Natural and forced air convection operation in a direct solar dryer assisted by photovoltaic module for drying of green onion

•A direct solar dryer self-sufficient in energy was developed with PV module.•Drying kinetics of green onions under natural and forced convection were evaluated.•The drying air convection controlled the constant rate period.•Colorimetic analysis were performed on green onions after solar drying.•The efficiency of the dryer and the specific energy consumption were assessed. The aim of this research was to develop a direct solar dryer assisted by photovoltaic module and evaluate its performance under natural and forced convection experiments. The photovoltaic module provides electrical power to eight coolers that allow the renewal of air inside the equipment. In this configuration, the dryer is capable of operating independently of electrical energy distribution grid. Moisture and colorimetric parameters were monitored during drying of green onion. Drying kinetics showed a constant rate period followed by a falling rate period for both operating conditions. The convection experiment showed a higher rate and external mechanisms may have controlled the process. Straight lines were adjusted to constants rate periods with R2 of 0.999. Page and Overhults models best described the falling rate periods, with R2 between 0.932 and 0.999. The effective diffusivity values were 5.15 × 10−9 m2/s and 1.15 × 10−8 m2/s for drying with natural and forced convection runs. The average efficiencies of the solar dryer and the specific energy consumption were 34.2%, 18.3 kWh/kg for the drying process with natural convection, and 38.3%, 16.4 kWh/kg for the drying process with forced convection. Little color variation was observed between fresh and dried green onions in the two operating conditions, which is consistent with the need to maintain the green color of the material.