Domestic Refrigerators-recent Developments

5/27/2018 Domestic Refrigerators-recent Developments

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E L S E V I E R

Int J. ReJr ig V ol. 19, No . 1, pp. 61 69, 1996Copyright c~ 1996. Published by Elsevier Science Ltd a nd IIRPrinted in G rea t Britain. All rights reserved0140 -7007 (95)00 069-0 0140-7007/96/$15.00 + .00

] R E V I E W P A P E R ]D o m e s t i c r e fr ig e r a to r s : r e c e nt de v e lo pm e nt s

R . R a d e r m a c h e r a n d K . K i m *C e n t e r f o r E n v i r o n m e n t a l E n e r g y E n g i n e e ri n g , U n i v e r s i t y o f M a r y l a n d , C o l le g e P a r k ,M D 2 07 42 , U S AThe refrigerator/freezer is one of the most im por tant an d the biggest energy-consuming ho me appliances.There are several literature references that discuss th e historical developm ent of refrigeration I~ lg. M ost o fthem, however, consider historical highlights up to several decades ago. This paper summarizes recentdevelopments in the field of domestic hou sehold refrigerators based o n a survey of publications and patents.Keywords: Refrigerator;dom estic review; refrigerants, cycles, charge optimization)

R6 f r ig 6 r a t e ur s do m e s t ique s : m i s e s a u po in t r 6 c e nt e sLe r~frig a teur-cong~lateur dom est ique es t I 'une des applications dom est iques les p lus impo rtantes e t les p luscon som ma trices d'Onergie. Plusie urs pub lication s (vo ir bibliographie 1 1 4) {tudie nt l 'histoire d u Jroid. Laplupart d ' entre e l les fo nt remo nter les grands m om ents h is tor iques i t p lus ieurs d~cennies . L 'ar t ic le r{su me h , sm i s s au point rdcentes d ans le dom aine des rOfrig~rateurs domest iqu es bas~s sur l ' k tude des publ icat ions e t desmar que s .(Mots-cl~s: ???;????)

In t he be g i nn i ngF r o m W i l l i am C u l l e n o f t h e U n i v e r s it y o f G l a s g o w ,S c o t l a n d , w h o w a s r e c o r d e d a s t h e f i r s t p e r s o n t od e m o n s t r a t e t h e m a n - m a d e p r o d u c t i o n o f c o l d w h e n h ee v a p o r a t e d e t h e r i n 1 7 4 8 , t o J a c o b P e r k i n s w h od e v e l o p e d t h e f i rs t p r a c ti c a l r e f r i g e r a t i o n m a c h i n e u s i n ga v a p o u r c o m p r e s s i o n c y c le in L o n d o n w i t h e th e r a s t h er e f r i g e r a n t i n 1 83 4, t h e r e f r i g e r a t i o n i n d u s t r y d e v e l o p e ds t e a d i l y . W h e n e l e c t r i c i t y b e c a m e g e n e r a l l y a v a i l a b l e ,W i l l i a m F . S i n g e r o f N e w Y o r k , p a t e n t e d t h e e a r l ie s ta u t o m a t i c e l e c t r i c u n i t f o r s m a l l - s i z e r e f r i g e r a t i n gsystems in 189715A s e l e c t r i c - g e n e r a t i n g c a p a c i t y g r e w a n d a s h o m e sw e r e b e g i n n i n g t o b e w i r e d f o r i t s u s e , h o u s e h o l dr e f r i g e r a t o r s b e c a m e m o r e p o p u l a r a n d b e g a n r e p l a c -i n g t h e c o m m o n w i n d o w a n d s t a n d i n g i c e b o x e s . T h ei n t e r e s t a n d d e m a n d f o r h o u s e h o l d r e f r i g e r a t o r s w a sa i d e d b y t h e d e s i g n a n d d e v e l o p m e n t o f f r a c ti o n a lh o r s e p o w e r m o t o r s , w h i c h w e r e u s e d i n r e f r i g e r a t o r s .T h e s e u n i t s b e g a n b e i n g p r o d u c e d i n l a r g e n u m b e r si n t h e e a r l y 1 9 2 0 s a n d h a v e b e c o m e a n e c e s s i t y f o rall .I s k o w a s p r o b a b l y t h e f i r s t r e a s o n a b l y s u c c e s s f u l a i r -c o o l e d u n it . F r e d W . W o l f d es i g n e d a n d m a r k e t e d ah o u s e h o l d s y st e m c a ll ed D O M E L R E , a c o n t ra c t io n o fD o m e s t i c _ _E le ctric R e f r i g e r a t o r . T h e W o l f s y s t e m w a sm a r k e t e d b y M e c h a n i c a l R e f r i g e r a t o r C o m p a n y a n dl a t e r b y I s k o u n t i l a b s o r b e d b y F r i g i d a i r e i n 1 9 2 2 . B u tt h e m o s t i m p o r t a n t t e c h n i c a l c o n t r i b u t i o n s w e r e m a d e

* Present address: L ivingSystems R&D Center, Sam sung ElectronicsCo. Ltd , 416 Maetan-3Dong, Suw on 442-742, Korea

b y G e n e r a l E l e c t ri c a n d t h e K e l v i n a t o r . G e n e r a l E l e c t ri ch a d b e g u n t o m a n u f a c t u r e t h e A u d i f f re n m a c h i n e in191116 and i n Februa ry o f 1918 , Ke lv ina to r so ld i t s f i r s tr e f r i g e ra to r 17. On e o f t h e f i rs t p rac t i c a l au tom a t i cc o n t r o l s w a s t h e t h e r m o s t a t i c s w i t c h d e v e l o p e d b yC o p e l a n d f o r t h e f ir s t K e l v i n a t o r s ~8. T h e s e a l ed u n i t ,w h i c h e l i m i n a t e d t h e b e l t , w a s i n t r o d u c e d i n 1 9 2 5 b yG en era l Elec t r ic 16 1s.A dvanc e s i n m at e r i a l s and de s i gnA s p l a s t i c s a n d t e c h n i q u e s i n w o r k i n g p l a s t i c s , a n d n e wi n s u l a t i n g m a t e r i a l s , w e r e d e v e l o p e d , t h e s m a l l d o m e s t i cr e f r i g e r a t o r b e c a m e h i g h l y s o p h i s t i c a t e d i n d e s i g n a n dc o n s t r u c t i o n , a n d t h e r a t i o o f u s e f u l s t o r a g e c a p a c i t y t ot o t a l v o l u m e i n c r e a s e d .A g r e a t i m p e t u s t o r e f r i g e r a t o r d e s ig n w a s p r o v i d e d b yt h e i n t r o d u c t i o n o f t h e h a l o g e n r e f r i g e r a n t s w i t h w h i c hn o n - f e r r o u s m e t a l s c o u l d b e e m p l o y e d . T h e f l u o r o c a r b o nr e f ri g e r an t s w e re a n n o u n c e d b y M i d g l e y a n d H e n n e i na91 93 0 a n d t h e i n t r o d u c t i o n o f d i c h l o r o d i f l u o r o m e t h a n e( R 1 2 ) a s a c o m m e r c i a l r e f r ig e r a n t i n 1 9 3 1 . F r o m t h e l a te1 9 20 s, a n e x t r e m e l y s m a l l b o r e w a s c o n s i d e r e d t o r e d u c et h e p r e s s u r e o f s u l p h u r d i o x i d e ( R 7 6 4 ) . W i t h t h ei n t r o d u c t i o n o f t h e p h y s i c a l l y s a f e , o i l - s o l u b l e , h a l o -g e n a t e d h y d r o c a r b o n s a s w o r k i n ~ f l u i d s , t h e a p p l i c a t i o n20 1o f c a p i l l a r y t u b e s i n c r e a s e d ' ~ . B e f o r e t h a t t i m ec l o g g i n g o f t h e c a p i l l a r y tu b e p o s e d a c h a l l e n g e a n du s u a l ly a n e x p a n s i o n v a l ve w a s u s e d . T h e i n t r o d u c t i o n o fR 1 2 r e m o v e d a n i m p o r t a n t o b s t a c l e t o c o m p l e t ea c c e p t a n c e o f d o m e s t i c m e c h a n i c a l r e f r i g e r a t i o n , f ore a r li e r m a n u f a c t u r e r s h a v e b e e n c o m p e l l e d t o d e p e n d o nr e f r i g e r a n t s s u c h a s s u l p h u r d i o x i d e ( R 7 6 4 ) , m e t h y l

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chloride (R40), and methyl formate (R611) and dichloro-methane (R30). All of these fluids are toxic.During the 1930s the domestic refrigerator wasstandardized. The hermetic compressor became thenorm, and with its adoption, rotary compressors beganto substitute the older reciprocating compressor tech-nology. Expansion valves were replaced by capillarytubes. The refrigerators, all self-contained, were built o fsteel and were well insulated. Finally, the mechanism wasplaced at the bottom of the unit.A more important development was the two-temper-ature refrigerator, which was introduced about 1939 andbegan to come into its own in the post-war period. As itsname implied, it consisted of two separate compartmentslike the current refrigerator/freezer unit 22.Between the 1950s and 1960s, domestic refrigeratorschanged considerably from the early designs. Multiplecompartments had made them more complex. Designshad been refined to meet exacting customer demands.Acceptance levels had been raised considerably23. In1944 almost 70% of American homes tha t hadrefrigerators were equipped with 'mechanicals 24 and in1958 94% of households owned refrigerators in theU.S. 25.

E n v i r o n m e n t a l c o n c e r n sIn 1974, Rowland and Molina 26 advanced the hypothesisthat antrhopogenic emission of certain chlorinated andbromate compounds, particularly chlorofluorocarbons(CFCs) and hydrochlorocarbons (HCFCs), couldaccumulate in the stratosphere and substantially depletethe ozone layer that shields the earth from cancer-causing ultraviolet-B solar radiation. Within a few yearsthe issue had moved onto the public policy agenda, andthe U.S. Congress in 1978 banned the use of CFCs asaerosol propellants, which accounted for the majority oftotal CFC emissions in the U.S. at the time27. Since theozone layer depletion is a global problem, an inter-national treaty to regulate the production and trade ofthe ozone-depleting substances, the Montreal Protocol,was signed by 24 nations and the European Com-munity 28'29. It required the U.S. and other signatories toreduce production of CFCs to 50% of 1986 levels by1998, and placed no restrictions on the production ofHCFCs. Refrigerator/freezers were one of the majortechnologies dependent on R12 as a refrigerant.

Besides contributing to the destruction of strato-spheric ozone, the Antarctic ozone hole and significantozone reductions in the Arctic, CFCs have beenimplicated as a major anthropogenic cause of globalwarming. As refrigerants are lost through leaks, equip-ment maintenance and retirement, they dispersethroughout the atmosphere and act as greenhousegases and contribute to global warming. In addition tothe accumulation of CFCs in the atmosphere, the carbondioxide emissions associated with energy used to operaterefrigeration equipment, such as the domestic refrigera-tor, reflect the greenhouse warming effect. Almost all thisenergy results from the combustion of fossil fuels andcreates emissions of CO2. Carbon dioxide is the largestcontributor to global warming3. Since refrigeratorsconsume about 2.9 1011 kWh of primary energy, or12% of the total residential energy budget annually,substantial improvements in domestic refrigerator/

freezer efficiency extended over the 15- to 20-year lifeof this appliance would significantly~l benefit nationalgoals for environmental progress .T e s t r e s u l t s w i t h n e w r e f r i g e r a n t sWhile new refrigerants were being developed and testedin response to the environmental challenges, one possiblesolution that was proposed for alterantive refrigerantsfocused on using zeotropic refrigerant mixtures, whosecomponents were environmentally safe and theircharacteristics as drop-in refrigerants appeared to bepromising.From the 1960s numerous publications throughoutthe world explored the use of refrigerant mixturesprimarily with two objectives in mind: (1) achieving alow evaporator temperature with a moderate pressureratio during single-stage compression and (2) conservingenergy when the refrigeration duty consists of cooling afluid stream through a large temperature range 32 37.

However, the refrigerant most commonly consideredas the future substitute for CFC12 in domestic equip-ment was HFC 134a. Many calculations based on simpleRankine cycle models, however, have concluded thatHFC134a is less energy efficient than CFC123s. Theseclaims were confirmed by experimental results whichshowed efficiencies for HFC134a which were 4-10% lessthan those for CFC1239'4.In order to overcome difficulties of pure alternativerefrigerants, a number of binary and ternary mixtureswere suggested34'41 43. Among the proposed fluids wasalso a mixture of R22 and R142b, with the latter being aflammable component. At this time, a mixture contain-ing R22 was still acceptable and had the advantage ofbeing compatible with the familiar alkylbenzene lubri-cant. Moreover, since the percentage of a flammablecomponent was relatively low, the flammability problemwould be eliminated. These mixtures had 'drop-in'characteristics for use as a substitute in existingrefrigerator systems. Test results showed that some39 44 46mixtures ' - consumed less energy when the systemwas optimized in terms of the length of the capillary tubeand the amoun t of charge while others consumed slightlymore energy3942 .ure fluorinated hydrocarbons

Subsequent revisions o f the Montreal Protocol requiredthe eventual phaseout of HCFCs as well as CFCs, andspecific schedules were established by statute in eachsignatory country. The most likely replacements at thistime are HFCs, although hydrocarbons and ammoniaare being considered because they also have a zero ozonedepletion potential (ODP). Further, modelling resultsdemonstrated that with appropriate superheat andsubcooling taken into consideration, HFC134a canprovide COP values essentially equivalent to those ofR1247.In terms of implementation however, manufacturerswere faced with significant challenges. In order to achieve

the commonly observed long lifetime of refrigerators,manufacturing standards have to be tightened consider-ably. R134a systems tolerate far less contaminants thanR12 systems did.At the same time, R152a was another potential


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replacement candidate for R12. It has a very low globalwarming potential (GWP) value compared to othercandidates. The flammability of R152a, however, com-pared to the nonflammabili ty of R134a is a majordrawback. Theoretically, it is expected that the energyconsumption of R152a is about 3-4% lower than that ofR134a. However, experimental results show that themeasured performance of a refrigerator charged withR152a and that of an equivalent system charged withR134a (with the same compressor's energy efficiencyratio, EER) did have essentially the same perform-ance4s'49. It was confirmed that the ester oils have goodmiscibility, insulating characteristics and reliability indurabili ty tests 5. However, the flammability aspecttogether with the liability concerns of refrigeratormanufacturers has not led to an application of R152a.HydrocarbonsNaturally occurring substances such as carbon dioxide,ammonia and hydrocarbons are considered to beenvironmentally safe refrigerants. An alternative solutionmay be provided by hydrocarbons which offer thepossibility of a low cost (at least for the refrigerantitself), and which are readily available and environ-mentally benign alternatives to CFCs and CFC sub-stitutes. The absence of chlorine results in zero ozonedepletion potential, and very low GWP. The transportproperties of hydrocarbons, e.g. propane (R290), aresuperior (lower viscosity, higher thermal conductivitythan other alternatives) while thermodynamic propertiesare similar to those of CFC12 and 22. Anotheradvantage of hydrocarbons is their solubility in mineraloil, which is traditionally used as a lubricant incompressors. The major drawback is their flammability,although it was suggested they would be unlikely tocreate an ignitable atmosphere in the refrigerator due tothe small charge quantity51-54.

Currently, the use of isobutane in domestic refriger-ators and freezers is well established in Europe forrefrigerators that have no automatic defrost. Studieshave shown energy savings with hydrocarbon refriger-ants 55'56. Experimental results with pure propane 55- orisobutane 57 as a drop-in substitute showed up to 2%improvement while tests with cyclopropane showedenergy savings of 6 o / o 4 6 . Binary mixtures of hydrocarbonrefrigerants showed improvements of up to 10% foroptimized systems58. Further, a ternary mixture with17% energy savings in the modified Lorenz-Meutznercycle proved to be better than the binary mixtures59.System and component efficiency improvementsStudies to measure the effect of different usage conditionson energy consumption revealed the sensitivities ofthermal performance and energy use to differentvariables60. Field test results with a highly efficienthermetic compressor utilizing a four-pole permanentsplit-capacitor motor showed that the energy-savingcompressor did permit a refrigerator unit to operate onless power than a standard compressor61 .The U.S. Congress established the National ApplianceEnergy Conservation Act (NAECA) in 1987 andrequired the Department of Energy to consider new oramended standards for refrigerators and freezers. In

order to meet the proposed standards several researchershave evaluated design options for improving the energyefficiency o f domestic refrigerator/freezers49,62-65. A fieldperformance test was conducted to compare the refrig-erator field performance with laboratory test resultsbased on the power consumption of 209 refrigerators31.Typical options to improve system efficiency ofconventional refrigerators were suggested in four generalareas: (1) improving the refrigeration cycle efficiency; (2)decreasing the cabinet heat load; (3) reducing parasiticelectrical loads; and (4) reducing on/off cycling losses. Arecent computer model that considers the entire systemdemonstated that the energy consumption for a20 ft3(5701) refrigerator with a top-mounted freezer andno through-the-door features could work out to con-sume.equal to or less than 1.00kW hday -~. It is notedthat each improvement exacts a penalty in terms ofincreased cost or system complexity/reliability 2'64.

ycling lossesThe cycling losses are defined as the difference betweenthe energy consumption of a system with a continuouslyoperating compressor and a system with a cyclingcompressor both having the same operating temper-atures and the same cooling load. The on/o ff cyclingmode of a refrigerator results in cycling losses whichdepend on the cycle length and the dimensions of theheat exchangers. The experimental studies66'67 indicatedtha t preventing refrigerant migration during the off cyclewould reduce the power consumption by 4% andincrease the cooling capacity. Some compressor com-panies introduced a liquid-line shut-of f valve (energy-saving valve) which operates without consuming anypower. It is operated through the .P6ressure change whenthe compressor switches on and off 8, and a fluid controlvalve which operates by using a peak electrical currentinduced when the compressor motor is switched on andof f 69InsulationIn the 1950s, urethane foam had been developed from alaboratory curiosity to an item o f commerce, and early in1960s rigid urethane foam produced with fluorinatedhydrocarbon expanding agents, such as Rl l and R12was introduced70 and had been accepted rapidly until the1996 ban on the manufacturing of foam-blowing agentscontaining CFCs. Under the amended Montreal Proto-col new suggestions and techologies to substitute thecurrent CFC-based technology were discussed71'72. It islikely that cyclopentane may be one of the nonfluoro-carbon options that may have a similar performance ascompared to CFC blowing agents in terms of energyefficiency. Although there is a penalty. As anotheralternative, FICs (flouro-iodine-carbons) are discussed.These fluids are environmentally safe because of theirshort atmospheric lifetime. In fact, they potentiallydecompose under the influence of sunlight. However,as long as they are used within a foam that is entirelyenclosed by metal and plastic, they may be quite feasible.Vacuum insulation, as part of foam insulation, was seenas an option with commercial potential. Initially anumber of concerns had to be addressed, such asvacuum reliability, and the weight of the insulation


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64 R Radermacher and K Kim

m a y p o t e n t i a ll y b e a d r a w b a c k . M o s t o f t h es e w e r ea d d r e s s e d a n d v a c u u m i n s u l a t io n i s n o w i n p r o d u c t i o n .H o w e v e r , t h e l a s t r e m a i n i n g c o n c e r n i s th e c o s t .

Defrost methodsE v a p o r a t o r s o f r e f ri g e r a t o r s m u s t b e d e s i g n e d t o w i t h -s t a n d t h e b u i l d - u p o f fr o s t, a n d f o r e as e o f d e f r o s t i n g b yt h e r m a l o r m e c h a n i c a l m e a n s . A s f r o s t a c c u m u l a t e s , i tc a n d e c r e a s e t h e r a t e o f h e a t t r a n s f e r i n t w o w a y s : (1 ) t h ef r o s t m a y a c t a s a n i n s u l a t o r t o h e a t f l o w ; a n d ( 2 ) t h ef r o s t g r o w t h w i l l d e c r e a s e t h e a m o u n t o f a i r f lo w o v e r t h ec o i l a n d t h u s d e c r e a s e t h e h e a t t r a n s f e r . T h e e f f ec t o ff r o s t o n t h e o v e r a l l h e a t t r a n s f e r c o e f f i ci e n t a n d t h e a i r -s i d e p r e s s u r e d r o p w a s e v a l u a t e d e x p e r i m e n t a l l y 73'74.W h e n t h e a i r fl o w ra t e w a s k e p t c o n s t a n t , t h e U A - v a l u ew a s f o u n d t o i n c r e a se s t e a d i l y o v e r a 1 0- h p e r i o d a s f r o s tw a s d e p o s i t e d o n t h e c o i l . I t w a s d e t e r m i n e d t h a t t h ei n c r e a s e w a s d u e t o a n i n c r e a s e d a i r - s i d e h e a t t r a n s f e rc o e f f ic i e n t a n d a n i n c r e a s e i n t h e s u r f a c e a r e a o f t h ee v a p o r a t o r d u e t o t h e s u r fa c e r o u g h n e s s o f th e f r o s t. F o rr e f r i g e r a t o r / f r e e z e r s e q u i p p e d w i t h v a r i a b l e d e f r o s tc o n t r o l s y s t e m s , a c a l c u l a t i o n m e t h o d o f t h e d a i l ye n e r g y c o n s u m p t i o n w a s e v a l u a t e d a n d s u g g e s t e d 75.C u r r e n t l y , t h e p r e f e r r e d m e t h o d f o r d e f r o s t i n g ar e f r i g e r a t o r e v a p o r a t o r i s t o u s e a n e l e c t r i c a l h e a t e r t oh e a t t h e e v a p o r a t o r i t s e l f , t h e s u r r o u n d i n g s u r f a c e s a n dt h e a i r o f t h e e v a p o r a t o r c o m p a r t m e n t , w h i c h u s u a l l ye l i m i n a t e s a l l f r o s t c o m p l e t e l y . I n t h i s c o n v e n t i o n a lt e c h n o l o g y , th e d e f r o s t h e a t e r i s o p e r a t e d b y a t i m e r a n df o r m o s t c l im a t e s it m a y c o m e o n m u c h m o r e o f t e n t h a nw o u l d b e r e q u i r e d . T o a v o i d t h i s , s o - c a l l e d a d a p t i v ed e f r o s t c o n t r o l s d e t e r m i n e w h e n i t i s n e c e s s a r y t o i n i t i a tet h e d e f r o s t c y c l e . I n t h i s w a y , t h e d e f r o s t h e a t e r i s o n l yt u r n e d o n w h e n n e c e s s a r y . T h i s m e t h o d i s a l r e a d ya p p l i e d i n t h e s o - c a l l e d S E R P r e f r i g e r a t o r , a n e n e r g ye f f ic i e nt r e f r i g e r a t o r t h a t o n l y r e c e n t l y w a s i n t r o d u c e din to th e m a rk et 65'76'134'135.

Charge optimizationC h a r g e m i n i m i z a t i o n i s a n i m p o r t a n t f a c t o r i n t h e d e s ig no f r e f r i g e r a t i o n s y s te m s , n o t o n l y t o r e d u c e t h e c o s t a n dt o a l l e v i a te t h e f l a m m a b i l i t y o r t o x i c i t y p r o b l e m s o fm a n y o f th e s u g g e s t e d a l t e r n a t i v e r e f r ig e r a n t s , b u t a l s ot o i m p r o v e t h e p a r t - l o a d e f fi c ie n c y o f e q u i p m e n t a s t h er e f r i g e r a n t c h a r g e b e c o m e s s m a l l e r . N u m e r o u s i n v e s t i -g a t i o n s w e r e d e s c r i b e d b a s e d o n t h e a s s u m p t i o n o f a z e r os li p 77 a n d a k n o w n v o i d f r a c t i o n 78 i n t he two-phaser e g i o n s i n h e a t e x c h a n g e r s . S i m p l e c o r r e l a t i o n s f o r t h ed e p e n d e n c e o f t h e o p t i m u m r e f r ig e r a n t c h a r g e o n t h ec a p a c i ty o f e v a p o r a t o r a n d c o n d e n s e r w e r e s u g g e s te d f o r79d o m e s t i c r e f r i g e r a t o r u n i t s . A s t u d y a b o u t th e i n f l u e n c eo f t h e a m b i e n t t e m p e r a t u r e o n t h e r e f r i g e r a n t c h a r g e80n e c e s s a r y i n r e f r i g e r a t o r u n i t s w a s p e r f o r m e d . D e t e r -m i n a t i o n o f t h e co r r e c t c h a r g e f o r n o r m a l o n / o f fo p e r a t i o n m o d e s w a s c a r r i e d o u t a t a l l a m b i e n tt e m p e r a t u r e s .

FansW h i l e c o n s i d e r a b l e p r o g r e s s h a s b e e n m a d e i n t h e o v e r a l le n e r g y e f f i c ie n c y i n r e f r i g e r a t o r s w i t h r e g a r d t o c y c le sa n d t h e c a b i n e t i n s u l a t i o n m a t e r i a l s , i t s h o u l d n o t b e

ove r look ed t h a t s i gn i f i c an t e f f ic i ency ga ins a re po ss ib l ef r o m i m p r o v e d c o m p o n e n t s .F o r e x a m p l e , t h e f a n s t h a t c i r c u l a t e a i r a c r o s s t h ec o n d e n s e r a n d t h e e v a p o r a t o r c o n t r i b u t e s i g n i f i c a n tl y t ot h e o v e r a l l p o w e r c o n s u m p t i o n . H e r e t h e e v a p o r a t o r f a ni s e s p e c ia l ly o f i m p o r t a n c e , s i nc e i ts p o w e r c o n s u m p t i o nl eads t o a doub le pena l t y . The en t i r e e l ec t r ic i npu t t o t h i sf a n i s c o n v e r t e d i n t o h e a t e v e n t u a l l y t h a t h a s t o b er e m o v e d b y t h e c o m p r e s s o r , i n c r e a s i n g th e c o o l i n g l o a d .M a n u f a c t u r e r s a r e n o w i n t h e p r o c es s o f i n t r o d u c i n gh i g h - e ff i c ie n c y f a n m o t o r s t h a t r e d u c e f a n p o w e rc o n s u m p t i o n fr o m o ri g in a l ly 1 0 - 1 5 W t o a b o u t 2 5 W ,a q u a n t i t y t h a t i s a l m o s t n e g l i g i b l e . A n i n t r o d u c t o r ya r t i c l e a b o u t t h e n e w E C M m o t o r s f o r t h e a p p l i a n c eindus t ry i s g iven i n r e f s . 81 and 82 .I n a d d i t i o n t o t h e b e t t e r f a n m o t o r s , i m p r o v e dcon t ro l s c an l e ad t o s i gn i f i c an t ene rgy sav ings a s we l l .O n e e x a m p l e i s g i v e n b e l o w w i t h t h e T a n d e m c y c l e .O t h e r s p e r t a i n t o b e t t e r d e f r o s t m e t h o d s . T h e a b o v e -m e n t i o n e d a d a p t i v e d e f r o s t c o n t r o ls a r e a n o t h e r m e a n so f r e d u c i n g e n e r g y c o n s u m p t i o n .CompressorsN e w c o m p r e s s o r d e v e l o p m e n t s a r e l e a d i n g t o c o n s i d e r -a b l e e f fi c ie n c y i m p r o v e m e n t s . F o r e x a m p l e , w o r k i sp r o c e e d i n g o n a c o n t i n u o u s b a s i s o n t h e v a l v e s a n de s p e c i al l y o n t h e e l e c tr i c m o t o r s . A n y i m p r o v e m e n t s f o rt h e l a t t e r p r o v i d e s f o r d o u b l e b e n e f i t . F i r s t , t h e p o w e rr e q u i r e m e n t i s r e d u c e d a n d s e c o n d l y t h e r e f r ig e r a n t t h a ti s u s e d t o c o o l t h e m o t o r i s l e s s s u p e r h e a t e d . T h i sinc rea se s c apac i t y and e f f i c iency a s w e lP 7 .R e c e n t l y a l i n e a r c o m p r e s s o r w a s i n t r o d u c e d t h a t h a sthe advan t age o f a ve ry e f f i c i en t l i nea r e l ec t r i c moto r .C o m b i n e d w i t h o t h e r i m p r o v e m e n t s i n v a l v i n g a n d t h ef a c t t h a t i t i s o il - fr e e ( n o o i l p u m p n e e d e d ) p r o v i d e s f o radd i t i on a l e f f i ci ency ga ins 83.Refrigeration c ycle alternativesT h e g r e a t e s t c h a l l e n g e s f o r t h e d e s i g n a n d d e v e l o p m e n to f r e f r i g e r a t o r / f r e e z e r s y s t e m s a r e t h e r e d u c t i o n o ft h e r m o d y n a m i c i r r e v e r s i b i l i t i e s r e s u l t i n g f r o m a ni n e f fi c ie n t o p e r a t i o n o f r e f r i g e r a t i o n c y c l e a n d t h ei n t r o d u c t i o n o f e n v i r o n m e n t a l l y s a f e r e f r i g e r a n t s . T h ef o l l o w i n g o p t i o n s w e r e i n v e s t i g a t ed .The Lorenz M eutzner cycleT h e L o r e n z - M e u t z n e r c y c le i s t h e m o s t c o n s e q u e n tm e a n s t o e x p l o i t t h e i n h e r e n t t h e r m o d y n a m i c a d v a n -t a g e s o f t h e t e m p e r a t u r e g l id e o f z e o t r o p i c m i x t u r e s .S i n c e L o r e n z a n d M e u t z n e r c l a i m e d t h a t a t w o -e v a p o r a t o r , t w o - s u b c o o l e r r e f r i g e r a t o r / f r e e z e r h a ds h o w n e n e r g y s av i n gs u p t o 2 0 % u s i n g a R 2 2 / R l lm i x t u r e i n t h e 1 9 7 5 I n t e r n a t i o n a l C o n g r e s s o f R e f r i g e r -a t i on ( I I ) 84, seve ra l s t ud i e s we re con du c t ed wi the n v i r o n m e n t a l l y b e n i g n r e f r i g e r a n t m i x t u r e s 8 5-9 3 a n dw i t h h y d r o c a r b o n m i x t u r e s 94.E x p e r i m e n t a l t e s t s w e r e r e p o r t e d t h a t a r e f r i g e r a t o rc o n v e r t e d t o t h e L o r e n z - M e u t z n e r c y c l e s h o w e d a 9 %r e d u c t i o n i n e n e r g y c o n s u m p t i o n w i t h a z e o t r o p i cm i x t u r e o f R 2 2 / R 1 2 3 c o m p a r e d w i t h a n o r ig i n a lp r o d u c t i o n u n i t o f a 2 0 f t 3 , s i d e -b y - s i d e u p r i g h t r e f ri g e r -a t o r 94 w h i l e c o m p u t e r s i m u l a t i o n r e s u l ts p r e d i c t e d a n


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ome st ic refrigerators 65improvement of the coefficient of performance (COP) by16-20%. In 1993 a modified version of the Lorenz-Meutzner cycle was patented by Radermacher andJung95. In the modified cycle, the freezer and foodcompartment evaporators serve as three-way heatexchangers. They are essential conventional units witha small tube (liquid-line) inserted into a larger tube(suction-line). The compartment air surrounding thelarger tube and subcooled liquid in the smaller tube rejectheat to the two-phase refrigerant flowing counterflowthrough the annulus of concentric tubes. Experimentaltest results were reported with 16.5% energy savings fora modified Lorenz-Meutzner cycle refrigerator with anHFC refrigerant mixture96, and 17.3% energy savingswith a hydrocarbon ternary mixture, propane/n-butane/pentane R290/R600/n-c5) TM A new investigation intoproviding an independent temperature control of thecompartments is made in a modified Lorenz-Meutznercycle which was tested using a bistabile solenoid valve 97.Dual-loop systemThe best way to reduce the thermodynamic irreversi-bilities resulting from the operation with a singleevaporator in domestic refrigerator/freezers is toemploy two separate refrigeration cycles. This so-calleddual-loop system has two completely separate refrigera-tion cycles which provide cooling for the freezer and foodcompartments independently. The efficiency for thissystem may be lower than theoretically expected. Theefficiency degradation is due to the use of twocompressors, instead of one, that are generally smallerthan the original one and that have generally a lowerefficiency. The major disadvantage with this system iscost. The increased volume due to the two refrigerationsystems would either increase the product's externaldimensions or decrease usable refrigerator volume. Sometheoretical and experimental results were published. Thetheoretical results predicted energy savings of typically20% 90 91 while the experimental work showed 16 % 98,99and 4% with a potential for 20% if better compressorswere available ll .Two-stage systemThere are several two-stage configurations under investi-gation. Generally speaking, this system has one con-denser, two evaporators, two compressors and at leastone suction-line heat exchanger. Two versions of a two-stage system for domestic refrigerator/freezers werepatented 11'12. The major gain of this system is a smallerwork requirement resulting from the low-pressure ratiofor each of the two compressors. Based on the same totalcooling capacity, this system promises an improvementof 48.6% in theory over the single-stage system,Control valve systemThe control valve system has two evaporators but onlyone compressor and one condenser. Two different-lengthcapillary tubes and a control valve are installed betweenthe food and freezer evaporator inlets and the condenseroutlet. Refrigerant flow is directed to either the freezerevaporator or the food evaporator according to thetemperature of each compartment while the compressor

is in operation. Experimental results by using a solenoidvalvel03j 4 show the energy savings over the single-stagesystem.Ejector refrigeratorOne of the intrinsic losses in the vapour compressionrefrigeration cycle is the throttling of refrigerant in thecapillary tube. In the ejector refrigeration system theenergy wasted in the capillary tube is used, at least tosome extent, productively by means of an ejector whichallows the raising of the suction pressure entering thecompressor. The performance of the ejector cycle witha varying cooling ratio (ratio between food compart-ment and freezer load) and a comparison of severalrefrigerants were analysed 15. An ejector expansionrefrigeration cycle with one evaporator was patentedand this system was expected to reach up to 20%improvement for refrigerator applications 16. Anotherejector-enhanced refrigeration cycle with two evapor-ators was suggested and the simulation results show anincrease of up to 12.4% in COP over the single-stagerefrigerator 107.Tandem systemA conventional refrigerator/freezer system which con-trols the temperature of each compartment by using athermo-damper was suggested ls. This system operateswith no thermodynamic advantages while it has anenergy-saving effect only for the defrost cycles which usethe food-compartment air during the off-cycle. Mean-while, a high-efficiency, automatic-defrost refrigerator/freezer which consists of a forced convection freezerevaporator and a natural convection food evaporatorconnected in series was demonstrated 19 111. The fieldtest o f this system showed energy savings of 58%compared with a baseline unit resulting from utilizingadvanced design features such as optimized thick wallinsulation and two evaporators. Recently, the tandemsystem which is a new technique for using twoevaporators and two fans was developed. This systemtakes advantage of the evaporator fan control such thateach evaporator fan operates one at a time while thecompressor is in operation. The two evaporators areconnected in series and at the beginning of thecompressor on-time, the food compartment fan isturned on first. Thus, the food compartment is suitablycooled before the system reaches steady state. At thistime, the system turns off the food compartment fan andturns on the freezer fan. As a result, this system utilizesthe pull -down period of each cycle, which is generally notsuitable for cooling, to cool the food compartment. It ismore efficient and provides substantial energy savings ofup to 18% as compared to the baseline unit ofconventiona l design. In addition, a new internal defrostmethod by using a thermosyphon phenomenon betweentwo evaporators was introduced j 12Alternative refrigeration systemAbsorption refrigeratorThe dif fusion-absorption (DA) cycle was patented and113put into practice in 1928 . This system is heat operated


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66 R Ra d e r m a ch e r a n d K K imand combines the circulation of an inert gas (hydrogen)between the evaporator and absorber with the use of thebubble pump to ensure the circulation of an aqua-ammonia mixture. The amonia evaporates and providesthe cooling capacity and the vapour is absorbed into anammonia/water mixture. This occurs at a low ammoniapartial pressure. Pure ammonia is generated by addingheat to the ammonia/water mixture at high temperaturesand at a high partial ammonia vapour pressure, theoverall pressure is kept constant by the added auxilarygas: hydrogen. The main advantages are the lack ofmoving parts and associated noise and vibration, as wellas the ability to operate wi thout any electric power input.This makes the unit ideal in today's niche markets suchas operation in remote locations or applications such asrecreational vehicles and hotel rooms.The ammonia-absorption system was not ignored indomestic refrigeration while the vapour-compressionsystem dominated in refrigerator industries. Themachine most important in the United States was animproved version manufac tured by Servel Inc. However,due to the success of the mechanical vapour-compressionsystems, the sale of DA units dropped consistently;although promoted vigorously by gas companies, saleswere small 114.

Further investigations of the DA system were pub-lished115-117. Recently, the role of the DA cycle has beenreconsidered because of its CFC-free operation 118-120. Astudy of the DA cycle with a greatly improved genera tordemonstrated a significant improvement in the coolingCOP compared with the baseline results TMHowever, based on primary energy consumption, thediffusion-absorption refrigerator is about 30% lessefficient than conventional vapour-compression systems.Th e rm o e le c t r i c re f r ig e ra to rSince the basic theory of thermoelectric refrigerators wasderived satisfactorily in 1909 and 1911 by Altenkirch TM,a number of applications to the refrigerator wasinitiatedlZt 128. Angrist 's work indicated that for thisapplication materials were needed with high Seebeckcoefficients, high electrical conductivity to minimizeJoule heating, and low thermal conductivities to reduceheat transfer through the devices. These requirementscontradict each other. High electric conductivity isgenerally accompanied by high thermal conductivity.Although Altenkirch enumerated the desirable proper-ties for materials to be used in thermoelectric devices, 50years passed before those materials (semiconductors)became known and widely available. This device has nomoving parts, does not age, and has an infinite shelf life.Currently, however, because i t has a very low efficiency itis suitable only in military applications, scientific andmedical instruments and applications when cooling loadsand/or temperature lifts are small.S t i r l i n g c y c l eOne of the most remarkable developments since WorldWar II was the resurgence of interest in the Stirling cycle,which was named after the inventors Robert and JamesStirling. A Stirling cycle machine is a device whichoperates on a closed regenerative thermodynamic cycle,with cyclic compression and expansion of the working

fluid at different temperature levels, and where the flow iscontrolled by volume changes, so that there is a netconversion of heat to work or vice versa With the adventof environmental concerns for refrigerants, this cycle hasreceived renewed interest. It is considered for refrigeratorapplications and has no direct impact on the environ-ment. Several investigations and suggestions have beenpublished129 132.R e v i e w a n d o u t l o o kDomestic refrigerator/freezers have undergone consis-tent and steady development worldwide over the years.Local preferences led to different designs, but the basicrefrigeration technology was the same. R12 was theworking fluid for all vapour-compression systems.However, recent environmental concerns led to aconsiderable boost in development efforts emphasizingtwo aspects: (1) environmenta lly safe fluids; and (2)reduced energy consumption. This led to a number ofnew refrigerants that are now under consideration and tothe introduction of several of them. While in the U.S.R134a is unquestionably the fluid of choice, othercountries are considering other alternatives and mostlyEuropean manufacturers are implementing hydrocar-bons. Thus a 'split' in working fluid technology hasoccurred for the first time in 50 or so years, and is here tostay for at least a number of years. The U.S. choice isdriven by safety and liability concerns. These areaggravated in U.S. technology because of the large sizeof the equipment. Typical refrigerant charges are in therange of 150 300 g. For most other refr igerators world-wide, these numbers are considerably smaller, reducingthe overall risk of the use of hydrocarbons. Further, inthe U.S., the use of electric heaters for automatic defrostunits represents a considerable safety hazard. Having anignition source attached to an evaporator that were tocontain flammable and explosive substances, if hydro-carbons were used, would sooner or later lead toaccidents. Another argument for R134a is that refrig-erators are very reliably sealed and the loss of charge isminute and rare. On the other hand, the R134atechnology is now well established and compressorsand other components are quite efficient. Thus, concernsabout the global warming impact are not very significant.This argument is even more important from the point ofview that the refrigerant contributes only several percentto the total global warming impact. The major contribu-tion results from the power consumption. Since R134aeliminates ozone depletion concerns, it also limits theglobal warming impact because of good efficiency.Hydrocarbons have an almost negligible direct globalwarming impact and may possibly reduce the indirectone. However, having to implement the required safetyfeatures will almost certainly compensate for anyefficiency gains the fluid may provide and increase cost.The trend to more efficient refrigerators will continuein the U.S. driven by more and more stringentgovernment standards and possibly by initiatives likethe SERP Program 134'135. This will eventually lead to theintroduction of new technology for the refrigerationsystem itself and for the insulation and the cabinet designas well. In terms of refrigeration system development, amajor step will be the introduction of the foodcompartment evaporator that operates at a significantly


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ome st ic refrigerators 67hi ghe r pr e s sur e l e ve l t han t he fr e e ze r e va po r a t o r or w i t ht h e u s e o f r e fr ig e r a n t m i x tu r e s . T h e r e a r e s e v e r a l w a y s o fi m p l e m e n t i n g t h i s f e a t u r e a s d e s c r i b e d a b o v e a n d o n l yt h e f u t u r e w i l l s h o w w h i c h o n e s w i l l b e s e l e c t e de v e n t u a l l y . R e g a r d i n g i n s u l a t i o n , v a c u u m i n s u l a t i o nw i l l b e u s e d s o o n e r o r l a t e r . T h i s h o l d s g r e a t p r o m i s ef o r in c r e as e d u s a b l e c o l d s t o r a g e v o l u m e a t p o s s i b l yr e d u c e d o u t s i d e d i m e n s i o n s b u t w i l l i n c r e a s e c o s t a n dw e i g h t . A n u m b e r o f in t e r es t in g o p t i o n s t o i m p r o v eene rgy ef f i c iency are l i s ted in 133.

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