ncrac 2011 inv 10 presentation
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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!!!
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