Radiative properties of ionic liquid-based nanofluids for medium-to-high-temperature direct absorption solar collectors

Here the radiative properties of the ionic liquid [HMIM][NTf2] and its nanofluids are investigated for the first time by the experimental and theoretical methods. [HMIM][NTf2] is almost transparent in the visible light range, while its optical absorption property can be significantly enhanced by dispersing a very low volume fraction of nanoparticles in it. At the volume fraction of 10 ppm, the extinction coefficient of the nanofluid containing the Ni nanoparticles with an average size of 40 nm is higher than that of the one containing the Cu nanoparticles with the similar average size, owing to their different complex refractive indexes. The nanofluid containing the carbon-coated Ni (Ni/C) nanoparticles exhibits lower transmittance and higher extinction coefficient, compared with the one containing the Ni nanoparticles with the similar average size. The radiative properties of the Ni/C nanofluids increase with the volume fraction of the nanoparticles. As the volume fraction is increased to 40 ppm, the absorbed energy fraction by the Ni/C nanofluid reaches up to almost 100% after the incident light only penetrate 1 cm. The excellent radiative properties of the IL-based nanofluids make them show promising to be used as the absorbers for medium-to-high-temperature direct absorption solar collectors.