Performance And Optimization Of Direct Expansion Solar Assisted Heat Pump Drying System

The drying industry involves many areas of people’s production and life.It is a very important production link in food,agricultural products,medicines,wood and other industries.The drying process requires a lot of energy.In the total energy consumption of the national economy,the proportion of energy consumption in the drying industry reaches about 12%.At present,most of the drying field uses coal,electricity,natural gas and other energy sources,and there are problems such as large drying energy consumption and low economic benefits of drying products.Therefore,how to improve the energy efficiency of the drying system is of great significance.This paper builds a direct expansion solar assisted heat pump drying system based on an energy-saving and environmentally friendly drying system using solar energy,and conducts research from theoretical analysis,experimental testing,and optimization evaluation.The main research contents are as follows:(1)In order to explore the drying characteristics when the heat pump is used in combination with solar energy,and design a reasonable material drying characteristic curve,a theoretical study on the heat and mass transfer mechanism involved in the drying process is carried out.The analysis shows that the types of moisture in the materials mainly include mechanically bound water,physical and chemical bound water and chemically bound water.The main factors affecting the drying rate include external environmental conditions and the internal conditions of the material.Controlling the temperature,humidity and flow rate of the drying medium,reasonable selection of the size and stacking method of the material are all effective ways to increase the drying rate.(2)In this study,a direct expansion solar assisted heat pump drying system was built,and the component composition and basic working principles of the drying system were described.Experimental research reveals the influence of parameters such as ambient temperature,solar radiation intensity and compressor operating frequency on system efficiency.The temperature and relative humidity of the air in the drying room are the most important factors affecting the drying effect,and the two basically show the opposite trend of change,which has an obvious negative correlation.The change trend of the heat pump heating performance coefficient(COP)is consistent with the change trend of the temperature in the drying room.The value of COP reflects the drying effect and capacity of the drying system to a certain extent.Factors affecting the value of COP include operating frequency of compressor,solar radiation intensity and outdoor ambient temperature.(3)In order to improve the utilization rate of solar energy,four different drying modes have been formulated for different seasons and climatic conditions,and the system performance under different drying modes has been compared and analyzed.The research shows that the average heating performance coefficient(COP)of the heat pump system under typical climatic conditions varies from 1.92 to 6.01.The average COP under the solar-assisted heat pump intermittent operation drying(SAHPD)mode in summer is the highest,reaching 6.01,which is higher than the COP of the conventional closed heat pump drying system.There is no frost during the operation of the system,the air temperature in the drying room is kept above 40 ℃,and the SMER of the system is 0.123 kg/kW·h.(4)In order to test the practical performance of the system,a drying experiment was carried out with mushroom as the material,and the sensory evaluation of mushroom products under different drying modes was carried out in terms of color,texture and flavor.Comparative analysis shows that the comprehensive score of mushrooms under SAHPD mode was the highest,is 92.5 points.In addition,the drying kinetics analysis was carried out on the experimental data of drying mushrooms.By comparing the fitting results of the seven drying models,the best drying kinetic model for mushrooms is the Page model for the direct expansion solar assisted heat pump drying system,with an R2 value of 0.996 and an RMSE value of 0.0211.The research results of this article show that the direct expansion solar assisted heat pump drying system has great application potential in reducing drying energy consumption and improving the quality of dried products.The research in this paper can provide a reference for the research and development design and optimized operation of the efficient and energy-saving solar heat pump combined drying systems.