Silicone oil envelope for enhancing the performance of nanofluid-based direct absorption solar collectors

Nanofluids inherently have high emissivity in the mid-infrared region making them unsuitable for satisfying the conditions of solar selectivity required for direct absorbers. In this study, a layer of silicone oil is located above nanofluid comprising of graphite nanoparticles and deionized water. Silicone oil layer is shown to act as a barrier against the infrared emissions from the nanofluid. High transparency of silicone oil in the visible solar spectrum and high absorptivity in the mid-infrared wavelength range made it an excellent candidate for this study. Solar selectively was achieved by using two different liquid layers (nanofluid and silicone oil) in direct thermal contact. Silicone oil layer on the top of the nanofluid resulted in an approximately 17% higher temperature rise in the nanofluid.