Analysis and application on thermal performance of direct solar drying system

Abstract: The direct solar drying system has the advantages of simple structure, cheapness, and protecting the dried materials from being damaged by dust, rain, dew, and so on; however, there are some disadvantages such as overheating of the dried materials surface, poor quality of the dried material and drying ability limited. In order to solve problems of drying product overheating and drying process uncontrollable in direct solar dryer, a direct solar drying system was designed and built to test system performance by using sweet potatoes as drying material. The system was mainly composed of air inlet, air outlet, dry material tray, transparent glass cover and stainless steel plate. The frame of drying chamber was made of aluminum alloy, and the black endothermic material was uniformly applied to the inner surface of the stainless steel plate of the drying chamber. The tilt angle of the roof glass of the drying chamber was 26.7°. The length, width and height of the drying chamber were 1 000, 800 and 800 mm, respectively. The insulation material of the drying chamber was made of 20 mm thick polyurethane insulation cotton to prevent the heat loss, and the transparent surface of drying chamber adopted ordinary glass with transmittance of 90%. The dry material used in this experiment was fresh sweet potato, purchased in Jingdong Mall, with the origin of Yuxi, Yunnan. Before the experiment, the sweet potatoes were cut into pieces of uniform size and thickness, and divided into 2 parts with the same mass by the balance; one part was used for the experimental group of direct solar drying and the other part was used for the comparison group of open sun drying. The experimental test was conducted on May 7, 2017, from 8:30 to 17:00. The initial mass of dry materials in the experimental group and the control group was 240 g. In addition, dynamic mathematical models of thermal performance of system were built and calculated with the help of MATLAB 2014a software, outdoor meteorological parameters, the characteristics of drying materials and building systems were given, and the correlations between experimental value and simulation value were analyzed. The results showed that the transmittance of drying chamber ranged from 51.7% to 89.6%. The highest air temperature in the drying chamber appeared half an hour later than the peak of outdoor solar radiation intensity. It indicated that the drying chamber had certain delay and resistance to the change of outdoor environment interference. The average drying rate of the direct solar drying system was 7.7 g/h higher than that of the open solar drying system. Time period with drying air temperature ranging from 50 to 70 ℃ accounted for 80% of the total drying time, which showed that the direct solar drying system can provide the most suitable drying temperature for most dry materials. The total thermal energy obtained by the drying system was 3.92 kW?h, and the average solar heat utilization efficiency was 21.23%; the correlation coefficient of the experimental value and the simulated value was 0.98, and the root mean square error was 1, which proves that the dynamic model of dynamic thermal performance can accurately predict the surface temperature of the dry materials in the drying chamber.