Liquid layer envelope for reducing radiative losses in nanofluid-based volumetric receivers

At high flux conditions, surface absorption based solar thermal systems underperform owing to high overheat temperatures of surface (temperature difference between the surface and the working fluid). One of the ways by which this drawback could be addressed is by allowing direct interaction of solar radiation with the working fluid. In this context, nanoparticles (NP) laden fluid (nanofluids) have been shown to be promising candidates as direct absorption volumetric solar energy absorbers owing to their enhanced thermo-physical properties and high solar weighted absorptivity. However, the nanofluids inherently have high emissivity in the mid- infrared region thus not satisfying the conditions of solar selectivity. In this study a layer of liquid (silicone oil) is placed above the nanofluid, which acts as a barrier against the infrared emissions from the nanofluid. Selection criterion for this liquid layer is high transparency in the visible solar spectrum and high absorption in the mid-infrared wavelength range. Thus the two conditions of solar selectively have been met by using two different liquid layers in direct thermal contact. On comparison of two results, it is seen that with silicone oil layer, the temperature rise is about 17% more as compare to without silicone oil layer