Renovation and improvement of direct and indirect clay- based solar dryers for drying medicinal and aromatic plants

Abstract

This thesis investigates the renovation and performance improvement of two clay-based solar dryers a direct configuration and an indirect configuration for drying medicinal and aromatic plants

(MAPs) in the arid climatic conditions of Ghardaia, Algeria. Although Clay-based dryers are low- cost, locally manufactured systems with favorable thermal mass properties, but existing designs

suffer from several shortcomings including degraded polycarbonate covers, uneven heat distribution, dust infiltration, and difficult product loading. Two experimental campaigns were conducted at the URAER research unit: a pre-renovation campaign in February 2026 and a post-renovation campaign in May 2026. Five targeted upgrades were implemented: replacement of degraded polycarbonate panels with 5 mm glass panes, installed of black floor tiles to improve heat absorption and ease of loading, application of air filtration screens to prevent dust ingress, installation of a one-piece matte black aluminum solar collector panel, and structural repair of the clay walls using clay & limestone paint. The results show that post-renovation, the indirect dryer achieved peak internal temperatures up to 11 °C higher than before renovation, with improved temperature uniformity between the sensors. Both dryers exhibited better alignment between their internal temperature profiles and solar irradiation patterns, confirming an enhanced thermal performance. The study also evaluated the drying kinetics of Asphodelus tenuifolius and mint (Menthe sp.) as test plants, validating the superior moisture-extraction capability of the renovated systems compared to open sun drying. The findings confirm that simple, cost-effective, locally available material upgrades can transform underperforming clay dryers into efficient, sustainable tools for preserving medicinal and aromatic plants while reducing post-harvest losses for small-scale farmers in arid regions.