Solar thermal energy technology: Experimental study on the design and performance of an indirect solar dryer for agricultural products

This chapter investigated the use of solar energy for drying. The objective of thestudy was to study a simple batch dryer for agriculture products in order to improve thedesign and to determine the most effective variables influencing its performance. For thispurpose, an indirect conventional solar dryer unit consisting of a solar air heater (i.e.,inclined solar collector with corrugations) and a drying chamber (i.e., basically a batchdryer) was designed and built at the College of Engineering, Sultan Qaboos University(SQU) in Muscat (Oman). This unit can be used for drying various agricultural productslike fruits and vegetables. In this chapter, the solar dryer unit was used to dry radish cropsas the test samples. The dryer unit was investigated experimentally and its performancewas evaluated under the climatic conditions of Oman (21° 00 N, 57° 00 E). To this effect,a series of experiments were performed on the developed solar dryer during summerconditions in Oman (between May 4 and June 8, 2008). The performance of the solardryer was computed and expressed in terms of the moisture evaporation (crop massduring drying). Several influencing design parameters, involved in the dehydrationprocess, that were expected to affect on this performance and on the drying time weretested and discussed. The design parameters studied were the number of glazing cover,the type of the absorber of the solar collector, the loading capacity, and the drying airflowrate. In terms of the number of glazing cover sheet, experiments were performed with asingle glazing and a double glazing. With regard to the type of the absorber plate, verticaland horizontal corrugated absorber plates were used. The thermal performance of thesolar dryer was also evaluated under no absorber plate conditions. In addition, thethermal performance of the solar dryer was evaluated under various load conditions andthe unit was also tested under various drying air velocity. The temperature and relative humidity data during the solar drying were also recorded for each experiment. Thecomparison revealed that the performance of the solar dryer was improved remarkablywith the use of the vertical corrugated duct collector and the drying time wasconsequently reduced. The results also affirmed that the performance increased withreducing the loading capacity. The other parameters were found to have a negligibleeffect on the performance.