Dehydration of food crops using a solar dryer with convective heat flow

A solar drying system designed on the principles of convective heat flow was constructed from local materials (wood, metals and glass sheets) and used to dry food crops (cassava, pepper, okro, groundnuts, etc.). The solar collector could transfer 118 W m−2 thermal power to the drying air. The thermal exchanges within the dryer were determined from a psychometric chart. Ambient air at 32°C and 80% relative humidity (RH) could be heated to 45°C at 40%. The crops were dried to a final moisture content of <14% and were preserved for a period of one year without deterioration. The low-temperature drying system ensured the viability of the seeds for planting. The drying process can be represented by an empirical equation of the form M(t)=MOexp (−kt) or dM/dt=−kM, where MO is the initial moisture content, M(t) is the moisture content at time t, and k is the drying constant. Under identical conditions, a high value of k was correlated with a shorter drying period. The drying process takes place in two phases: constant rate and falling rate periods, and the drying equation was solved to predict the total drying time. The mechanisms for the dehydration are the removal of unbound “free” water in the cell cavities and of “bound” water (water films) trapped within cells or chemically bound with solids as water of crystallization.