Simulation Research On Air-to-water Heat Pump Systems With Large Temperature Lift

In the current situation of tight energy supply and increasing environmental protection requirements,it is the trend to use heat pump technology to achieve efficient heating.Due to the heating temperature lift of the traditional single-stage heat pump is below 55℃,when the heat source temperature is low,if the heating temperature is above 75℃,the traditional single-stage heat pump system will face problems such as high discharge pressure（above 2.5 MPa）,discharge temperature（above 120℃）,compressor ratio（above 8）,low coefficient of performance.In order to solve the problems of heat pump large temperature heating,the thermodynamic cycle construction and performance analysis of different heat pump systems are carried out in this paper,and builds the experimental bench to verify the accuracy of the simulation software.The variable concentration characteristics and recuperation mechanism of the zeotropic mixture heat pump system are deeply investigated,and the influence of temperature matching of hot and cold fluids on the system performance is revealed.Based on the application background of direct-heating condition and space heating condition,the adaptability laws of different heat pump systems to different operating conditions is described,meanwhile,the economy and environmental evaluation is made.The research contents of the work are as follows:1)Based on the current research status of heat pump with large temperature lift heating,the new zeotropic mixture recuperative heat pump system has great potential to achieve large temperature heating.Therefore,the zeotropic mixture component selection and thermodynamic analysis of variable zeotropic mixture components are carried out for the new zeotropic mixture recuperative heat pump system.The COP is used as the optimization target function,and the genetic algorithm in MATLAB is used to optimize the system parameters.According to the relationship between the temperature matchπ_glide reaction temperature match and exergy loss,the more the temperature matchπ_glide tends to 1,it indicates that the temperature match between hot and cold fluids in the heat transfer process is better.The results show that when the working fluid of the three-stream regenerative heat pump system is propylene/n-hexane（0.9/0.1）,its maximum COP is 4.08,and its efficiency is 45.4%,the temperature matching indicatorsπglide for evaporator,recuperator,and condenser are 3.56,2.71 and1.88,respectively;The working fluid of the two-stream recuperative heat pump system is propane/n-hexane（0.9/0.1）,its maximum COP is 3.67,and its efficiency is 40.9%,the temperature matching indicatorsπglide for evaporator,recuperator,and condenser are 4.51,3.16,and 2.98,respectively.At this time,the operating parameters of the compressor meet the requirements for good operation.2)Select the direct-heating condition（ambient temperature-35℃to 25℃,cold/hot water temperature 20℃/80℃）and space heating condition（ambient temperature-30℃to 0℃,return/supply water temperature 50℃/80℃）as the application background,conduct in-depth thermodynamic quantitative comparative analysis on the recuperative heat pump system with large temperature lift,cascade heat pump system with large temperature lift and CO₂heat pump system with large temperature lift,to explore the performance difference between the direct-heating condition and the space heating condition;From the relationship of exergy loss and the temperature matching of hot and cold fluids,this research in depth discusses the influence on the heating performance of the heat pump system.The results show that the cycle performance of the recuperative heat pump system is better than that of the cascade heat pump and the CO₂heat pump in the direct-heating.When the ambient temperature is 20℃,the COP of the recuperative heat pump system is up to 4.81.The main reason for the high COP of recuperative heat pump is that the exergy loss of its system is smaller than that of cascade heat pump and CO₂ heat pump;Under space heating conditions,the COP of cascade heat pump is higher than that of recuperative heat pump and CO₂ heat pump.When the ambient temperature is 20℃,the maximum COP of the cascade heat pump can reach 2.47.3)The temperature matching of hot and cold fluids is the key to reduce the exergy loss of heat pump and improving system performance.In the direct-heating,due to the phase change temperature glide of zeotropic mixture,the temperature matching between the hot and cold fluids in the recuperative heat pump is good,which will reduce the exergy loss in the heat transfer process.However,the refrigerant in the cascade heat pump has no phase change temperature glide,so the COP is low in the large span heating;In the space heating,the large throttling loss of recuperative heat pump and CO₂ heat pump is the main reason for their lower COP.Measures should be taken to increase the high-pressure side pressure to COP with the large throttling loss.4)In terms of economy and environment,the minimum payback period of the recuperative heat pump under direct-heating condition is 1.69 years,which is superior to the cascade heat pump and CO₂ heat pump,and the minimum payback period of the cascade heat pump under space heating condition is 5.53 years,which is superior to the regenerative heat pump and CO₂ heat pump;In terms of exergoeconomic,the exergoeconomic factor of the recuperative heat pump compressor is less than 7%,indicating that the investment of the compressor should be increased to improve its exergy loss.In terms of CO₂ and SO₂ emissions,the recuperative heat pump has the lowest CO₂ of 4431 kg and SO₂ of 130 kg annual emissions for direct-heating.The cascade heat pump has the lowest 4234 kg CO₂ and 127kg SO₂ annual emissions for space heating.