Energy and exergy analysis of forced and natural convection indirect solar dryers: Estimation of exergy inflow, outflow, losses, exergy efficiencies and sustainability indicators from drying experiments

Abstract

In this study, energy and exergy analysis (EEA) was performed during drying of okra (Abelmoschus esculentus) in an indirect solar dryer (ISD) under forced and natural convection and results were compared for better assessment of the performance of ISD and optimize the drying process by considering the quantity of energy. A divergent duct integrated with central processing unit (CPU) fans was attached to solar air collector (SAC) in forced convection ISD and this fitting was removed during natural convection experiments. The average SAC and drying efficiencies were 74.98% and 24.95% under forced convection and the same were 61.49% and 20.13% under natural convection, respectively. The exergy inflow, outflow and losses were determined for the collector and drying chamber for both setups. The exergy outflow of SAC was in a range of 1.04–46.85 W and 1.13–50.94 W, in forced and natural convection mode, respectively. The exergy loss for the drying chamber varied from 0.062 to 21.99 W and 0.394–24.99 W, under forced and natural convection, respectively. The values of average exergy efficiencies for SAC and drying chamber in forced convection ISD were 2.03 and 59.32%, while in natural convection, these were 2.44 and 55.45%, respectively. In forced convection ISD, the improvement potential was in the range of 0.0095–10.51 W while the sustainability index and waste exergy ratio varied from 1.19 to 17.05 and 0.06 to 0.83, respectively. EEA analysis revealed that the performance of forced convection was better than the natural convection ISD. Uncertainty of all parameters was estimated and updated in all plots in terms of error bars.