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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 °C 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. [ABSTRACT FROM AUTHOR]

Open sun drying has been widely applied in the developing country. It is a popular, effective and economic method for drying and preservation of agricultural products and food, but it has some drawbacks such as degradation of materials quality and larger drying period in ambient environment. In order to solve the problem of uncontrollability and randomness in the process of open sun drying, heat and moisture transfer model of open sun drying materials was studied with theoretical knowledge of heat and mass transfer in this study, and heat and moisture transfer mechanisms of open sun drying materials were analyzed. The heat exchange process of dry materials in the open sun drying process mainly included the absorption of solar radiation heat, the convective heat exchange with the surrounding environment, the heat transfer of long wave radiation and latent heat by water evaporation. Thermal resistance and thermal energy diagram of drying materials was built. Heat balance equation of materials was also built based on energy balance principle, and model parameters were determined by comprehensively considering solar radiation intensity, ambient air temperature and humidity, and ambient wind velocity; the dynamic temperature change of material surface and the water transfer rate of material were predicted, the program was solved by using MATLAB software, and the initial values required for the solution of the model were derived from the experiment data. To verify the accuracy of prediction model, sweet potato was selected as drying material in the open sun drying test. The tested samples of sweet potato were sliced uniformly in terms of size and thickness, the initial mass of sweet potato was 240 g, and the thermal performances and drying characteristics of sweet potato in the open sun drying were investigated in the Qianfo mountain campus of Shandong University. The test period was from 8:30 to 17:00 on May 7, 2017. The test parameters included solar radiation intensity, outdoor air temperature and humidity, surface temperature and mass change of sweet potato during the drying process. Fourteen mathematical models were tested to specify the suitable model for describing the drying behavior of the tested samples, and it was found that the Page model was convenient to describe the drying characteristics of sweet potato in open sun drying. During the test period, the solar radiation intensity ranged from 398.91 to 979.9 W/m2, the air temperature ranged from 27.2 to 41.2 ℃, and the sweet potato mass decreased from 240 to 57 g. The determination coefficient were respectively 0.96, 0.89 between the simulated and experimental values in terms of drying materials temperature and mass, and root mean square error were respectively 0.97 ℃, 28.35 g, and the change trend of simulated and experimental value was consistent, but there were certain differences between them, and especially the difference of dry material mass was large. The main reason for this phenomenon was the model calculation parameters and experimental data had some irrationality. It is concluded that the model can predict accurately the material dynamic change of surface temperature and drying mass rate in the process of open sun drying of materials, and this model can be used in the process control of open sun drying. [ABSTRACT FROM AUTHOR]