Experimental study of a cabinet solar dryer in mixed and indirect modes

In the present study, the performance of a forced convection solar dryer with a solar air collector for providing hot air and a cabinet dryer has been experimentally investigated. The solar collector was made up of a covering glass; a black painted aluminum sheet absorber and a wooden wall were thermally insulated by glass wool. A wooden cabinet with glass covered was attached to the solar collector. A centrifugal blower with adjustable air flow rate was used. The assembly could be applied in two different modes namely indirect and mixed mode. Grape was selected to be dried in thin layers at this dryer. The effect of air flow rates and different modes of dryer on drying parameters such as drying time, moisture reduction trends, drying efficiency and solar collector efficiency were investigated. For this purpose, experiments with three levels of the air flow rates (0.085, 0.126 and 0.171 m³/s) and two modes of the dryer were carried out. The results showed that the collector efficiency increased as the air flow rate increased. Parametric analysis of the solar collector indicated that the collector heat removal factor and the overall heat loss coefficient of the collector were 0.74 and 35.4 W/m² K respectively. The drying efficiency was lower in case of indirect drying than mixed mode drying. Also it was deduced that the drying efficiency increased when the air flow rate decreased. However, a maximum collector efficiency of 63.5% at 0.171 kg m⁻² s⁻¹ air flow rate was achieved. Also, the drying efficiency have a maximum amount of 30% for air flow rate of 0.085 kg m⁻² s⁻¹ in the mixed mode type of drying which led to a minimum drying time of 22 hours.