Transcritical CO2 for Mobile Air-Conditioning and Heat

Transcritical CO2 for Mobile Air-Conditioning and Heat Pump Systems

Prof. Stefan ElbelAssociate Director

Air Conditioning and Refrigeration Center (ACRC)University of Illinois at Urbana-Champaign

Chief EngineerCreative Thermal Solutions, Inc.

Urbana IL, USACreative Thermal Solutions, Inc.

Sustainable Technologies for Air Conditioning WorkshopInternational Civil Aviation Organization Headquarters, Montreal, Canada

November 18, 2017

S. ElbelSustainable Technologies for Air Conditioning Workshop

Creative Thermal Solutions, Inc.

Need for heat pump in electric vehicles

Before We Get Started…

Cost of CO2 system

Cost of R1234yf

Low CO2 production volumes (initially)

Technology to improve efficiency exists, but adds complexity (cost)

Safety concerns• Flammability• Combustion byproducts (HF) • Long-term environmental

concerns (TFA)

Efficiency of CO2 at high ambient temperatures

CO2 systems requires re-design of each component (not a drop-in)

Service & maintenance (training) Patent situation

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Presentation Outline• Brief History of CO2 as Refrigerant

- CO2 used to be very popular. What made it disappear?- Why did it reappear? Why does transcritical CO2 make sense for cars?

• What is so Different About CO2?- What are the strengths of CO2?- What technical challenges need to be overcome?

• Technology Examples that Paved the Road for CO2- Microchannel gas cooler- Internal heat exchanger- Ejector for expansion work recovery- Heat pump for low ambient temperatures (hybrid / electric vehicles)

• Path to Successful Implementation• Summary & Conclusions

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CO2 Used to be a Very Popular Refrigerant!

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Stera A.C., 1992, Ammonia Refrigerating Plant on Reefer Ships. Introduction to Ammonia as a Marine Refrigerant, Lloyd's Register Technical Seminar, London, UK

Percentage Use of Primary Refrigerants in Existing Marine Cargo Transport Classed by Lloyd’s Register

Advertisement in Ice & Refrigeration, 1922, Vol. 63

Fish Freezing, 1915

Pettersen J., 2002, CO2 Vapour Compression Cycle in a Historical Perspective, Public Lecture (PhD Defense)



But What Made CO2 Refrigeration Disappear?

•Containment problems (leaks)•Perceived problems of high pressure

- Production and material technology did not offer good options for high pressures

•Loss of capacity and efficiency at high temperatures

- Cold cooling water not availabe or too hot in tropical seas

•CFC refrigerant development (”Freon”)

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Why Did CO2 Reappear?Why Does it Make Sense for Cars?

• Environmental issues: it appears CO2 is the only ASHRAE A1 (non-toxic, non-flammable) low-GWP refrigerant available for mobile air-conditioning (not considering blends which have additional technical issues)

- CO2 is non-toxic to humans and plants (no phytotoxic breakdown products)

• Improved materials and technologies:- High pressure and containment issues have been resolved - Microchannel HXs offer great compactness at reasonable cost. Instead of thick

walls use tubes with small diameter. Weight of HXs does not increase for CO2!- Improvements in compressor technology and significantly smaller displacements

make CO2 systems more compact

• Increased need for heat pump and combined AC/HP applications where CO2 has advantages(electric / hybrid vehicles)

Sanyo, 2009

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What is so Different About CO2? • Has excellent thermophysical properties

- High thermal conductivity, low viscosity, …

• Has low critical temperature (only 31oC)- Cannot differentiate between vapor and liquid above

critical point, i.e. no condensation but instead gas cooling

• Runs as a transcritical cycle at elevated ambient temperatures- That is what’s causing the efficiency loss in AC operation

Additional losses due to isenthalpic expansion

Loss due to temperature glide

Rankine cycle with R134a looks more like Carnot than with R744

40

0

T[oC]

s[J/kg-K]

R134aR744

Source: www.Danfoss.com

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http://www.danfoss.com/



Initial Expectations Were Not Very High! But…

• Although initial modeling showed inferior performance for CO2, even our first results (1996) showed surprisingly good results!

- CO2 outperformed R134a! (R134a only better at idle and > 35oC)

- Good prototype compressors and microchannel HX

R744V=3.71 l

Face area 408 cm2

A air = 4.2 m2

A ref = 0.66 m2

R134aV=3.72 l

Face area 405 cm2

A air = 3.5 m2

A ref = 0.55 2

R134a

V=155cm3

R744

V=20.7cm3

R744V=3.71 l

Face area 408 cm2

A air = 4.2 m2

A ref = 0.66 m2

R134aV=3.72 l

Face area 405 cm2

A air = 3.5 m2

A ref = 0.55 m2

V = 4.32 lFace area 1964 cm2

A air = 7.2 m2

A ref = 0.40 m2

V = 3.32 lFace area 1950 cm2

A air = 5.2 m2

A ref = 0.49 m2

R744gas cooler

R134acondenser

Te0=10 oC Control or Equal Capacity / 900 rpm and 2500

0

1

2

3

4

5

6

7

8

10 15 20 25 30 35 40 45 50

CO

P [-

]

900 rpm

R134a

R744

2500 rpm

Equal COP Equal COP

COP @ 35 oC, idling

From: Pettersen et. al (1998)

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What Explains the Good CO2 Performance?

20oC

10oC0oC

R134a (estimated)

-20oC

CO2

From: Pettersen et al. (2000)

Optimized high-side pressure for maximum performance

Cross-counter-flow micro-channel gas cooler to minimize ∆Tapproach

∆Tapproach = Tref,out – Tair,in

Tair,in

Higher ηcompressordue to lower pressure ratio

CO2

R134a

Much higher CO2convective heat transfer coefficients

Tref,out

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20+ Years of Automotive CO2 Research Resulted in Dramatically Improved COPs

• But R134a (R1234yf) COPs also improved over time!

d

• For each condition: COP improvement in time larger than COP difference between R134a and CO2

• Twenty years of intense research efforts lead to COP improvements of almost 100% for both fluids (at some conditions)

idling

0

5

then now then now

CO

P

driving

0

5

then now then nowC

OP

R134aR744

25oC35oC 35oC25oC

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Technology Developed for CO2 That is Now Used to Improve HFC/HFO

0.80

0.85

0.90

0.95

1.00

1.05

1.10

Expansion valve Ejector

CO

P [-

]

~ 70Hz 60Hz

COP ~ +18%Qmatched = 4.8kW

.

Internal Heat Exchanger

Outer tube

Inner tube

Ejector (instead of expansion valve)

From

: Den

so(2

010)

From

: Den

so (2

006)

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Advantages for CO2 in Heat Pump Mode• Increased need for heat pump in electric / hybrid

vehicles• Due to higher pressures, CO2 can cover wider

temperature range than R134a or R1234yf (CO2operates at -40oC with good COP)

• Temperature glide in gas cooler makes good heat exchanger for heat pumping

0

1

2

3

4

5

-50 -40 -30 -20 -10 0 10 20 30

Hea

ting

Cap

acity

[kW

], C

OP

[-]

Indoor air temperature [°C]

0°C-10°C

-20°C

-40°C

Q -Heating capacityCOP

Compressor

D

S

IHX

Accumulator

Airflow

Airflow

3 Way Valve

Ball Valve

EXV

Mr

Mr

Indoor HX

Outdoor HX

Secondary Indoor HX

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DX reversible AC / HP

CO2



Long Path to Successful Implementation of CO2 Air Conditioning in Cars

Images from: Hafner A., 2017, International Symposium on Natural Working Fluid CO2 Refrigeration and Heat Pump technology, Tianjin, China, Sep. 17-18

1989: First CO2systems at University labs

1998: OEMs start equipping cars with CO2 AC for wind tunnel and field testing

2006: Numerous Asian and European OEMs and suppliers work on 2nd

generation CO2 with much improved performance

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Long Path to Successful Implementation of CO2 Air Conditioning in Cars

Sour

ce: w

ww

.uba

.de

2009: CO2vehicles for long term fleet tests under real conditions

CO2 car operated by German UBA between 2009 and 2017, accumulates more than 165,000 km

2017: First mass produced cars with CO2 AC systems

Sour

ce: w

ww

.dai

mle

r.com

Source: www.sanden.jp

Happy End for CO2… More OEMs to

Follow?

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http://www.uba.de/

http://www.daimler.de/

http://www.sanden.jp/



Conclusions• Miracle refrigerant that can solve all our problems does not exist!

- All fluids have pros & cons: environment, flammability, cost, performance, …

• For the past 75 years, our industry did not challenge mechanical engineers much, because chemists were able to provide refrigerants that allowed simple cycles

• CO2 could be (for many applications) the “final” refrigerant selection, but it requires more effort to get good results

• Good news is that technology to make CO2 work at high ambient temps exists

- Microchannel HX, internal heat exchangers, ejectors, …- Eventually, prices will come down, but CO2 systems are more complex

• Many examples of successful implementation of transcritical CO2exist

• Heat pumps for electric cars could bring the breakthrough for CO2

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In 1995, a Wise Man from Norway Wrote:

From: Hafner (2004)

Prof. Lorentzen’s first sketch of transcritical CO2 cycle

Prof. Gustav Lorentzen (1915- 1995)

From: Int. J. Refrig., 1995, 18(3):190-197

Thank you for your attention!

[email protected]

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Transcritical CO2 for Mobile Air-Conditioning and Heat