Efficiency improvement of a solar direct volumetric receiver utilizing aqueous suspensions of CuO

Summary In this paper, an experimental study was performed to investigate the photothermal conversion properties of CuO‐H 2 O nanofluid‐based volumetric receiver mainly considering the effects of nanoparticle (NP) concentration, irradiation time, and receiver depth. First, stable aqueous suspensions of CuO with NPs having average diameter close to 10 nm were produced by the precursor transformation method. The spectral transmittances of CuO‐H 2 O nanofluids decrease with increasing the NP concentration (0.01‐0.25 wt%) at wavelengths of 200 to 1350 nm. The photothermal conversion performance of CuO‐H 2 O nanofluids is sensitive to the receiver depth, irradiation time, and NP concentration. The higher NP concentration causes stronger optical absorption in the upper part and reduces the temperature at the bottom accordingly. The temperature difference between CuO‐H 2 O nanofluid and distilled water increased initially and then decreased with the increase of penetration depth, and there existed an optimal depth of 1 cm with respect to the best photothermal conversion performance. The receiver efficiency decreased with increasing the light irradiation time, and an efficiency improvement up to 30.4% was achieved for the 0.25 wt% nanofluid at the optimal depth of 1 cm as compared with water. This work shows that volumetric receivers provide a potential alternative for solar thermal energy utilization versus surface‐based absorber especially under concentrated solar radiation.