Research On Design Method Of The Low Ambient Temperature Air-Source Heat Pump Unit And The Vapor-Injection Mechanism

With the implementation of the clean heating policy,the low temperature adaptability of air-source heat pumps applied in north region for heating has gradually eased owing to the improvement of vapor-injection technology and frequency conversion technology for compressors.It is possible to use air source heat pumps in severe cold regions and emerges the low ambient temperature inverter air-source heat pump unit.The low ambient temperature inverter air-source heat pump unit usually employs the vapor-injection technology of compressor and frequency conversion technology to adapt to a wide ambient temperature range.However,to integrate building load characteristics well with compressor frequency conversion performance,the design method of low ambient temperature inverter air-source heat pump unit still needs to be researched in-depth.At the same time,the low temperature adaptability problem has always been the focus of attention,and the reason for the difference is not clear between vapor-injection by a flash tank and vapor-injection by an intermediate heat exchanger.Hence,this paper adopts a combination of theory and experiments to study and discuss the design method and two vapor-injection technologies difference for the low ambient temperature inverter air-source heat pump.The purposes are to provide a reference for developing and applying a low ambient temperature inverter air-source heat pump.Research indicated that the air-source heat pump heating has the optimal heating energy balance point.According to the reverse thinking mode of the optimal heating energy balance point,this paper establishes the supply-demand balanced relationship between the air-source heat pump heating system and the building heating load.It proposes a heating load partition design method.The building heating load is divided into three districts: cooperating district,following-load district,and trust district.Combined with compressor frequency conversion performance,the optimal heating energy balance point is confirmed to expand the unit’s temperature range for efficient operation.As a result,five design operating points of the low ambient temperature inverter air-source heat pump are determined.Based on a DC speed-regulated horizontal two-stage scroll compressor,combined with vapor-injection by a flash tank and vapor-injection by an intermediate heat exchanger,we produced a two-stage compressed inverter air-source heat pump with vapor-injection by a flash tank(TCIAHP-IF)prototype and a two-stage compressed inverter air-source heat pump with vapor-injection by an intermediate heat exchanger(TCIAHP-IHX)prototype.The experiments test the operating parameters and heating performance of the TCIAHP-IF unit when the compressor frequency is 60.0 Hz～90.0 Hz,and the outdoor ambient temperature is-20.0～7.0 ℃ to produce 41.0 ℃ of hot water.The results show that the TCIAHP-IF unit can produce heating stably in a wide temperature range.When the ambient temperature is-20.0 °C,the maximum heating capacity of the TCIAHP-IF unit is 11.3 k W,which reaches 77.2% of the maximum heating capacity of the unit under nominal operating conditions,and the coefficient of performance(COP)is not less than 2.0.Compared with the standard limit of COP for both nominal and low temperature conditions,the COPs of the TCIAHP-IF unit improved by 9.0% at least.That confirms the reliability of the heating load partition design method.In addition,the scroll compressor designed with a variable volume ratio and two-stage compression effectively improve the heating performance of the air-source heat pump in low ambient conditions.A combination of experiments and theory analyze the difference between the two vapor-injection mechanisms.It finds that vapor-injection by a flash tank is superior to that by an intermediate heat exchanger in improving the heating performance.The reason is the different pre-throttle subcooling caused by difference between the two vapor-injection mechanisms.Because the pre-throttle subcooling of the TCIAHP-IF unit is greater than that of the TCIAHP-IHX unit under the same operating conditions,the TCIAHP-IF unit obtains a higher relative vapor-injection ratio of the refrigerant.Hence,the heating performance of the TCIAHP-IF unit is better.The vapor-injection process employs entropy analysis to show that the effectiveness of vapor-injection lies to reduce the entropy generation in the compression process.It means that reduces the irreversibility of the compression process.Besides,the effectiveness of vapor-injection is related to the location of vapor-injection or vapor-return.Based on the technical feasibility and economic benefits,a throttling device of a solenoid valve coupled with capillary is proposed to control the vapor-injection.The experiments compare the effect of vapor-injection controlled by a thermostatic expansion valve on the heating performance of the TCIHAP-IHX unit.The results show that the solenoid valve should be located behind the capillary.The smaller the on-off period of the solenoid valve and the longer the opening time is,the more favorable it is for the stable operation of the TCIHAP-IHX unit.When the unit operates stably at low temperature,the vapor injection controlled by the capillary tube coupled with solenoid valve is very close to that of thermostatic expansion valve for the heating performance of the air-source heat pump.The research results provide a theoretical basis for designing and optimizing the low ambient temperature inverter air-source heat pump and improving the understanding of the vapor-injection mechanism.