Abstract:
Typical drying process can consume a large amount of energy in recent years. Conventional solar drying systems generally suffer from low heat-exchange efficiency, uneven heat distribution, and high susceptibility to weather fluctuations. Existing studies focus mainly on the equipment to convert solar energy into electricity. It is still lacking on structural optimization of heat exchangers during drying. Furthermore, single-layer heat-exchange structures are prone to local overheating, thus limiting the large-scale deployment of solar drying for agricultural products. Multi-layer structures can be expected for solar drying applications. In this study, system simulation and experiment were performed on the solar heat exchanger with the curved surface for concentrating sunlight. Accordingly,
Zanthoxylum armatum was taken as the drying material. According to the non-imaging optics, Light Tools software was adopted to simulate the optical path for structural optimization of the reflector assembly. A parabolic reflector was obtained with a curvature radius of 0.88 m and an effective reflection area of 1.84 m2. The optimal structural parameters were determined for light concentration: heat-exchanger height of 135.6 cm, reflector inclination angle of 48.5°, and light channel length of 50.4 cm. Numerical simulation was conducted on copper heat exchanger with fin-free and three-layer logarithmic spiral fins using Fluent software. Heat-transfer characteristics of two configurations were compared after simulation. The total heat-transfer area of the finned heat exchanger was 1.07 times that of the fin-free counterpart. An experimental platform was established to simulate solar irradiation. Drying tests were conducted at constant temperature and humidity (ambient temperature: (24±1) °C, relative humidity: 45%±2%). Heat-transfer performance of the heat exchanger and drying performance of
Zanthoxylum armatum were analyzed at heating-surface temperatures of 60~80 °C and inlet air velocities of 0.2~0.6 m/s. Scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP) were combined to characterize the evolution of microstructural porosity during drying. The results show that the three-layer finned structure improved the temperature uniformity of internal airflow inside the heat exchanger; Its average temperature of the air at the outlet was 7.41% higher than that of the finless heat exchanger, effectively mitigating the common local overheating defect of single-layer structures. The steady-state heat-exchange efficiency of the fin heat exchanger ranged from 24.16% to 56.28% under experimental conditions, and exergy efficiency varied from 6.37% to 18.14%. Heat-exchange efficiency achieved the maximum increment of 68.75% at an air velocity of 0.6 m/s, compared with that at 0.2 m/s. Irradiation power shared a strong linear correlation with the steady-state temperature of the heat-exchanger heating surface; Irradiation power levels of 0.9, 1.3 and 1.7 kW were corresponded to steady-state heating-surface temperatures of 60 °C, 70 °C and 80 °C, respectively. The minimum thermal equilibrium time of the system was 59 min. The final splitting rate of Zanthoxylum armatum reached 93.2%~98.7% at the heating-surface temperatures of 60~80 °C and inlet air velocity of 0.6 m/s. The drying process was divided into preheating, constant-rate drying and stabilization stage. The moisture stabilization stage reached within only 120 min at 80 °C, with a 40% reduction in drying duration, compared with the 60 °C condition. Microstructural analysis revealed that the peel porosity of
Zanthoxylum armatum increased first and then decreased during drying. The porosity of an individual pericarp reached 67.82% in the constant-rate drying stage. This findings can provide numerical and experimental support for high-efficiency heat exchangers during solar drying. The curved-surface concentrating solar drying can be expected for the heat-sensitive agricultural products. High efficiency and quality drying can be realized to serve as the technical solutions and theoretical references for the low energy consumption in modern agriculture.