Research On Capacity Matching And Optimization Of Compressors For Two-stage Compression Heat Pump System

Air-source heat pump,as the most suitable method for the "coal-to-power" program of the northern clean heating project in China during the "Thirteenth Five-Year Plan" period,has received extensive attention due to its features of high efficiency,energy conservation,and environmental protection.However,when the air-source heat pump is operated at low temperature,there will be some problems such as the increase of the compression ratio,the increase of the exhaust temperature,the decrease of heating performance,etc.So,scholars in the field put forward a variety of improvement measures.Among them,two-stage compression technology can effectively solve the problem of adaptability of air-source at low temperature,which has attracted much attention.The deep research of this technology can enhance the low temperature heating performance of air source heat pump,improve the comprehensive energy efficiency ratio of the system heating season,and effectively expand the application scope of the air source heat pump system.In order to further improve the double stage compression technology,the capacity matching characteristic of the compressor in the two-stage compression heat pump system is studied in this paper.First of all,R410 A is selected as the most suitable refrigerant for the two-stage compression air source heat pump system in cold regions through the comparative analysis of the characteristics.In addition,a two-stage compression system with a compressor capacity ratio fixed is considered as the research object.Based on the energy balance method and the enthalpy analysis method,the performance comparison of a two-stage compression cycle with no inter-stage structure,inter-stage structure with intermediate heat exchanger and inter-stage structure with flash tank was performed.The exergy efficiency of each cycle process system and the exergy loss of each component are analyzed,and then the first-stage throttling intermediate incomplete cooling cycle with the intermediate heat exchanger is determined to be the best circulation process for the two-stage compression heat pump system in the cold region.Secondly,taking the best circulation process as a research object,the dynamic coupling model of the injection characteristic compressor with variable frequency and controllable capacity is constructed,and then the dynamic simulation model of the two-stage compression heat pump system with injection characteristics is constructed based on the structural parameters and operating characteristics of the single-cylinder variable-frequency rolling-rotor compressor.At the same time,a two-stage compression heat pump with injection test system is constructed.And experimental test system is used to verify the simulation model.The results show that the deviations between the simulated and experimental values of the intermediate pressure of the system,the intermediate gas temperature,the high-stage exhaust temperature,and the mass flowrate of inter-stage refrigerant are-9%~+5%,-6%~11%,-1%~7% and-4%~+8%,respectively.Thirdly,two-stage compression heat pump systems with different compressor capacity ratios were analyzed under variable conditions through simulation calculations and experimental testing methods.The results show that when the evaporating conditions and the condensing conditions are constant,the intermediate gas temperature,inter-stage pressure,and power consumption of the low-stage compressor in the system with no injection are increased as the capacity ratio of the low-and high-stage compressor increases.But the high-stage exhaust temperature and power consumption of the high-stage compressor is decreased,the heating performance coefficient rose first and then decreased.The coefficient of heating performance is first raised and then decreased.When the injection structure is set in the inter-stage,with the increase of the injection quantity,the heating capacity and heating performance parameters of the system are increased first and then decreased.The maximum increase in heating capacity is 7%,and the heating performance coefficient is 8%.Finally,scroll-rotary multi-stage variable capacity compressor is proposed,that is an optimized structure of the two-stage compressor.In order to achieve the optimal operation of the compressor and improve system heating performance,the compressor can adjust the capacity of the low-and high-stage according to operating conditions to realize the optimal operation of the compressor.The scroll-rotary multi-stage variable capacity compressor is suitable for popularization in air-source heat pump systems in cold regions.