Simulation-Based Design and Performance Evaluation of a Mixed-Mode Solar Dryer for Maize (ZEA Mays L.) Preservation in the Guinea Savannah Agro-Ecological Zone of Taraba State, Nigeria

Abstract

Post-harvest losses arising from delayed or inadequate drying remain a major constraint to maize (Zea mays L.) preservation among smallholder farmers in the Guinea savannah belt of Nigeria, including Bali Local Government Area of Taraba State. This study presents an extended simulation-based design and performance evaluation of a mixed-mode solar dryer intended for medium-scale maize drying at the polytechnic teaching-and-research level. Ahead of the construction of a physical prototype, the drying kinetics, thin-layer model performance, effective moisture diffusivity, collector thermal and exergetic performance, specific energy consumption and indicative economics reported here were generated through engineering-design simulation, using governing equations of heat and mass transfer that are well established in the literature, namely the Page, Newton , Henderson-Pabis and Logarithmic thin-layer drying models, Fick's second law of diffusion with an Arrhenius temperature dependence, and the Hottel-Whillier flat-plate collector energy balance. Model coefficients were selected within ranges reported for maize and comparable cereal thin-layer drying kinetics in the literature, and were then used to generate synthetic drying-curve data against which the four thin-layer models were fitted and statistically compared. Simulation results show that maize at an initial moisture content of 24.5% (wet basis) could be reduced to the recommended safe storage moisture content of 13% (wet basis) within approximately 8, 6 and 4 hours at drying-air temperatures of 50, 60 and 70 °C respectively; the Page model gave the closest fit to the generated drying curve (R2 = 1.00) followed by the Logarithmic model (R2 = 0.999), with effective moisture diffusivity estimated in the range of 1.08 x 10−9 to 2.67 x 10−9 m2/s and an Arrhenius activation energy of about 41.4 kJ/mol. Collector thermal efficiency, computed from the full Hottel-Whillier energy balance across a mass-flow-rate range of 0.01-0.05 kg/s, rose from 34.6% to 55.7%, with a corresponding system drying efficiency of 24.2-47.9% and specific moisture extraction rate of 1.12-1.37 kg water per kWh. An indicative payback-period estimate, following the techno-economic approach used for comparable mixed-mode solar dryers elsewhere, suggests recovery of the assumed capital cost within about three years. Collectively, these design-stage results, several of them illustrated graphically, are consistent with the range of values reported for comparable solar maize and grain dryers evaluated experimentally in Nigeria and beyond, and support the technical soundness of the proposed design as a basis for fabrication, instructional use and further field-based validation at Federal Polytechnic Bali.

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