Development and Evaluation of a Solar-Powered, Pneumatic-Assisted Cowpea Grading Machine for Improving Postharvest Processing in Smallholder Agro-Enterprises

Smallholder agro-enterprises in sub-Saharan Africa continue to face major constraints in postharvest cowpea processing due to the absence of reliable electricity, limited mechanization, and the high cost of commercial grading equipment. This study reports the development and performance evaluation of a solar-powered, pneumatic-assisted cowpea grading machine designed to improve seed cleaning, defect removal, and throughput in off-grid rural environments. The system integrates a vibration-based mechanical grader and a pneumatic module configured to deliver an airflow velocity of 6–8 m/s, corresponding to the reported terminal velocity range of cowpea seeds to effectively separate shriveled, lightweight, and low-density grains. Power requirements were matched with a 200 W photovoltaic module and a 12 V, 100 Ah deep-cycle battery, sized through energy-balance calculations covering the 0.5 hp DC motor, blower load, and control circuitry to ensure sustained off-grid operation. A Box–Behnken design under Response Surface Methodology was employed to optimize feed rate (500–1500 g/min), seed moisture content (6–8%), and vibration speed (65–85 rpm) for two cowpea varieties (Efe Brown and Niger White). Under optimal conditions, the machine achieved a throughput of 91.6 kg/h, mechanical grading efficiency of 81.2%, and pneumatic sorting accuracy of 88.6%, outperforming conventional manual and small-scale mechanical methods. The dual-stage solar-powered system significantly reduced labor input, enhanced seed quality, and demonstrated strong potential for scalable deployment in energy-constrained communities. This work provides a low-cost, clean-energy solution that strengthens cowpea value chains and contributes to improved food security across rural sub-Saharan Africa.