WSe2 Nanosheets Synthesized by a Solvothermal Process as Advanced Nanofluids for Thermal Solar Energy

Nanofluids are colloidal systems based on the suspension of nanoparticles in a fluid. Their thermal properties mean that they are promising heat transfer fluids with possible applications in different fields, concentrating solar energy being one of particular interest. Thus, this study presents the preparation of nanofluids based on WSe2 nanoparticles suspended in the eutectic mixture of biphenyl and diphenyl oxide, which is a heat transfer fluid widely used in concentrating solar power plants. To this end, solvothermal synthesis was used to prepare WSe2 nanosheets, which were characterized by means of scanning transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and Raman spectroscopy. The physical and chemical stability of the nanofluids was analyzed, observing that they became more stable when Triton X-100 was used as the surfactant. The presence of WSe2 nanosheets did not result in a significant increase in density or viscosity, but, by contrast, improvements were obtained in their isobaric specific heat and thermal conductivity, up to 4.7 and 64%, respectively. Spectral optical properties were investigated as well, showing a significant effect of the WSe2 nanosheet addition to the extinction coefficient of the base fluid in the wavelength range of the solar spectrum, promising for direct absorption solar collectors. Finally, the efficiency of the nanofluids was analyzed in a solar collector considering the Ur parameter, obtaining a remarkable increase in the efficiency of up to 34% with respect to the pure heat transfer fluid. This proves the possibility to obtain a sustainable production of energy from the sun using these WSe2-based nanofluids.