The aim of this work was to study different hybrid photovoltaic-thermal (PVT) collectors. Numerical simulation of the collectors was carried out using a 3-dimensional computational fluid dynamic (CFD) model. A set of experiments was conducted using a box-type PVT collector to validate the obtained computed data. Simulation results for water outlet temperature, especially at the lower flow rates, were closer to the measured data compared with average module surface temperature. Based on the simulation results, the spiral flow designs of heat exchanger provided the most uniform temperature distribution for module surface and the highest temperature gradient was observed with the direct flow designs. A hot zone was generated on the module surface of the direct flow box-type collector and the results indicated that the hot zone was considerably declined using cross-fins inside the box-type heat exchanger. The highest heat transfer to the ambient occurred with the spiral flow design and the box type heat exchanger presented a relatively low heat losses. The cross-fined box type and the spiral flow designs of the PVT collector had the highest electrical efficiencies at the different test conditions.
Ultraviolet radiation is one of the environmentally friendly methods that was used in the present study to process lemon juice. The designed UV system had five UV-C lamps and irradiation was performed based on the distance of the lamps (10, 20, and 30 cm) from the fluid layer and the circulation time (10, 20, and 30 minutes) of lemon juice in the system. Non-irradiated sample was also considered as a control treatment. The control and irradiated samples were filled in dark polyethylene bottles and stored at ambient temperature for 90 days. Immediately after irradiation and during the storage period, parameters such as pH, titratable acidity, total soluble solids, ascorbic acid content, color indices (L*, a*, and b*) and browning index, as well as sensory attributes (color, taste, aroma, and overall acceptance) were evaluated. The pH, acidity, and total soluble solids did not show a definite trend. The results showed that among the color indices, L* decreased significantly, but a* and b* increased significantly during the storage period. The rate of color changes in irradiated samples from a distance of 10 cm was higher than other treatments. Irradiation also significantly reduced the ascorbic acid content of irradiated samples from a distance of 10 cm compared to other samples and control. However, the browning of irradiated samples from 10 cm distance was significantly less than that of irradiated samples from 20 and 30 cm distance. The score of the panelists indicated that irradiation and storage time altered the quality characteristics of lemon juice. The panelists' scores showed more acceptance of irradiated samples from 20 and 30 cm distance compared to irradiated samples from 10 cm distance. In general, the designed system in this study is not recommended for lemon juice processing due to the destructive effect of UV-C radiation on the content of ascorbic acid.
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