Development of a photovoltaic powered forced circulation grain dryer for use in the tropics

In tropical Africa, a high percentage of crop loss, of the order of 10–50%, occurs during the drying period due to either permanently high relative humidity and sudden rainfall periods, or such periods coinciding with the harvesting season. The traditional sun-drying technique is labour-intensive and requires a lot of land per unit throughput. Due to lack of any control, sun-dried grain is usually prone to overheating, cracking, vermin and dust contamination, rewetting during sudden rains, predation by animals and vandalism by humans. All this leads to significant product loss and impaired quality. A solar maize dryer incorporating a directly coupled photovoltaic (PV) powered d.c. fan was developed and field-tested for small scale use in Malawi, central Africa. The dryer has a capacity of 90 kg and it has been designed to utilise forced air circulation without the use of external power supplies like grid electricity, fossil fuel and batteries, which are either very expensive or non-existent in the rural areas of central Africa. A main design constraint was that the drying air temperature should not exceed 60°C, which is the international drying standard for maize grain used for human consumption. Temperatures in excess of 60°C lead to grain overheating, cracking and subsequent microbial attack. Results showed that the incorporation of a PV-driven d.c. fan provided some form of passive control over the air flow and hence the drying air temperature. The dryer was coupled to a solar air heater having a sun-tracking facility and optimised blackened sisal rope grids for improved energy collection efficiency of the order of 80%. Grain drying with this solar dryer technology, compared with sun drying, reduced the drying time by over 70%. Grain quality, texture, and flour quality and flavour improved significantly with the dryer, as grain was permanently protected from sudden rains, vermin and dust contamination. Although the capital cost of the solar dryer was found to be high (about US$900), the dryer was found to be cost-effective with a payback period of less than one year if it is used to dry grain for purchasing by the Cereal Boards.