Development of Solar-Assisted Dryers for Food and Farm Crops

"Solar energy is free but not cheap. " That thought has driven research at Nong-Lam University to design, fabricate, and test mechanical dryers of different capacities, using solar energy as the heat source, the aim being to reduce the drying costs due to fossil fuels. The dryers are based on the air reversal principle. The machine has three components: an axial-flow fan; a drying bin containing the agricultural material, wherein the airflow can be set upwards or downward; and a solar collector consisting of two parallel horizontal cylinders made from polyethylene (PE) transparent sheet. Work started with a laboratory dryer for 5 kg of material per batch, from which basic data on the solar collector were obtained. Next, a full-scale 500 kg macaroni dryer was designed, and drying tests showed that the solar collector could replace the coal furnace of an existing macaroni dryer, which had consumed 6 kg of coal per hour for drying a batch in 5 h. With solar radiation ranging from 500 to 900 W/m 2 , the drying temperature reached 37°C to 52 °C, averaging 14 °C above ambient temperature. The collected power was 44 kW, and the collector efficiency was 40%. By substituting coal with solar energy, the macaroni dryer has saved US$120/month, or 100% of the fuel cost. Compared to its initial cost and the replacement cost for the PE tube every seven months, the payback period is less than one year. Use of clean solar energy meets the hygienic requirements for food drying. Drying tests in the 2007 dry-season harvest using a popular 4-ton paddy dryer with a solar collector showed that solar energy could contribute to a cost saving of 43% to 78% by reducing coal consumption, while maintaining the capacity and quality as normally practiced with conventional fuel.