1

An Assessment of Alternative Refrigerants

for Automotive Applications

based on Environmental Impact

V. Sumantran

Bahram Khalighi

Kevin Saka

General Motors R&D Center

Steve Fischer

Oak Ridge National Laboratory

ABSTRACT

It is evident that societies around the globe are demonstrating growing interest and

concern for the environment. This trend has manifested itself in demand for products

that meet societies' expectations for environmental friendliness. Automakers, globally,

have rallied to devise better designs and solutions that meet these expectations. In the

area of automotive air-conditioning systems, the technology has evolved to a reliance

on HFC-134a as a stable non-corrosive, non-toxic refrigerant that avoids adverse impact

on the ozone layer. More recently, the industry has been involved in assessment of

refrigerants other than HFC-134a, motivated primarily by efforts to minimize

greenhouse gas emissions. Typical candidates include carbon-dioxide as well as

members of the hydrocarbon group (usually, propane and iso-butane).

The present study was undertaken to assess the relative advantages of these

alternative refrigerants, with specific emphasis on carbon-dioxide systems. To do so,

the study employs the Total Environmental Warming Impact (TEWI) index as a holistic

measure of the system. The analysis was undertaken with carefully defined conditions

involving two standard production vehicles representing small and mid-size cars. The

simulations were run to represent vehicle and air-conditioning use in six cities around

the globe using standard vehicle operation cycles. Key assumptions such as refrigerant

emission were made using a range of values cited in references. In the case of CO2

systems, given lack of adequate on-road measurements, the effect of approach

temperature was also evaluated with a range of values.
2

The results reveal that when the total picture is considered, HFC-134a systems

demonstrate advantages compared to carbon-dioxide and hydrocarbon systems on the

TEWI scale. Looking ahead, there is opportunity for further reduction of TEWI values

for HFC-134a systems if their emission rates can be reduced. The advantage for HFC134a systems is pronounced in the warmer regions around the globe. The advantages

derive mainly from the fact that the HFC-134a systems offer better cycle efficiencies at

high ambient temperature. For some of the cooler regions, carbon-dioxide systems may

offer better TEWI performance. This advantage may be eroded if further progress is

made to reduce lifetime emission for HFC-134a systems. Also, the hydrocarbon systems

are broadly comparable to carbon-dioxide systems on the TEWI scale for the conditions

evaluated.

To maximize beneficial impact on the environment, it may be necessary to analyze the

distribution of global vehicle population densities and the efficacy of any technical

solution.

1. INTRODUCTION

The Montreal Protocol was signed in 1987 amidst growing concern about ozone

depletion effects of various refrigerants used in air-conditioning systems.

Chlorofluorocarbons (CFCs) which were extensively used at that time in air-conditioning

systems were identified as contributing to ozone depletion. The potential dangers of

ozone-producing materials, such as CFC’s, which affect the earth’s stratosphere, have

been documented in various publications [1-6]. Since the signing of the Montreal

Protocol, a broad search was undertaken for alternatives that would replace CFCs.

Among the several groups of alternatives the hydrochlorofluorocarbons (HCFCs) and the

hydrofluorocarbons (HFC’s) were considered to be the most obvious. The choices were

motivated by the urge to develop safe (non-toxic, non-flammable) refrigerants that

would be efficient for the intended air-conditioning cycle. It should be noted, however,

that HCFCs, unlike HFCs, contain chlorine and therefore are still ozone depleting

materials. Consequently, the automotive industry made the switch to HFCs from CFCs

as a long-term solution to this environmental issue [5-7].

More recently, the Kyoto Protocol (1997) has caused environmental focus to shift from

ozone depletion to global warming and equivalent emission of carbon dioxide into the

atmosphere. All emissions, attributable to automobiles and their effect on global

warming were identified and this, in the area of refrigerants, led to the inclusion of

HFC-134a among the "basket" of gases monitored by the Kyoto Protocol.

As a gas, HFC-134a has a significantly larger global warming potential (GWP=1300)

than non-halocarbon working fluids. This means that emission of HFC-134a causes a

significantly larger effect on global warming, compared to an equivalent amount of

carbon dioxide (GWP=1 by definition). However, it is also important to study the

problem in its larger context. The total effect of mobile refrigeration systems on global
3

warming is more accurately described by the Total Environmental Warming Impact

(TEWI) [5,7]. The TEWI concept takes into account the contributions to global warming

of:

1) the overall efficiency of the air-conditioning system which directly affects fuel burned

to power the system and thereby, the c from that related combustion,

2) the emission of CO2 from burning fuel to transport the (mass of the) air-conditioning

system;

3) the result of refrigerant being released to the atmosphere due to leakage, servicing

and accidents.

The TEWI concept has gained favor as the preferred method for considering, in a

holistic manner, the multiple environmental effects of alternative refrigerants in mobile

air-conditioning systems.

Focus on the effect of refrigerant emission has triggered a move to advocate use of

working fluids like carbon dioxide. Indeed, CO2 and hydrocarbons (HCs) are viable

refrigerants that, solely from an emission perspective, offer advantages compared to

HFC-134a. Furthermore, carbon dioxide being a non-flammable, enjoys popular

preference over hydrocarbons.

The present study was undertaken to investigate the relative advantages of these

alternative refrigerants employing the TEWI index. The analysis was undertaken with

carefully defined conditions involving two standard production vehicles representing

small and mid-size cars. The simulations were run to represent vehicle and airconditioning use in six cities around the globe using standard vehicle operation cycles.

2. ANALYSIS TOOL

Measured compressor performance data were used with a commercial software

package called EES (developed by F-Chart Inc.) [8] to evaluate system coefficient of

performance (COP) for the different air-conditioning cycles. The basic function provided

by EES is the numerical solution of a set of algebraic equations. The EES software was

used in conjunction with REFPROP6 [8] from the National Institute of Standards and

Technology (NIST) in order to calculate refrigerant thermodynamic and thermophysical

properties. REFPROP6 computes high accuracy properties for many pure and mixtures

of liquids and gases. Laboratory and wind tunnel measurements were used to specify

optimal high side pressure of the CO2 systems [9-11] and condensing pressure for the

other systems.

In addition to the EES program an Excel spreadsheet was developed to calculate TEWI

values in each case. This spreadsheet takes into account many factors:

ß annual vehicle driving hours and distance

ß annual air-conditioning usage, the vehicle driving cycle (regional driving schedules),

the average lifetime of the vehicle

ß refrigerant emission values
4

ß incremental (vehicle) fuel consumption

ß refrigerant type

ß vehicle electrical requirements to operate the A/C system (blower) for the different

regions

ß other factors

For HFC134a systems, the variable displacement compressor information was obtained

from the manufacturer and uses measured data. For CO2 systems, data was provided

by sources cited in the list of references [9, 10].

3. VEHICLES MODELED AND CONDITIONS

The analysis was performed for two standard production vehicles representing small

and mid-size cars. The vehicle and the engine specifications as well as the important

parameters adopted in the present study are summarized in Table 1.

The performance comparisons for different refrigerants (HFC-134a, CO2, and HCs) were

made under identical conditions. Air temperature and the air flow leaving the

evaporator core were set at 55 °F and 250 CFM, respectively.

4. ANALYSIS CONDITIONS AND ASSUMPTIONS

4.1 Ambient Temperature and Relative Humidity Distributions

The simulations were run to represent vehicle and air-conditioning use in six cities

around the globe using standard vehicle operation cycles as well as different vehicle

characteristics. The six cities that were selected represent diverse climates around the

globe. For example, European climates (Frankfurt) tend to be cooler, with airconditioning usage at lower energy consumption, while Pacific Rim, and tropical

climates (Miami) are warmer and more humid, with more frequent air-conditioning

usage at higher consumption. Typical meteorological weather information was used for

the different cities around the globe [12]. Figure 1 shows the ambient temperature and

the corresponding relative humidity distributions for the regions (six cities) considered

in our investigation. For each region these meteorological data, in conjunction with the

assumed comfort levels, were used to obtain the required cooling loads for calculation

of the system COP.

4.2 Vehicle Operational Cycles

For cities in the US the Environmental Protection Agency (EPA) driving cycle was

adopted to determine the air-conditioning compressor usage profile and consequently

the TEWI index for running the air-conditioning system. Similarly, for Europe the

calculations involved the European ECE 93/116 driving schedule (Figure 2a). For Tokyo

a driving cycle shown in Figure 2b was used in the analysis [13].
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A variable displacement compressor model was used for the HFC-134a system. The

power input to the compressor was a function of ambient temperature. Also, the

compressor was turned on at and above 40 °F with cooling output and power input

changing to meet the load.

4.3 HFC-134a Cycle Systems

Figure 3 depicts the schematic of the HFC-134a compression used in the present study.

The subcritical systems were evaluated using calculated refrigerant properties at: the

compressor inlet, the compressor exit, the condenser exit, the evaporator inlet, and

saturated vapor leaving the evaporator.

4.3.1 Emission Assumption for HFC-134a System

Refrigerant emissions from leakage, servicing, end-of-life procedures and accidents are

critical components for HFC-134a air conditioning systems. An estimate in automobile

air-conditioning system has been given by [14,15]. To address this issue, four different

emission rates were used for the HFC-134a refrigerant system. Henceforth these four

emission rates will be referred to as: E1, E2, E3 and E4 refrigerant emissions. The

values for HFC-134a systems in different cars are described in Table 2. The E3 and E4

values for each vehicle are based on 1.5 and 2 recharges (for refrigerant) during the

operating lifetime of the air-conditioning system (11 years), each requiring refrigerant

addition at 40% of the original charge [14]. The range of values for refrigerant

emission, used in this study, spans values used by other studies [3, 14-16].

4.4 Carbon Dioxide (CO2) Cycle

Figure 4 illustrates the schematic of the CO2 cycle for the two vehicles. For the CO2

cycle, the analysis of the supercritical refrigerant includes the use of calculated

refrigerant properties at the evaporator inlet, saturated vapor at the evaporator exit,

the inlet and exits of a suction line heat exchanger, compressor suction and discharge

ports, and the gas cooler exit. Refrigerant temperature leaving the gas cooler is

determined from the ambient temperature and the approach temperature.

4.4.1 Approach Temperature Assumptions for CO2 System

It is convenient to compute TEWI using cooling loads and effeciencies as they vary with

ambient temperature, since that is how climate data are organized. The performance of

CO2 systems, however, depends heavily on the temperature of the air entering the gas

cooler. At idle conditions or low vehicle speeds the air temperature (air entering the gas

cooler) can be significantly higher than the ambient temperature due to the

entrainment of engine heat. The temperature of the refrigerant leaving the gas cooler

will be hotter than the air entering the gas cooler. The temperature difference across
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the gas cooler can be very small depending on the design conditions (3 to 5 °K) for a

well-built gas cooler but it varies with heat exchanger loading. These two temperature

differences (ΔTs) and the gas cooler approach temperature are illustrated in Figure 5.

The approach temperatures used in this analysis are based on the various reentrainment modes for the CO2 systems (Figure 6a). These values are used in

conjunction with the vehicle operation cycles. Experimental measurements conducted in

a climatic wind tunnel confirmed this behavior, both as a function of vehicle speed and

ambient temperatures (Figure 6b).

4.4.2 Emission Assumptions for CO2 System

Emission rates for CO2 air-conditioning systems are not readily available compared to

that of HFC-134a systems. However, gases such as CO2 and hydrocarbons have

extremely low global warming effects and thus the impact of CO2 leakage on TEWI is

insignificant.

4.5 Hydrocarbon systems

The refrigeration cycles adopted for propane and iso-butane are shown in Figure 7

schematically. This system adopts a secondary loop, compared to HFC-134a systems.

TEWI evaluations for the hydrocarbons were performed only for the mid-size car. The

secondary loop pump power (for the secondary loop) is assumed to be 200 W

coincident with the compressor operation and there is an assumed temperature

difference across the secondary heat exchanger.

5. TEWI CALCULATIONS

Figure 8 summarizes the steps that are taken to generate TEWI values in each analysis.

For each vehicle, the regional climates (Figure 1) and the required passenger

compartment comfort levels were used as inputs to the EES program. This step

generated the COP and the cooling load required for each air-conditioning system for

the respective vehicle and the region. The COP and the cooling load were then fed into

an Excel spreadsheet utilizing the various usage profile assumptions, which in turn

resulted in a TEWI index.

In this analysis the equivalent CO2 emission due to the transportation of the weight of

the air conditioning system is computed using 57 liters/100 kg/10,000 km for

incremental fuel use for weight increases [7]. A lifetime fuel consumption was then

computed by using the vehicle usage time for the different regions (cities) and the airconditioning lifetime. This lifetime fuel consumption for the weight of the airconditioning was converted to an equivalent CO2 emission using 2.32 kg CO2/liter of

fuel [3,7,14].
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6. RESULTS

In each section, results are shown as bar charts. Each bar consists of TEWI

contributions from the energy input to the compressor, the fuel consumption for

transporting the weight of the air-conditioning system, and the direct global

warming effect of refrigerant emissions.

6.1 Effect of Different Emission Rates on TEWI for the HFC-134a System

Figure 9 illustrates the effect of the emission rates on TEWI index for the mid-size car in

all six cities for the HFC-134a system. It is evident from this figure that the TEWI

contribution from refrigerant emission is the dominant portion of the total TEWI in the

cooler climates while the power consumption for the air-conditioning system is

dominant in other regions. The following table compares the contributions of the

various components that comprise the total TEWI values for Phoenix and Frankfurt for

the mid-size vehicle. These values are based on the E2 emission level (55 g/year).

City A/C compressor

power consumption

fuel consumption for

 transporting the weight

of the air-conditioning system

Direct Emission

Phoenix 72% 8% 20%

Frankfurt 22% 15% 63%

Similar results were observed for the small car (not shown here).

As expected the TEWI values increase as the emission rate increases. Also shown in the

figure are significant variations of total TEWI for different regions. This result is

consistent with the previous published investigations [3,7].

6.2 Effect of Approach Temperature on TEWI for CO2 System

As discussed earlier, three different approach temperatures (Figure 6) were used for

the CO2 system. These approach temperatures represent:

ß A1 re-entrainment,

ß A2 re-entrainment and

ß A3 re-entrainment modes, respectively.

Figure 10 shows the variation of TEWI index as a function of approach temperature in

different climates. It is apparent from the figure that the approach temperature

significantly affects the TEWI index particularly at higher hot air re-entrainment modes

during idle. This will impact performance of CO2 system significantly while the vehicle is
8

stationary in the warmer regions. The direct emission (leakage) effects are so small for

the CO2 system that they are not visible in the figure.

6.3 TEWI Comparisons for Vehicle and Refrigerant Types in Different

Climates

As described in the previous sections, TEWI values for HFC-134a systems vary

significantly, depending on the assumptions of emissions from such systems. Likewise,

for CO2 systems, assumptions of approach temperature can strongly influence TEWI

values. This section will depict these variations as a range, using the values described

above.

Figures 11 and 12 show the TEWI variations as a function of vehicles (small and mid

size cars) and refrigerant types (HFC-134a and CO2) in different climates (six cities). In

these figures for CO2 systems, the third segment is labeled as “approach temperature

uncertainty” which represents emissions from re-entrainment ranging from A1 to A3 reentrainment values. For the HFC-134a system the last segment (in the bar chart) is the

uncertainties due to direct emission levels (E2-E4 emissions).

It appears that the HFC-134a system is superior to the CO2 system, particularly in hot

and humid regions (Phoenix and Miami.). In cooler climates, like Frankfurt, however,

CO2 systems reveal an advantage in terms of TEWI index.

Also, comparing Figures 11 and 12, it is evident that the value of TEWI (for both HFC134a and CO2 systems) increases with vehicle size while maintaining the same level of

occupant comfort. This is also understandable in the sense that the larger vehicle

requires greater cooling capacity.

Figure13 illustrates the variation of TEWI index for propane and iso-butane in different

cities. Similar to the CO2 systems, the emission effects of the hydrocarbons are very

small (third segment almost zero). It should be noted that the effect of re-entrained

approach temperature was not performed for the hydrocarbon systems.

7. CONCLUSIONS

In summary, it is evident that the total environmental impact of a mobile refrigeration

system derives from several factors in addition to refrigerant GWP. Any assessment

based solely on the GWP of the refrigerant is therefore incomplete. The TEWI index

appears to comprehend these additional factors successfully. Furthermore, calculating

the TEWI index depends on several key assumptions of local climate, vehicle use,

system performance and, etc. Our data reveals that it is possible to arrive at different

conclusions depending upon particular choice of assumptions. To achieve robust

conclusions, it was necessary to evaluate the TEWI index for a selection of vehicles, in
9

different global cities using standard reference values for local climates, driving cycles,

vehicle use, etc.

From the set of calculations performed for this study, it appears that HFC-134a systems

enjoy an advantage (over CO2 systems) in the warmer regions around the globe. The

advantages derive mainly from the fact that the HFC-134a systems offer better cycle

efficiencies emissions when actual heat exchanger air temperatures are considered. For

some of the cooler regions, CO2 systems may offer better TEWI performance. As was

observed in the results, for cooler climates, TEWI contributions are often dominated by

emission of refrigerant.

This advantage, for CO2 systems in the cooler climates, may be eroded if further

progress is made to reduce lifetime emissions for HFC-134a systems. This may be

impacted by local (technical) improvements to fittings, system sealing, etc. as well as

procedural changes for recovery and recycling during service and at end-of-life.

Likewise, it is possible to arrive at very different conclusions on the TEWI performance

of CO2 systems depending on how one accounts for re-entrainment of engine and

pavement heat and for gas-cooler approach temperatures. Given limited test data for

this effect, it is an important area for further study. Current experience reveals that CO2

systems suffer significant performance degradation when the temperature of the air

passing through the gas cooler is elevated. This translates to much higher values of

TEWI under these conditions.

Finally, this study briefly examined the TEWI performance of hydrocarbon systems.

Overall, they offered a comparable level of TEWI performance as HFC-134a systems for

the assumptions chosen regarding secondary loop performance and pumping power.

Additional work is underway to study this system in more detail.

To maximize beneficial impact on the environment, it may be necessary to analyze the

distribution of global vehicle population densities and the efficacy of any technical

solution.
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ACKNOWLEGEMENTS

The authors would like to thank Mike Meloeny, Don Cassidy and Dwight Blaser for their

support and encouragement during the course of this study. For providing the various

data and information needed for this study, we thank Bill Hill, Walter Schlueter, Jeff

Wright, and Lou Savich. Also, the authors thank Prof. Hrnjak from the University of

Illinois for numerous discussions on the CO2 system.

REFERENCES

1. M. J. Molina and F. S. Rowland, “Stratospheric Sink for Chlorofluoromethanes:

Chlorine Atom-Catalyzed Destruction of Ozone,” Nature, Vol. 249, pp. 810 – 812,

1974.

2. M. J. Molina, “Polar Ozone Depletion,” 1995 Nobel Lecture, Angewandte Chemie,

Vol. 35, No. 16, International English Edition, pp. 1778 – 1777, September 6, 1996.

3. M. S. Bahatti, “A Critical Look at R-744 and R-134a Mobile Air Conditioning

Systems,” SAE paper no. 970527, February 1997.

4. P. A. Newman, “Preserving Earth’s Stratosphere,” Mechanical Engineering, October

1998, pp. 88 – 91.

5. S. Gopalnarayanan, “Choosing the Right Refrigerant,” Mechanical Engineering,

October 1998, pp. 92 – 95.

6. Y. Hwang, M. Ohadi, and R. Radermacher, “Natural Refrigerants,” Mechanical

Engineering, October 1998, pp. 96 – 99.

7. S. K. Fischer and J. R. Sand, “Total Environmental Warming Impact (TEWI)

Calculations for Alternative Automotive Air-Conditioning Systems,” SAE paper no.

970526, February 1997.

8. EES (Engineering Equation Solver), User Manual, F-Chart Software, Middleton,

Wisconsin.

9. R. P. McEnaney, Y. C. Park, J. M. Yin, and P. S. Hrnjak, “Performance of the

Prototype of a Transcritical R744 Mobile A/C System,” SAE paper no. 990872, 1999.

10.D. Boewe, J. Yin, Y. C. Park, C. W. Bullard, and P. S. Hrnjak, “ The Role of

the Suction Line Heat Exchanger in Transcritical R744 Mobile A/C Systems,”

SAE paper no. 990583, 1999.
11

11.R. P. McEnaney, D. E. Boewe, Yin, J. M., Y. C. Park, C. W. Bullard, and P. S.

Hrnjak, “Experimental comparison of mobile a/c systems when operated with

transcritical CO2 versus conventional R134a”, Purdue conference

Proceedings, pp. 145-150, 1998.

12.Engineering Weather data, AFM 88-29, TM 5-785, NAVFAC P-89, Department of the

Air Force, the Army and the Navy, July 1978.

13.T. Hirata, Private communication to S. Fischer of Oak Ridge National

Laboratory, 1997.

14. J. R. Sand, S. K. Fischer and V. D. Baxter, “Energy and Global Warming Impacts of

HFC Refrigerants and Emerging Technologies,” Oak Ridge National Laboratory,

1997.

15. James A. Baker, “Mobile Air Conditioning: HFC-134a Emissionsand Emission

Reduction Strategies” Joint IPCC/TEAP Expert Meeting on Options for the

Limitation of Emissions of HFCs and PFCs", Energieonderzoek Centrum

Nederland (ECN), Petten, The Netherlands, May 26-28, 1999.

16. J. M. Yin, J. Pettersen, R. McEnaney, and A. Beaver, “TEWI Comparison of

R744 and R134a Systems for Mobile Air Conditioning”, SAE paper no. 990582,

1999.
12

Table 1. Vehicles and A/C system information

Small Car Mid-size Car

2.4L 4-Cylinder Engine 3.8L V6 Engine

Automatic Transmission Automatic Transmission

Refrigerant Charge = 682 g (24 oz) HFC134aRefrigerant Charge = 964 g (34 oz) HFC134a

Compressor = Variable displacement, Mfr.

data

Compressor = Variable displacement, Mfr.

data
13

 Table 2. Refrigerant emissions assumptions for HFC-134a systems

Small Car E1 E2 E3 E4

Vehicle lifetime (years) 11 11 11 11

Full initial charge (g) 682 682 682 682

Recharges over lifetime 0.5 1 1.5 2

EOL residual charge (g) 450 450 450 450

EOL emission (g) 45 45 45 45

Recovery at EOL (%) 90 90 90 90

Effective emissions per year (g) 25 40 55 70

Mid-size Car E1 E2 E3 E4

Vehicle lifetime (years) 11 11 11 11

Full initial charge (g) 964 964 964 964

Recharges over lifetime 0.5 1 1.5 2

EOL residual charge (g) 630 630 630 630

EOL emissions (g) 63 63 63 63

Recovery at EOL (%) 90 90 90 90

Effective emissions per year (g) 35 55 78 98
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 (a) Temperature Distributions

(b) Relative humidity distributions

Figure 1. Regional temperature and relative humidity distributions [12].

-5

0

5

10

15

20

25

30

0 20 40 60 80 100 120 140

Ambient Temperature, Tambient [°F]

Percent Annual Incidence, TI [%]

Phoenix

Miami

Tokyo

Frankfurt

Boston

Sydney

0

10

20

30

40

50

60

0 20 40 60 80 100 120

Relative Humidity, RHambient [%]

Humidity IncidenceE, HI [%]

Phoenix

Miami

Boston

Frankfurt Tokyo

Sydney
15

0

20

40

60

80

100

120

0 200 400 600 800 1000 1200

Time (s)

Velocity (km/h)

0

10

20

30

40

50

60

70

80

90

100

0 200 400 600 800 1000 1200 1400

Time (s)

Velocity (km/h)

0

10

20

30

40

50

60

70

80

90

100

0 100 200 300 400 500 600 700 800

Time (s)

Velocity (km/h)

Driving cycle for

European

Union (93/116)

Length: 11.007 km

Total duration: 1220 s

Max. speed: 120 km/h

Average speed: 33.6 km/h

US Federal Test

Procedure

(EPA) City Cycle

Length: 17.8 km

Total duration: 1877 s

Max. speed: 91.2 km/h

Average speed: 34.1 km/h

US Federal Test

Procedure

(EPA) Highway Cycle

Length: 16.5 km

Total duration: 77.4

Max. speed: 96.4 km/h

Average speed: 77.4 km/h

Figure 2a: Vehicle test cycles used for evaluation of refrigerants
16

0

20

40

60

80

0 20 40 60 80 100 120 140 160

Time (sec)

Speed (km/h)

Japan test cycle

Mode 10 

0

20

40

60

80

0 50 100 150 200 250

Speed (km/h)

Time (sec)

Japan test cycle

Mode 15

Figure 2b. Vehicle test cycle for Japan [13]
17

 Figure 3. Schematic of the HFC-134a cycle.

 

 Figure 4. Schematic of the CO2 cycle.

CONDENSER

EVAPORATOR

COMPRESSOR

2 1

3

ACCUMULATOR

5

4

- .

Expansion

device

GAS-COOLER

EVAPORATOR

COMPRESSOR

1

2

3

ACCUMULATOR

6

5

.

face/area 2

l

s

SLHEX

4

7

Expansion

device
18

 where, ΔTre-entrainment = Tair in - Tair ambient

 ΔTgas cooler = Trefrigerant out –Tair in

Figure 5. Schematic of a CO2 gas cooler and the approach temperature

definition.

Trefrigerant out

Trefrigerant in

Tair ambient

Tair in

CO2 Gas Cooler

Tair in

ΔTapproach = ΔTre-entrainment + ΔTgas cooler

Hot-air re-entrainment
19

Figure 6a. Three approach temperature distributions used for the CO2 cycle.

Figure 6b. Wind tunnel data

0

10

20

30

40

50

60

70

80

Idle 2500 3500

Compressor Speed (rpm)

Approach Temperatue (F)

A3

A2

A1

0

10

20

30

40

50

60

IDLE 2500 3500

High re-entrainment

Medium re-entrainment

Low re-entrainment

0

10

20

30

40

50

60

IDLE 2500 3500

Ambient Temperature 115 °F

Ambient Temperature 65 °F
20

 

 Figure 7. Schematic of a typical hydrocarbon cycle for the Mid-size car.

GLYCOL PUMP

CONDENSER

EVAPORATOR

counterflow HEX

CHILLER

HEAT-EXCHANGER

COMPRESSOR

1

2

3

ACCUMULATOR

7

5

4

8

6

Expansion

device
21

Figure 8. TEWI calculation program flow chart.

REGIONAL CLIMATE

Temperature & Humidity

Distributions

COMFORT-LEVEL SET TO

55F Air from Evap-Core

and 250 cfm Air Flow

USAGE PROFILES

Annual Driving Hours

Annual Driving Mileage

Air Conditioner Operating Time

Vehicle Driving Profile : EPA & European Cycles

Average Vehicle Lifetime

REFRIGERANT EMISSIONS

leakage assumptions based on cited sources for HFC-134A

CO2 has been determined to be insignificant with regard to emissions relative to 134A

OTHER PARAMETERS

incremental fuel consumption

global warming potential of each fluid (GWP)

engine efficiency

vehicle electrical requirements to operate the A/C system (fan, blower, etc.)

TEWI#

EES

(F-Chart Software)

COP’s Cooling Loads

Excel Program

A/C SYSTEM

Compressor Specs

Heat Exchanger Specs

Plumbing Info

WindTunnel Data
22

Figure 9. Effects of HFC-134a emission rates on TEWI for the Mid-size car in

different climates.

0 2000 4000 6000

E1

E2

E3

E4

E1

E2

E3

E4

E1

E2

E3

E4

E1

E2

E3

E4

E1

E2

E3

E4

E1

E2

E3

E4

HFC-134a Emission Senario 

TEWI

A/C Energy Effect

Transportation Effect

Emission

Phoenix

Miami

Boston

Tokyo

Frankfurt

Sydney
23

0 2000 4000 6000 8000

A1

A2

A3

A1

A2

A3

A1

A2

A3

A1

A2

A3

A1

A2

A3

A1

A2

A3

Approach Temperature (re-entrainment)

TEWI

A/C Energy

Transportation Energy

Emission

Phoenix

Miami

Boston

Tokyo

Frankfurt

Sydney

Figure 10. Effects of approach temperature (hot air re-entrainment) on TEWI 

for CO2 system for the Mid-size car in different climates.
24

0 2000 4000 6000 8000

HFC-134a

CO2

HFC-134a

CO2

HFC-134a

CO2

HFC-134a

CO2

HFC-134a

CO2

HFC-134a

CO2

Refrigerant

TEWI

A/C Energy

Transportation Effects

Emissions Uncertainty (E2-E4)

Approach Temp. Uncertainty 

E1 Emission

Phoenix

Miami

Boston

Tokyo

Frankfurt

Sydney

 Figure 11. TEWI variations for the HFC-134a and CO2 systems for the small car.
25

0 2000 4000 6000 8000

HFC-134a

CO2

HFC-134a

CO2

HFC-134a

CO2

HFC-134a

CO2

HFC-134a

CO2

HFC-134a

CO2

Refrigerant

TEWI

A/C Energy

Transportation Effects

Emissions Uncertainty (E2-E4)

 Approach Temp. Uncertainty

E1 Emission

Phoenix

Miami

Boston

Tokyo

Frankfurt

Sydney

Figure 12. TEWI variations for the HFC-134a and CO2 systems for the Mid-size car.
26

Propane

(a) Iso-butane

Figure 13. TEWI results for two hydrocarbons in different climates for the

mid-size car.

0 2000 4000 6000 8000

Sydney

Frankfurt

Tokyo

Boston

Miami

Phoenix

TEWI 

A/C Energy

Transportation Effects

0 2000 4000 6000 8000

Sydney

Frankfurt

Tokyo

Boston

Miami

Phoenix

TEWI 

A/C Energy

Transportation Effects