A simple, non-toxic, environmentally benign nanofluid with high photothermal conversion efficiency and high stability is requirable for DASCs. In this work, a novel deep eutectic solvent composed of glycerol and betaine is used as working fluids, which exhibit excellent stability and repeatable performance at the working temperature. The highest photothermal conversion efficiency can reach 96.59% for 50 ppm nanofluids, which is 39.57% higher than the pure working fluids. An outdoor experiment is conducted utilizing a heat exchanger to extract warm water (50 degrees C) from the nanofluids under flowing conditions, which can fully meet people's daily hot water needs. This study paves a new avenue for seeking for stable and environmentally benign nanofluids in the solar thermal conversion technique.