Thermal performance, drying kinetics, and thin-layer model evaluation of a cocoa pod husk briquette-assisted passive solar dryer

Abstract

Inadequate drying rates and weather dependency remain significant postharvest challenges in conventional solar drying of cocoa (Theobroma cacao L.). This study evaluated the thermal performance, drying kinetics, and thin-layer drying model goodness-of-fit of a passive hybrid solar dryer utilizing cocoa pod husk (CPH) briquettes as a renewable auxiliary heat source. Drying trials were conducted under three operating configurations: solar-only, hybrid solar–biomass, and biomass-assisted drying. Temperature, relative humidity, and sample weight were recorded at 30-minute intervals, and dimensionless moisture ratios were fitted to thirteen empirical and semi-empirical thin-layer drying models using nonlinear regression. Model suitability was evaluated using the coefficient of determination (R²), sum of squared errors (SSE), and root mean square error (RMSE). The hybrid solar–biomass mode established superior thermal performance, generating the highest mean maximum chamber temperature (59.70+1.06^oC) and system drying efficiency (57.80%). While solar-only drying achieved 56+2.83% moisture reduction over an extended period of 25 h (average drying rate of 0.011 kg/h), the hybrid and biomass-assisted modes reached comparable moisture reduction (51.00+1.41% and 50%, respectively) in only 7 h at an accelerated rate of 0.036 kg/h. Among the evaluated models, the Hii model exhibited the best overall fit under solar and hybrid drying regimes (R² up to 0.9988, with correspondingly low SSE and RMSE values). The results demonstrate that incorporating CPH briquettes into a passive solar dryer provides effective thermal supplementation, substantially accelerates moisture removal, and promotes circular agricultural waste utilization in cocoa postharvest operations.