Alternative Refrigerant AC6 

Performance Evaluation and 

Optimization Potential

Sept 28, 2011

By 

Xinzhong Li 

Manager of Engineering

Creative Thermal Solutions, Inc
2

Presentation Objectives

• CTS introduction

• System and facility used for the evaluation

• Performance of new refrigerant – AC6 as a drop in 

replacement for R134a

• Different from R134a:

• High glide zeotropic refrigerant blend 

• Analysis of results

• Focus on evaporator distribution

• Discussion of possible improvements??
3

CTS - Creative Thermal Solutions, Inc.

Located in Urbana, Illinois near 

the Univ. of Illinois

 Thermal lab covering 6,000 m2

 50+ Engineers and technicians

 100+ publications in the thermal 

area

 17 environmental chambers for test of Mobile Air 

Conditioning Systems 

 18 environmental chambers for test of Stationary Systems

 MAC leak test facilities following EU and SAE standards

 Various specialized MAC facilities

 In-house support for fabrication and instrument calibration

 Involved in four previous SAE CRP development activities
4

MAC system hardware

Mid-size Sedan 

system Features

Core size

W*H*D [mm]

Overall size

W*H*D [mm] Face Area

[m2]

Evaporators

Plate-fin 

two slab

4 passes

258*211*58 263*250*58 0.054

Condenser

Micro-channel 

condenser with IRD 630*385*16 668*397*16 0.243

IHX Extruded tube type

Compressor ext 

controlled var disp

Control based on 

average nozzle 

temperature out of 

evaporator

163 cc [3-100%]
5

Evaluation framework: 

single evaporator TXV system in environmental chambers

All tests per 

SAE J2765
6

Three energy balances within ±3%
7 9/28/2011

AC6 performance equal to R134a at 

Tamb 35°C (high Load)

Performance
8 9/28/2011

AC6 efficiency equal to R134a at 

Tamb 35°C (high load)
9

Performance - 25°C x 80% (low load)

Capacity controlled to be equal
10

Efficiency - 25°C x 80% (low load)

AC6 efficiency lower than R134a
11

Performance summary

 At high loads (high ambient temperatures):

 Compressor run full stroke

 Capacity equal to R134a 

 COP equal to R134a

 At low loads (low ambient temperatures):

 Compressor run at reduced stroke to match the target temp setting

 Capacity controlled to be equal to R134a 

 COP lower than R134a

 Difficulties in achieving good distribution with AC6 clearly 

identified as the main reason

 We have seen similar distribution issues in almost every new 

refrigerant when used as a drop-in alternate
12

What is happening at low loads with 

the evaporator !?

Use modeling for analysis
13

Good agreement 

@ high load H35

R134a and AC6 show 

very good agreement

Model (dashed) vs. 

experiment (solid) lines

Good agreement for 

different fluids show that 

refrigerant and circuiting 

effects are predicted well
14

Model agrees well with test data

Since evaporator models capture the effects of glides 

and pressure drops but not mal-distribution, excellent 

agreement in previous high load conditions indicates 

good distribution for both R134a and AC6. 

Let’s look at low load
15

Worse model 

prediction for AC6 

than for R134a 

at low load L25

R134a model shows 

small over-prediction

Mal-distribution with 

AC6 is left as the 

only suspect for the 

difference (2oC) 

between the model 

and the experiment 
16

Can we see effects of distribution 

in the test data?
17

Look at the refrigerant circuit 

first

Understanding evaporator

1 3 2 4

• Four passes

• Two slabs
18

TC grid (5X4) at the air outlet

Understanding evaporator

1 3 2 4 AIR

AIR 15 14 13 12 11

25 24 23 22 21

35 34 33 32 31

45 44 43 42 41

Differences due to GLIDE

Difference from 

mal-distribution
19

Actual Teao at high load indicate good 

distribution
20

Temperature distribution at low load 

indicates mal-distribution forAC6

Is there any effect of glide in variation?
21

0

5

10

15

20

25 Teao11 Teao12 Teao13 Teao14 Teao15 Teao21 Teao22 Teao23 Teao24 Teao25 Teao31 Teao32 Teao33 Teao34 Teao35 Teao41 Teao42 Teao43 Teao44 Teao45 Teao_avg

R134a w/IHX AC6 w/IHX

R134a model AC6 model

Actual vs. predicted exit air temperature: 

mal-distribution @ 25oC (low load) is more 

pronounced for AC6

Top Bottom
22

So, what we learned

• Experiments show very good agreements with CTS 

model for R134a at high loads where distribution is 

good

• The same model provides the same good 

agreement with experiments for AC6 at high loads 

(good distribution for AC6)

• Measured data agree with the model with R134a at 

low load (acceptable distribution)

• Model over-predicts experiments with AC6 at low 

load – mal-distribution is a reasonable explanation
23

Assessment of potential 

improvement: COP increase 6.6%

At equal capacity the 

system model shows that 

with 2°C increase in 

average evaporating 

temperature:

COP will increase 6.6%

A new design for the 

evaporator has a big 

potential 
24

Conclusions

9/28/2011

 At high loads (high ambient temperatures):

 Capacity equal to R134a 

 COP equal to R134a

 At low loads (low ambient temperatures):

 Controlled to be equal capacity to R134a 

 COP lower than R134a

 Refrigerant mal-distribution identified as a main reason for 

lower efficiency with AC6 at low loads

 Improving refrigerant distribution in evaporator could 

improve efficiency for 6.6% at low loads
25

Other opportunities to make AC6 

equal to or better than R-1234yf at 

low load 

• Cycle optimization

• SH control

• Charge optimization (SC control)

• Condenser optimization

• The condenser model indicated potentials for improvement 

of system efficiency and reduction of high discharge 

pressure for AC6

• Will test new condensers with different pass configurations 

in the next steps