Performance and sustainability assessment of a greenhouse dryer with integrated sunlight protection for mint drying

Abstract

Herbs dried in conventional systems often suffer quality degradation due to direct sunlight exposure and uneven drying. To address this, a novel greenhouse dryer (NGHD) equipped with an integrated drying chamber was evaluated, where the walls consist of two plates separated by a thermal insulating material (sandwich type). The metal plate exposed to the sun is painted black to act as an absorber, while the insulating material helps retain heat inside the drying chamber. This innovative design combines the large capacity of direct dryers with the superior quality of dried materials characteristic of indirect dryers. Experiments, conducted at the University of El Oued, assessed mint drying kinetics, moisture removal, and effective moisture diffusivity. Among nine tested models, the two-term model best described the drying kinetics. The NGHD achieved higher internal temperatures (up to 14 °C without load, 10 °C with load) than the conventional greenhouse dryer (CGHD), reducing drying time by 32 %. Mint's moisture content dropped from 5.45 to 0.03 kg water/kg dry basis in 390 min (NGHD), compared to 570 min (CGHD) and 920 min (sun drying). Effective diffusivity improved by 27 % in the NGHD. Superior mint quality, retaining color and shape, was achieved in the NGHD. Economic analysis showed payback periods of 0.925 years (NGHD) and 1.004 years (CGHD), with carbon credits of 1927.40 $ over 15 years. The NGHD demonstrates its novelty and sustainability by preserving product quality while offering economic and environmental benefits.