The Characteristics Study Of Cascade Middle-High Temperature Air Source Heat Pump System

As the conventional air source heat pump cannot meet the needs of high temperature heating under low temperature condition, this paper puts forward a kind of system named cascade middle-high temperature air source heat pump system, which can adapt to higher heating temperature and changing ambient temperature. Under a complex and variable operating condition, both of the two compressors of the system work within the safe operating limitation, and simultaneously, the system works safely and efficiently. After setting the 2-channel finned tube evaporator and 3-channel tube shell condensate-evaporator, the low-temperature level and high-temperature level loop cycle of this system run independently. This system can work under a variety of operating modes, such as cascade mode, one-stage single-operating mode and one-stage joint-operating mode, so as to meet different conditions and user requirements.Based on the one-stage cycle operationg limitations, like compression ratio ranges from 2 to 8, temperature of the exhaust gas should be less than 110℃and condensing temperature should be higher than 20 ℃, a thermodynamic analysis was done to determine the operating range of the two one-stage loop cycle. The thermodynamic analysis shows the following points. Firstly, if the maximum condensing temperature of the one-stage cycle is low, the evaporating temperature will be low as well. In addition, the maximum condensing temperature decreases, while the evaporating temperature decreases. Secondly, the intermediate temperature has a large effect on the operating limitation of the cascade heat pump cycle. When the pump is operating under the optimal intermediate temperature corresponding to the optimal energy efficiency ratio, it can only operate saftly on a relatively narrow range. If the intermediate temperature is set constant in a appropriate range, the operating limitation of the pump can be significantly expanded. Thirdly, when the intermediate temperature is set as 20℃, the operating limitations refer to that the evaporating and condensing temperature of the low-temperature level loop cycle ranges from-20℃to 0℃ and 40℃to 75℃, respectively.Numerical modeling and simulation of the system is done, according to the design and selection of the units. The simulation indicates that the cascade heat pump is highly applied, while the ambient temperature ranges from-10℃ to 12 ℃ and the temperature of inlet water ranges from 35 ℃ to 65 ℃. Based on the analytical result of the single cycle and cascade cycle, the unit can meet all of the requirements of the operating conditions of the air source heat pump through switching the operating state of single cycle or cascade cycle. Then, several conclusions are drawn. The energy efficiency ratio increases with the decrease of the temperature of the inlet water and the increase of the ambient temperature. When the ambient temperature and the temperature of the inlet water are respectively-10℃ and 65 ℃, the energy efficiency ratio is 2.05. Under the design conditons, the intermediate temperature of the cascade pump ranges from 12℃ to 15℃, and the heating capacity and energy efficiency ratio increase while the intermediate temperature increases.On the basis of the above analysis, this paper develops 3 operating strategies of the unit, respectively when heating capacity, energy efficiency ratio and life span of the unit is set as priority target. Moreover, performance parameters like heating time, energy consumption and integrated heating energy efficiency ratio are analized under these 3 operating conditions.