Thermo-exergetic optimization and local temperature field behavior in multi-tray solar drying systems with double-pass absorbers

Solar drying technology represents a sustainable solution for post-harvest food preservation in tropical regions; however, uneven internal airflow and poor thermal distribution across multi-tray chambers often cause inconsistent product quality and substantial thermodynamic losses. To address these limitations, this study presents a comprehensive thermodynamic, exergy, and tray-level thermal environment evaluation of a multi-tray solar dryer numerically investigated under the climatic conditions of Yaoundé, Cameroon. Three distinct collector and glazing architectures were analyzed: a double-absorber double-glass design (Design 1), a single-absorber double-glass design (Design 2), and a single-absorber single-glass design (Design 3). Using parametric inlet air velocity simulations ( 0.1 − 1.0 m/s), system performance was evaluated in terms of thermal energy efficiency ( η th ), exergy efficiency ( η ex ), exergy destruction rate ( E dest ), Sustainability Index ( S I ), and spatial temperature profiles across bottom ( y = 0.50 m), middle ( y = 0.80 m), and top ( y = 1.10 m) trays. Results demonstrate that Design 1 substantially outperforms the other configurations, reaching a maximum thermal efficiency of 79.6% and an exergy efficiency of 3.12% at 1.0 m/s, compared to 70.2% ( η th ) and 2.35% ( η ex ) for Design 2, and 62.1% ( η th ) and 1.81% ( η ex ) for Design 3. Although all designs exhibit comparable mean exergy destruction rates ( ∼ 840 − 850 W ), Design 1 achieves the highest Sustainability Index ( S I = 1.032 ). Tray-level thermal environment analyses reveal that Design 1 effectively mitigates cold spots across all tray levels, achieving high thermal uniformity with transverse temperature spans of 300.4-303.8 K (bottom, y = 0.50 m), 301.0-303.8 K (middle, y = 0.80 m), and 301.6-303.8 K (top, y = 1.10 m), corresponding to a maximum transverse variation of only 3.4 K. In contrast, Design 2 and Design 3 exhibit pronounced central cold spots with maximum transverse thermal gradients of up to 6.3 K and 8.8 K, respectively, reaching a severe temperature drop down to 293.1 K at the bottom tray of Design 3. These quantitative insights establish the double-pass absorber approach as an effective strategy for optimizing both global thermodynamic efficiency and localized tray-level drying uniformity in equatorial climates like Yaoundé.