Efficiency of a volumetric receiver using aqueous suspensions of multi-walled carbon nanotubes for absorbing solar thermal energy

This paper analytically investigates the efficiency of a nanofluid volumetric receiver (NVR) for absorbing solar thermal energy considering the experimentally measured extinction coefficient of aqueous suspensions of multi-walled carbon nanotubes (MWCNT) according to the wavelength from 200 to 2000 nm. For this purpose, considering the spectral behavior of nanofluids, we obtained analytical solutions of temperature fields as well as the efficiency of the NVR based on the condition of fully developed flow between the two plates. The aqueous MWCNT nanofluids were prepared using the two-step method, and their extinction coefficients were experimentally measured by the UV/Vis/NIR spectrophotometer according to the wavelength. With the analytical equations, we identified those key engineering parameters that affect the efficiency of an NVR: the Nusselt number of heat loss, the concentration of nanoparticles, the Peclet number, and aspect ratio. Also, we systematically observed the effects of key engineering parameters on the temperature fields and on the efficiency of the NVR. The current results clearly show that the efficiency calculated under the assumption of plug-flow through an NVR reported by previous researchers is overestimated in the case of high heat loss. Moreover, the present results show that NVR efficiency is proportional to the Peclet number as well as to the concentration of nanoparticles, while it is inversely proportional to the Nusselt number of heat loss and aspect ratio. The results of this study may be helpful to design and predict the efficiency of an NVR.