TRANSCRITICAL CO2 BASED SYSTEMS

FOR REFRIGERATION AND AIR

CONDITIONING

Dr. M. Ram Gopal

Department of Mechanical Engineering

Indian Institute of Technology Kharagpur

Kharagpur, India, PIN: 721 302

Introduction

• Due to ozone depletion and global warming,

environment friendly refrigerants are needed in

refrigeration and air conditioning applications

• Most of the proposed non-ODS, synthetic

refrigerants have high GWP – future use

uncertain?

• Natural refrigerants such as air, water, carbon

dioxide and hydrocarbons offer a permanent

solution to environmental problems

• Of these natural refrigerants, only CO2 is non-

flammable, non-toxic with sub-zero normal

boiling point

Background

• Carbon dioxide (R744) was widely used duringlate 19th and early 20th centuries primarily for:

• marine refrigeration, cold storages, comfort cooling etc.

• Invention of synthetic refrigerants in 1930sreplaced most of the older fluids, including CO2

• Factors responsible for replacement of CO2 are:

– Failure to differentiate CO2 from other refrigerants

– Problems due to high operating pressures

– Rapid loss of capacity and efficiency at high heat

sink temperatures

– Aggressive marketing and low cost of CFC systems

– Failure of the CO2 system manufacturers to adapt

improved designs

Revival of CO2 as refrigerant

• Prof. Lorentzen patented a transcritical CO2

system with high pressure control in 1989

• Lorentzen and Pettersen published results on an

automobile air conditioning system based on

transcritical CO2 cycle in 1993

• CO2 prototype performance found to be

comparable or better than the baseline CFC12

system

• Soon after, many potential applications of CO2

based systems for cooling and heating applications

are identified

• Many systems are successfully commercialized

Current status of CO2 refrigeration systems

a) Systems developed & commercialized

a) Beverage coolers (e.g. Coca Cola)

b) Heat pump water heaters (domestic and commercial)

c) Supermarket refrigeration systems

d) Chest freezers

e) Transport refrigeration (bus, train)

b) Systems developed, but not commercialized

a) Mobile air conditioning (MAC) systems

c) Systems under development

a) Mobile heat pumps

b) Air conditioning (residential & non-residential)

c) Heat pumps for combined air and water heating

d) Transport refrigeration (containers, trucks)

e) Heat pump dryers (residential & commercial)

CO2 as refrigerant

Advantages:

• Environment friendly (ODP =0, GWP=1)

• Non-toxic and non-flammable

• Sub-zero normal boiling point

• Excellent thermo-physical properties

• Material compatibility

• Low cost and easy availability

Disadvantages:

• Low critical temperature ( 31.0oC)

• High operating pressures

• Low theoretical efficiency of the basic cycle

CO2 in mobile air conditioning

• Problems and requirements

– Relatively large refrigerant leakage

– Need for compact and light-weight systems

– Future need for heating, independent of engine heat

• R134a is the currently used refrigerant – to be

replaced due to high GWP

• Currently proposed alternatives are:

– HFO-1234yf (CF3CF=CH2)

• Low GWP (=4)

• Synthetic with relatively unknown impacts

• Mildly flammable

– R744 (CO2)

• Need for performance improvement

Thermodynamics of transcritical CO2 cycles

• For high sink temperatures, sub-critical cycle

has to be replaced by a transcritical cycle

– Heat rejection is non-isothermal

– Gas cooler to replace traditional condenser

– Discharge pressure independent of refrigerant

temperature – depends on CO2 charged

– Possibility of optimizing the discharge pressure

to maximize COP, exergetic efficiency or capacity

– Optimum discharge pressure depends on several

parameters

– Need for modifications in cycles, component

design and controls

Variation of refrigeration effect and specific work with discharge

pressure [Danfoss, 2004]

Typical COP variation with discharge pressure and gas cooler

exit temperature [Bullard, 2004]

Effect of discharge pressure on a simple CO2 cycle performance

te=7oC, tgc,exit = 43oC

• Optimum discharge pressure for maximum COP

depends on several parameters

• Large number of studies are carried out to

estimate optimum pressure and maximum COP

• For example, for a given compressor it is shown

that [Sarkar et al., 2004]

Where t3 = gas cooler temperature (30oC to 50oC)

t4 = evaporator temperature (-10oC to +10oC)

Discharge pressure in CO2 systems can be varied in a number of

ways

Comparison between R134a & CO2 (R744)

• Input parameters:

– Single stage cycle with no LSHX

– Evaporator temperature =7oC (for R134a & R744)

– Heat sink temperature = 35oC

– Condenser temperature (R134a) = 54.4oC

– Condenser exit temperature (R134a) = 43oC

– Gas cooler exit temperature (R744) = 43oC

– Saturated condition at evaporator exit (R134a & R744)

– Irreversible but adiabatic compression

– Isenthalpic expansion

– No pressure drops in connecting lines & HXs

– Design refrigeration capacity = 1 TR (3.517 kW)

1. Operating pressures are an order-of-magnitude higher and the

displacement rate is an order-of-magnitude lower compared to R134a

2. Discharge temperatures are higher than that of R134a (85oC vs 63oC)

3. COP is only about 60 % that of R134a (2.52 vs 4.258)

4. Losses due to throttling are much higher for R744 compared to that of

R134a (37 % vs 13 %) Opportunity to recover throttling losses?

Methods to reduce throttling losses

1. Cooling the refrigerant before throttling using

an internal heat exchanger (IHX)

2. Use of an expander in place of throttle valve

3. Use of an ejector in place of throttle valve

4. Use of multi-expansion and flash gas removal

• Reduction in throttling losses also reduces

losses in other components

• Performance improvement of the suggested

methods varies depending on operating

conditions

Improvement with internal heat exchanger (effectiveness = 90%)

Near the optimum discharge pressure:

•Throttling loss decreases from 0.515 kW to 0.24 kW

•Loss in gas cooler increases from 0.214 kW to 0.334 kW

•Improvement in COP is about 8.33 % (2.73 vs 2.52)

Improvement using an expander

• Expanders may be more

more beneficial in CO2

systems as most of the

expansion takes place in

single phase region

• Expander increases the

refrigeration effect and reduces the net work input

• Expanders may be economically viable in larger

systems

• Combined expander-compressor have also been

developed for transcritical CO2 systems

Improvement using a 2-phase ejector

Use of an ejector in place of a throttle valve:

1. Increases refrigeration effect and improves evaporator performance

2. Increases suction pressure thereby improving compressor performance

3. Actual improvement in performance depends on ejector efficiency

4. Ejectors are preferable as they are inexpensive and do not consist of

any moving components

Comparative performance with throttle valve, expander

and ejector

is,exp = 0.8is,comp

ejector = 10 %

At optimum discharge pressure:

1. Use of expander improves the COP by about 42 %

2. Use of ejector improves the COP by about 4.5 %

Multi-stage cycle

At evaporator and gas cooler exit temperatures of 7oC and 43oC,

respectively:

•Use of 2-stage system improves the COP by about 12.3 %

•Optimum discharge pressure also reduces with 2-stage system

•Other schemes with multi-expansion and multi-compression can be

envisaged

Comparison of with baseline R134a system with and

without improved heat exchangers

1. Due to excellent thermo-physical properties, highly efficient heat

exchangers can be developed for CO2 systems (terminal T 2 to 3 K)

2. With improved heat exchangers the theoretical performance of a basic

cycle can approach that of R134a

3. Theoretical performance of CO2 system with expander and improved

HXs can exceed that of R134a

Actual performance of refrigeration cycles

• Actual performance can be far away from

theoretical cycle performance due to various

losses in actual components

• Difference between actual and theoretical

performance is much higher in case of R134a

• Studies show that with suitable design

modification and optimization, actual CO2

systems can outperform synthetic refrigerants

• The study by Lorentzen is a classic example of

how CO2 systems can be made to perform better

than the then state-of-art R12 system

Performance under high ambient temperatures

Neksa et al. [2010]

•CO2 systems tend to be less efficient at higher ambient temperatures

•Data shows that 90% of the time the ambient temperature in most of the

cities is less than 35oC

•Hence seasonal performance of CO2 systems can be better

•If the system is expected to operate at high sink temperatures for

longer periods, then it may be necessary to use advanced cycles or

concepts such as expanders etc.

35oC

Life Cycle Climate Performance of R134a and R744based car air conditioners

• The equivalent greenhouse gas emission is dividedinto:

– 1) Impact due to transportation of the system due to its mass

– 2) Impact due to release of refrigerant into atmosphere, and

– 3) Indirect impact due to system performance

• Studies carried out by Hafner and Neksa [2006] showthat even for typically high ambient temperatureconditions, the LCCP of CO2 systems is much betterthan that of R134a

• The analysis shows that by using CO2 systems thegreenhouse gas emissions can be reduced by about:

• 40 % for Indian conditions, and

• 55 % for Chinese conditions

Prospects for CO2 based MAC

• Studies available in open literature clearly show that CO2

offers the best possible long term solution for car air

conditioning [B-Cool Project, Malvicino et al., 2009]

• With properly matched components, the efficiency of

CO2 system can be comparable or better than R134a

• At projected 2011 costs, the cost of CO2 system may

be much higher than the currently used R134a system

• Though with volume production costs are likely to come

down, still CO2 system may remain costlier

• Further studies are required on the issues of reliability

and system costs

Safety and other issues

• Operating pressures of CO2 systems are an order-of-

magnitude higher than R 134a systems

• However, internal volume of CO2 systems will be an order-

of-magnitude lower compared to R 134a

• Hence, explosive energy (depends on the product of

pressure and volume) is almost same for both

• For refrigeration systems, water content in CO2 should be

less than 10 ppm

• Manufacturers recommend specially developed lubricant,

filter-driers for CO2

• CO2 is compatible with all common metals and alloys

• Since in transcritical systems, CO2 can dissolve in some

polymers, suitable polymers should be used

Availability of systems & components

• Compressors: Dorin, Bock, Bitzer, Mayekawa, Sanyo,

Danfoss, Embraco, Obrist

• Heat exchangers: Alfa Laval, Frascold, Swep etc.

• Control valve, expansion devices and other

accessories: Danfoss, Johnson Controls, Grundfos,

etc.

• The web portal R744.com is developed to showcase

manufacturers exclusively for transcritical and sub-

critical CO2 (R 744) systems and components

• Transcritical CO2 based systems for other cooling

and heating applications are available from Sanyo,

Denso, Daikin, Mitsubishi, Hitachi, Matsushita etc.

Conclusions

• CO2 along with other natural refrigerants offer a

permanent solution to most of the environmental

problems caused by synthetic refrigerants

• To make CO2 systems competitive, the unique

properties of CO2 should be recognized and used in the

design of operating cycles and components

• Results obtained so far are very encouraging

• However, a large scale promotion is needed to

alleviate the various, and mostly unfounded

apprehensions about this high pressure refrigerant

from the minds of the various stakeholders

CO2 related activities at IIT Kharagpur

• Design and development of a transcritical CO2 based

heat pump for simultaneous water cooling and heating

• Theoretical studies on transcritical CO2 based heat

pump dryers

• Studies on heat exchangers for CO2 applications

• Theoretical studies on natural refrigerant based

cascade systems with:

a) CO2 as low temperature fluid in subcritical cycle

b) CO2 as high temperature fluid in transcritical cycle

• Theoretical and experimental studies on CO2 based

natural circulation loops

Thank you for your attention!

Questions are welcome!!!