Influence of wall insulations and orientation on spatial heat distribution coefficient and thermal time constant for passive solar drying of yellow pepper

Conventional solar greenhouse dryers have transparent walls, and the north wall effect can prevent the north wall from receiving solar radiation, making it a conduit for heat dissipation. To reduce heat loss, opaque walls are used on the north side of the greenhouse. However, a comprehensive comparative analysis of solar greenhouses with insulated opaque north walls is lacking. Therefore, this study aimed to investigate the effect of opaque walls on certain thermal performance indicators of solar greenhouse dryers. Three configurations were tested: transparent, insulated northern, and insulated eastern walls. The results show that the north insulated wall configuration exhibits superior thermal performance, with a higher temperature increase (6.67 % to 11 %) and lower thermal time constant (τ = 8.21 s) compared to the transparent wall (τ = 8.38 s) and insulated eastern wall (τ = 9.03 s). The transparent wall configuration displays the highest solar fraction values (97.81 % to 99.31 %) but compromises energy efficiency due to higher heat loss. The insulated eastern wall configuration shows improved thermal cycling stability (TCS = 82.04 %) but lower solar fraction values (95.87 % to 98.2 %). The spatial heat distribution coefficient (h) was highest for the insulated northern wall (2.2868 W/m²K), indicating improved heat distribution. The thermal effusivity (σ) was lowest for the insulated northern wall (8.656 W⋅ S^1/2⋅ m⁻²⋅ C⁻¹), indicating improved heat retention. The coefficient of performance (COP) was highest for the insulated northern wall (0.52), indicating improved efficiency. The study demonstrates the importance of careful design and optimization of passive solar greenhouse dryers to achieve energy efficiency and controlled temperature conditions. This study provides a scientific basis for the optimal configuration of opaque walls in solar greenhouse dryers.