Refrigerators and Freezers
Refrigerators and Freezers
REFRIGERATORS AND FREEZERS
With 120 million household refrigerators and freezers in operation in the United States, their consumption of electrical energy is of major concern not only to the consumer, but also to the power generating utilities that have to provide the power. The government, charged by Congress with guarding against air pollution, protecting the Earth's ozone layer, and fighting global warming, has a keen interest in refrigerator energy consumption.
Vapor compression, absorption refrigeration (instead of electric power, uses heat as the source of energy), and thermoelectric refrigeration (the direct conversion of electrical energy to cooling effect), are the principal means of refrigeration. Of the three methods, vapor compression, often referred to as mechanical refrigeration, is the most energy efficient, approximately two times more efficient than absorption refrigeration, and four times more efficient than thermoelectric refrigeration. Vapor compression is by far the most popular means for refrigerating household refrigerators and freezers, although the other two technologies have unique advantages in some specific applications.
VAPOR COMPRESSION BASICS
Figure 1 shows the four basic elements of the vapor compression system: (1) the evaporator, where the refrigerant vaporizes, and thus absorbs heat from the surroundings; (2) the compressor, where the refrigerant vapor is compressed (typically in the ratio of ten to one); (3) the condenser, where the refrigerant vapor of high pressure and high temperature is condensed by rejecting the heat absorbed by the evaporator, together with the heat of compression, to the atmosphere; and (4) the expansion device, be it an expansion valve or a capillary tube, that allows the liquid refrigerant arriving from the condenser at high pressure and at room temperature, to enter the evaporator, and repeat the refrigeration cycle.
Figure 2, the pressure-enthalpy plot of the standard vapor compression cycle, traces the state of the refrigerant through the refrigeration system. (Enthalpy represents the energy of the refrigerant as it circulates through the various components.) Note that the evaporator absorbs heat at low pressure, and the condenser rejects the heat absorbed by the evaporator and the work of compression, at high pressure. The curvature is the saturation curve of the refrigerant. It delineates the various statesthat the refrigerant passes through during the cycle, liquid to the left of the curve, two-phase mixture within the curve, vapor to the right of the curve.
The efficiency of any device is the ratio between what we are after to what we have to pay for it. In the case of a refrigerated appliance, we are after the refrigeration effect, (i.e., heat absorbed by the evaporator) and we pay for it with electric power that runs the compressor. In Figure 2, the evaporator absorbs more energy in the form of heat than the energy supplied to the compressor in the form of electricity. The ratio of the two is called the coefficient of performance (COP), a dimensionless number. Compressor efficiency is commonly expressed as the energy efficiency ratio (EER), the ratio of the refrigeration effect in Btu per lb of refrigerant, to the energy input to the compressor, in watt-hour per lb of refrigerant. The EER of a typical refrigerator compressor in the 1990s was about 5.5 Btu per watt-hour.
THE HISTORY OF VAPOR COMPRESSION
In 1748, William Cullen of the University of Glasgow in Scotland made the earliest demonstration of man-made cold when he evaporated ether in a partial vacuum. In 1824, Michael Faraday of the Royal Institute in London, found that ammonia vapor, when condensed by compression, would boil violently and become very cold when the pressure is removed. Only ten years later, in 1834, Jacob Perkins, an American living in London, patented the first closed vapor compression system that included a compressor, condenser, expansion device, and evaporator. This is the same basic system as in today's domestic refrigerator. He used ethyl ether as the refrigerant, and received British Patent 6662, dated 1834.
In 1844, John Gorrie, Florida, described in the Apalachicola Commercial Advertiser his new machine for making ice. The delays in ice delivery from the Boston lakes forced Gorrie to build his ice machine so that his hospital fever patients could be assured that ice would always be available.
In the early days ethyl ether and methyl ether were the refrigerants of choice. In 1869 the first ammonia system was introduced. Today ammonia is still a very common refrigerant not only in commercial vapor compression systems, but also in absorption refrigerators.
In 1915 Kelvinator marketed the first mechanical domestic refrigerator, followed shortly by Frigidaire. These models used belted compressors underneath wooden ice boxes that required frequent maintenance because of leaky shaft seals. In 1910 General Electric took out a license to build a sealed compressor that eliminated the need for shaft seals. The machine was originally invented in 1894 by Abby Audiffren, a French monk, who developed it for the purpose of cooling the monastery wine. After a major redesign it was introduced as the Monitor Top refrigerator in 1927, and was an instant success. Up to this time refrigerator cabinets were made principally of wood, with a great deal of carpentry, and not suitable for mass production. The Monitor Top had a steel cabinet, fabricated by punch presses and welding. The steel inner liner was attached to the outer shell by phenolic plastic breakerstrips. A more current production is the 27 cu ft refrigerator with through-the-door water and ice dispenser.
The Monitor Top used a toxic refrigerant, sulfur dioxide. In the late 1920s, Frigidaire Corporation, then a leading manufacturer of household refrigerators, asked the General Motors research laboratory to develop a refrigerant that is non-toxic and non-flammable. The result was a chlorofluorocarbon (CFC), namely dichlorodifluoromethane, commonly known as Refrigerant 12 of the Freon family. By the time of World War II, it completely replaced sulfur dioxide. In the 1980s, it was discovered that CFCs deplete the ozone layer surrounding the Earth, thus increasing the likelihood of skin cancer. In 1987 the United States joined other industrial nations in signing the "Montreal Protocol on Substances that Deplete the Ozone Layer." The phase-out of CFCs began on July 1, 1989, and by 1997, a hydrofluorcarbon, HFC134a, with zero ozone depletion potential, became the dominant refrigerant in the United States. The phase-out of CFCs in developing countries is on a slower schedule.
In the years after World War II many new customer-oriented features were introduced : plastic food liners, foam insulation, combination refrigerator-freezer, automatic defrosting, ice makers, and child-safe door closures. In the 1940s, the typical refrigerator was a single door cabinet, requiring periodic defrosting, with a storage volume of about 8 cu ft. By the 1990s, it developed into a 20 cu ft automatic defrosting two door combination refrigerator-freezer, with an ice maker.
The energy consumption of refrigerators and freezers is regulated by the Congress of the United States. The "National Appliance Energy Conservation Act of 1987" (NAECA) established an energy conservation standard "which prescribes a minimum level of energy efficiency or a maximum quantity of energy use". The Energy Standards were revised effective January 1, 1993, and again, effective July 2001. The first revision resulted in a cumulative 40 percent reduction in energy consumption when added to the initial standards. The result of the second revision in 2001 is an additional 30 percent reduction. A historical chart, Figure 3, shows actual and projected improvements in the use of electrical energy for refrigerator-freezers. In 1978 one manufacturer offered a 20 cu ft refrigerator-freezer that consumed 1,548 kWh per year. In 1997, the same manufacturer had a 22 cu ft refrigeratorfreezer on the market that consumed 767 kWh per year. The 2001 target for the same product is 535 kWh per year. The latest mandatory energy reduction will add about $80 to the cost of the product, and result in an annual saving of $20 for the customer. Based on an annual production of 8.5 million refrigerators and freezers in the United States, this significant reduction of energy use will eliminate the need for building eight new power plants.
The energy consumption of a refrigerator is a function of two distinct elements: the heat that the evaporator needs to absorb to maintain the specified storage temperatures and the efficiency of the refrigeration system to reject that heat, and also the heat of compression, by the condenser. The heat load is determined by the storage volume of the appliance, the interior temperatures to be maintained, and the effectiveness of the insulation surrounding the storage space. The engineering challenge resides in reducing the heat load, and at the same time, improving the efficiency of the refrigeration system.
THE HEAT LOAD
Figure 4 shows the cabinet cross section of a typical automatic defrosting refrigerator. To defrost a refrigerator, forced convection heat transfer is needed between the evaporator and the load, thus the electric fan. The role of external heaters is to prevent the condensation of water vapor (sweating) on external cabinet surfaces. As the figure indicates, the heat flow through the walls of the cabinet represents 52 percent of the total heat load. Consequently, the thickness and thermal conductivity of the insulation has a major impact on the energy consumption of the appliance. Until the early 1960s the insulating material was glass fiber. Then the development of polyurethane insulation revolutionized the refrigeration industry. With an insulating value twice as good as glass fiber, wall thickness could be reduced by one-half, resulting in additional cubic feet of useful storage. In contrast to applying the glass fiber by hand, the assembly line for polyurethane foam is completely automated. The foam adds structural strength to the cabinet, allowing significant reduction in the thickness of the materials used on the inner and outer surfaces.
In the 1980s, with the discovery of the ozone depletion potential of CFC refrigerants, the R11 blowing agent used in polyurethane foam was destined for phase-out in the United States by 1996, just like refrigerant R12 of the refrigeration system. A frantic search began for environmentally acceptable substitutes and Table 1 lists some of the more promising candidates. Note that in addition to ozone depletion potential (ODP), a relatively recent environmental concern, global warming potential (GWP) (sometimes referred to as the "greenhouse effect") is also listed. The numbers are all relative to the properties of CFC 11, the worst offender on the list. In their search for an alternative to R11, the United States and Western Europe went their separate ways. The American choice was hydrochlorofluorocarbon (HCFC) refrigerant R141b. Europe, with an annual production of some 17 million refrigerators and freezers, and under pressure from environmentalists,
|Blowing Agent||ODP 1 G||WP 2|
|1Ozone Depletion Potential (ODP)|
2Global Warming Potential (GWP)
went to pentane.. While both substances have zero ODP, it is the difference in GWPs that tilted the Europeans toward pentane. As a matter of fact, R141b is scheduled to be phased out in the United States by January 2003, due to its residual ODP. Pentane is a hydrocarbon, like gasoline, and highly flammable. Due to the mandated safety measures, the conversion from R11 to pentane at the manufacturing sites is very extensive and expensive. Both foams, whether blown with R141b or with cyclopentane, have a higher thermal conductivity (poorer insulating value) than R11 blown foam. Consequently, these new formulations, require considerable redesign. For the energy consumption to remain the same, either the cabinet wall thickness needs to be increased or the efficiency of the refrigeration system needs to be improved.
To meet the 2001 U.S. energy standards and the 2003 phase-out of HCFCs, there is a great incentive to develop a significantly better thermal insulation. The most dramatic approach would use vacuum panels for insulating the cabinet. A number of U.S. and Japanese manufacturers have developed such panels and placed these kinds of refrigerators in homes. The panels consist of multilayer plastic envelopes filled with precipitated (fumed) silica. The claimed thermal conductivity is one-fourth that of polyurethane foam. The two major obstacles are cost and the maintenance of vacuum for twenty years.
THE REFRIGERATION SYSTEM
To meet the 1993 Energy Standards, the industry undertook , at considerable cost, the optimization of the various refrigeration system components. The most significant improvement was the increase in compressor efficiency, from an EER of about 4 to about 5.5. Other system improvements included more efficient fan motors, more effective heat transfer by the evaporator and the condenser, and less defrost energy. In the early 1980s, both the Whirlpool Corporation and White Consolidate Industries introduced electronic defrost controls. Heretofore, an electric timer initiated the defrost cycle, typically every twelve hours, whether the evaporator needed it or not. With the electronic control the defrost interval is more a function of frost accumulation than of time, and thus referred to as a "variable defrost control" or as "adaptive defrost."It saves energy by being activated only when needed.
Further improvements in system efficiency will be difficult to achieve with evolutionary changes. Following are some of the more promising areas of development:
Rotary Compressor. Inherently a rotary compressor is more efficient than the current reciprocating compressor. (room air-conditioners have been using rotary compressors for decades.) Several manufacturers in the U.S. and Japan have produced refrigerators with rotary compressors, but experienced long-term quality problems.
Dual Evaporators. A typical refrigerator-freezer, with automatic defrost, has a single evaporator for refrigerating both compartments, at a pressure that is determined by the temperature of the freezer. With separate evaporators in each compartment, the compressor would alternate between the two evaporators in providing refrigeration. The fresh food compartment has a significantly higher temperature than the freezer compartment, so it would requier less pressure to refrigerate, resulting in energy savings.
Variable Speed Compressor. Every time the compressor cycles on during the first few minutes of running, the compressor works on building up the pressure difference between the evaporator and the condenser, and is extremely inefficient. By letting the compressor run all the time, and modulating its speed according to the refrigeration needs, these periods of inefficiency can be eliminated.
Sonic Compression. Sonic compression is appealing as a potential low-cost, high efficiency oil-free technology. It should work with a wide range of refrigerants, including hydrocarbons, fluorocarbons, and ammonia. Theoretical COP is comparable with current vapor compression refrigeration cycles. The initial prototypes have a target cooling capacity of 110 to 250 W.
Absorption refrigeration is based on the great affinity between certain liquids and gases. For example, 1 cu ft of water is capable of absorbing 800 cu ft of ammonia gas. One might look at this process as compression of a sort. Starting out with a small volume of absorbent (water) and a large volume of refrigerant gas (ammonia), the process ends up with a small volume of liquid solution of the refrigerant in the absorbent. In comparison to the vapor compression system (Figure 1), the compressor is replaced by two components, the absorber and the generator (Figure 5). The condenser and the evaporator function just as in the vapor compression cycle. Upon entering the generator, the aqua-ammonia solution is heated, and the ammonia in the solution vaporizes first, due to it's lower boiling point. The ammonia vapor then enters the condenser as the water drains back to the absorber. The condensed liquid ammonia is allowed to expand into the evaporator. As it vaporizes it absorbs heat from the load, and then it is reabsorbed by the water in the absorber. The cycle is ready to repeat itself.
Vapor compression uses the highest form of energy, namely electrical energy. In absorption refrigeration, the energy input is any source of heat (e.g., electrical energy, bottled gas, kerosene, or solar energy).
The first practical absorption refrigerator was developed in 1850 by Edmund Carre of France, who was granted a U.S. patent in 1860. During the Civil War, the supply of ice from the North was cut off and Edmund's brother, Ferdinand Carre, shipped a 500 lb per day ice machine through the federal blockade to Augusta, Georgia, to be used in the convalescent hospital of the Confederate Army. Absorption refrigeration enjoyed it's heyday in the United States in the 1920s, with several manufacturers, like Servel and Norge, providing products. As larger refrigerators were demanded by the public, absorption refrigerators were unable to compete due to their inefficiency. The only remaining role for absorption refrigeration in the United States is in remote areas that are without electricity, and in boats and recreational vehicles that have bottled gas. There are many under-developed countries where absorption refrigeration is still the principal means of food preservation, due to lack of electricity.
In 1821, Thomas Seebeck, an Estonian physician, discovered the existence of an electric current in a closed circuit consisting of unlike conductors, when the junctions between the conductors were at different temperatures. This discovery is the basis for thermocouples, used for all kinds of temperature measurements. In 1834, Jean Peltier, a French watchmaker, discovered the reverse of the "Seebeck Effect"; namely, if a direct current passes through a junction of two dissimilar metals in the appropriate direction, the junction will get cold. When thermocouples are put in series, one face of the assembly will get cold, the other hot.
The "Peltier Effect" was a laboratory curiosity until 1954, when Goldsmid and Douglass of England achieved a temperature difference of 47°F between the two junctions using a semiconductor, bismuth telluride, types negative and positive. It was predicted at that time that by the 1970s thermoelectric refrigeration would replace the vapor compression system in refrigerators and freezers. The 1966 Sears catalog offered a "Coldspot Thermo-electric Buffet Bar" that "chills foods and drinks to 40 degrees...." The buffet bar took advantage of an exciting feature of thermoelectric devices; namely, by reversing the electric current flow, the cold junctions will get hot, and the hot junctions cold. By the flick of a switch, the 2 cu ft compartment would change from a refrigerator to an oven. Due to its inefficiency, the thermoelectric refrigerator did not replace vapor compression.
The key to future advances in thermoelectric refrigeration is the discovery of a semiconductor more efficient than bismuth telluride. Until then, the only marketable applications are small portable refrigerators for cars and boats. They take advantage of the readily available direct current provided by the battery. Just plug it into the cigarette lighter and it begins to refrigerate.
J. Benjamin Horvay
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