ECO-FRIENDLY REFRIGRATION SYSTEMS AND CRYOGENICS

ABSTRACT

The earth's ozone layer in the upper atmosphere is needed for the absorption of harmful ultraviolet rays from the sun. These rays can cause skin cancer. CFSs have been linked to the depletion of this ozone layer. They have varying degree of Ozone Depletion Potential (ODP). In addition, they act as greenhouse gases. Hence they have Global Warming Potential (GWP) as well. According to 'Montreal Protocol', the use of fully haloganated CFCs (no hydrogen at all in the molecule).they are considered to have high ODP viz. R11,R12,R113,R114 and R502 have been phased out by the year 2000A.D. R22 , which is an HCFC is not covered under the original Montreal Protocol as its ODP is only 5% of that of R12,but because of its GWP, it will be phased out by year 2030 AD.
Thus, for environment , efforts are now being directed to develop Eco-friendly alternative refrigeration systems .Search for alternative refrigerants is now necessary. Some alternative are R123,R125, R-134a,R407a,R408,R143a,R152a,R32.Among these R-134a is the best alternative for R-12.However application of R-134a is not without any problems. One more attractive alternative is R152-a. Its magnitude of GWP is less than R-134a and it also has lower energy consumption. Hence, the Environment Protection Agency of Europe prefers R152a over R-134a.to the extent that broader use of conventional refrigerants including ammonia and carbon dioxide are considered as additional alternative.
Liquefied gases (e.g. nitrogen gas) can also be used as alternative refrigerants or as Eco-friendly refrigerants, because they does not have O.D.P and G.W.P. Stirling cryocooler refrigerators used for small units are also to be considered.

INTRODUCTION

Refrigerants and air conditioning play an important role in modern human life. The accelerated technical development and economic growth of most countries during the last century has produced severs environmental problems. we have recognized the fact that man-made products contributing to human comfort as a result of harming the environmental :ozone depletion and global warming. These concerns are the field of refrigeration and air conditioning.
Choroflurocarbons (CFCs) and hydrochloroflurocarbons(HCFCs)are being used as refrigerants .They posses most of the desirable characteristics, such as thermal and chemical stability, thermodynamic suitability, non toxicity, non flammability, material compatibility, low cost etc. CFCs and HCFCs are now being regulated because of ozone depletion. Refrigerants such as R11,R12,R113,R114,R502 have to be phased out by year 2030 AD.

WHY ALTERNATE REFRIGERANTS ?
CFCs have dominated the refrigeration industry because of their favorable thermodynamic and transport properties. There is no denial of the fact that these are still the best from the performance point of view. Still we are talking of replacing them. Recent studies have established that CFCs are instrumental in depleting the ozone layer(stratosphere) which is 20km away from the earth and also contribute towards global warming. The ozone layer is needed for the absorption of harmful ultraviolet rays from the sun. These rays can cause health hazards on human and living creatures and other ecological and environmental problems such as global warming, sea level rise, increased floods, drought and storms etc. A hole was detected in ozone layer for the first time in 1956. Recent studies shows that this hole has expanded.

Mechanism of ozone layer depletion:
As these CFC molecules reach stratosphere they are dissociated by sunlight in to active chlorine compounds which attack ozone, generating a chain reactions. One molecule of CFC is sufficient to destroy 100000 molecules of ozone.
Equations are as follows:
CCl2F2 = CClF2 +Cl
Cl +O3 = ClO +O2
O+Cl2 = Cl +O2 .
O +O3 = 2O2 .

CFCs AND GLOBAL WARMING
Green house effect refers to the trapping of infrared radiation by atmosphere and subsequent warming of the earth. Although the green house effect is the primarily due to carbon dioxide and the concentration of CFCs are very low compared to CO2, CFCs absorb strongly in the I-R region particularly in the wavelength between 7 and 13 microns where atmosphere is largely transparent. This absorption is due to the C-CI and C-F bonds present in the CFCs. If the greenhouse gases keep on emitting at present rate, the average global temperature will increase. This would cause melting of large polar ice caps and hence submerging the coastal areas and also lead to ecological unbalance.

MONTREAL PROTOCOL:
Montreal Protocol (MP) on substances that deplete the ozone layer was established to phase out the consumption and production of ozone depleting substances(ODP)within a specified time frame both for developed and developing countries, reviewed time to time based on the advice from Scientific Technology and Economic Assessment Panels. Montreal Protocol does not address non _ODS. The original control schedule is contained was a 50% reduction in emission of CFCs by the year 1998 compared to 1986 levels and grace period of 10 years for developing countries. The protocol has been adjusted and many ozone depleting chemicals were added in the period of 1990-1995.

KYOTO PROTOCOL:
The Kyoto-Protocol (KP) adopted at the third conference of parties on the frame work convention of the global climate changes in the Kyoto in 1997 has been decided to put HCFCs together with CO2, N2O,CH4, PFCs AND SF6 in the basket of controlled substances. The Kyoto Protocol aims at the reduction and control of GHG emission and it is obligatory only for developed countries. While developing countries have no such commitments.
Although CFCs , HCFCs contribute to global warming, the Kyoto Protocol does not address these substances since these are already controlled under Montreal Protocol. Montreal Protocol and Kyoto-Protocols are interconnected.

ALTERNATIVES TO CFCS:
All around the world there is tremendous effort to develop CFC alternatives. Any alternative has to develop CFC alternatives. Any alternative has to posses all usual desirable characteristics. In addition they should have zero Ozone Depletion Potential(ODP), relatively low Global Warming Potential (GWP) and they should not be a volatile organic compound.
Hydrocarbons(HCs) and hydroflurocarbons(HCFCs) provide an alternative to fully haloganated CFC refrigerants. They have zero ODP. However HCFCs have a level of GWP. Hence these also have to be phased out ultimately. Till then, they can be used as transitional refrigerants.

SUBSTITUTES FOR CFC 12:
It is considered to be the most preferred substitute for R12 in USA. It's Normal Boiling Point (N.B.P.) of -26.15 is quite close to R12's N.B.P.(-29.8). But its one of big disadvantage is that it has relatively high GWP. The use of oil in R134 a systems requires a very stringent quality control. It is not soluble in mineral oil. The polyester based synthetic oil that is used with it should be totally dry. Moreover, it is non-reactive with Cu-based materials of construction, winding enamel etc.

HYDROCARBONS:
Hydrocarbons have zero ODP and negligible GWP.
Earlier researches for alternatives had excluded hydrocarbons because of their flammability. However, hydrocarbons are readily available, much cheaper and thermodynamically very suitable. The use of hydrocarbons in domestic refrigerator does increase the risk when compared to CFCs. Therefore possible means of minimizing the risk must be attempted.
Hydrocarbon refrigerator was tested and a safety mark award certificate was issued stating that the product meets the safety requirement of their Equipment Safety Law. The published reports on the performance of HC refrigerator have been encouraging and no adverse effect with respect to safety have been reported so far.

R152a(DIFUROETHANE)
One more attractive alternative is R152a. Its GWP is less and it has zero depletion potential. Also it has lower energy consumption. Hence Environmental Protection Agency of Europe prefers R152a over R134a. It is flammable. Inconsistencies exist among efficiency results reported by various investigators. However, results appear comparable to CFC-12 and HFC-134a. Comprehensive thermal stability information are not available for HCF-152a, but no significant issues have been identified to date.

ALTERNATIVE REFRIGERANTS TO R-502
R-502 is very widely used for low temperature applications. R-502 is being azeotropic mixture with one of its constituents CFC-115 and thus it is to be phased out along with other CFCs. The leading alternative to R-502 are R-507,R-404a,
R-407. These are HCF based azeotropic mixtures with a small temperature glide.
The performance of various alternatives to R-502 have been studied and relative performance is given in the table

Test refrigerant Condensing Temp Evaporating Temp. System Capacity (KW) COP

R-502

40.6
-33.4
31.2
1.00

R-507
40.6

-35.8
31.2
0.92

R-404
40.6
-36.8
28.9
0.90

R-407
40.6
-43.0
18.5
0.80

ALTERNATIVE REFRIGERANTS TO HCFC-22
HCFC-22 is very widely used refrigerant both for refrigeration and air conditioning. HCFC-22 is also considered as an alternative to CFCs for many applications. However HCFCs are also controlled substances due to their ozone depleting potential. The alternative refrigerants to HCFC-22 are discussed in the following table.

Refrigerant N.B.P Characteristics
R-23 -82 Critical temp. is too low
R-32 -51.7 Flammable
R-125 -48.1 High GWP, low theoretical efficiency
R-143a -47.2 Flammable, high GWP
R-134a -26.1 Low volumetric capacity
R-152a -24 Flammable

As shown in table R-32 has good properties as a good refrigerant, such as a high latent heat and a high thermal conductivity. But it is flammable and has high vapor pressure.

AMMONIA AS A REFRIGRANT:
In future, the environmental impact and higher cost of CFC's or their alternatives would favor the use of ammonia. COP of ammonia is 3% better than R-22 and 7% better than R-50. Ammonia's sensible heat capacity and conductivity are 4 to 5 times better than R-22 and R-12 and it is marginally less viscous. Thermodynamic and thermophysical properties of ammonia is nearly as a perfect refrigerant. Ammonia's physical and transport properties are much better than R-134a and it is recognized as a heat transfer fluid of very high order.
Advantages:
The advantages of low molecular weight of ammonia is of very high latent heat capacity. Thus, very less mass flow are required
to provide a given refrigeration effect. Ammonia has zero global warming potential & offers superior thermodynamic properties.
Disadvantages:
The low molecular weight of ammonia results pumping capacity for the compression equipments.
Ammonia has a relatively high specific heat ratio, therefore the compression heat results in very high discharge temperature of the vapour particularly at high pressure ratios.
One of the most serious drawback is ammonia`s toxicity. It is dangerous in excessively high concentrations & is considered as a hazardous chemical. Ammonia is also moderately flammable.


CRYOCOOLERS:
As CFC & HCFCs compounds have to be phased out a need has been created for a replacement refrigerant or a new cooling device. The stirling cycle , though already invented as an effective prime
mover, has become widely used in the cryogenic industry to obtain low temperatures & for production of liquid gases since the cycle uses only inert gas its working fluid, the environmental issues concerning the refrigeration are avoided altogether. It is well known that at low temperatures, the stirling cycle is superior to vapour compression cycle from an efficiency stand point.
Stirling cycle was invented by a Scottish minister Robert stirling for use in a hot air engine. Attempts have been made by investigator to revive stirling cycle as a refrigeration device. The best overall performance achieved so far is 35% of carnot. With continued development it is expected that at least 40% of carnot is easily within reach & greater than 50% may also be attained.
Efforts towards development of long life domestic refrigerator stirling cycle coolers are necessary and such coolers have a tremendous potential as a replacement for CFC based refrigeration system. Long life coolers will find variety of applications such as in hospitals, refrigerated transportation, electronics etc.

CONCLUSION:
As CFC`s & HCFC`s are to be phase out, so best suitable alternative refrigerants should be made available as early as possible. Various tests and investigations are being carried out and the property information is readily available for new refrigerants.
Research has to be done to find best alternative refrigerants. The future refrigerant should be low cost, early availability, high COP, non flammable, non toxic and most important low GWP & zero ODP.