EP A/600/A-97/008
EVALUATION OF ALTERNATIVES FOR HFC-134A REFRIGERANT
IN MOTOR VEHICLE AIR CONDITIONING
James J. Jetter and N. Dean Smith
U.S. Environmental Protection Agency, MD-63
National Risk Management Research Laboratory
Highway 54 and Alexander Drive
Research Triangle Park, NC 27711, USA
Krich Ratanaphruks, Angelita S. Ng, Michael W. Tufts, and Francis R. Delafield
Acurex Environmental Corporation
4915 Prospectus Drive
Durham, NC 27713-4401, USA
ABSTRACT
HFC (hydrofluorocarbon)-134a is currently used as the refrigerant in motor vehicle air conditioners.
Although HFC-134a has no ozone depletion potential, it has a GWP (global warming potential) approximately
1300 times that of carbon dioxide over a 100 year time horizon. For this reason, the EPA's (Environmental
Protection Agency's) NRMRL (National Risk Management Research Laboratory) has evaluated lower global
warming alternatives for HFC-134a in motor vehicle air conditioning. Alternative technologies were
evaluated. Four pure chemicals and one azeotropic mixture were identified and evaluated as alternative
refrigerants for conventional systems, but were found to be unsuitable. The four pure chemicals were HFC-
227ca, HFC-227ea, HFC-245cb, and HFE (hydrofluoroether)-143a. The azeotropic mixture was 51.7 weight
percent HFC-134 and 48.3 weight percent HFC-245cb. The evaluation process and results are described in
this paper.
INTRODUCTION/BACKGROUND
HFC-134a is currently used as the refrigerant in new and retrofitted motor vehicle air conditioners,
replacing the stratospheric ozone depleting refrigerant CFC (chlorofluorocarbon)-12. Although HFC-134a
has no ozone depletion potential, it has a GWP approximately 3400 times that of C02 (carbon dioxide) over
a 20 year time horizon, 1300 times that of C02 over a 100 year time horizon, or 420 times that of COj over
a 500 year time horizon.1 By the year 2005, approximately 260 million vehicles with HFC-134a air
conditioners will be in service in developed countries and will require approximately 75,000 metric tons of
HFC-134a annually.2 This represents an annual, greenhouse gas, carbon-equivalent of 27 million metric tons,
based on a 100 year time horizon. In 1994 the estimated total U.S. greenhouse gas emissions were 1666
million metric tons carbon-equivalent.3
ALTERNATIVE TECHNOLOGIES
Alternative technologies could potentially reduce the global warming impact of motor vehicle air
conditioning. Studies sponsored by EPA and others have examined the potential for alternative technologies.4"
10 A brief summary of the most promising known alternatives is presented in Tables 1 and 2, listing alternative
vapor compression and non-vapor compression technologies, respectively. Technologies listed in the tables
are in various stages of development, but none are likely to be commercialized in the near future.
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Table 1. Alternative vapor compression technologies
Technology
Advantages
Disadvantages
Carbon dioxide refrigerant
• Working fluid (carbon dioxide.)
has low direct GWP
• High pressure
Flammable refrigerant
• High energy efficiency
• Low direct GWP
• Safety issue
• Liability issue
Flammable refrigerant with
nonflammable secondary fluid
• Low direct GWP
• Secondary fluid reduces safety risk
• Secondary fluid increases
production cost and reduces
efficiency
Hermetic system
• Low emissions of refrigerant
• Low energy efficiency due to
losses in compressor drive system
• High production cost
Ejector
• Low emissions of refrigerant
• Low energy efficiency
• Waste heat from small engines
may be inadequate to drive system
Table 2. Alternative non-vapor compression technologies
Technology
Advantages
Disadvantages
Adsorption
• Working fluids, such as water and
hydrogen, are environmentally
benign
• High weight
• Large equipment
Stirling cycle
• Working fluid, such as helium, is
environmentally benign
• Low energy efficiency
Desiccant cooling (open cycle)
• Air and water are the cooling
media
• Low energy efficiency
• Large equipment
Air cycle
• Working fluid (air) is
environmentally benign
• Low energy efficiency
• High production cost
Absorption
• Fluids, such as water and
ammonia, are environmentally
benign
• Low energy efficiency
• High weight
• Large equipment
Thermoelectric
• No refrigerant needed
• Low maintenance
• Low energy efficiency
• Materials breakthrough needed
Although alternative technologies might be implemented in the future, the focus of this paper is on
potential alternative refrigerants for conventional, vapor compression, motor vehicle air conditioners.
Conventional vapor compression technology is well developed and has desirable attributes. Modern motor
vehicle air conditioners are relatively simple, reliable, easy to maintain, and inexpensive to manufacture. A
suitable replacement for HFC-134a would maintain the advantages of current systems and would reduce the
global warming impact associated with refrigerant emissions.
ALTERNATIVE PURE CHEMICALS
EPA's NRMRL established a program to identify alternatives for ozone depicting substances. In 1987,
the EPA convened a panel of international experts to advise on chemical alternatives for ozone-depleting
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CFCs. Included in the panel's formal findings was a recommendation that governments sponsor research of
new chemicals, particularly of those not under active consideration by chemical companies." In a cooperative
program with the Flectric Power Research Institute, Clemson University, and the University of Tennessee, 37
new chemicals were synthesized and tested to determine relevant thermophysical properties. After the initial
testing, 11 of the new chemicals were selected as leading candidates for various applications.12 Each of the
11 new chemicals was evaluated to determine atmospheric lifetime, toxicity, flammability, thermodynamic
properties, lubricant miscibiiity, materials compatibility/stability, and vapor thermal conductivity. Four of the
] 1 chemicals have normal boiling points close to that of HFC-134a. The four chemicals are listed in Table
3 with normal boiling points, and HFC-134a is included for comparison.
Table 3. Normal boiling points of new chemicals
Chemical Code
Chemical Name
Chemical Formula
Tnh (°C)
HFC-227ca
1,1,1,2,2,3,3-Heptafluoropropane
CFrCF2-CF2H
-16
HFC-227ea
1,1,1,2,3,3,3-Heptafluoropropane
CF3-CHF-CF3
-16"
HFC-245cb
1,1,1,2,2-Pentafluoropropane
CF3-CFrCH3
-18
HFE-143a
1,1,1 -Trifluorodimethyl ether
CFrO-CH3
-24
HFC-134a
1,1,1,2-Tetrafluoroethane
CFrCHjF
-26
For a new chemical to be suitable as an alternative for HFC-134a refrigerant in conventional motor
vehicle air conditioners, the chemical must meet certain acceptability criteria. Primary criteria are listed in
Table 4. The four new chemicals selected as potential alternatives to HFC-134a do not deplete stratospheric
ozone, but further evaluations revealed their unsuitability for use in conventional systems. Results from
evaluations are summarized in Table 5.
Atmospheric lifetimes were determined from measured or calculated hydroxyl (OH) reaction rate
constants. In Table 5, (meas) indicates an atmospheric lifetime determined from a measured OH rate constant,
and (calc) indicates a lifetime determined from a calculated constant Rate constants were measured by NIST
(National Institute of Standards and Technology).
The calculated rate constant for HFC-227ca
provides only a rough approximation, and a
measured rate constant will eventually be
obtained. Atmospheric lifetime is a primary
factor in determining GWP and indicates a
chemical's likely effect on global climate.
Infrared absorbance is also a factor in
determining GWP, but because it is less
important, it was not included in the preliminary
evaluations. A chemical with an atmospheric
lifetime close to or exceeding that of HFC-134a
is an unlikely candidate for use in motor vehicle
air conditioning. HFC-227ca has an atmospheric
lifetime of 15 years, approximately the same as
the 14.6* year lifetime of HFC-134a. The
atmospheric lifetime that will be determined from
the measured rate constant for HFC-227ca is
unlikely to be significantly shorter. HFC-227ea
Table 4. Refrigerant acceptability criteria
for conventional motor vehicle air conditioning
• No stratospheric ozone depletion
• Reduced direct GWP compared to HFC-134a
• Low toxicity
• Not flammable
• Energy efficiency similar to or better than HFC-
134a
• Refrigeration capacity similar to HFC-134a
• Critical temperature: 93 °C minimum
• Normal boiling point similar to HFC-134a
• Latent heat of vaporization at 0°C: 150 kJ/kg
minimum
• Specific volume of saturated vapor at 0°C: 75 1/kg
maximum
• Acceptable miscibiiity with lubricant
• Compatible with engineering materials
• Minimal fractionation
• Acceptable cost to manufacture
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and HFC-245cb have atmospheric lifetimes of 36.51 and 44 years, respectively, so these two chemicals are not
considered viable substitutes for HFC-134a. HFE-143a has a lifetime of 5 years, considerably shorter than
HFC-134a.
Table 5. Evaluation of new chemicals
HFC-227ca
HFC-227ea
HFC-245cb
HFE-143a
HFC-134a
Atmospheric Lifetime (years)
15 (calc)
36.5(meas)
44 (meas)
5 (meas)
14.6 (meas)
Toxicity
Low, screen
Low
Low, screen
Untested
Low
Flammability
No
No
Marginal
Yes
No
Critical Temperature (°C)
106
104
109
105
101
Predicted Energy Efficiency
0.60
0.62
0.73
1.00
1 (baseline)
Predicted Volumetric Capacity
0.56
0.61
0.62
0.86
1 (baseline)
Latent Heat of Vap. at 0°C (kJ/kg)
122
124
155
202
201
Sp. Vol. of Sat. Vap. at 0°C (1/kg)
64
62
78
81
70
Preliminary toxicity tests were conducted for three of the four new chemicals. Toxicity was assessed
for HFC-227ca with a 4-hour inhalation screening test at one exposure level, 1000 ppm, and no adverse effects
were found. Toxicity was assessed for HFC-227ea by a chemical manufacturer.13 In a 4-hour inhalation test,
HFC-227ea had a lethal concentration for 50 percent of the population (LC^) greater than 800,000 ppm.
Toxicity for HFC-245cb was assessed with a 4-hour inhalation screening test at one exposure level, 50,000
ppm, and no adverse effects were found. Toxicity for HFE-143a was not assessed because of unavailability
of test quantities of the chemical.
Flammability of new chemicals was determined at room temperature in accordance with the ASTM
(American Society for Testing and Materials) E681-85 procedure.14 Humidity of the air was not controlled
in the preliminary tests. HFC-227ca and HFC-227ea were found to be nonflammable at room temperature.
HFC-245cb was found to be marginally flammable with weak combustion observed in the test flask. The
flammability range was 8.4 to 12.6 volume percent HFC-245cb in air at room temperature and ambient
pressure using an electric spark for ignition. HFE-143a was found to be flammable; however, the range was
not determined due to limited quantities of the chemical.
Thermodynamic properties for die four new chemicals were determined by using the CSD (Carnahan-
Starling-DeSantis) equation of state. Coefficients for the CSD equation of state were regressed from PVT
(pressure/volume/temperature) data by NIST.'5 From the thermodynamic properties, energy efficiency and
refrigeration capacity were predicted for a basic
vapor compression refrigeration cycle. Conditions
for the performance predictions are listed in Table
6. Simplifying assumptions included: zero pressure
losses in heat exchangers and an isenthalpic
expansion process. As shown in Table 5, predicted
energy efficiency for HFE-143a was equal to HFC-
134a, but predicted efficiencies for the other three
new chemicals were lower. Predicted volumetric
capacities for all four new chemicals were lower
than for HFC-134a.
Table 6. Conditions for performance predicUons
Evaporating Temperature (°C)
4.4
Superheat (C°)
5.6
Compressor Isentropic Efficiency
0.7
Condensing Temperature (°C)
60.0
Subcooling (C°j
11.1
-------
Latent heats of vaporization at 0UC for the four new chemicals were determined by using the CSD
equation of state. As shown in Table 5, latent heats for HFC-227ca and HFC-227ea were below the minimum
criterion of 150 kJ/kg. Specific volumes of saturated vapor at 03C were also determined with the CSD
equation of state. As shown in Table 5, specific volumes for HFC-245cb and HFE-143a were above the
maximum criterion of 75 1/kg.
In summary, the four pure chemicals were considered to be unsuitable for conventional motor vehicle
air conditioners for various reasons. HFC-227ca and HFC-227ea were considered unsuitable because of their
long atmospheric lifetimes, low predicted efficiencies, low predicted volumetric capacities, and low latent heats
of vaporization. HFC-245cb and HFE-143a were considered unsuitable because of their flammabilities and
high specific volumes of saturated vapor. HFC-245cb was also considered unsuitable because of its long
atmospheric lifetime.
ALTERNATIVE AZEOTROPIC REFRIGERANT MIXTURE
Following the evaluation of the pure chemicals, NRMRL predicted seven new azeotropes formed from
mixtures of new chemicals.16 Several of the predicted azeotropes were confirmed by laboratory tests. One of
the confirmed azeotropes has a normal boiling point close to that of HFC-134a. The azeotrope is 51.7 weight
percent HFC-134 and 48.3 weight percent HFC-245cb at room temperature. Preliminary evaluation of the
azeotropic mixture indicated that it might be suitable for use in conventional motor vehicle air conditioners.
However, further evaluation revealed that it too was unsuitable for use as an HFC-134a alternative. Evaluation
results are summarized in Table 7. Data for the components of the azeotrope and for HFC-134a are shown
for comparison.
Tabic 7. Evaluation of HFC-134/HFC-245cb azeotrope
HFC-134/HFC-245cb (51.7/48.3 weight percent)
HFC-134a
Azeo trope
HFC-134
HFC-245cb
Atmospheric Lifetime (years)
—
10.6
44
14.6
Toxicity
Untested
Low
Low, screen
Low
Flammability
No
No
Marginal
No
Normal Boiling Temperature (°C)
-22
-20
-18
-26
Critical Temperature (°C)
99
11917
109
101
Predicted Energy Efficiency
0.97
1.03
0.73
1 (baseline)
Predicted Volumetric Capacity
0.82
0.82
0.62
L (baseline)
Latent Heat of Vap. at 0°C (kJ/kg)
178
212
155
201
Spec. Volume of Sat. Vapor at 0°C (l/kg)
73
93
78
70
Lubricant with Acceptable Miscibility
POEa
POE
Untested
PAG", POE
aPOE = polyolester
h PAG = polyalkylene glycol
HFC-134 has a reported atmospheric lifetime of 10.6 years, although the uncertainty of this lifetime
has recently been reported to be 200 percent.18 The atmospheric lifetime of HFC-245cb was estimated to be
-------
much shorter than HFC-134a based on a preliminary, calculated value for the OH reaction rate constant. With
short apparent atmospheric lifetimes for both components, the azeotrope appeared to be an attractive alternative
for HFC-134a, and consequently more extensive evaluations were undertaken. However, the measured value
for the OH reaction rate constant for HFC-245cb was recently obtained from NIST, and the atmospheric
lifetime based on this value was 44 years. Because of the long lifetime, the azeotrope was no longer
considered a viable alternative for HFC-134a. Results from the evaluations are described below so that
duplication of this work may be avoided and so that the azeotrope could be reassessed in the future if the
scientific understanding of atmospheric chemistry or other factors change.
Toxicity for HFC-245cb was assessed with a 4-hour inhalation screening test at one exposure level,
50,000 ppm, and no adverse effects were found. Toxicity for HFC-134 has been extensively assessed by a
chemical manufacturer.19 In a 4-hour ALC (approximate lethal concentration) determination in rats, the ALC
for HFC-134 was greater than 460,000 ppm, indicating very low toxicity. HFC-134 was tested for mutagenic
activity in Salmonella typhimurium strains TA1535, TA97, TA98, and TA100 with and without an activation
system. Results were negative. Results from testing HFC-134 in an in vitro chromosomal aberration study
with human lymphocytes were negative. In a 4-week subchronic inhalation study in rats, no clinical chemistry
changes were considered to be biologically or toxicologicaily significant. In a pilot rat teratology study, no
fetal body weight changes or fetal malformations were noted. In a cardiac sensitization study in dogs, HFC-
134 was found to have potential to cause cardiac sensitization with 33 percent positive responses at an
exposure level of 10 percent in air. The results from the cardiac sensitization study compare favorably to those
from studies of other refrigerants currently in widespread use.
Although HFC-245cb was marginally flammable, the azeotropic mixture of HFC-134 and HFC-245cb
was not flammable at room temperature by definition of the ASTM E681-85 test standard. Because the
mixture was an azeotrope at room temperature, it was not expected to fractionate in motor vehicle air-
conditioning systems.
Energy efficiency and volumetric capacity were predicted for the azeotrope under the conditions given
in Table 6 using the CSD equation of state. The interaction parameter used for the azeotropic mixture was
0.039. Predicted efficiency for the azeotrope was nearly the same as for HFC-134a. Predicted capacity for
the azeotrope was 18 percent less than the capacity for HFC-134a.
The latent heat of vaporization and the specific volume of saturated vapor at 0°C were determined
using the CSD equation of state. Latent heat of vaporization at 0°C was 178 kJ/kg for the azeotrope, above
the minimum criterion of 150 kJ/kg. Specific volume of saturated vapor at 0°C was 73 1/kg, below the
maximum criterion of 75 1/kg.
Miscibility was tested for the HFC-134/HFC-245cb azeotrope with a PAG (polyalkylene glycol)
lubricant and a POE (polyolester) lubricant. The PAG lubricant was supplied by an OEM (original equipment
manufacturer). Its viscosity was reported to be 135 cSt at 40°C and 25 cSt at 100°C. It was reported to
contain proprietary additives. The POE lubricant was supplied by a chemical manufacturer. Its viscosity was
reported to be 134cStat40°Cand 15cStat 100°C. It also contained proprietary additives. Sealed glass tubes
were prepared, with the HFC-134/HFC-245cb azeotrope and both lubricants at lubricant concentrations of 5,
10, 20, and 30 weight percent. The PAG lubricant was partially miscible with the azeotrope as shown in
Figure 1. At a lubricant concentration of 5 percent by weight, phase separation of the azeotrope and PAG
lubricant occurred at 47°C. The azeotrope was completely miscible with the POE lubricant over a temperature
range of -35 to +100°C at all the test concentrations.
Materials compatibility and stability were tested for the HFC-134/HFC-245cb azeotrope in accordance
with the methods described in ANSI/ASHRAE Standard 97-1989.K Materials were selected in preparation for
6
-------
testing the azeotrope as a refrigerant in an
automotive air-conditioning system
designed for HFC-134a. Results for the
materials compatibility tests are
summarized in Table 8. Compatibility
tests were conducted by heating the
materials with the refrigerant at 125°C in
sealed glass tubes for 2 weeks. Tests were
done both with and without POE lubricant.
Two glass tubes were prepared for each
test, and average values for each pair of
tubes are reported in Table 8. Neoprene
and nylon with lubricant had measured
shrinkage and visually observed
extractabies in the liquid phase following
the aging process. Volume change for the
neoprene without lubricant was excessive
compared to generally accepted criteria of
volume change from -5 to +25 percent.
Hardness values were not obtained for nylon and Teflon since these "hard" materials were not amenable to the
test method. Copper, aluminum, and steel were heated with the refrigerant and lubricant at 175°C for 2 weeks.
Cast iron and brass were heated at 125°C for 2 weeks. Volatiles released upon breaking the tubes under
vacuum were analyzed by gas chromatography, mass spectrometry, and infrared spectrometry. No chemical
degradation was observed for either component of the azeotrope or the lubricant with any of the elastomers,
plastics, or metals tested. The presence of a small amount of water did not alter the results. Thus, it may be
said that the azeotrope was thermally and hydrolytically stable up to the test temperatures even in the presence
of possible metallic catalysts.
Table 8. Materials compatibility test results for HFC-134/HFC-245cb azeotrope
Material
Wt. Change
Vol. Change
Linear Swell
Hardness (Durometer)
(Percent)
(Percent)
(Percent)
Initial
Final
.Buna-N-"
+ 8.7; >
::ffiWSItSS§
-0.6
69.0 - - -
Buna-N (with lube)
+ 13.9
+11.8
+ 0.7
77.5
67.4
Neoprene
-ji.3
- 1,6/
¦76.8,. ~
Neoprene (with lube)
-0.8
-0.6
- 1.2
75.0
80.5
Nylon 6/6
¦>;0;j:|: j|S fill
Nylon 6/6 (with lube)
- 12.1
-5.4
-5.2
—
—
Teflon i"
'z'L'i--
Teflon (with lube)
+ 3.0
+ 3.7
+ 0.6
—
—
Energy efficiency and refrigeration capacity were measured for the azeotrope in an instrumented
automotive air-conditioning system. Instrumentation was similar to that described in a previous paper.21 The
automotive system was an OEM unit designed for HFC-134a For tests with the azeotrope, only the orifice
tube expansion device was changed to optimize performance. Test results are shown in Figure 2.
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Immiscible
Miscible
0 5 10 15 20 25 30 35
Lubricant/Refrigerant Mixture (Weight Percent)
Figure 1. Miscibility forHFC-134/HFC-245cb
azeotrope with PAG lubricant
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-------
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Temperature (°C)
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Refrigeration capacity and COP (coefficient of performance) of the azeotrope and HFC-134a were measured
at three compressor rotational speeds, three evaporating temperatures, and three condensing temperatures.
Both refrigeration capacity and COP of the azeotrope were lower than predicted by the thermodynamic model,
but performance would likely improve with further hardware optimization. Compressor discharge
temperatures for the azeotrope ranged from 4 to 16 C° lower than for HFC-134a. The average compressor
discharge temperature was 10C° lower than for HFC-134a over the range of test conditions.
CONCLUSIONS
In the long-term, alternative technologies could reduce or eliminate the use of HFC-134a in motor
vehicle air conditioning, or alternative chemicals could replace HFC-134a in conventional systems. Four pure
chemicals and one azeotropic mixture were evaluated for use as refrigerants in conventional systems, but were
found to be unsuitable. The pure chemicals were HFC-227ca, HFC-227ea, HFC-245cb, and HFE-143a. The
azeotropic mixture was 51.7 weight percent HFC-134 and 48.3 weight percent HFC-245cb. Initial evaluation
of the azeotropic mixture indicated that it might be suitable for use in conventional motor vehicle air
conditioners. However, a recently obtained OH rate constant for HFC-245cb indicated that the azeotrope has
a long atmospheric lifetime and is unsuitable for use as an HFC-134a alternative. The search for low global
wanning alternatives for HFC-134a continues as global climate change remains an important issue.
ACKNOWLEDGMENTS
The authors wish to acknowledge the contributions of Cynthia Gage of NRMRL-RTP, who developed
computer codes and predicted the thermodynamic performance for refrigerants, and Georgi S. Kazachki of
Acurex Environmental Corporation, who provided technical consultation for performance testing.
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