Experimental investigation and economic analysis of a novel clay-based solar dryers for drying medicinal and aromatic plants under semi-arid climate

This study presents a novel, low-cost material used for solar drying of medicinal and aromatic plants, namely clay. Two solar dryers, one direct and one indirect, were designed, constructed, and experimentally investigated under identical conditions. A set of experiments was conducted under load and no-load conditions. The results show that at relatively low ambient temperatures, an optimum drying temperature of herbs of 49.6°C was achieved. Additionally, at high ambient temperatures, the dryers were capable of maintaining an ideal drying environment below 50°C, attributed to the good insulation provided by the novel material and forced ventilation. Both moringa and mint were dried in the two solar dryers, taking 5.33 and 8.66 hours in the direct dryer and 31 and 41.85 hours in the indirect dryer, respectively, to reach a moisture content below 10% (kg water/kg wet basis). Furthermore, a high drying rate was observed in favor of the direct solar dryer. Additionally, economic efficiency increased by 100%, favoring the use of the direct solar dryer with a low payback period of 6 months compared to 14 months for the indirect solar dryer.