A Solar-Regenerated Desiccant Dryer for One Tonne of Smallholder Grain: Water Budget, Sizing and Failure Limits

Grain harvested wet in humid climates moulds within days, and farmers without fuel or grid power lose the harvest at the drying step. This paper designs a dryer in which a silica-gel desiccant bed dries the air and the day's sun regenerates the bed. For one tonne of maize dried from 25.0% to 13.0% moisture (wet basis), 137.93 kg of water must be removed. Drying in three 24-hour cycles requires 191.6 kg of silica gel, 163.2 MJ of regeneration heat per cycle, and 20.2 m2 of solar collector at 5.0 kWh/m2/day insolation (24.0 m2 specified), with an elapsed drying time of 3.0 days. The design fails on schedule below 4.20 kWh/m2/day, fails against a four-day mould-risk window below 3.15 kWh/m2/day, stalls if the drying air rises above 65.6% relative humidity, and overruns for grain harvested wetter than 25.0%. Design performance is SIMULATED, NOT MEASURED; nothing has been built or hardware-tested. The computation script and its results accompany the paper.