Thermal performance of a nanofluid-based flat plate solar collector: A transient numerical study

Flat plate solar collectors (FPSCs) are commonly used devices to convert solar radiation into useful heat for a variety of thermal applications. Due to the lower thermal efficiencies of these systems, recently, nanofluids are suggested to be used in FPSCs as the working fluid to enhance their energy harvesting potential. This study introduces a transient heat transfer approach for determining the thermal inertia of each component such as glass, trapped air, absorber and working fluid for nanofluid based flat plate solar collectors. The analyses were carried out with water and three different volumetric concentrations of Al2O3 nanoparticles as 1%, 2% and 3%. Mass flow rate of the heat transfer fluid is varied in a wide range, between 0.004 and 0.06 kg/s, to demonstrate the effect of thermophysical properties at different flow Reynolds numbers. The results indicate that the maximum increase of the outlet temperature is obtained by 7.20% at 0.004 kg/s and 3% (vol.) mass flow rate and volumetric concentration, respectively, in July. On the other hand, the highest thermal efficiency is obtained as 83.90% at 0.06 kg/s mass flow rate for 1% (vol.) in October. It is worthy of note that nanofluids can increase the thermal efficiency of the FPSCs at lower flow rates and beyond a critical flow rate the base fluid becomes effective working fluid. For the current study, the critical flow rate is determined to be 0.016 kg/s.