In-line measurement of absorbed solar irradiance using a volumetric collector with SWCNH nanofluid

Direct absorption solar collectors operating with nanofluids represent a promising technology in the field of solar thermal systems. However, the stability and the reliability of such fluids in real time operation is still an open issue, since their absorption performance has been mainly evaluated at lab-scale under stagnant conditions and their optical properties can be subject to degradation due to multiple reasons. A novel technique based on the combined use of pyranometers is here presented for measuring the absorption rate of nanofluids circulating in a volumetric solar receiver. In the present work, the absorption capability of a Single-Wall-Carbon-NanoHorns (SWCNHs) based nanofluid is experimentally investigated when varying temperature (between 25 °C and 45 °C) and mass flow rate (between 5 kg h−1 and 315 kg h−1). The optical efficiency of the nanofluid is found to be slightly affected by the temperature of the circulating fluid. The optical efficiency is checked for more than 80 hours of operation.