Optimal Selection Of HFC125/HCs Refrigerant Mixtures For Heat Pump Applications And Their System Characteristics Based On Pinch Point

The refrigerant mixtures provide an important direction in selecting newenvironment-friendly alternative to match the desirable properties with the existinghalogenated refrigerants or future use in the new devices, in which, HFCs refrigerants withzero ODP combined with HCs refrigerants with zero ODP and lower GWP are of importantvalue in the fields of application. In the present work, aiming at heat pump systems usingHFC125/HCs binary refrigerant mixtures, the behavior of the structure type of the heatpump using such refrigerant blends, optimal compositions of the mixtures and heat transfermatching between secondary fluid and refrigerant are studied theoretically andexperimentally, which can be taken as references for large-scale applications of therefrigerant blends and system design in future. Main conclusions are summarized asfollows:1）Selection of the refrigerant mixturesBased on flame retardant and excellent thermodynamic properties, HFC125refrigerant is selected from HFCs as basic component, and combined with HC290,HC600,HC600a and HC1270, respectively. It is found that, by mixing, the GWP of theHFC125is decreased effectively, and the flammability of the HCs is inhibited significantly,which will be formed promising alternative refrigerants for heat pumps.2）Dtermination of the optimal compositon of the refrigerant mixturesA thermodynamic model is presented based on the pinch point of heat transfer fluids,and the effects of mass fraction of mixtures on the heat pump cycle performance are investigated. Under given conditions, compared with HCFC22, the results show that theCOPs of the refrigerant mixtures HFC125/HC600and HFC125/HC600a are higher atHFC125concentration6-66%and13-55%, respectively, and compared with HFC134a, thedominant composition span are10-46%,0-75%,5-68%and0-35%for refrigerant mixturesHFC125/HC290, HFC125/HC600, HFC125/HC600a and HFC125/HC1270, respectively.And at the same time, the variation of the performance parameters with mass fraction ofHFC125exhibit the similarity between HFC125/HC290and HFC125/HC1270,HFC125/HC600and HFC125/HC600a, respectively, based on the characteristics andavailability, the refrigerant mixture HFC125/HC290and HFC125/HC600a are chosen forfurther study.The influence of mass fraction of refrigerant blends and outlet temperature of heatsink on the shift of pinch point in the condenser is analyzed. Simulated results show that,for mixture HFC125/HC290, the pinch point is always stabilized on the outlet of thecondenser under mass fraction40%, and above it, the pinch point slightly shift to themiddle of the condenser. And between35oC to55oC of outlet temperature of heat sink, thepinch point shift to outlet of the condenser with the temperature increasing. The refrigerantmixtures HFC125/HC600a exhibits the similarity trends. Which can provide some usefulguidelines for the heat transfer design and optimal operation of the system.3）Characteristics of the heat pump using HFC125/HCs under variable compositionsThrough the test of variable composition for refrigerant mixture HFC125/HC290andHFC125/HC600a under different outlet temperature of heat sink, the behavior of heatpump under different conditions is obtained and compared with simulated results. ForHFC125/HC290mixtures, all the measured values are departure from the calculationvalues, but the trends of them are consistency, and a peak value of COP is obtained nearcomposition25/75. As for HFC125/HC600a, because of the wet compression due to thehigh temperature glide, the measured values are completely inconsistency with thecalculation ones, the peak vale of COP is gain around composition10/90. Moreover, it isfound that, the poor heat transfer matching occurs at the concentration25/75for refrigerantmixture HFC125/HC290, and for refrigerant mixture HFC125/HC600a, the good heattransfer matching appears at the concentration20-40%. Finally, the ways for improvingthe heat transfer matching are analyzed.4）Characteristics of the heat pump using HFC125/HCs under variable conditions The experimental study on heat pump for refrigerant mixture HFC125/HC290(25/75)and HFC125/HC600a(10/90) is carried out, the results show that, compared withHFC125/HC600a, refrigerant mixtures HFC125/HC290is dominant under lowertemperature lift conditions, in other words, HFC125/HC600a blends are suitable for highertemperature lift conditions. In addition, the performance decreases20%, while therefrigerant charge reduces50%, which means the mixture is suitable for special placewhere the refrigerant charge is strictly limited.Under variable flowrate of heat sink, the pinch point for refrigerant mixtureHFC125/HC290and HFC125/HC600a are all moving from middle to inlet of thecondenser with increasing of the flowrate, however, compared with HFC125/HC290, theshift of pinch point for HFC125/HC600a is more sensitive with mass flowrate of heat sink.Which means that the heat transfer matching can be improved by adjusting mass flowrateof heat sink, and so as to decrease the irreversible loss and ultimately improve theefficiency of the system.Finally, the performance of refrigerant mixtures and substitute refrigerants arecompared under given conditions, the results show that, under outlet temperature of heatsink of55oC, the performance of HFC125/HC290is superior than HFC134a at theconcentration20-25%, and superior than HCFC22at the concentration20-35%; as forHFC125/HC600a, at the full range of compositions, it is poorer than substitute refrigerants.And compared each other under variable mass flowrate of heat sink for specialcomposition mixtures HFC125/HC290(25/75) and HFC125/HC600a(10/90), therefrigerant mixture HFC125/HC290(25/75) seems to be promising alternative refrigerant，and for mixture HFC125/HC600a(10/90), although it’s COP is very high in theory, it isimpractical due to high temperature glide in the case.