Study on environmentally friendly refrigerant R13I1/R152a as an alternative for R134a in automotive air conditioning system

Nuochen Zhang,Yuande Dai,Linghao Feng,Biao Li

School of Mechanical and Electrical Engineering,Nanchang University,Nanchang 330031,China

Keywords:Binary mixture Vapor liquid equilibria Thermodynamic properties R13I1/R152a Cycle performance

ABSTRACT Aiming at solving the problem of high global warming potential of R134a,a new mixed refrigerant R13I1/R152a(molar fraction ratio of 35:65)with no ozone depletion potential and low global warming potential was proposed as a substitute for R134a in automotive air conditioning.The computational models for the thermodynamic properties of R13I1/R152a were established by using the PR(Peng-Robinson)equation of state combined with the vdW mixing rule.Based on these models,the cycle performance of this working fluid was calculated,which was also compared with that of R134a and R1234yf under the different operating conditions.The results show that R13I1/R152a is a near azeotropic refrigerant whose temperature glide is approximately 0,and the saturated vapor pressure curve of which is equivalent to that of R134a.Moreover,compared to R134a,R13I1/R152a has an average 5.7% improvement in coefficient of performance as well as similar volumetric cooling capacity.The average coefficient of performance and volumetric cooling capacity of R13I1/R152a are significantly higher than those of R1234yf by 13.8% and 12.0%,respectively.However,the average discharge temperature of R13I1/R152a is approximately 13.3 K higher than that of R134a,but it is also within reasonable limits.Hence,the application of the proposed refrigerant R13I1/R152a in automotive air conditioning system is technically feasible.

1.Introduction

Over the past decades,R134a has been used extensively as a working fluid in automotive air conditioning systems (AACs) due to its excellent thermodynamic properties and cycle performances.With the rapid industrial development,the destruction of the ozone layer and the rise of the atmospheric temperature have attracted international attention.The ozone depletion potential(ODP) of R134a is 0,but it has a relatively high global warming potential (GWP) of 1430 [1].The F-gas regulation issued by the European Union stipulated that refrigerants with the GWP greater than 150 in AACs will be phased out [2,3].Therefore,the replacement of R134a has become the hot spot in the refrigeration and air conditioning industry.

In recent years,R1234yf(GWP is 4)as a substitute for R134a in AACs was proposed [4–6],the results presented that compared with R134a,R1234yf had similar thermodynamic properties,but the lower latent heat of vaporization and volumetric cooling capacity of R1234yf hinder its further promotion and use in AACs.Meanwhile,Carbon dioxide (CO2) used for AACs was studied by Brownet al.and Wanget al.[7,8],the results indicated the system of CO2was usually worked in the high-pressure transcritical region.

R152a,which belongs to hydrofluorocarbon (HFC) refrigerant,has excellent environmental characteristics with zero ODP and low GWP of 124 [9].Thus,the interest in the application of R152a has grown continuously.Such as the condensation heat transfer and pressure drop performances of low-GWP refrigerants(R152a,R1234yf,and R1234ze(E)) were tested by Longoet al.[9],and the results showed those characteristics of refrigerants were similar to that of R134a.Maet al.[10] compared the performance of R134a and R152a in the gas-engine heat pump and found that R152a could achieve higher heating performance than R134a.Hasheer and Srinivas [11] compared the performance of low GWP refrigerants as alternatives to R134a in a refrigerator and indicated R152a could be replaced R134a directly.Furthermore,the research on the application of R152a in AACs was conducted[12],and the results revealed the main advantages of R152a were lower GWP and higher coefficient of performance(COP)than those of R134a,however,the main drawback of R152a was strong flammability,which needed to be specially considered.As a result,some scholars were committed to adding some flame-retardant components to R152a to reduce its flammability.For example,the ratio range of R1234yf/R152a used as the refrigerant in AACs was presented by Sunet al.[13],the results exhibited the cycle performance of R1234yf/R152a was similar to that of R134a when the molar fraction of R1234yf was 0.65–0.67.Huoet al.and Menget al.[14,15] added R1234ze(E) to R152a,and the feasibilities of R1234ze(E)/R152a mixture as an alternative for R22 and R134a were studied respectively.The results demonstrated that compared with R22 and R134a,this mixture had the advantages of higher COP as well as lower discharge temperature.Qiet al.[16]added R245fa to R152a,and thepvTxproperties of this mixture for the organic Rankine cycle were measured.Similarly,Zhaoet al.[17] investigated the characteristics of vapor–liquid equilibrium (VLE) for R600a/R134a/R152a.Leeet al.[18] added R32 into R152a to form a mixture of R32/R152a,and the performance of it for water source heat pumps was measured.In addition,the performance of R152a/R125/R32 in domestic air-conditioners was studied by Wuet al.[19].In conclusion,with the addition of the above refrigerants,the flammability of R152a will be weakened,however,R1234yf,R1234ze(E),R600a,and R32 still have some flammability.For R245fa and R125,while not flammable,the GWP is relatively high.Consequently,these refrigerants are not available as long-term alternatives to R134a.

R13I1 which is used as a flame-retardant has excellent environmental indicators with low GWP(about 1)and zero ODP,as well as good behavior in thermophysical and chemical properties,however,R13I1 is hard to apply to AACs directly owing to the poor cycle performance [20–22].Thus,the addition of hydrocarbon(HC) or HFC refrigerants to R13I1 has been proposed in recent years,such as Fenget al.[23] analyzed the performance of R1270/RE170/R13I1,and showed the COP and volumetric cooling capacity of the mixture were equivalent to those of R22.A refrigerant mixture R290/R600a/R13I1 as a drop-in replacement for R134a in air source heat pump water heater was proposed by Xiaoet al.[24],which presented that compared with R134a,this mixture had lower condensing pressure and better cycle performances.Simultaneously,the experiment of R290/R13I1 applied in AACs indicated the charge amount of this mixture was lower than that of R134a,and the COP of this mixture was similar to that of R134a [25].Furthermore,the VLE characteristics of binary mixtures consisting of R13I1 and hydrofluoroolefins (HFO),HC,or HFC have been investigated [26–30].

In order to obtain the refrigerant with eco-friendly,high safety,and excellent cycle performance,a refrigerant mixture composed of R13I1 and R152a as an alternative for R134a was proposed.There is no published literature about R13I1/R152a mixed refrigerant applied in AACs at present.In this work,the feasibility of R13I1/R152a as an alternative for R134a was investigated from the aspects of environmental indicators,thermodynamic properties,and safety characteristics.Meanwhile,the computational models for the thermodynamic properties of R13I1/R152a were established.Based on these models,the theoretical cycle performance of this mixture in AACs under the variable working conditions was analyzed.Moreover,R1234yf was also cited as an alternative for R134a in comparison with R13I1/R152a.

2.Thermodynamic Properties

2.1.Physical and environmental properties

Table 1 shows the main physical and environmental properties of R134a,R1234yf,R152a,and R13I1.As illustrated in Table 1,the main physical properties of R152a and R13I1 are close to those of R134a.The mixture consisting of R152a and R13I1 has an ODP of zero since the ODP of R152a and R13I1 are both 0,and the GWP of the mixture can be estimated by:

whereX1andX2represent the mass fraction of component 1 and 2,respectively.Therefore,the GWP of R13I1/R152a is less than 124,which conforms to the European Union standard.

2.2.Temperature glide characteristics

Temperature glide refers to the difference between bubble and dew point temperature of mixed refrigerant which is undergone phase change during constant pressure,it has an important effect on refrigerant system performance.In this work,the VLE characteristic of R152a/R13I1 was studied to obtain the temperature glide characteristics of this mixture.In the equation of state (EOS) of vapor and liquid,Peng-Robinson(PR)EOS has good accuracy in calculating the saturated vapor pressure and saturated liquid density.However,when EOS is used to calculate the thermodynamic properties of the mixture,appropriate mixing rules should be added.For example,the vdW mixing rule is widely used because of its simple form and high accuracy.Therefore,the PR EOS combined with vdW mixing rule was selected to correlate the experimental data [29] to obtain the temperature glide characteristics of R13I1/R152a.

The PR EOS [32] is:

wherep,T,and v are the pressure,temperature,and molar volume,respectively.TheRis the general gas constant,which value is 8.3145 J?mol-1?K-1,pc,Tcand ω are the critical pressure,critical temperature,and acentric factor of pure refrigerant,respectively.The critical properties and the acentric factors of pure refrigerants are presented in Table 2.

Table 1 Physical and environmental properties of refrigerants [31]

Table 2 The critical properties and acentric factors of pure refrigerants [31]

The vdW[33]mixing rule for binary mixtures can be written as:

wherekijis the binary interaction parameter,andkij=kji,kii=kjj=0.

When the mixture is in vapor–liquid equilibrium,the fugacity of each component in the vapor phase is equal to that in the liquid phase,which can be expressed as:

The binary interaction parameter was calculated by minimizing the following objective function (OF):

whereNis the number of the experiment points,and the subscripts‘‘exp”and ‘‘cal”represent the experimental and calculated values,respectively.

The result of the calculation program for VLE characteristics of the R13I1/R152a mixture exhibits that the binary interaction parameter (kij) of R13I1/R152a is 0.0523.To verify the accuracy of this calculation model,the relative deviations of bubble pressures and the absolute deviations of vapor compositions between the experimental[29]and calculated values were plotted in Figs.1 and 2,respectively.As can be seen,the calculated data agree well with the experimental data.The maximum relative deviation between the experimental and calculated values of bubble pressure is 0.93%,and the maximum absolute deviation of the vapor phase molar fraction is 0.0076.

Fig.3 presents the experimental and calculated values of bubble and dew pressures.From Fig.3,it should be noted that the R13I1/R152a is a near azeotropic refrigerant when the molar fraction of R13I1 is near 0.35,which can be used as a pure refrigerant in the application as well as significantly avoid the change of compositions and properties of the mixture under the condition of leakage.Therefore,a mixture of R13I1/R152a(molar fraction ratio of 35:65)was chosen to analyze the feasibility of replacing R134a.

2.3.Saturated vapor pressures

The saturated vapor pressure curve of R13I1/R152a was calculated by the dew point pressures at different temperatures,and which was also compared with that of R134a and R1234yf [31],were shown in Fig.4.As can be seen,R13I1/R152a has a similar saturated pressure curve with R134a and R1234yf,indicating that the system of R13I1/R152a does not require high-pressure treatment during the replacement process,which can reduce the replacement cost.

2.4.Safety characteristics

The safety of refrigerant mainly includes the aspects of flammability and toxicity.According to the safety classification in ASHARE,R152a is classified as A2 with low toxic but inflammable,which has 4.9% for lower flammability limit (LFL),and 18% for upper flammability limit(UFL)in volumetric concentration[9,35].Moreover,the heat of combustion (HOC) of R152a is higher than 17400 kJ?kg-1[36].However,R13I1 is commonly used as aflame-retardant.When R13I1 with a molar fraction of 35%is added to R152a,the flammability will be reduced greatly.On the other hand,the results of the toxicity test on R13I1 were also encouraged,and it was found that R13I1 has no acute toxicity [37].

2.5.Lubricating oil and stability

At present,polyol ester (POE) lubricating oil has been widely used in air conditioning systems that use HFC as working fluids,such as R134a and R152a,which have good compatibility with POE [38–39].Moreover,the literature [21–22,24] shows R13I1 has good oil solubility and material compatibility,and R13I1 does not decompose when heated to 443.15 K on metal.Thus,it is possible to use the POE oil for R13I1/R152a system.Daiet al.[40]established a chemical thermodynamic prediction method for the thermal stability of the working fluid and showed that the thermal decomposition of R152a ranged from 395.15 to 623.15 K.Therefore,a refrigerant mixture consisting of R13I1 and R152a is not susceptible to thermal decomposition under the automotive air conditioning conditions.

3.System Description

Besides physical properties and environmental characteristics,the cycle performance of the new refrigerant should be evaluated.Fig.5 shows a diagram of the conventional vapor compression refrigeration cycle in AACs,which essentially consists of an evaporator,compressor,condenser,expansion valve,and internal heat exchanger.In order to obtain the performance of refrigerants in AACs,simulation work was conducted at evaporating temperature of 263.15–283.15 K and condensing temperature of 313.15–333.15 K,also based on the following assumptions:

(1) The pressure drop of refrigerants is negligible in the evaporator,condenser,and pipes.

Fig.1.The relative deviations of pressures between the experimental and calculated values for R13I1 (1)+R152a (2).

Fig.2.The absolute deviations of vapor phase molar fractions between the experimental and calculated values for R13I1 (1)+R152a (2).

Fig.3. P–x–y diagram for the R13I1 (1)+R152a (2) mixture:(●) liquid phase experimental data;(○) vapor phase experimental data;(--) calculated values.

Fig.4.Saturated vapor pressure curves of R134a,R1234yf,and R13I1/R152a.

(2) Compression process is adiabatic but not isentropic,with an isentropic efficiency (η) of 0.8.

(3) The superheat temperature at the compressor inlet is 5 K.

(4) The throttling process of the expansion valve is isenthalpic.

The corresponding pressure-enthalpy (p-h) diagram is shown in Fig.6.

The thermodynamic parameters such as enthalpy and entropy should be known first when calculating and analyzing the performance of the refrigerant cycle,which can be calculated by the residual functions accurately.The residual functions derived from PR EOS can be obtained as follows:

wherefr,sr,andhrare the residual free energy,residual entropy,and residual enthalpy,respectively.

The enthalpy and entropy expressions for the actual state of working fluid are as follows:

wherecp0is the specific heat capacity of the ideal gas,the subscript‘‘0′′indicates the values of the ideal gas at the same temperature and pressure as the actual state,and the superscript ‘‘r”represents the state under reference conditions.

The main parameters to be considered in the system include COP,volumetric cooling capacity,specific cooling capacity,pressure ratio,specific power consumption and discharge temperature of compressor,which can be calculated as follows:

Fig.5.Schematic diagram of AACs.

Specific cooling capacity:

Neglecting the heat losses in the internal heat transfer,according to the balance of energy:

Volumetric cooling capacity:

where v3is the suction specific volume of the compressor.

Specific power consumption of compressor:

where η is the isentropic efficiency of the compressor,which is defined as:

whereh4sis the specific enthalpy of refrigerant at the compressor outlet under isentropic compression,andh4is the specific enthalpy of the refrigerant at the compressor outlet under the actual compression process.

The pressure ratio of the compressor:

The coefficient of performance (COP) of the refrigeration system:

4.Results and Discussions

4.1.Variable evaporation temperature working conditions

Figs.7–9 shows the variation of volumetric cooling capacity,COP,specific cooling capacity,power consumption,pressure ratio and discharge temperature of the compressor with the evaporation temperature at a condensing temperature of 323.15 K.

Fig.6.Pressure-enthalpy diagram of AACs.

As indicated in Fig.7,the compression ratio decreases while the volumetric cooling capacity increases as the evaporation temperature increases.The pressure ratios of the proposed refrigerant R13I1/R152a and R1234yf are 4.66%–8.13% and 6.36%–10.62%lower than that of R134a,respectively.A lower compression ratio means that the compressor has a higher volumetric efficiency in operation.Meanwhile,it can be seen that the difference of volumetric cooling capacity between R13I1/R152a and R134a is relatively small under the different working conditions,which demonstrates that R13I1/R152a can be a direct replacement for R134a without modifying the size of compressor.Furthermore,R13I1/R152a has an 8.25%–15.36% improvement in volumetric cooling capacity compared with that of R1234yf at the given range ofTevap.From this point of view,R13I1/R152a is more suitable than R1234yf as an alternative for R134a.

Fig.8 shows the variation of discharge temperature and power consumption of compressor with the evaporation temperature,which indicates compared with R134a,the system energy consumptions of R1234yf and R13I1/R152a are lowered by 19.38%–20.23% and 8.81%–9.12%,respectively.The discharge temperature of R13I1/R152a varies from 347.5 to 359.8 K,while that of R134a and R1234yf is in the range of 337.0–343.7 K and 329.1–331.1 K.It should be noted that the discharge temperatures of the above refrigerants are within the safe range of the compressor.

Fig.9 displays the COP and specific cooling capacity of the above refrigerants under different evaporation temperatures.As shown in Fig.9,the specific cooling capacity of R13I1/R152a is slightly lower than that of R134a by 3.05%–4.76%,however,it has a 24.97%–33.06% improvement in specific cooling capacity compared with that of R1234yf.Therefore,the refrigerant charge amount of R13I1/R152a is significantly lower than that of R1234yf.In addition,R1234yf is classified as A2L,which has some flammability while the component R13I1 in R13I1/R152a has a flame-retardant effect.Thus,R13I1/R152a is safer than R1234yf as an alternative for R134a in AACs.On the other hand,the results show that R13I1/R152a has a significant advantage in terms of energy efficiency,with a COP variation of 2.65–4.65 in the evaporating temperature range of 263.15–283.15 K,which is 4.45–6.68% and 10.49%–16.79% higher than of R134a and R1234yf,respectively.This is mainly due to the significantly higher specific cooling capacity and lower compressor power consumption of R13I1/R152a compared to R1234yf and R134a.

4.2.Variable condensation temperature working conditions

Fig.7.The effect of evaporating temperature on the pressure ratio and volumetric cooling capacity at condensing temperature of 323.15 K.

Fig.8.The effect of evaporating temperature on the power consumption and discharge temperature of compressor at condensing temperature of 323.15 K.

Fig.9.The effect of evaporating temperature on COP and specific cooling capacity at condensing temperature of 323.15 K.

Similarly,Figs.10–12 present the pressure ratio,volumetric cooling capacity,COP,specific cooling capacity,power consumption and discharge temperature of the system at different condensing temperatures with an evaporating temperature of 273.15 K.Fig.10 reveals the pressure ratio increases while the volumetric cooling capacity decreases as the condensing temperature increases.The pressure ratios of R1234yf and R13I1/R152a are 7.12%–9.49% and 5.53%–7.40% lower than that of R134a,respectively.In addition,the volumetric cooling capacity of R13I1/R152a is 5.65%–20.28% higher than that of R1234yf while the volumetric cooling capacity of R13I1/R152a is completely approaching that of R134a when the condensation temperature is in the range of 313.15–333.15 K.Fig.11 indicates the power consumption and discharge temperature of the compressor increase with the increase of the condensation temperature,and the power consumptions of R1234yf and R13I1/R152a are 19.27%–20.38% and 8.46%–9.72% lower than that of R134a,respectively.Fig.12 shows the COP and specific cooling capacity of the system decrease with the increase of condensation temperature,and it can be seen that the COP of R13I1/R152a is 4.00–8.04% and 8.95%–20.65% higher than that of R134a and R1234yf,respectively.The specific cooling capacity of R13I1/R152a is 1.10%–6.11%slightly lower than that of R134a,but 21.84%–38.71% higher than that of R1234yf.

Fig.10.The effect of condensing temperature on pressure ratio and volumetric cooling capacity at evaporating temperature of 273.15 K.

Fig.11.The effect of condensing temperature on power consumption and discharge temperature of the compressor at evaporating temperature of 273.15 K.

Fig.12.The effect of condensing temperature on COP and specific cooling capacity at evaporating temperature of 273.15 K.

The above results prove that the proposed refrigerant R13I1/R152a has obvious advantages over R1234yf as a substitute for R134a in AACs.However,it also has a disadvantage in the discharge temperature of compressor,such as the discharge temperature of R13I1/R152a changes in the range of 338.7–367.5 K at the given range ofTcon,which is 19.0–27.7 K and 10.2–16.1 K higher than that of R1234yf and R134a,respectively.

5.Conclusions

In the present work,a new refrigerant mixture R13I1/R152a(molar fraction ratio of 35:65) as an alternative to R134a in AACs at different working conditions was investigated theoretically,and the results were also compared with those of R134a and R1234yf.The following conclusions can be drawn:

(1) The GWP of R13I1/R152a is about 48,which is much lower than that of R134a (1430).Moreover,R13I1/R152a can be made near azeotropic by adding R13I1 with a molar fraction of 35%,as well as reduce the flammability of R152a effectively.

(2) The saturated vapor pressure curve of R13I1/R152a is similar to that of R134a and R1234yf.On the other hand,the theoretical cycle performance of the above refrigerants under different operating conditions shows compared with R134a,R13I1/R152a can achieve higher cycle COP by an average of 5.7% as well as similar volumetric cooling capacity.Furthermore,R13I1/R152a has an apparent improvement in average COP and volumetric cooling capacity by 13.8% and 12.0% compared to R1234yf.

(3) The high discharge temperature of the compressor is obtained using R13I1/R152a,and the average of which is approximately 13.3 K higher than that of R134a.Still,it is within the safe operating temperature range of the compressor.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (22068024).

Nomenclature

agravitational parameter,Pa?m6?mol-2

bvolume parameter,m3?mol-1

cpspecific heat capacity,kJ?kg-1?K-1

ffugacity,MPa or free energy,kJ?kg-1

hspecific enthalpy,kJ?kg-1

kijbinary interaction parameter

ppressure,MPa

qvvolumetric refrigeration capacity,kJ?m-3

q0specific refrigeration capacity,kJ?kg-1

sspecific entropy,kJ?kg-1?K-1

Ttemperature,K

vmolar volume,cm3?mol-1

wspecific compressor work,kJ?kg-1

xmolar fraction of liquid phase

ymolar fraction of vapor phase

γ pressure ratio

η isentropic efficiency

φ fugacity coefficient

ω acentric factor

Subscripts

c critical point

cal calculated

con condensation

dis discharge

evap evaporation

exp experimental

m mixture

R contrast state

r residual

0 ideal state

1–6 state points of refrigerant or components

lliquid phase

r reference state

vvapor phase