Impact of nanofluids, radiation spectrum, and hydrodynamics on the performance of direct absorption solar collectors

The coupled radiative transfer and thermal energy equations were numerically solved for a nanofluid-based, direct absorption solar collector (DASC) using a Rayleigh scattering approximation for the optical properties of the nanofluid. The flow field was solved using a vorticity-stream function formulation. The numerical model was validated against published results in the literature. The effects of collector aspect ratio, incident radiative flux, heat loss coefficient, inlet flow velocity, nanoparticle size, and nanoparticle concentration were determined for four different types of nanoparticles. It was established that the impact of these parameters on collector performance was determined by the extent to which they affected the outlet temperature gain and thermal losses to the ambiance. Through the exact solution of the flow field, it was established that performance deviations from the fully-developed assumption were encountered for small collector aspect ratios, especially with high Reynolds numbers (Re > 1500). Moreover, with the exception of cases with very low Reynolds numbers (Re < 40), assuming a plug flow profile throughout the collector was shown to cause performance overestimations. Sensitivity of DASC performance to the type of incident radiation spectrum was also investigated and it was found to be largely dependent on spectral distribution of the extinction coefficient of the nanoparticle. The common assumption of a blackbody incident spectrum results in performance overestimations for certain types of nanoparticle suspensions (e.g., silver) more than others (e.g., graphite). Using a higher nanoparticle volume fraction causes the efficiency of the collector to be more insensitive to the incident spectrum type. For a given nanofluid material and thickness, it was established that an optimum nanoparticles concentration existed.