Evaluation the Performance of Drying Efficiency and Energy Efficiency of a Solar Dryer Quipped with Phase Change Materials and Air Recirculation System

IntroductionSolar energy is one of the sources of renewable energy that can be used in both buildings, industry and agriculture in the form of heat or electrical energy. According to previous researches, energy consumption in the world is doubling every 20 years. However, the use of renewable energy is still less than fossil fuels, which has caused environmental problems in the world. In recent decades, the tendency to use renewable energy, especially solar energy, has increased. A significant portion of the world's energy (about 30%) is spent on agriculture, and about 3.62% is used to dry agricultural products (Iranmanesh et al., 2020). However, thermal and drying efficiency of the solar collectors are not in acceptable range. Applying different ways to improve the performance of solar dryers such as using thermal energy storage system, air recirculation mechanism and using desiccant system. In this paper, phase change materials were placed vertically in consecutive rows at different distances inside the collector and the thermal performance of the collector was investigated. Also, the drying process of Oleaster were evaluated using PCM and air recirculation system. Materials and MethodsThe indirect solar dryer used in this study includes the chassis, flat plate collector, electric fan, drying cabinet, pipes containing PCM and sensors. 25 copper tubes containing PCM are placed on the absorber plate with fixed intervals. The insulated cabinet of the dryer has three trays. A 220 volt 60 W electric fan is placed in the inlet of the collector and causes to flow air inside the system. The process of drying Oleaster in a solar dryer was carried out for 9 consecutive days in August 1401. The drying process was performed at three positions of PCM pipes at 5, 10 and 15 cm intervals with air flow rate of 0.5, 1 and 2 m/s. The drying kinetics of Oleaster was investigated using five mathematical models considering drying time and related constants. The selected model is selected based on the degree of fit (the highest R2 and the lowest RMSE) on the experimental data. Thermal efficiency was calculated according to ASHRAE standard 2003 (Eltawil et al., 2018). Moreover, to determine the drying efficiency the amount of energy required to heat the dryer and the product and extract water from the Oleaster and the total energy (electrical and thermal) input to the dryer was considered. SCE is defined as the energy required to dry one kilogram of the product. Results and DiscussionThe drying time of the product by the dryer is reduced from 2.09 to 4.16% on average by changing the position of PCM from 5 cm to 15 cm. On the other hand, with the increase of air velocity from 0.5 to 2 m/s, the drying time decreased from 8.32% to 16.64%. Henderson and Pabis model was the best model to describe and define the drying process of Oleaster with solar dryer. The curves of the drying rate against the time in different conditions illustrated that in the initial stage of drying of samples. The amount of moisture evaporation is high due to the high water content in Oleaster, and a major part of the drying process took place in this period. The value of SEC for the dryer without PCM was 4.26 MJ/kg, while for the case with PCM, it was 2.04 MJ/kg with a distance of 15 cm. By increasing the distance between the tubes, the drying efficiency increases due to the reduction of drying time and energy consumption. In this case, the consumption of electrical energy by the fan (for fluid flow in the dryer and collector) and the thermal energy input to the dryer are reduced. However, with the increase of air speed from 1 m/s to m/s2, there is a significant reduction in drying efficiency. The highest drying efficiency was 36.72% and the lowest was 25.65% for distance 15 cm, air velocity of 1 m/s and distance 5 cm, air speed 2 m/s, respectively. Drying efficiency was improved by at least 12% using PCM. ConclusionIn this research, the analysis of the thermal process in the solar dryer system in three positions of the tubes containing PCM inside the flat plate collector for the distances of 5, 10 and 15 cm between the tubes at three air velocities of 0.5, 1 and 2 m/s was investigated. Using the return flow system and the phase changing material at the same time improved the thermal efficiency of the flat plate collector by 19.12%.