Research On Thermal Throttling Characteristics And Variable Frequency Operation Performance Of CO₂ Heat Pump With Electronic Expansion Valve Throttling

Considering environmental protection and energy conservation,realizing the sustainable development of building and building a clean and low carbon energy system have become the public focus of attention.Under the guidance of"13th Five Year Plan","coal to electricity"and a series of policies,reusing natural working CO2 as refrigerant has been a significant measure.Because CO2 trans-critical cycle heat pump system is energy efficient and environment friendly,it is followed with interests.CO₂ air source heat pump hot water system is running year-round,and the range of ambient temperature changes is relatively large.Therefore,it is difficult to balance thermal performance in low temperature,medium temperature,and high temperature environments.In this context,the experimental platform of variable frequency CO₂ air source heat pump system with throttling of electronic expansion valve was constructed,which is suited to study technical approaches to solve this problem and improve thermal performance.In order to meet the operating conditions of the CO₂ air source heat pump system throughout the year,electronic expansion valves are used for the throttling devices.Research shows that the effect of electronic expansion valve throttling on the performance of a CO₂ air source heat pump system is closely related to the control strategy.In order to improve the performance of the CO₂ air source heat pump system,the study of the electronic expansion valve control strategy is significant.The study of the superheat control strategy is optimized in detail.Plackett-Burman?PB?test was first used to select two significant influencing factors of affecting heating and COP from the four factors.On this basis,through establish response surface models of superheat of electronic expansion valve optimized for analysis.By means of the center composite design in response surface optimized and analyzed significant factor superheat.Test results provide theoretical and experimental basis for the electronic throttle expansion control strategy of CO₂ air source heat pump system.Based on the optimization control strategy of the electronic expansion valve of the CO₂air source heat pump hot water system,the influence of the opening of the electronic expansion valve on the system performance was studied.Superheat is controlled by the change of the opening of the electronic expansion valve.Experimental tests that the influence of the opening degree of the electronic expansion valve on the system performance such as suction and exhaust pressure parameters,heating power and COP.Regarding the fixed frequency CO₂ heat pump system can't run in the condition of wide temperature and wide load,the variable frequency compressor CO₂ heat pump system which is based on PLC on sub-critical cycle is putted forward.According to the theoretical analysis and experimental testing,the impact frequency of the suction pressure,discharge pressure,heating power and the COP under high ambient temperature and high inlet water temperature conditions were analyzed.Text results show that,reasonably decreasing the frequency of compressor can effectively control gas cooler pressure,and guarantee compressor stable operation.The gas cooler pressure sustainably decreases with decreasing compressor frequency.The COP increases first and then drops with decreasing compressor frequency.There is an optimum value for compressor frequency.The suitable operating frequency of air source carbon dioxide heat pump water heater is 35 Hz⁴5 Hz under high ambient temperature and high inlet water temperature conditions.