Study On The Thermal Performance Of A Solar Assisted Heat Pump Drying System

Drying operations are widely used in various fields of the national economy.The energy used for drying in China accounts for about 12%of the total energy consumption.Therefore,using existing technologies to reduce the energy consumption of drying operations is important for China's energy conservation and emission reduction strategies.As a composite drying system,the solar assisted heat pump drying system has the advantages of solar drying and heat pump drying.It can be switched to a more energy saving mode operation according to environmental conditions while keeping the system stable and reliable.Based on the previous research,a small solar assisted heat pump drying system was proposed in the paper,and a small-scale solar assisted heat pump drying device was built according to the system.Based on this,experimental research and simulation analysis were carried out for the performance of the device.According to the proposed system form,the system design calculation was carried out,and the components in the system were designed and selected according to the calculation results.The system mainly consists of DC inverter compressor with rated power of 420 W,external balanced thermal expansion valve of TEN2 type,evaporator-collector of 1.2 m~2,soalr flat plate air collector of 2.0 m~2 and self-designed evaporator and condensor.On this basis,the data measurement and acquisition system in the experimental process was determined,and the error analysis of the experimental system was carried out.Then the construction of the test platform was completed.The performance of the solar assisted heat pump drying device was investigated under different environmental conditions.The results show that the system performs well in solar drying,heat pump drying and solar assisted heat pump drying mode.In the solar drying mode,the inlet air temperature of the drying chamber can reach above 50?,the average collector efficiency of the air collector was about 0.562,and the SMER was as high as 3.56 kg/kWh.In the heat pump drying mode,the influence of air temperature,wind speed and loading on the system was studied by the control variable method.The results show that as the air temperature increases,the drying rate of the mushroom increases,the evaporation temperature and condensation temperature of the heat pump will increase,and the average COP will decreases;As the wind speed increases,the drying rate of the mushroom increases slightly,the evaporation temperature and the condensation temperature of the heat pump also increase slightly,and the average COP increases;As the loading increases,the drying rate of the mushroom decreases slightly,the evaporation temperature and the condensation temperature of the heat pump do not change much,and the average COP decreases.In the solar assisted heat pump drying mode(only assisted by solar air collector),the average air temperatures obtained in the open,semi-open and closed modes were 59.93?,60.58?and 61.95?respectively,and the COPs were 3.25,3.14 and 2.79,respectively,which is significantly more energy efficient than the heat pump drying mode that achieve the same air temperature.In the late drying stage,switching to the solar assisted heat pump closed drying mode(only assisted by evaporator-collector),the energy saving rate can reach 37.96%compared with the heat pump closed drying mode under the same conditions.The mathematical model of the solar assisted heat pump drying system was established.The simulation program of the system was written using MATLAB and verified by experimental data.The validated model was used to simulate the parameters that affecting the performance of the system.The simulation results show that under the heat pump drying mode,with the compressor speed increases,the inlet air temperature of the drying chamber increases while the COP decreases.As the refrigerant charge increases within a certain range,the COP will rise.Under the condition that the inlet air temperature of the drying chamber is determined and the drying mode is solar assisted heat pump drying mode,with the solar radiation intensity or the ambient temperature increases,the COP of the heat pump also increases,and the compressor speed decreases.