Thermal efficiency comparison: Surface-based solar receivers with conventional fluids and volumetric solar receivers with nanofluids

This paper reports on a comparative study of the difference in thermal efficiency between surface-based solar receivers (SRs) with conventional base fluids and volumetric solar receivers (VRs) with water-based multi-walled carbon nanotubes (MWCNT) nanofluids. The analytical solutions for temperature distribution and thermal efficiency of SRs and VRs are theoretically obtained to identify the key engineering parameters that affect the thermal efficiency of both solar receivers. In order to confirm the analytical solutions, we experimentally measured the thermal efficiency of both solar receivers according to the volume fraction and the Peclet Number. Moreover, the experimental results are compared with the analytical solutions. Based on the comparison, we show that the analytical solutions can reasonably estimate the thermal efficiency with respect to the volume fraction and the Peclet number. Furthermore, the analytical and experimental results indicate that the efficiency of both solar receivers are proportional to the volume fraction (ϕ) (not applicable for SRs) and the Peclet number (Pe), while it is inversely proportional to the Nusselt number of heat loss (Nu) and the aspect ratio (AR). Finally, this study systematically demonstrates the nanofluid-based VRs can be achieved higher efficiency compared to the conventional SRs over 10%.