Solar drying for some agricultural in Northern of Sumatra-My personal recent investigation

A preliminary identification of the community needs for the solar drying application of agricultural products in the Province of North Sumatra, Indonesia, had been made by means field survey and literature reviews. Farmer need to dry selected agricultural products; For this solar dryer could be applied. The products are candle nuts, coffee, chillies, soybeans, ground nuts, and mung beans.

Drying characteristics for the selected agricultural products were studied for the purpose of designing a solar dryer. In particular, the drying characteristics of candle nuts, including its sorption isotherms and drying rates, were investigated in a laboratory. It was found that the unshelled kernels of candle nuts had a higher equilibrium moisture content than shelled kernels. In both cases hysteresis was pronounced, and decreased as the temperature was increased. The isosteric heat of desorption was higher than the isosteric heat of adsorption. The GAB equation gave a satisfactory goodness of fit to the unshelled kernel data. For the drying-rate experiment, the measurements were taken for unshelled kernels of candle nuts, from both fresh and stored samples. Measurements were made at the temperatures of 40, 50, and 60^oC and a drying air velocity of 0.67 m/s, and about 1 m/s using a forced convection dryer. The total drying time was substantially reduced with an increase in temperature, but the effect of the air velocity was relatively small. Page’s model adequately describes the drying behavior for the range of temperatures measured. The dependence of the drying constant on air temperature can be described by the Arrhenius model.

A mixed mode natural convection solar dryer, combined with a simple biomass burner and heat storage as a back-up heating system, was designed. The design considerations covered type of product to be dried, physical drying characteristics, capacity, construction material, type of dryer, and included prediction through modeling and numerical simulation.

A pilot solar dryer was constructed in the Department of Mechanical Engineering, Prince of Songkla University Hat Yai, Thailand. The type of dryer was mixed mode, natural convection, with biomass and heat storage back-up heating system. Serial tests for performance evaluation of the dryer were conducted from June 2005 through March 2006. From the tests, the dryer was found to be satisfactory for drying selected agricultural products, including candle nuts, chillies, coffee cherries, soybean, and ground nuts. The overall thermal efficiency of the dryer, tested for drying of the listed products, was found to be in the range of 3 - 13%. The overall thermal efficiency of the biomass back-up heater was found to be about 20%. Solar dried chilies were used for sensory evaluation of the product quality. The results of sensory evaluation showed that the solar dried product gained good acceptance for its color, aroma, and texture. The cost for construction of the dryer was estimated to be 50,000 Thailand Bath (US$1,250). The payback period of the dryer was estimated to be 2.5 - 3 years.