Performance Study Of Closed Transcritical CO₂ Heat Pump Drying System

With the continuous development of human society,the massive consumption of energy and the destruction of the environment have become one of the great problems faced by mankind in the past decades.Drying has been widely used in industry and agriculture,but it is also one of the most energy-consuming units.Compared with traditional drying methods,heat pump drying that based on the principle of heat pump can save a lot of energy,and no harmful gas is emitted into the environment during the drying process,which is a relatively green and energy-saving drying method.The natural working medium CO₂ is green and has superior thermodynamic properties compared to conventional refrigerants.Therefore,it is important to study the performance of transcritical CO₂ heat pump drying system for its application in practice.The study establishes a basic theoretical model of a closed transcritical CO₂ heat pump drying system that analyzes the thermodynamic and environmental performance of the system at different drying chamber temperatures.It is calculated and analyzed to obtain the optimal system performance coefficient COP,dehumidification capacity per unit energy consumption SMER,carbon emission over the life cycle climate performance（LCCP）and the emission of environmental pollutants(SO₂,NO_X,PM_2.5,PM₁₀)at different drying chamber inlet temperatures.The study finds that the COP value decreased from 5.24 to4.08 when the drying inlet temperature was changed from 45°C to 70°C.When the inlet of the drying chamber is 60°C,the optimal value of SMER is as 3.49 kg/（k W·h）,and the emissions of LCCP and SO₂,NO_X,PM_2.5,PM₁₀ are the smallest with 5401.85 t CO_2e,2296kg,2119 kg,1198 kg,2049 kg,respectively.This study investigates an experimental study of a basic transcritical CO₂ heat pump drying system.White radish was used as the drying material to analyze the system performance under different compressor frequencies.According to the results,the COP of the system improves with the increase of compressor frequency,and when the compressor frequency is 105 Hz,the maximum COP of the system is 3.35 in the drying process.For the COP values under different operating conditions of the experiment and the COP values calculated by the theoretical model,the maximum deviation is within 20%,which indicates that the theoretical model is more reasonable.During the whole drying process,the system SMER decreases with the increase of the compressor frequency.When the frequency of compressor is higher and the inlet temperature of drying chamber is higher in the same drying time,the moisture content of dried white radish base is lower.This study establishes a model of a closed transcritical CO₂ heat pump drying system with an auxiliary air cooler,which quantitatively analyzes the influence of the cooling water temperature and flow rate of the auxiliary air cooler on the system.The value of COP of the system decreases continuously as the cooling water inlet temperature T_B2 rises from4°C to 22°C,and the value of COP decreases from 4.38 to 3.25;When the mass flow rate m_w is 60 kg/h and the cooling water inlet temperature T_B2 increases from 4°C to 22°C,the SMER decreases from 2.79 kg/（k W·h）to 2.12 kg/（k W·h）;As the cooling water inlet temperature T_B2 is 5°C and the cooling water mass flow rate m_w increases from 40 kg/h to80 kg/h,the SMER value increases from 2.20 kg/（k W·h）to 3.05 kg/（k W·h）.Therefore,the drying chamber inlet temperature is controlled by adjusting the cooling water mass flow rate and the cooling water inlet temperature.In the condition of achieving the material drying,the cooling water inlet temperature should be reduced and the cooling water flow rate should be increased to improve the COP and SMER of the system.This study also compares the carbon emissions and environmental pollutant emissions over the full life cycle of different drying scenarios under the same drying conditions.The results show that the CO₂ heat pump drying system has low carbon and pollutant emissions.The LCCP value of the CO₂ heat pump drying system with auxiliary air cooler is 2922.55t CO_2e,which is about 46.06%lower than the LCCP of the basic CO₂ heat pump drying system.As compared to the basic system,the emissions of SO₂,NO_X,PM_2.5 and PM₁₀ were reduced by about 43.28%,41.05%,42.20%and 44.18%,respectively.