Drying Kinetics of Coffee Cherries From Two Varieties Using a Solar Cabinet Dryer Coupled With Underground Thermal Energy Storage

Abstract

Coffee is a vital cash crop in Ethiopia, yet substantial postharvest losses and quality degradation which limit its market value and export potential. This study explores drying of coffee cherries in a solar cabinet dryer integrated with an underground rock bed thermal energy storage system (TES) as a novel approach. Experiments were conducted using two beds with different particle sizes (36 and 56 mm) at an airflow rate of 0.75 m³/s. Results showed that the solar cabinet dryer with TES considerably reduced drying time, maintained higher and more consistent drying temperatures during off-sunshine periods, and improved overall drying efficiency. Bed 1's smaller rocks enhanced heat retention, increasing drying rates by 6%–11% despite slightly lower efficiency (32%) than Bed 2 (33.88%). Nine thin-layer drying models were evaluated based on R², RMSE, and SSE values. The Modified Midilli model best fits Zegie cherries, while the Midilli-Kucuk model fits Dangela cherries. Zegie cherries exhibited higher moisture diffusivity (6.16 × 10⁻¹¹ m²/s), mass transfer coefficients (4.71 × 10⁻⁸ m/s), and heat transfer coefficients (1.20 × 10⁻⁵ W/m²K). The activation energy was 23.91 kJ/mol for Zegie and 46.27 kJ/mol for Dangela. Statistical analysis revealed significant (p < 0.05) effects of coffee variety and bed effect. This integrated system provides a sustainable, energy-efficient solution for agricultural drying, reducing fossil fuel reliance while enhancing decentralized renewable energy and supporting smallholder farmers in agro-industrial value chains. Future research should focus on system scale-up, cost–benefit analysis, and integration with other postharvest technologies to maximize impact.