E L S E V I E R

Int J. ReJrig. Vol. 19, No. 1, pp. 61 69, 1996 Copyright c t~ 1996. Published by Elsevier Science Ltd and IIR

Printed in Great Britain. All rights reserved 0140-7007(95)00069-0 0140-7007/96/$15.00 + .00

] R E V I E W P A P E R ]

Domestic refrigerators: recent developments

R. Radermacher and K. Kim* Center for Env i ronmenta l Energy Engineering, Universi ty of Mary land , College Park,

M D 20742, U S A

The refrigerator/freezer is one of the most important and the biggest energy-consuming home appliances. There are several literature references that discuss the historical development of refrigeration I~ lg. Most of them, however, consider historical highlights up to several decades ago. This paper summarizes recent developments in the field of domestic household refrigerators based on a survey of publications and patents. (Keywords: Refrigerator; domestic review; refrigerants, cycles, charge optimization)

R6frig6rateurs domestiques: mises au point r6centes Le r~frig&ateur-cong~lateur domestique est I'une des applications domestiques les plus importantes et les plus consommatrices d'Onergie. Plusieurs publications (voir bibliographie 1 14) {tudient l'histoire du Jroid. La plupart d'entre elles font remonter les grands moments historiques it plusieurs d~cennies. L'article r{sume h,s mises au point rdcentes dans le domaine des rOfrig~rateurs domestiques bas~s sur l'ktude des publications et des marques. (Mots-cl~s: ???; ????)

In the beginning

From William Cullen of the University of Glasgow, Scotland, who was recorded as the first person to demonstrate the man-made production of cold when he evaporated ether in 1748, to Jacob Perkins who developed the first practical refrigeration machine using a vapour compression cycle in London with ether as the refrigerant in 1834, the refrigeration industry developed steadily. When electricity became generally available, William F. Singer of New York, patented the earliest automatic electric unit for small-size refrigerating systems in 189715 .

As electric-generating capacity grew and as homes were beginning to be wired for its use, household refrigerators became more popular and began replac- ing the common window and standing iceboxes. The interest and demand for household refrigerators was aided by the design and development of fractional horsepower motors, which were used in refrigerators. These units began being produced in large numbers in the early 1920s and have become a necessity for all.

Isko was probably the first reasonably successful air- cooled unit. Fred W. Wolf designed and marketed a household system called DOMELRE, a contraction of Domestic __Electric Refrigerator. The Wolf system was marketed by Mechanical Refrigerator Company and later by Isko until absorbed by Frigidaire in 1922. But the most important technical contributions were made

* Present address: Living Systems R&D Center, Samsung Electronics Co. Ltd, 416 Maetan-3Dong, Suwon 442-742, Korea

by General Electric and the Kelvinator. General Electric had begun to manufacture the Audiffren machine in 191116 and in February of 1918, Kelvinator sold its first refrigerator 17. One of the first practical automatic controls was the thermostatic switch developed by Copeland for the first Kelvinators ~8. The sealed unit, which eliminated the belt, was introduced in 1925 by General Electric 16"1s.

Advances in materials and design

As plastics and techniques in working plastics, and new insulating materials, were developed, the small domestic refrigerator became highly sophisticated in design and construction, and the ratio of useful storage capacity to total volume increased.

A great impetus to refrigerator design was provided by the introduction of the halogen refrigerants with which non-ferrous metals could be employed. The fluorocarbon refrigerants were announced by Midgley and Henne in

a9 1930 and the introduction ofdichlorodifluoromethane (R12) as a commercial refrigerant in 1931. From the late 1920s, an extremely small bore was considered to reduce the pressure of sulphur dioxide (R764). With the introduction of the physically safe, oil-soluble, halo- genated hydrocarbons as workin~ fluids, the application

20 1 of capillary tubes increased '~. Before that time clogging of the capillary tube posed a challenge and usually an expansion valve was used. The introduction of R12 removed an important obstacle to complete acceptance of domestic mechanical refrigeration, for earlier manufacturers have been compelled to depend on refrigerants such as sulphur dioxide (R764), methyl

61

62 R. Radermacher and K. Kim

chloride (R40), and methyl formate (R611) and dichloro- methane (R30). All of these fluids are toxic.

During the 1930s the domestic refrigerator was standardized. The hermetic compressor became the norm, and with its adoption, rotary compressors began to substitute the older reciprocating compressor tech- nology. Expansion valves were replaced by capillary tubes. The refrigerators, all self-contained, were built of steel and were well insulated. Finally, the mechanism was placed at the bottom of the unit.

A more important development was the two-temper- ature refrigerator, which was introduced about 1939 and began to come into its own in the post-war period. As its name implied, it consisted of two separate compartments like the current refrigerator/freezer unit 22.

Between the 1950s and 1960s, domestic refrigerators changed considerably from the early designs. Multiple compartments had made them more complex. Designs had been refined to meet exacting customer demands. Acceptance levels had been raised considerably 23. In 1944 almost 70% of American homes that had refrigerators were equipped with 'mechanicals '24 and in 1958 94% of households owned refrigerators in the U.S. 25.

Environmental concerns

In 1974, Rowland and Molina 26 advanced the hypothesis that antrhopogenic emission of certain chlorinated and bromate compounds, particularly chlorofluorocarbons (CFCs) and hydrochlorocarbons (HCFCs), could accumulate in the stratosphere and substantially deplete the ozone layer that shields the earth from cancer- causing ultraviolet-B solar radiation. Within a few years the issue had moved onto the public policy agenda, and the U.S. Congress in 1978 banned the use of CFCs as aerosol propellants, which accounted for the majority of total CFC emissions in the U.S. at the time 27. Since the ozone layer depletion is a global problem, an inter- national treaty to regulate the production and trade of the 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 to reduce production of CFCs to 50% of 1986 levels by 1998, and placed no restrictions on the production of HCFCs. Refrigerator/freezers were one of the major technologies dependent on R12 as a refrigerant.

Besides contributing to the destruction of strato- spheric ozone, the Antarctic ozone hole and significant ozone reductions in the Arctic, CFCs have been implicated as a major anthropogenic cause of global warming. As refrigerants are lost through leaks, equip- ment maintenance and retirement, they disperse throughout the atmosphere and act as greenhouse gases and contribute to global warming. In addition to the accumulation of CFCs in the atmosphere, the carbon dioxide emissions associated with energy used to operate refrigeration equipment, such as the domestic refrigera- tor, reflect the greenhouse warming effect. Almost all this energy results from the combustion of fossil fuels and creates emissions of CO2. Carbon dioxide is the largest contributor to global warming 3°. Since refrigerators consume about 2.9 × 1011 kWh of primary energy, or 12% of the total residential energy budget annually, substantial improvements in domestic refrigerator/

freezer efficiency extended over the 15- to 20-year life of this" appliance would significantly~l benefit national goals for environmental progress .

Test results with new refrigerants

While new refrigerants were being developed and tested in response to the environmental challenges, one possible solution that was proposed for alterantive refrigerants focused on using zeotropic refrigerant mixtures, whose components were environmentally safe and their characteristics as drop-in refrigerants appeared to be promising.

From the 1960s numerous publications throughout the world explored the use of refrigerant mixtures primarily with two objectives in mind: (1) achieving a low evaporator temperature with a moderate pressure ratio during single-stage compression and (2) conserving energy when the refrigeration duty consists of cooling a fluid stream through a large temperature range 32 37.

However, the refrigerant most commonly considered as the future substitute for CFC12 in domestic equip- ment was HFC 134a. Many calculations based on simple Rankine cycle models, however, have concluded that HFC134a is less energy efficient than CFC123s. These claims were confirmed by experimental results which showed efficiencies for HFC134a which were 4-10% less than those for CFC1239'4°.

In order to overcome difficulties of pure alternative refrigerants, a number of binary and ternary mixtures were suggested 34'41 43. Among the proposed fluids was also a mixture of R22 and R142b, with the latter being a flammable component. At this time, a mixture contain- ing R22 was still acceptable and had the advantage of being compatible with the familiar alkylbenzene lubri- cant. Moreover, since the percentage of a flammable component was relatively low, the flammability problem would be eliminated. These mixtures had 'drop-in' characteristics for use as a substitute in existing refrigerator systems. Test results showed that some

39 44 46 mixtures ' - consumed less energy when the system was optimized in terms of the length of the capillary tube and the amount of charge while others consumed slightly more energy 3942 .

Pure fluorinated hydrocarbons

Subsequent revisions of the Montreal Protocol required the eventual phaseout of HCFCs as well as CFCs, and specific schedules were established by statute in each signatory country. The most likely replacements at this time are HFCs, although hydrocarbons and ammonia are being considered because they also have a zero ozone depletion potential (ODP). Further, modelling results demonstrated that with appropriate superheat and subcooling taken into consideration, HFC134a can provide COP values essentially equivalent to those of R1247.

In terms of implementation however, manufacturers were 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 than R12 systems did.

At the same time, R152a was another potential

Domestic refrigerators 63

replacement candidate for R12. It has a very low global warming potential (GWP) value compared to other candidates. The flammability of R152a, however, com- pared to the nonflammability of R134a is a major drawback. Theoretically, it is expected that the energy consumption of R152a is about 3-4% lower than that of R134a. However, experimental results show that the measured performance of a refrigerator charged with R152a and that of an equivalent system charged with R134a (with the same compressor's energy efficiency ratio, EER) did have essentially the same perform- ance 4s'49. It was confirmed that the ester oils have good miscibility, insulating characteristics and reliability in durability tests 5°. However, the flammability aspect together with the liability concerns of refrigerator manufacturers has not led to an application of R152a.

Hydrocarbons

Naturally occurring substances such as carbon dioxide, ammonia and hydrocarbons are considered to be environmentally safe refrigerants. An alternative solution may be provided by hydrocarbons which offer the possibility of a low cost (at least for the refrigerant itself), and which are readily available and environ- mentally benign alternatives to CFCs and CFC sub- stitutes. The absence of chlorine results in zero ozone depletion potential, and very low GWP. The transport properties of hydrocarbons, e.g. propane (R290), are superior (lower viscosity, higher thermal conductivity than other alternatives) while thermodynamic properties are similar to those of CFC12 and 22. Another advantage of hydrocarbons is their solubility in mineral oil, which is traditionally used as a lubricant in compressors. The major drawback is their flammability, although it was suggested they would be unlikely to create an ignitable atmosphere in the refrigerator due to the small charge quantity 51-54.

Currently, the use of isobutane in domestic refriger- ators and freezers is well established in Europe for refrigerators that have no automatic defrost. Studies have shown energy savings with hydrocarbon refriger- ants 55'56. Experimental results with pure propane 55- or isobutane 57 as a drop-in substitute showed up to 2% improvement while tests with cyclopropane showed energy savings of 6 o / o 46 . Binary mixtures of hydrocarbon refrigerants showed improvements of up to 10% for optimized systems 58. Further, a ternary mixture with 17% energy savings in the modified Lorenz-Meutzner cycle proved to be better than the binary mixtures 59.

System and component efficiency improvements Studies to measure the effect of different usage conditions on energy consumption revealed the sensitivities of thermal performance and energy use to different variables 60. Field test results with a highly efficient hermetic compressor utilizing a four-pole permanent split-capacitor motor showed that the energy-saving compressor did permit a refrigerator unit to operate on less power than a standard compressor 61 .

The U.S. Congress established the National Appliance Energy Conservation Act (NAECA) in 1987 and required the Department of Energy to consider new or amended standards for refrigerators and freezers. In

order to meet the proposed standards several researchers have evaluated design options for improving the energy efficiency of domestic refrigerator/freezers 49,62-65. A field performance test was conducted to compare the refrig- erator field performance with laboratory test results based on the power consumption of 209 refrigerators 31.

Typical options to improve system efficiency of conventional refrigerators were suggested in four general areas: (1) improving the refrigeration cycle efficiency; (2) decreasing the cabinet heat load; (3) reducing parasitic electrical loads; and (4) reducing on/off cycling losses. A recent computer model that considers the entire system demonstated that the energy consumption for a 20 ft3(5701) refrigerator with a top-mounted freezer and no through-the-door features could work out to con- sume.equal to or less than 1.00kWhday -~. It is noted that each improvement exacts a penalty in terms of increased cost or system complexity/reliability 62'64.

Cycling losses

The cycling losses are defined as the difference between the energy consumption of a system with a continuously operating compressor and a system with a cycling compressor both having the same operating temper- atures and the same cooling load. The on/off cycling mode of a refrigerator results in cycling losses which depend on the cycle length and the dimensions of the heat exchangers. The experimental studies 66'67 indicated that preventing refrigerant migration during the off cycle would reduce the power consumption by 4% and increase the cooling capacity. Some compressor com- panies introduced a liquid-line shut-off valve (energy- saving valve) which operates without consuming any power. It is operated through the .P6ressure change when the compressor switches on and off 8, and a fluid control valve which operates by using a peak electrical current induced when the compressor motor is switched on and off 69"

Insulation

In the 1950s, urethane foam had been developed from a laboratory curiosity to an item of commerce, and early in 1960s rigid urethane foam produced with fluorinated hydrocarbon expanding agents, such as Rl l and R12 was introduced 70 and had been accepted rapidly until the 1996 ban on the manufacturing of foam-blowing agents containing CFCs. Under the amended Montreal Proto- col new suggestions and techologies to substitute the current CFC-based technology were discussed 71'72. It is likely that cyclopentane may be one of the nonfluoro- carbon options that may have a similar performance as compared to CFC blowing agents in terms of energy efficiency. Although there is a penalty. As another alternative, FICs (flouro-iodine-carbons) are discussed. These fluids are environmentally safe because of their short atmospheric lifetime. In fact, they potentially decompose under the influence of sunlight. However, as long as they are used within a foam that is entirely enclosed by metal and plastic, they may be quite feasible. Vacuum insulation, as part of foam insulation, was seen as an option with commercial potential. Initially a number of concerns had to be addressed, such as vacuum reliability, and the weight of the insulation

64 R. Radermacher and K. Kim

may potentially be a drawback. Most of these were addressed and vacuum insulation is now in production. However, the last remaining concern is the cost.

Defrost methods

Evaporators of refrigerators must be designed to with- stand the build-up of frost, and for ease ofdefrosting by thermal or mechanical means. As frost accumulates, it can decrease the rate of heat transfer in two ways: (1) the frost may act as an insulator to heat flow; and (2) the frost growth will decrease the amount of air flow over the coil and thus decrease the heat transfer. The effect of frost on the overall heat transfer coefficient and the air- side pressure drop was evaluated experimentally 73'74. When the air flow rate was kept constant, the UA-value was found to increase steadily over a 10-h period as frost was deposited on the coil. It was determined that the increase was due to an increased air-side heat transfer coefficient and an increase in the surface area of the evaporator due to the surface roughness of the frost. For refrigerator/freezers equipped with variable defrost control systems, a calculation method of the daily energy consumption was evaluated and suggested 75.

Currently, the preferred method for defrosting a refrigerator evaporator is to use an electrical heater to heat the evaporator itself, the surrounding surfaces and the air of the evaporator compartment, which usually eliminates all frost completely. In this conventional technology, the defrost heater is operated by a timer and for most climates it may come on much more often than would be required. To avoid this, so-called adaptive defrost controls determine when it is necessary to initiate the defrost cycle. In this way, the defrost heater is only turned on when necessary. This method is already applied in the so-called SERP refrigerator, an energy efficient refrigerator that only recently was introduced into the market 65'76'134'135.

Charge optimization

Charge minimization is an important factor in the design of refrigeration systems, not only to reduce the cost and to alleviate the flammability or toxicity problems of many of the suggested alternative refrigerants, but also to improve the part-load efficiency of equipment as the refrigerant charge becomes smaller. Numerous investi- gations were described based on the assumption of a zero slip 77 and a known void fraction 78 in the two-phase regions in heat exchangers. Simple correlations for the dependence of the opt imum refrigerant charge on the capacity of evaporator and condenser were suggested for

79 domestic refrigerator units . A study about the influence of the ambient temperature on the refrigerant charge

80 necessary in refrigerator units was performed . Deter- mination of the correct charge for normal on/off operation modes was carried out at all ambient temperatures.

Fans

While considerable progress has been made in the overall energy efficiency in refrigerators with regard to cycles and the cabinet insulation materials, it should not be

overlooked that significant efficiency gains are possible from improved components.

For example, the fans that circulate air across the condenser and the evaporator contribute significantly to the overall power consumption. Here the evaporator fan is especially of importance, since its power consumption leads to a double penalty. The entire electric input to this fan is converted into heat eventually that has to be removed by the compressor, increasing the cooling load. Manufacturers are now in the process of introducing high-efficiency fan motors that reduce fan power consumption from originally 10-15W to about 2 5W, a quantity that is almost negligible. An introductory article about the new ECM motors for the appliance industry is given in refs. 81 and 82.

In addition to the better fan motors, improved controls can lead to significant energy savings as well. One example is given below with the Tandem cycle. Others pertain to better defrost methods. The above- mentioned adaptive defrost controls are another means of reducing energy consumption.

Compressors

New compressor developments are leading to consider- able efficiency improvements. For example, work is proceeding on a continuous basis on the valves and especially on the electric motors. Any improvements for the latter provides for double benefit. First, the power requirement is reduced and secondly the refrigerant that is used to cool the motor is less superheated. This increases capacity and efficiency as welP 7.

Recently a linear compressor was introduced that has the advantage of a very efficient linear electric motor. Combined with other improvements in valving and the fact that it is oil-free (no oil pump needed) provides for additional efficiency gains 83.

Refrigeration cycle alternatives

The greatest challenges for the design and development of refrigerator/freezer systems are the reduction of thermodynamic irreversibilities resulting from an inefficient operation of refrigeration cycle and the introduction of environmentally safe refrigerants. The following options were investigated.

The Lorenz-Meutzner cycle

The Lorenz-Meutzner cycle is the most consequent means to exploit the inherent thermodynamic advan- tages of the temperature glide of zeotropic mixtures. Since Lorenz and Meutzner claimed that a two- evaporator, two-subcooler refrigerator/freezer had shown energy savings up to 20% using a R22/Rll mixture in the 1975 International Congress of Refriger- ation (II) 84, several studies were conducted with environmentally benign refrigerant mixtures 85-93 and with hydrocarbon mixtures 94.

Experimental tests were reported that a refrigerator converted to the Lorenz-Meutzner cycle showed a 9% reduction in energy consumption with a zeotropic mixture of R22/R123 compared with an original production unit of a 20 ft 3, side-by-side upright refriger- ator 94 while computer simulation results predicted an

Domestic refrigerators 65

improvement of the coefficient of performance (COP) by 16-20%. In 1993 a modified version of the Lorenz- Meutzner cycle was patented by Radermacher and Jung 95. In the modified cycle, the freezer and food compartment evaporators serve as three-way heat exchangers. They are essential conventional units with a small tube (liquid-line) inserted into a larger tube (suction-line). The compartment air surrounding the larger tube and subcooled liquid in the smaller tube reject heat to the two-phase refrigerant flowing counterflow through the annulus of concentric tubes. Experimental test results were reported with 16.5% energy savings for a modified Lorenz-Meutzner cycle refrigerator with an HFC refrigerant mixture 96, and 17.3% energy savings with a hydrocarbon ternary mixture, propane/n-butane/ pentane (R290/R600/n-c5) TM. A new investigation into providing an independent temperature control of the compartments is made in a modified Lorenz-Meutzner cycle which was tested using a bistabile solenoid valve 97.

Dual-loop system

The best way to reduce the thermodynamic irreversi- bilities resulting from the operation with a single evaporator in domestic refrigerator/freezers is to employ two separate refrigeration cycles. This so-called dual-loop system has two completely separate refrigera- tion cycles which provide cooling for the freezer and food compartments independently. The efficiency for this system may be lower than theoretically expected. The efficiency degradation is due to the use of two compressors, instead of one, that are generally smaller than the original one and that have generally a lower efficiency. The major disadvantage with this system is cost. The increased volume due to the two refrigeration systems would either increase the product's external dimensions or decrease usable refrigerator volume. Some theoretical and experimental results were published. The theoretical results predicted energy savings of typically 20% 90"91 while the experimental work showed 16% 98,99

and 4% with a potential for 20% if better compressors were available l°l .

Two-stage system

There are several two-stage configurations under investi- gation. Generally speaking, this system has one con- denser, two evaporators, two compressors and at least one suction-line heat exchanger. Two versions of a two- stage system for domestic refrigerator/freezers were patented 1°1'1°2. The major gain of this system is a smaller work requirement resulting from the low-pressure ratio for each of the two compressors. Based on the same total cooling capacity, this system promises an improvement of 48.6% in theory over the single-stage system,

Control valve system

The control valve system has two evaporators but only one compressor and one condenser. Two different-length capillary tubes and a control valve are installed between the food and freezer evaporator inlets and the condenser outlet. Refrigerant flow is directed to either the freezer evaporator or the food evaporator according to the temperature of each compartment while the compressor

is in operation. Experimental results by using a solenoid valvel03j °4 show the energy savings over the single-stage system.

Ejector refrigerator

One of the intrinsic losses in the vapour compression refrigeration cycle is the throttling of refrigerant in the capillary tube. In the ejector refrigeration system the energy wasted in the capillary tube is used, at least to some extent, productively by means of an ejector which allows the raising of the suction pressure entering the compressor. The performance of the ejector cycle with a varying cooling ratio (ratio between food compart- ment and freezer load) and a comparison of several refrigerants were analysed 1°5. An ejector expansion refrigeration cycle with one evaporator was patented and this system was expected to reach up to 20% improvement for refrigerator applications 1°6. Another ejector-enhanced refrigeration cycle with two evapor- ators was suggested and the simulation results show an increase of up to 12.4% in COP over the single-stage refrigerator 107.

Tandem system

A conventional refrigerator/freezer system which con- trols the temperature of each compartment by using a thermo-damper was suggested l°s. This system operates with no thermodynamic advantages while it has an energy-saving effect only for the defrost cycles which use the food-compartment air during the off-cycle. Mean- while, a high-efficiency, automatic-defrost refrigerator/ freezer which consists of a forced convection freezer evaporator and a natural convection food evaporator connected in series was demonstrated 1°9 111. The field test of this system showed energy savings of 58% compared with a baseline unit resulting from utilizing advanced design features such as optimized thick wall insulation and two evaporators. Recently, the tandem system which is a new technique for using two evaporators and two fans was developed. This system takes advantage of the evaporator fan control such that each evaporator fan operates one at a time while the compressor is in operation. The two evaporators are connected in series and at the beginning of the compressor on-time, the food compartment fan is turned on first. Thus, the food compartment is suitably cooled before the system reaches steady state. At this time, the system turns off the food compartment fan and turns on the freezer fan. As a result, this system utilizes the pull-down period of each cycle, which is generally not suitable for cooling, to cool the food compartment. It is more efficient and provides substantial energy savings of up to 18% as compared to the baseline unit of conventional design. In addition, a new internal defrost method by using a thermosyphon phenomenon between two evaporators was introduced j 12

Alternative refrigeration system

Absorption refrigerator

The diffusion-absorption (DA) cycle was patented and 113 put into practice in 1928 . This system is heat operated

66 R. Radermacher and K. Kim

and combines the circulation of an inert gas (hydrogen) between the evaporator and absorber with the use of the bubble pump to ensure the circulation of an aqua- ammonia mixture. The amonia evaporates and provides the cooling capacity and the vapour is absorbed into an ammonia/water mixture. This occurs at a low ammonia partial pressure. Pure ammonia is generated by adding heat to the ammonia/water mixture at high temperatures and at a high partial ammonia vapour pressure, the overall pressure is kept constant by the added auxilary gas: hydrogen. The main advantages are the lack of moving parts and associated noise and vibration, as well as the ability to operate without any electric power input. This makes the unit ideal in today's niche markets such as operation in remote locations or applications such as recreational vehicles and hotel rooms.

The ammonia-absorption system was not ignored in domestic refrigeration while the vapour-compression system dominated in refrigerator industries. The machine most important in the United States was an improved version manufactured by Servel Inc. However, due to the success of the mechanical vapour-compression systems, the sale of DA units dropped consistently; although promoted vigorously by gas companies, sales were small 114.

Further investigations of the DA system were pub- lished115-117. Recently, the role of the DA cycle has been reconsidered because of its CFC-free operation 118-120. A study of the DA cycle with a greatly improved generator demonstrated a significant improvement in the cooling COP compared with the baseline results TM .

However, based on primary energy consumption, the diffusion-absorption refrigerator is about 30% less efficient than conventional vapour-compression systems.

Thermoelectric refrigerator

Since the basic theory of thermoelectric refrigerators was derived satisfactorily in 1909 and 1911 by Altenkirch TM, a number of applications to the refrigerator was initiatedlZt 128. Angrist's work indicated that for this application materials were needed with high Seebeck coefficients, high electrical conductivity to minimize Joule heating, and low thermal conductivities to reduce heat transfer through the devices. These requirements contradict each other. High electric conductivity is generally accompanied by high thermal conductivity. Although Altenkirch enumerated the desirable proper- ties for materials to be used in thermoelectric devices, 50 years passed before those materials (semiconductors) became known and widely available. This device has no moving parts, does not age, and has an infinite shelf life. Currently, however, because it has a very low efficiency it is suitable only in military applications, scientific and medical instruments and applications when cooling loads and/or temperature lifts are small.

Stirling cycle

One of the most remarkable developments since World War II was the resurgence of interest in the Stirling cycle, which was named after the inventors Robert and James Stirling. A Stirling cycle machine is a device which operates on a closed regenerative thermodynamic cycle, with cyclic compression and expansion of the working

fluid at different temperature levels, and where the flow is controlled by volume changes, so that there is a net conversion of heat to work or vice versa. With the advent of environmental concerns for refrigerants, this cycle has received renewed interest. It is considered for refrigerator applications and has no direct impact on the environ- ment. Several investigations and suggestions have been published129 132.

Review and outlook

Domestic refrigerator/freezers have undergone consis- tent and steady development worldwide over the years. Local preferences led to different designs, but the basic refrigeration technology was the same. R12 was the working fluid for all vapour-compression systems. However, recent environmental concerns led to a considerable boost in development efforts emphasizing two aspects: (1) environmentally safe fluids; and (2) reduced energy consumption. This led to a number of new refrigerants that are now under consideration and to the introduction of several of them. While in the U.S. R134a is unquestionably the fluid of choice, other countries are considering other alternatives and mostly European manufacturers are implementing hydrocar- bons. Thus a 'split' in working fluid technology has occurred for the first time in 50 or so years, and is here to stay for at least a number of years. The U.S. choice is driven by safety and liability concerns. These are aggravated in U.S. technology because of the large size of the equipment. Typical refrigerant charges are in the range of 150 300 g. For most other refrigerators world- wide, these numbers are considerably smaller, reducing the overall risk of the use of hydrocarbons. Further, in the U.S., the use of electric heaters for automatic defrost units represents a considerable safety hazard. Having an ignition source attached to an evaporator that were to contain flammable and explosive substances, if hydro- carbons were used, would sooner or later lead to accidents. Another argument for R134a is that refrig- erators are very reliably sealed and the loss of charge is minute and rare. On the other hand, the R134a technology is now well established and compressors and other components are quite efficient. Thus, concerns about the global warming impact are not very significant. This argument is even more important from the point of view that the refrigerant contributes only several percent to the total global warming impact. The major contribu- tion results from the power consumption. Since R134a eliminates ozone depletion concerns, it also limits the global warming impact because of good efficiency. Hydrocarbons have an almost negligible direct global warming impact and may possibly reduce the indirect one. However, having to implement the required safety features will almost certainly compensate for any efficiency gains the fluid may provide and increase cost.

The trend to more efficient refrigerators will continue in the U.S. driven by more and more stringent government standards and possibly by initiatives like the SERP Program 134'135. This will eventually lead to the introduction of new technology for the refrigeration system itself and for the insulation and the cabinet design as well. In terms of refrigeration system development, a major step will be the introduction of the food compartment evaporator that operates at a significantly

Domestic refrigerators 67

higher pressure level than the freezer evaporator or with the use of refrigerant mixtures. There are several ways of implementing this feature as described above and only the future will show which ones will be selected eventually. Regarding insulation, vacuum insulation will be used sooner or later. This holds great promise for increased usable cold storage volume at possibly reduced outside dimensions but will increase cost and weight. A number of interesting options to improve energy efficiency are listed in 133.

References

1 Lloyd, E. W. Electric household refrigeration Ice Refrig (1926) LXX 656

2 Ewell, A. W. The post-war domestic refrigerator Ice Refrig (1944) CVII 25

3 Worstrel, J. F., Praetz, J. G. Household Electric Refrigeration, McGraw-Hill, New York (1948)

4 Anderson, O. E., Jr. Refrigeration in America Princeton Univ. Press, Princeton, NJ (1953)

5 Oldham, B. C. The history of refrigeration J Refrig (1965) 8 147-149

6 Oldham, B. C. The history of refrigeration J Refrig (1968) 11 58-60

7 Woolrleh, W. R. The Men who Created Cold, a History of Refrigeration 1st edn, Exposition Press, New York (1967)

8 Woolrieh, W. R. The history of refrigeration; 220 years of mechanical and chemical cold: 1748-1968 ASHRAE J (1969) July 31-39

9 Nagengast, B. A. Small size automatic electric refrigeration When did it start? ASHRAE Trans (1982) 88 953-955

10 Downing, R. Development of chlorofluorocarbon refrigerants ASHRAE Trans (1984) 90 481-491

11 Johnson, V. J. A history of cryogenics ASHRAE Trans (1984) 90 492-507

12 Sehulz, U. W. Start of the art: the capillary tube for, and in, vapor compression systems ASHRAE Trans (1985) 91 92 105

13 Spauchus, H. O., Speaker, L. M. A review of viscosity data for oil-refrigerant solutions ASHRAE Trans (1987) 93 667-681

14 Bnllard, C. W., Radermacher, R. New technologies for air conditioning and refrigeration Ann Rev Energy Envir (1994) 19 113-152

15 Singer, W. F. Refrigerating apparatus U.S. Patent No. 577.328 (1897)

16 Holladay, W. L. The General Electric Monitor Top refrigerator ASHRAE J (1994) September 49 55

17 Long, W. H. Gross relates story of Kelvinator's early history. Air Cond Refrig News (1938) XXV 26 October 11

18 Multty, G. Twenty-five years of household electric refrigeration development Ice Refrig (194l) C1 40

19 Midgley, T., Jr, Henne, A. L. Organic fluorides as refrigerants Ind Eng Chem (1930) 22 542

20 Karmazin, J. U.S. Patent No. 2.199,631 (1940) 21 Staebler, L. A. U.S. Patent No. 2,291,565 (1942) 22 Roider, R. F., Timmerman, H. W. The two-temperature

refrigerator - design and construction Refrig Eng (1948) 56 August 134-136

23 Tarleton, F. L. A discussion: complete hermetic system replace- ment Refr~g Eng (1955) 63 60 63

24 U.S. Department of Commerce, Bureau of the Census Charac- teristics of occupied dwelling units, for the United States: October. 1944 (Series H-45, N o 2) 8

25 The current economic position of domestic refrigerators J Refrig (1959) 2 93-94

26 Rowland, F. S., Moliua, M. J. Stratospheric for chlorofluoro- methanes: chlorine atom-catalyzed destruction of ozone Nature (1974) 249 810 812

27 U.S. Environmental Protection Agency Regulatory Impact Analysis: protection of stratospheric ozone, Volume I: Regulatory impact analysis document, 3-6

28 United Nations Environment Programme Montreal Protocol on Substances that Deplete the Ozone Layer, Final Act United Nations, New York (1987)

29 Benediek, R. E. Ozone diplomacy &s Sci. Tech (1989) Fall 43-50

30 Fischer, S. K. Total equivalent earming impact: a measure of the global warming impact of CFC alternatives in refrigerating equipment Int J Refrig (1993) 16 423 428

31 Meier, A. K., Jansky, R. Field performance of residential refrigerators: a comparison with the laboratory test ASHRAE Trans (1993) 99 704-713

32 Arora, C. P. Power savings in refrigerating machines using mixed refrigerants XII lnt Cong Refrig Madrid (1967)

33 Tschaikovsky, A. F., Kuznetsov, A. P., Vodyanitskaya, N. i. Investigation of refrigerating machines operating with mixtures of refrigerants XII Int Cong Refrig Madrid (1967)

34 McLinden, M. O., Radermaeher, R. Method of comparing the performance of pure and mixed refrigerants in the vapor compression cycle Int J Refr(g (1987) 10 318 325

35 Boot, J. L. Overview of alternatives to CFC in domestic refrigerators and freezers Int J RefrLg (1990) 13 100 105

36 Didiou, D. A., Bivens, D. B. Role of refrigerant mixtures as alternatives to CFCs Int J Refrig (1990) 13 163 175

37 Vineyard, E. A. The alternative refrigerant dilemma for refrigerator-freezer: truth or consequences ASHRAE Tech Data Bull (1991) 7 63-68

38 Wilson, D. P., Basu, R. S. Thermodynamic properties of a new stratospherically safe working fluid-refrigerant 134a ASHRA E Trans (1988) 94 2094-2118

39 Vineyard, E. A., Sand, J. R., Miller, W. A. Refrigerator freezer energy testing with alternative refrigerants ASHRAE Trans (1989) 95 295-299

40 Zhu, M. S., Han, L. Z., Lin, Z. Z., Fu, Y. D. The modeling evaluation & experimental researches on domestic refrigerators using HFC-134a as refrigerant Proc lnt CFC and Hahm Alternatives Conference (1992) 197 204

41 Jung, D., S., Radermaeher, R. Performance simulation of single evaporator refrigerator with pure and mixed refrigerants Int J Refrig (1991) 14 223-232

42 Won, S., Radermacher, R. Energy consumption of a mixture of RI42b/R22/R12 in a domestic refrigerator/freezer ASHRAE Trans (1993) 99 104-108

43 He, X, Spindler, E., Jung, D. S., Radermacher, R. Investigation of R22/RI42b mixture as a substitute for RI2 in single- evaporator domestic refrigerators ASHRAE Trans (1992) 98 150-159

44 Kuijpers, L. J. M., de Wit, J. A., Bensehop A. A. J., Janssen, M. J. P. Experimental investigation into the ternary blend HCFC22/124/152a as a substitute in domestic refrigeration Proc USNC/IIR Purdue RefFigeration Collie'fence (July. 1990) pp. 314-324.

45 Robin, M. L., likubo, Y. Performance of alternative refrigerants: refrigerator freezer energy testing with HFC- 152a/HFC-227ea mixtures Proc Int CFC and Halon Alternatives Conference (1992) pp. 99-104

46 Kim, K., Spindler, U., Jung, D. S., Radermaeber R. R22/R152a mixtures and cyclopropane (RC270) as substitutes for R12 in single-evaporator refrigerators: simulation and experiments ASHRAE Trans (1993) 99 1439-1446

47 Behrens, N., Dekleva, T. W., Hartley, J. G., Murphy, F. T., Powell, R. L. The R-134a energy efficiency problem, fact or fic- tion. USNC/IIR-Purdue Refrigeration Conference (1990) pp. 365 372

48 Pannock, J., Liu, Z., Yu, K., Radermaeher, R. Evaluation of R134a and R152a as working fluid in a domestic refrigerator/ freezer ASHRAE Trans (1994) 100 1344-1350

49 Vineyard, E. A., Sand, J. R., Bohman, R. H. Evaluation of design options for improving the energy efficiency of an environmentally safe domestic refrigerator freezer ASHRAE Trans (1995) in press

50 Komatsuzaki, S., lizuka T. Ester oils as lubricant for HFC- 134a refrigerator in domestic appliance Proc Int CFC and Halon Alternatives Conference (1992) pp 189 195

51 Camporese, R., Bigolaro, G., Cortella, G., Seattolini, M. Flammable refrigerants in domestic refrigeration XVIl l lnt Cong Refrig Montreal (August 1991) Paper No. 273

52 Bodin, E., Chorowski, M., Wilezek Working parameters of domestic refrigerators filled with propane-butane mixture Int. J Refrig (1993) 16 353-356

53 Buskin, E,, Perry, R. B. The performance of hydrocarbons in a household refrigerator/freezer Proc Int Refrig Conference at Purdue (1994) pp 237-244

68 R. Radermacher and K. Kim

54 Liu, Z., Haider, I., Liu, B. Y., Radermacher, R. Test results of hydrocarbon mixtures in domestic refrigerator/freezers lnt CFC and Halon Alternatives Conference (1995) pp 22 31

55 James, R. W., Missenden, J. F. The use of propane in domestic refrigerators Int. J Refrig (1992) 15 95-98

56 Richardson, R. N., Butterworth, J. S. The performance of propane/isobutane mixtures in a vapor-compression refrigeration system lnt J Refrig (1995) 18 58-62

57 Rifle, D. R. Isobutane as a refrigerator freezer refrigerant Proc Int Refrig Conference at Purdue (1994) pp 245-254

58 Doeldinger, M. The success of hydrocarbons in domestic refrigeration: energy efficient and environmentally friendly. ORNL-6797. Oak Ridge Nat Lab (1993) pp C1-C19

59 Liu, B. Y., Tomasek, M.-L., Radermacher, R. Experimental research with hydrocarbon mixtures in domestic refrigerator/ freezer ASHRAE Trans (1994) 101 in press

60 Grimes, J. W., Mniroy, W., Shomaker, B. L. Effect of usage conditions on household refrigerator-freezer and freezer energy consumption ASHRAE Trans (1977) 83 818-828

61 Farley, K. J., Alissi, M. S. Effects of ambient temperature and control settings on thermal performance and energy consump- tion of household refrigerator-freezer ASHRAE Trans (1987) 93 1578-1590

62 Turiel, I., Heydari, A. Analysis of design options to improve the efficiency of refrigerator-freezers and freezers ASHRAE Trans (1988) 94 1699-1712

63 Abramson, D. S., Turiel, I., Heydari, A. Analysis of refrigerator-freezer design and energy efficiency by computer modeling: a DOE perspective ASHRAE Trans (1990) 96 1354-1358

64 Granryd, E. Energy savings in vapor compression refrigerating systems Proc Int Symposium on refrigeration, Energy and Environment at Trondheim, Norway (June 1992) pp 43-60

65 Sand, J. R., Vineyard, E. A., Bohman, R. H. Investigation of design options for improving the energy efficiency of con- ventionally designed refrigerator-freezer ASHRAE Trans (1994) 100 1359-1368

66 Wang, J., Wu, Y. Start-up and shut-down operation in a reciprocating compressor refrigeration system with capillary tubes Int J Refrig (1990) 13 187 190

67 Fanssen, M. J. P., de Wit, J. A., Knijpers, L. J. M. Cycling losses in domestic appliances; an experimental and theoretical analysis Proc USNC/IIR Purdue Refrigeration Conference (July 1990) pp 90-98

68 Matsushita refrigeration company. Energy-saving valve for refrigerator (1992)

69 Berwanger, E., Sehwarz, M. G. Blocking valve for refrigeration on air conditioning systems (1990) U.S. Patent No. 4,970,872

70 Knox, R. E. Insulation properties'of fluorocarbon expanded rigid urethane foam ASHRAE Trans (1963) 69 150-159

71 Smith, M. K., Heun, M. C., Crawford, R. R., Neweii, T. A. Thermodynamic performance limit considerations for dual- evaporator, non-azeotropic refrigerant mixture-based domestic refrigerator-freezer system Int J Refrig (1990) 13 237-242

72 Proceedings of the 1993 Non-Fluorocarbon Insulation, Refrigeration and Air-Conditioning Technology Workshop. (1993) ORNL-6805. Oak Ridge Nat. Lab

73 Rite, R. W., Crawford, R. R. The effect of frost accumulation on the performance of domestic refrigerator-freezer finned- tube evaporator coils ASHRAE Trans (1991) 97 428-437

74 Rite, R. W., Crawford, R. R. A parametric study of the factors governing the rate of frost accumulation on domestic refrigerator-freezer finned-tube evaporator coils ASHRAE Trans (1991) 97 438-446

75 Mahajan, B. M. Energy rating of refrigerators with variable defrost controls ASHRAE Trans (1988) 94 330-339

76 Behrendsen. 1994, New Product: high-tech fridge. Popular Mechanics Oct. 1994, Vol 171 no. 10, p108

77 Daniels, T. C., Davies, A. The relationship between the refrigerant charge and the performance of a vapor- compression refrigeration system ASHRAE Trans (1975) 81 212-234

78 Rice, C. K. The effect of void fraction correlation and heat flux assumption on refrigerant charge inventory predictions ASHRAE Trans (1987) 93 341-357

79 Dmitriyev, V. I., Pisarenko, V. E. Determination of optimum refrigerant charge for domestic refrigerator unit Int J Refrig (1984) 7 178-180

80 Kuijpers, L. J. M., Janssen, M. J. P., Verboven, P. J. M. The influence of the refrigerant charge on the functioning of small refrigerating appliances ASHRAE Trans (1988) 94 813 828

81 Zimmerman, P. Electronically commutated DC feed drives for machine tools Drives & Controls International, Oct. Nov 1982, 13-19

82 Graham, D. E., Savage, J. W. Brushless DC motor technology Int J Vehicle Des (1988) 6 671 686 (a) Newborough, L. Electronically commutated direct-current motor for driving tube-axial fans: a cost-effective design Appl Energy (1990) 36 167 190

83 van der Wait, N. R., Unger, R. Linear compressors, a maturing technology International Appliance Technology Conference, May 9--11, 1994, University of Wisconsin, Madison, Wisconsin

84 Lorenz, A., Meutzocr. On application of non-azeotropic two- component refrigerants in domestic refrigerators and home freezers XIV lnt Cong Refrig (1975) Moscow

85 Stoccker, W. F. Improving the energy effectiveness of domestic refrigerators by the application of refrigerant mixtures ORNL/ Sub/78-5463/1 (September, 1978) Oak Ridge Nat Lab

86 Stoccker, W. F., Walukas, D. J. Conserving energy in domestic refrigerators through the use of refrigerant mixtures ASHRAE Trans (1981) 87 279-291

87 Arthur, D., Little Inc An evaluation of a two-evaporator refrig- erator-freezer using non-azeotropic mixtures ORNL/Sub/82- 47952/1 (1984) Oak Ridge Nat Lab

88 Rice, C. K., Sand, J. R. Initial parametric results using CYCLE-Z- An LMTD specified, Lorenz Meutzner cycle refrigerator freezer model Proc USNC/IIR Purdu Refrigeration Conference (1990) pp 448-458

89 Bare, J. C. Preliminary selection of refrigerants for dual-circuit and Lorenz refrigerator/freezers EPA Technical Report Data (1990) EPA/600/D-90/104

90 Smith, L. K., Potter, T. F. Assessment of the cost-effective energy conservation potential of advanced refrigerator insulation ASHRAE Trans (1990) 96 1341-1348

91 Bare, J. C., Gage, C. L., Radermacher, R., Jung, D. S. Simulation of nonazeotropic refrigerant mixtures for use in a dual-circuit refrigerator/freezer with countercurrent heat exchangers ASHRAE Trans (1991) 97 447-454

92 Jung, D. S., Radermacher, R. Performance simulation of two-evaporator freezer/refrigerator with pure and mixed refrigerants Int J Refrig (1991) 14 254-263

93 Rose, R. J., Jung, D. S., Radermacher, R. Testing of domestic two-evaporator refrigerators with zeotropic refrigerant mixtures ASHRAE Trans (1992) 98 216-226

94 Liu, Z., Haider, I., Radermacher, R. Simulation and test results of hydrocarbon mixtures in a Modified-Lorenz-Meutzner cycle domestic refrigerator submitted to ASHRAE lnt J HVA C&R Research (1995)

95 Radermacher, R., Jung, D. Subeooling system for refrigeration system U.S Patent No. 5.243,837 (1993)

96 Zhou, Q., Pannock, J., Radermacher, R. Development and testing of a high efficiency refrigerator ASHRAE Trans (1994) 100 1351-1358

97 Simmons, K. E., Kim, K., Radermacher, R. Experimental study of independent temperature control of refrigerator com- partments in a modified Lorenz-Meutzner cycle submitted to A S M E Int Mech Eng Cong Exposition (1995) San Francisco

98 Pedersen, P. H., Schjaer-Jacobson, J., Norgard, J. Reducing electricity consumption in American type combined refrigerator/freezer 37th Annual International Appliance Technology Conference at Purdue University (May 1986)

99 Pedersen, P. H., Galster, G., Guibrandsen, T., Norgard, J. S. Design and construction of an efficient US-type combined refrigerator/freezer XVllth International Congress of Refrigeration (1987) vol. B, Vienna

100 Won, S., Jung, D. S., Radermacher, R. An experimental study of the performance of a dual-loop refrigerator/freezer system Int J Refrig (1994) 17 411 416

101 Jaster, H. Refrigerator system with dual evaporators for household refrigerators U.S. Patent No. 4,910,972 (1990)

102 Jaster, H. Refrigerator system with dual evaporators and suction line heating U.S. Patent No. 4,918,942 (1990)

103 Onishi, T. A New power saving household refrigerator-freezer ASHRAE Trans (1980) 86 299-310

104 Fengyun, J. Experimental research on Hangtian household refrigerator using HCF-152a Int CFC and Halon Alternatives Conference (1992) 183

Domestic refrigerators 69

105 Guo, J. X., Tan, L. C., Chen, Z. Q. A new compression/ injection hybrid cycle for domestic refrigerators Proc of the sec- ond annual Sino-American refrigeration workshop at Indiana (November, 1992)

106 Silvetti, B. Ejector expansion refrigeration cycle Refrig and Air- Cond Technol Workshop (1993) ORNL-6797, Oak Ridge Nat Lab pp C229-C238

107 Tomasek, M.-L., Radermacher, R. Analysis of a domestic refrigerator cycle with an ejector ASHRAE Trans (1994) 101

108 Lee, J., Cho, K. Y., Lee, K. T. A new control system of a house- hold refrigerator--freezer Proc Int Refrig Conference at Purdue (1994) pp 109-114

109 Lee, W. D. Development of a high efficiency, automatic defrosting refrigerator/freezer, Phase I design and development (1980) ORNL/Sub-7255/1. Oak Ridge Nat Lab

110 Bohman, R. H., Harrison, R. L. The engineering and manu- facture of a high efficiency, automatic defrost refrigerator- freezer ASHRAE Trans (1982) 88 1053-1063

111 Topping, R. F., Vineyard, E. A. Field test of a high efficiency, automatic defrost refrigerator-freezer ASHRAE Trans (1982) 88 1064 1073

112 Kim, K., Kopko, B., Radermacher, R. Tandem system domestic refrigerator/freezer Proc of lnt Appliance Technical Conference at Chicago (1995)

113 Platen, B. C. V., Munters, C. G. Refrigerator (1928) U.S. Patent No. 1.685,764

114 Taylor, R. S. Progress in household refrigeration American Gas Association Monthly (1945) XXVII 25-30

115 Sellerio, U. Device to speed the circulation of water solutions in household absorption refrigerators Proc VIII Int Cong Refrig London, UK (1951) pp 453 459

116 Watts, F. G., Gulland, C. K. Triple-fluid vapor-absorption refrigerators J Refrig (July/August, 1958) 1 107-115

117 Stierlin, H. Latest developments in domestic absorption refrigerators and the future outlook Proc XIII Int Cong Refrig Madrid, Spain (1967) No. 3.43

118 Narayankhedkar, K. G., Prakash Maiya, M. Investigation on triple fluid vapor absorption refrigerator Int J Refrig (1985) 8 335 342

119 Kouremenos, D. A., Stegou-Sagia, A., Antonopoulos, K. A. Three-dimensional evaporation process in aqua-ammonia absorption refrigerators using helium as inert gas Int J Refrig (1994) 17 58-67

120 Chen, J., Kim, K. J., Heroid, K. E. Performance enhancement

of a diffusion-absorption refrigerator lnt J Refrig (1995) submitted

121 Angrist, S. W. Direct Energy Conversion Allyn and Bacon. Inc. (1982) pp 121-176

122 Elfving, T. M. Study of design problems and mode of operation for thermoelectric refrigerators ASHRAE Trans (1963) 69 198-206

123 Stoeeker, W. F., Chaddock, J. B. Transient performance of a thermoelectric refrigerator under step-current control ASHRAE Trans (1963) 69 380-387

124 Foster, A. R. Two-stage thermoelectric refrigeration ASHRAE Trans (1964) 70 312 318

125 Feldman, C. L. Optimization of cascade Peltier cooler ASHRAE Trans (1965) 71 176-182

126 Neild, A. B., Schneider, W. E., Henneke, E. G. Application study of submarine thermoelectric refrigeration systems ASHRAE Trans (1965) 71 183-191

127 Foster, A. R., Yamamura, A. Thermoelectric generator- refrigerator ASHRAE Trans (1970) 76 157-163

128 Mathiprakasam, B. Thermoelectric cooling technology Re#ig and Air-Cond Technol Workshop (1993) ORNL-6797, Oak Ridge Nat. Lab. pp. C211-C214

129 Walker, G. Reversed Stirling cycle refrigeration machines some aspects of design ASHRAE Trans (1965) 71 123-.132

130 Berchowitz, D. M., Shonder, J. Experimental performance of a free-piston Stirling cycle cooler for non-CFC domestic refrigeration applications Proc of the XVlll th lnt Conference of Refrig, Montreal (1991b)

l 31 Manor, E., Kushnir, M., Vaknin, D. Experimental results of a refrigerator operated by a rotary Stirling cycle cooling unit Proc Int CFC and Halon Alternatives Conference (1992) pp 165-173

132 Lundquist, P. G. Stirling cycle refrigerators and heat pump Proc of the 1993 Non-Fluorocarbon Insulation, Refrig and Air- Cond Technol Workshop (1993) ORNL-6805, Oak Ridge Nat Lab pp. 185-192

133 EPA 1993 Multiple Pathways to Super-efficient Refrigerators, United States Environmental Protection Agency, Office of Atmospheric and Indoor Air Programs, EPA-430-R-93-008, June 1993

134 Thornton, R. C. History and Growth of HVAC&R Since the 1930's ASHRAE J Jan 95 (1995) 37 S-68 S-80

135 Smith, L. A., Tatsutani, M. Market Transformation Strategies for high efficiency International Appliance Technology Conference May, 1995, Urbana, IL