Optimal Selection Of R744 Blends For Heat Pump Water Heaters And Study On Their System Performances At Steady And Transient States

The application of heat pump technology is an effective measure to realize energy saving and emission reduction.Finding the suitable null ODP and low GWP substitutes becomes a research hotspot of heat pump water heater technology because the conventional HCFCs and HFCs used in heat pump system have been banned or phased out.In this paper,the optimal selection of refrigerant mixtures and the steady and transient state performances of heat pump water heater system are investigated theoretically and experimentally on R744 mixtures with null ODP and low GWP as research object.Based on the comprehensive analysis of the refrigerant properties such as environmental,safety,thermodynamic,transport and oil-solubility performances,the preliminary optimal selection of the candidates R744/HCs and R744/HFOs is carried out.The appropriate mixtures for heat pump water heaters are R744/R290（0/100³5/65）,R744/R1270（0/100³0/70）,R744/R1234yf（0/100⁴0/60）and R744/R1234ze（E）（0/100⁴5/55）.The system performance prediction model was established for predicting the cycle performances of heat pump water heaters with four preliminary selected R744blends to further achieve the theoretical optimal selection of R744 blends.By the comparison on the cycle performances(heating COP_th,heating capacity and discharge pressure,etc),the predicting results show that the R744/R290 in the dominant concentration range of 5/95²0/80 is the most attractive alternative for heat pump water heater system,particularly R744/R290（14/86）,whose heating COP_th and heating capacity are obviously superior to those of R22.The instant heat pump water heater test system with R744/R290 is designed and constructed based on the equipment selections and heat exchanger designs.By established a heat pump test rig,the effects of charging concentration of R744/R290mixture and the heat sink temperature lift on the steady state cycle performance and the temperature distributions of heat transfer fluids in the heat exchangers of the heat pump water heater were investigated.The test results reveal that the R744/R290 blend with optimal charging concentration of 12/88,M_opt for short,achieves much higher heating COP_ex and heating capacity than those of R22 under both the standard and high temperature conditions.Hence,M_opt is the most suitable substitute for heat pump water heater.The heat sink temperature lift has outstanding effect on its heating COP_ex but little effect on heating capacity of heat pump water heater with M_opt.Compared with the R22 system,the heat pump water heater system with M_opt obtains much higher heating COP_ex and heating capacity but much lower discharge temperature at the investigated range of heat sink temperature lift.The results also indicate that,as the substitute for R22,the use of M_opt markedly promotes the system energy efficiency,which is mainly attributed to the obvious reduction of the irreversible losses in the condenser causing by the prominent improvement of temperature matching characteristic between the heat transfer fluids,and both the charging concentration and heat sink temperature lift have the significant impacts on the temperature matching characteristic between the heat transfer fluids in the condenser.And the location migration of pinch point in the condenser is not affected by the charging concentration and heat sink temperature lift.The normal startup and shutdown characteristics of heat pump water heater with M_opt under the standard condition are experimentally studied.The results show that the starting mode has outstanding impact on the startup times of the startup performance parameters during the normal startup processes.The startup times of M_opt system during both the normal cold and warm startup processes are quite long,especially the former.During both the normal startup processes,the starting mode has obvious effect on the change trends of the discharge temperature,inlet temperature of expansion valve and suction temperature but little effect on those of the other startup performance parameters.The minimum suction pressure during the normal cold startup is much lower than that during the normal warm startup.During the normal shutdown,the two high side and two low side refrigerant pressures show the similar tendency respectively,but the investigated temperatures at different positions present significantly different trends.In order to solve the problem of long startup time during the normal cold startup,the quick startup scheme based on the step flow of heat sink is proposed.The experimental study was carried out on the effects of the step flow of heat sink on the quick startup characteristics of heat pump water heater with M_optpt under the standard condition,and the feasibility of the proposed quick startup scheme was verified.The testing results indicate that,there exists the optimal ratio of heat sink step flow,at which the startup time of heat pump water heater with M_opt during the quick startup processe is the shortest,which is tremendously shorter than that of the normal cold startup.Hence,the quick startup of heat pump water heater with M_optpt can be realized by adopting the method of heat sink step flow.The ratio of the heat sink step flow has remarkable effect on the startup times of the quick startup performance parameters compared with the normal cold startup.The heat sink step flow has little effect on the variation trends of the discharge temperature and inlet temperature of expansion valve but obvious effect on those of the other quick startup performance parameters in comparison to the normal cold startup.The difference of the minimum suction pressures is very small between the quick and normal cold startup processes.