Investigation Of Multimode Single-fluid Cascade Air Source Heat Pump System

Air source heat pump is mainly used in building heating,which has advantages of environmental protection,energy saving and comfort.However,the traditional single-stage compression air source heat pump is difficult to be popularized to cold and severe cold areas owing to its poor adaptability in ambient temperature.In recent years,researchers have done a lot of studies to improve adaptability of low-temperature air source heat pump.The main direction includes two-stage compression and cascade heat pump system.This paper breaks the independence between the high and low temperature cycles in a cascade system,and proposes a multimode single-fluid cascade air source heat pump(MSCASHP)for low temperature environment,which can realize the mode switch among three modes.In summer,the system operates in single-stage refrigeration cycle(SRC)mode;in winter,the system operates in single-stage heating cycle(SHC)mode in medium climate and cascade compression heating cycle(CHC)mode in cold climate.By comparing the properties of various refrigerants and analyzing different configuration forms of high and low temperature compressors,a single refrigerant R410 A is selected as the research object of this paper,which is a constant speed compressor in high temperature cycle and a variable speed compressor MSC-ASHP system in low temperature cycle.Based on the previous research results,the mathematical model of MSC-ASHP system is established,and the experimental platform of MSC-ASHP system is built.Firstly,the system model is verified,and then through the system simulation model and experimental platform,the compression ratio,exhaust gas temperature,heating capacity and system energy efficiency ratio(COP)of the system in two different heating modes are simulated,calculated and tested.The results are shown in the table Ming: in the low temperature environment,the compression ratio and exhaust temperature of CHC mode are far lower than those of SHC mode;in the condition of 46 ℃ condensation temperature and-35 ℃ evaporation temperature,the heating efficiency of CHC mode is higher than 1.8,the exhaust temperature of compressor is lower than 120 ℃,and the compression ratio of high and low temperature compressors is no more than 5.0,so the system can operate stably and reliably;in addition,in the CHC mode,the speed of low temperature compressor can be increased by increasing the speed of low temperature compressor In order to continuously improve the heating capacity of the system and meet the heating demand in low temperature environment,finally,the heating capacity and heating coefficient of the two heating modes under different evaporation temperatures are compared,and the control strategy of Shc and CHC mode switching with the heating capacity and heating coefficient as the control objective and the two evaporation temperatures as the judgment conditions is proposed.A novel MSC-ASHP system for residential application in the northern regions of China was proposed,variable-speed characteristics and control strategy of the two heating modes have been investigated in this paper.It not only enriches low-temperature air source heat pump products,improves equipment utilization,but also expands the operation period and application area of air source heat pump.