Experimental Analysis of a Solar Dryer Equipped with a Novel Heat Recovery System for Onion Drying

In hot air dryers, only a small percentage of the provided thermal energy is used for the drying process, while a large fraction is lost via the exhaust air. To recycle waste heat from the exhaust air, the present study aimed to develop a solar dryer equipped with a novel heat recovery system. The designed dryer comprised of a solar air collector, a drying chamber, an internal closed-loop air circuit and an open-duct heat recovery system. The evaluation tests were conducted at different allowable relative humidities (RH) and mass flow rates of the recirculating air. The results indicated that the best solar fraction was at the highest RH and air flow rate. Increasing the RH from 7 to 17% caused a reduction of 51% in electricity consumption. Furthermore, electrical energy needed for drying increased by 24% with raising the air flow rate from 0.008 to 0.016 kg s-1. A minimum specific energy consumption of 7.54 MJ kg-1 was observed at the highest RH and the lowest air flow rate. At a constant RH, reduction of the air flow rate led to an increasing trend in lightness and decreasing trends in browning index of the products. Moreover, increasing the RH from 7 to 17% increased lightness and decreased browning index. In general, it can be stated that the best colour quality was achieved when the minimum air flow rate and the maximum RH were used for the solar drying.