Study On The Characteristics Of A Transcritical CO₂ Heat Pump Hot Water System

The natural refrigerant CO₂has a good application prospect.In recent years,great progress has been made in applying CO₂as refrigerant in heat pump water heaters.Transcritical CO₂heat pump water heater system has been paid more attention because of its high efficiency,energy saving and environmental protection.However,the system is difficult to accurately control the operating pressure of the high pressure side,and the system performance is low when making high temperature hot water,which brings great difficulty to the application of the system.In view of the above problems,this paper takes the transcritical CO₂heat pump water heater system as the research object,adopts MATLAB software to conduct simulation and optimization research on the optimal exhaust pressure of the system and the main components of the air cooler,and uses the transcritical CO₂heat pump water heater system test bench to carry out experimental research.The main research contents and conclusions of this paper are as follows:(1)The simulation model of transcritical CO₂heat pump water heater system with and without regenerator is established,and the influence of system variables on the optimal exhaust pressure is analyzed.The results showed that:When the regenerator is added into the system,the increase of evaporation temperature has a favorable effect on COP of the system performance within the range of the change of exhaust pressure.The maximum increase of evaporation temperature is 16.7%,and the maximum change of exhaust pressure is 2.3%when the maximum COP is reached.As CO₂air cooler outlet temperature rise,the system COP fell sharply,join the regenerator,the COP is a sharp decline and under the same outlet temperature of the air cooler,than no regenerator system COP smaller,until it reached a certain temperature,the COP compared to the absence of regenerator is slightly increased but the trend is still downward trend.With the increase of CO₂temperature at the outlet of the air cooler,the optimal exhaust pressure corresponding to the maximum COP of the system also increases.The effect of superheat on COP and the optimal exhaust pressure is negligible.When the regenerator efficiency?_is?0.8,the refrigerant dryness has little effect on the COP of the system,and its effect on the optimal exhaust pressure of the system can be ignored.The optimal exhaust pressure is mainly related to the evaporation temperature and the CO₂temperature at the outlet of the air cooler.Based on this,the correlation formula of the optimal exhaust pressure with the evaporation temperature and the CO₂temperature at the outlet of the air cooler as independent variables is fitted.(2)An improved simplified algorithm,the optimal conditions for the optimal formula of exhaust pressure,exhaust temperature formula and the pseudocritical temperature formula three simultaneous solution,get preparation at different temperature of hot water system for optimal operation condition,and gas cooler distributed parameter simulation model is established,simulated the optimal working conditions is the flow of the CO₂and water in the gas cooler heat transfer conditions.The results showed that:When the CO₂temperature in the tube is greater than the quasicritical temperature,the decreasing range of CO₂temperature decreases along the tube length,leading to the heat transfer temperature difference between CO₂and water gradually increasing along the tube length,and then making the irreversible loss caused by the heat transfer temperature difference gradually increase along the tube length.At this time,the entransy dissipation number increases along the tube length gradually.When the temperature of CO₂reaches near the quasicritical temperature point,the physical properties of CO₂change most dramatically,the decrease range of CO₂temperature is the smallest,and the irreversible loss of the system is the largest.At this time,the entransy dissipation number reaches the maximum near the quasicritical temperature point.When the temperature of CO₂is less than the quasicritical temperature point,the specific heat capacity at constant pressure of CO₂decreases gradually,and the decrease range of CO₂temperature increases gradually,so that the irreversible loss caused by the heat transfer temperature difference of the system decreases gradually along the length of the pipe,and the amount of entransy dissipation number decreases along the length of the pipe.(3)The influence of exhaust pressure on COP under different working conditions was studied by using the crosscritical CO₂heat pump water heater system test bed,and the optimal exhaust pressure values under different working conditions were obtained,which were compared and verified with the fitting correlation formula of optimal exhaust pressure in Chapter 2.The improved algorithm is verified and the effect of different effluent temperature on the system performance is studied.The results showed that:Under different working conditions,the maximum deviation between the optimal exhaust pressure value obtained by experiment and that obtained by simulation is 3%,the average relative error is 1.3%,and the errors are all less than5%.The fitted formula matches well with the actual operation process.When preparing hot water at different temperatures,the maximum errors of the experimental optimal conditions(exhaust pressure and temperature)and the calculated optimal conditions(exhaust pressure and temperature)are 6.2%and 8.7%respectively,and the average relative errors are 1.6%and 5.5%respectively,which are all less than 10%.The simplified algorithm is true and effective within the error range.With the increase of the outlet temperature of the air cooler,the heat production of the system decreases,and its maximum attenuation reaches 18.3%,while the power consumption of the compressor increases with the increase of the outlet temperature,and the heating performance of the system COP decreases with the increase of the outlet temperature.