| According to the statistics in 2020,as an agricultural country,the total output of grain,fruit and Chinese herbal medicine of China is 669.5 million tons,274 million tons and 4.5 million tons,respectively.However,lots of agricultural products are produced from the underdeveloped national poverty-stricken areas.The tough geographical environment and blocked transportation conditions have caused the unsalable agricultural and sideline products,which seriously limits the development of local economy.In this context,agricultural and sideline products need to be dried to improve the shelf life,reduce packaging costs,alleviate transportation weight and retain nutritional value.The natural air drying has been widely used since ancient times among many drying technologies,but it has some problems such as low efficiency,long period,poor effect and serious pollution.Other technologies like the hot-air drying also have the disadvantages of high investment and operation costs,severe energy loss,and uneven drying.Therefore,the research of green drying equipment suitable for different production areas is of great significance to decrease the processing cost,improve the quality and added-value,realize the energy-saving and efficiency of agricultural products processing.In order to improve the energy efficiency,reduce the energy consumption and improve the drying effect,a new type of PV/T solar heat pump combined drying system is proposed.The system is mainly composed of air and refrigerant circulation.The system adopts PV/T collector,which can be both used as the first stage heating device of air and evaporator of refrigerant.The power generated by the solar collector PV module can be applied by the compressor,wind fans and other electrical equipment.Solar energy can be used more efficiently and directly,and the power consumption can be greatly reduced compared to the existed solar energy heat pump combined drying system.Regenerator is employed to absorb the waste heat from the waste gas and improve the energy efficiency of the system.It can avoid the environmental and material pollution caused by improper use of phase change materials,and reduce the cost,fundamentally.The air flow rate and mass flow can be flexibly adjusted to meet the material drying demand under different conditions,and expand the application range of the system.To verify the effect of different kinds of air on the system performance during drying,the pipeline between the regenerator and the collector is designed as detachable.When the pipeline is removed and the ambient air enters the collector,it is the open drying mode,otherwise it is the closed drying mode.A large number of drying experiments are carried out at the different environmental conditions.The main conclusions are as follows:(1)Among the factors affecting the performance of PV/T collector,the increase of air mass flow will evidently improve the photo-thermal efficiency.When the photo-thermal efficiency reaches to a certain value,it will not change notably with the increase of air mass flow.On the contrary,the overall economy of the system will be debased due to the continuous improvement of fans power.The optimal mass air flow is 0.042kg/s of the proposed system.A negative influence on the photo-thermal efficiency emerged with the increase of inlet air temperature,while the photo-electric efficiency is not significant.The overall variation trend of the heat supply and photovoltaic power generation of the collector is close to the solar irradiation intensity.But the photovoltaic power generation will be negatively affected by the temperature of photovoltaic module.During the experiment,the average photo-thermal efficiency is 34.04%,and the comprehensive efficiency is 55.45%.The utilization ratio of solar energy is improved remarkably by using PV/T collector.(2)The drying results will be affected by different material treatment processes.The materials are divided into four parts with different treatments.The thickness of the first slice is 5mm,and the skin of the three remaining materials is removed with the slice thicknesses are 5mm,3mm,and 8mm,respectively.The existence of material skin will affect the water removal and drying rate in the drying process.The thinner the slice is,the better the water evaporation.(3)The residual heat in the waste dry gas can be absorbed by the regenerator stably.The maximum heat absorbed by the regenerator is 1.99 k W?h during the experiment,which greatly improves the energy efficiency of the system.(4)For verifying the performance of the two drying modes,comparative experiments are carried out under the different environmental conditions.The results reveal that the performance of the closed drying mode is more stable,and the ability to overcome the influence of environmental factors is stronger.The power generation of PV module can reduce the power consumption of the system by 28.40%,and the average COP value of the heat pump system is 4.05.The average temperature of drying chamber is 55.40℃,the specific moisture extraction rate(SMER)is3.73kg/(k W?h),and the final moisture content of the material is 13.68%.The system can improve the energy utilization,reduce the power consumption,achieve the purpose of energy saving and emission reduction,and provide a practical scheme for the drying of materials at the medium and low temperature.The promotion of this system can help rural revitalization and promote the modernization of agriculture with Chinese characteristics. |
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