‘Green’ refrigerators Not since the electric r@ir@?rator rqkced the icebox hm home r@r@eration technology unhgone so dradic an overhaul

he humble refrigerator today is one of the home’s main users of both ozonedepleting chemicals and energy. But by the end of 1995, chbrofluoro- carbons, the ozonedepleting compound used in refrig- erators, will be banned worldwide, according to the

Montreal Protocol (established in 1987 under the auspices of the United Nations). Mean- while, governments are taking steps to es- tablish new standards or tighten existing ones on refrigerator energy use.

These twin forces are driving a wave of research and development in refrigeration technologies. Existing technologies are being adapted to new chemicals, processes are being refined to become more energy efficient, and innovative approaches to re- frigeration are being tested. OZONE ANXIETY. In the 1970s, scientists found that chlorofluorocarbons (CFCs) were contributing to the destruction of the earth’s ozone layer. The problem stems from the chlorine in CFCs, which binds with oxygen to dismantle ozone molecules in the upper atmosphere. As the ozone protects the earth from the sun’s harmful ultraviolet radiation, less of it means more radiation passing through, causing a substantial in- crease in skin cancer, as well as increases in cataracts and immune system problems.

In 1986, with the discovery of a hole in the ozone over Antarctica, addressing the problem of ozone destruction became a worldwide imperative. As a result, an inter- national agreement has decreed that after Dec. 31, 1995, CFCs will no longer be produced by anybody for anything.

Already efforts to eliminate them are in progress. They have long been replaced as an aerosol propellant in the United States. Most printed-circuit board manufacturers, who used CFCs for solvents, have exchanged them for ozone-friendly substitutes. And re- frigerator manufacturers are on track to

. ___ Tekla S. Perry Senior Editor

IEEE SPECTRUM AUGUST 1S.t

eliminate CFCs from their products. In fact, the first CFC-free refrigerators have been on the market for some time.

Developed in the 1930s, CFCs were quickly embraced by the refrigeration in- dustry as a coolant. They seemed notoxic- so safe, it was thought, people could wash in them. Indeed, they were welcome sub- stitutes for the somewhat dangerous sulfur dioxide, ammonia, and methyl chloride then used as coolants. In the 1960s, the industry found another use for CFCs: as a blowing agent to create foam inkulation that was more effectiveand therefore more energy efficient-than the fiberglass insulation re- frigerators were using.

But eliminating CFC coolants today will require a lot more effort than just returning to the use of their predecessors. Safety reg- ulations and product liability standards now proscribe employing any toxic coolants, and energy conservation concerns prohibit working with less efficient insulation.

“We have been forced to look at other chemicals,” said Leonard Swatkowski, vice president of the Appliance Research Con- sortium for the Association of Home Appli- ance Manufacturers 0, Chicago. In its search for alternatives, the group is con- sidering five areas of concern detailed by the Environmental Protection Agency (EPA), Washington, D.C.-ozone depletion, global warming, toxicity, flammability, and energy efficiency-and three areas of concern to industry-reliability, economics, and manufacturability.

Given these criteria, two replacement coolants have been found so far. (Others are, in all likelihood, still being investigated, though refrigerator manufacturers are re- luctant to reveal any information that could give them a leg up on their competitors.) One is a hydrofluorocarbon (HFC) called HFC-134a. The other is hydrocarbon in the form of propane and isobutane.

HFC-l34a, manufactured by E. I. W o n t de Nemours, ICI, Allied, and others worldwide, was developed in the late 1980s and first marketed in 1990. According to DuPont technical service consultant Curt Lawson, when refrigerator compressors are optimized for the new chemical, HFC-134a yields the same energy efficiency as the CFC- 12 currently used. Since it lacks the prob- lematic chlorine atom, it cannot deplete the ozone. It is also nonflammable and nontoxic, but it has enough global-warming potential to exacerbate the earth‘s greenhouse effect.

001S-9235/94/$4.0001994 IEEE

Most refrigerator manufacturers in the United States, perhaps happy to be able to simply change their DuPont purchase order from CFC-12 to HFC-134a and make the pre- scribed design modifications, have selected HFC-134a as their new coolant. They expect to complete the switch by next year. (The design modifications, according to Hitachi Ltd., Tokyo, included finding a new lubricant, because HFC-134a is insoluble in conven- tional oil and is itself a poor lubricant.)

For the foam insulation blowing agent, most US. companies have selected HCFC- 141b and will complete that changeover by year-end. This chemical does deplete the ozone, though at a much slower rate than CFCs, and it is scheduled for phaseout in the United States by the year 2003. It also has some global-warming potential.

HCFC-141b is at best an interim solution, and, according to AHAM’s Swatkowski, US. manufacturers are currently assessing eight to 10 other possible substitutes. Japanese manufacturers appear to be making similar choices, although Matsushita Electric Industrial Co., Kadoma City, Osaka, has an- nounced that it will have a foot in each camp, using foam blown with a hydro- carbon-cyclopentane-in combination with an HFC coolant in some models. HYDROCARBONS I N EUROPE. Manufacturers in Europe, though, have turned almost com- pletely to hydrocarbon alternatives, much to the delight of Greenpeace International, the environmental organization based in Amsterdam, the Netherlands. In the group’s Washington, D.C., office, Jacques Rosas, Greenpeace’s ozone campaigner, said hydre carbons have no ozone-depleting potential. Their effect on global warming is minimal, equal to that of carbon dioxide, while HFC- 134a has 3200 times the effect of carbon dioxide (although US. laws require recla- mation of most of the HFC used> and HCFC- 141b has 1200 times the effect. Rosas also pointed out that hydrocarbons are a more cost-effective alternative, not under cor- porate patent, and can be produced fairly easily, even in developing countries.

The first hydrocarbon-based home re- frigerator was designed by an East German company, Foron, in 1993. It used a 50150 mixture of propane and isobutane as a coolant and expanded polystyrene as an in- sulation. The company was on the verge of going out of business, and other European manufacturers were leaning strongly toward the HFC and HCFC replacements

25

for CFCs, when Greenpeace started a grass roots campaign by asking people to place orders for an environmentally friendly re- frigerator. Acobrdihg to Rosas, that cam- paign yielded 70 000 direct orders.

(Greenpeace also has plans for a similar U.S. grass roots effort. According to Rosas, the first US. manufacturer to announce a hydrocarbon-based refrigerator will be handed the names of at least 75 000 people who have already told Greenpeace they would like to buy such a product.)

Several European manufacturers are now producing, or have committed to producing, hydrocarbon-based refrigerators, including Mh-Siemens, Liebherr, AEG, and Electro- lux. Said Dieter Barr“, managing director of the refrigerator division for Germany’s Fksch-Siemens Hausgerate Gmbh in Munich: “we produced absolutely CFC-free products in 1992 using FCs and HFCs, but because of the relatively high greenhouse potential of these agents, we searched strongly to get

I ~chemistrylesson r- 1 Understandino how chlorofluorocar- 1

better ones. In 1993 we replaced all the CFG in the foam with cyclopentane and in the re frigeration circuit with HFC-l34a, getting tht ozone depletion to zero and reducing tht greenhouse-warming potential drastically. A the end of 1993, to get the greenhouse warming potential to an absolute minimum we decided to replace HFC with the hydro carbon isobutane, even though we had jus made investments one year earlier to changf to HFC. We did this to save the environment and to keep ourselves from criticism by thf press and environmental associations.”

So the world has chosen up sides for CF( replacement technology, and the argument! for each case can be fairly heated.

When asked why they refused to choost hydrocarbons, US. refrigerator manufac turers first point to its flammability: hydro carbon coolants are flammable, HFC coolan is not, According to Curt Lawson, a technica service consultant at W o n t Co., Wilmington Del., the manufacturer of HFC-l34a, “The po

E e P

bons (CFCs),-hydrocarbons, hydro- [ fluorocarbons (HFCs), and hydrochlo-

luorocarbons (HCFCs) work-and ir various permutations and chemi- alternatives-by no means requires 1

6i ‘6

f a chemistry degree. The chemicals being used or considered as possible refrigerants in conventional refrigerator

agents for foam refrige

Chemists tinker with

‘If

of hydrogen atoms plus

two each of fluorine and chlorine red, and green in the illustration,

26

tential environmental risk with 1% whieh gives the same efficiency as CFC-12, must be evaluated against the detyrisks and product liability costs of a flammable refrigerant.”

Rosas at Greenpeace responds that hy- drocarbon refrigerators use the same amount of flammable fluid as two Bic lighters-and no one objects to someone carrying two Bic lighters in their pocket. “A can of Pam nonstick cooking spray has more hydrocarbons,” said Larry Schlussler, owner of Sun Frost, a refrigerator manu- facturer in Arcata, Cali.

Besides, hydrocarbon proponents argue, the refrigerator is sealed. Should a leak occur, light switches and evaporators in hy- drocarbon-based refrigerators have been moved into the insulation where they are unlikely to create a spark that would reach any leaking coolant. (Whirlpool Corp., Benton Harbor, Mich., indicated that sealing problematic components in an automatic defrost refrigerator would be costly.)

At Bosch-Siemens, the company’s risk as- sessments show that the chance of a hydro- ” c h a r g e d refrigerator’s exploding can be reduced below 1 in 6 OOO OOO by making a few technical adjustments. Tests have shown that explosion results in a modest boom, the opening of the refrigerator door, and a small short-lived flame inside the cabinet.

As for hydrocarbon-blown foam, US. manufacturers point out that it is less ef- fective an insulator than hydrochlorofluoro- carbon (HCFC), so that refrigerators will use more energy and contribute to global warming. “European companies went strictly by global warming potential; they didn‘t consider energy efficiency,” AHAM’s Swatkowski told IEEE Spectmm.

Hydrocarbon proponents respond that the energy consumption over the life of a refrigerator is about the same, given the better aging properties of hydrocarbon- blown foams. Initial energy consumption is 2-4 percent higher, but that can be remedied by making the insulation slightly thicker. (Bosch-Siemens expanded its insu- lation to 90 mm for some models.)

But Swatkowski disagrees, saying that the energy penalty is more l i e 10-12 percent, and the lower figures were obtained by com- parison with a CFC-reduced foam, not with standard US. CFC foam. (The energy con- sumption of refrigerators is notoriously hard to compare, because units vary in size, features, and location-a refrigerator in a warmer room uses more energy than that in a cooler room. Also, scientific tests of dif- ferent technologies in prototypes with the same sizes and features are rare.) VACUUM IN THE WINGS. While refrigerator man- ufacturers on both sides of the hydrocarbon issue are comfortable with ule available cooling agents-HFC-l34aor isobutane--bothgroups are seeking new possibiities for insulation.

The CFC replacement, blowing agent HCFC-14lb, is being phased out, and cy- clopentane’s insulation value is lower. Work

IEEE SPECTRUM AUGUST 1994

on other blowing agents continues, but the best solution may not be discovering yet another blowing agent, but turning instead to a completely different insulation.

The technology with the most potential appears to be vacuum insulation. It works on the same principle as a thermos jar-a case of plastic or steel panels with a vacuum created between them. The insulation value of the vacuum panels is enhanced by adding a filling substance. Owens-Corning Fiber- glass Corp. uses fiberglass within steel panels in its Aura insulation, while di- atomaceous earth in steel boxes is being used in the Swiss E o Refrigerator Project, and silica in plastic panels is beiig used by DeGussa AG of Germany.

The Swis version, which uses a box within a box instead of welded panels and thereby reduces edge losses, has an insulation value that is about five to six times that of standard foam insulation, according to Dick Mennink, a consultant with Innovatiebureau Mennink, Zutphen, the Netherlands. 0wens.COrning in- dicates its Aura insulation has eight to 10 times the insulation value of foam.

But this technology is not yet ready to be introduced as a product, though it will begin showing up in products late this year or in 1995. One concern refrigerator manufac- turers have is reliability: the vacuum must last for the 15-20-year life of a refrigerator. Testing is now under way to determine how fast the vacuum will degrade and how that would affect the insulation value.

As an interim step, Electrolux AB in Stockholm, Sweden, is reportedly starting production this summer of a refrigerator that combines foam insulation with silica- filled plastic vacuum panels. This design is said to consume 23 percent less energy than standard foam insulation and can be produced on current production lines.

According to David Goldstein, energy program directorfor the Natural Resources Defeme Council Inc, New York City, vacuum panel refrigerators could be on the market within three to five years, given strong in- centives for industry. “OwensGrning expects to have panels in some quantities for manufacturers this summer, so it is a question of how well those panels, and others b e i i de- veloped more secretly, work out in reliabity and durability tests, how fast production volume is increased, and what the costs will be,” he said. Barmann at Bosch indicated that that company is putting much research effort into meum technology. And W’hirlpool is r e portedly planning to start selling vacuum panel refrigerators next year.

“We’re watching the development of vacuum panels closely,” said Rochas at Greenpeace. “They use no hydrocarbons at all to produce, have no global-warming po- tential, and no ozone-depleting potential. And the energy savings are amazing.” ENERGY EFFICIENCY. Refrigerators in the ag- gregate use large amounts of electricity compared to other appliances. In fact, refrig-

Perry-‘Green’ refrigerators

erators and freezers use 16 percent or more of electricity in US. households and about 25 percent of household electricity in Europe.

This was not a concern until the ’705 when the Federal Trade commission, Washington, D.C., mandated energy use labeling in the United States. For the fiist time, consumers became aware of the impact their choice of refrigerators could have on their electric bills.

In 1976 California issued refrigerator energy standards, mandating a 30 percent reduction in average refrigerator energy use. “Until these were passed, energy use of

refrigerators was going up at the compound rate of 6 percent a year,” said Goldstein. ‘The California law reversed the trend.”

So, even as models were getting bigger and more feature laden, energy consumption

Actual Projected I 2000

-I With

’L

‘80 ‘85 ‘90 ‘95 2000 ‘05 ‘10 ‘Saleweighted average from Association of Home Appliance Manufacturers NAECA = National Appliance Energy Consewation Act. 1987 DOE = Department of Energy

While refrigerators have increased in size and features, the average energy use of home re- frigerators in the United States has steadily declined in the last 20 years because of the es- tablishment and tightening of refrigerator efficiency standards.

- eezer chamber I

Imreasingly, microcontrollers and various sensm are being wed to improve the energy efiiency of rejiigerators. A filly “loaded” electronic refrigerator might contain the deukes shown.

27

began declining. Then the US. government mandated that in 1990 refrigerator energy consumption must decline 10 percent from 1987 levels, with another 31.25 percent drop in 1993. Further energy consumption cuts are expected to be in place for 1998, probably a 25-35 percent reduction from 1993 levels.

Europe at present has no energy effi- ciency standards in effect for refrigerators, although the European Union, Brussels, Belgium, is considering a directive that would mandate a 10 percent energy usage cut from 1992 levels by 1997, which some complain is too lax Denmark and the Neth- erlands are currently petitioning the Union to either establish Europe-wide standards or let those countries establish their own do- mestic standards. Japan’s effort to set energy standards has been deactivated until the CFC replacement issue is resolved. The Ministry of International Trade and Industry plans to reexamine the energy standards issue in the year 2000.

Supporting reductions in reftigerator energy consumption are the electric utilities, which have gone so far as to offer a bounty of as much as US $100 for less efficient re- frigerators that are replaced and taken out of use, and rebates for the purchase of refrig-

Cabinet foam insulation HCFC-141 b increased in thickness (vacuum insulation available on 1995 models) Operation controls

timized compressor operation logic-based adaptive defrost nt anti-sweat strategies ved temperature controls

Heat exchangers, with enhanced surface area for improved heat transfer

Fans High-efficiency, Iow- energy motors Aerodynamically eff tclent

heat leakage) ’ Compressor with high efficiency

optimized for HFC-134a

,Thicker insulation in door

Gasket system with improved materials and design to reduce leakage

. Cabinet foam insulation HCFC-141 b increased in thickness (vacuum insulation available on 1995 models)

the refrigerator electricity bill by one half, which can be done by using advanced tech- nologies, we can cut electricity usage in the United States by 5.7 billion kilowatthours per year-equivalent to 12 base-level power plants.” Such an effect would clearly benefit the environment as well as the economy.

Greater efficiency could be achieved through a variety of means, some of which, unfortunately, conflict with the efforts to get CFCs out of the product. For example, moving from fiberglass insulation to CFC- blown foam in the ’70s greatly boasted refrig- erator efficiency and reduced energy usage RAISIN6 THE 1.0. Improvements that raise a refrigerator’s energy efficiency include better motors, variablespeed compressors, and compressors better designed to work at tighter tolerances; better seals; fans that run at lower power and so generate less heac and redesigned wiring and drain paths to reduce the influx of warm air. It is ba- sically an incremental game, with manufac- turers adding advances to a unit. But, in addition to evolutionary im-

provements in existing technologies, a new player is contriiting to energy savings: the “controller. Refrigerator energy use con- tinued to climb steadily upward in the ’60s and7oswiththegrowingpopularityoflarger, automatically defrosting refrigerators, in which a heater goes on at preset intervals, melts the frost buildup, and drains it into an evaporating tray. Microcontrollers, which began entering refrigerators in the mid-’80s, reduce the energy used by allowing adaptive

defrost cycles. At their simplest, such con- trollers monitor the length of the defrost cycle and use that information to schedule the next defrost cycle; more sophisticated versions monitor door openings and other factors.

Two or three years ago, controllers began using fuzzy logic and more extensive sensors to control cooling cycles as well as defrost cycles. According to Ebb Salem, a staff prodwt marketing engineer at Toshiba America Electronic Components Inc., Irvine, Calif., fuzzy logic controls make judgments based on use of appliances. Were a refrigerator’s door, for ex- ample, opened frequentlybetween ll a m and noon, the controller would not run a full- strength cooling cycle during this period, but anticipate the heavy use period by running a more moderate cooling cycle beforehand. Sensors can also monitor current con- sumption, to prevent it from peaking beyond set levels, as well as monitor the humidity balance between internal and external air.

Fme-tuning a refrigerator’s energy usage through sensors and microcontrollers can greatly enhance efficiency, but, said Salem, designers at many refrigerator manufac- turers are focusing more on physical aspects of refrigeration, like insulation, because they are less familiar with the po- tential of electronics.

”You will eventually reach the theoretical limit of physics when focusing on the physical aspects,” Salem told Spectrum, “and electronics will be needed to gain further improvements. But some com- panies appear to be intimidated by elec-

tronics, and the application of electronics to energy efficiency is lagging behind.” GOLDEN CARROT. This spring, Whirlpool, a company Salem admits is not electronics shy and is generously staffed with electrical en- gineers, introduced its Golden Carrot refrig- erator. The model puts together many of the known ways of incrementally improving energy efficiency, along with HFC-134a coolant and HCFC-141b insulation, to achieve in its 1994 models a 30 percent reduction in energy usage from the 1993 standards.

This design was sparked by a contest, the Super Efficient Refrigerator Program (SERP), created by 24 US. utilities, the Natural Resources Defense Council, the EPA, EPRI, the Washington state energy office, and others. The group in July 1992 invited companies to submit proposals for energy-efficient, CFC-free refrigerators. The proposals would be judged on a number of factors, including the companies’ abilities to produce, market, and sell completed units, and the winner would receive $30 million to subsidize the development and manufacture of the refrigerator so units could be cost- competitive with existing models.

Whirlpool was announced as the winner in June 1993, and so far has shipped more than 10 OOO units at $l200-$1300 each. But Whirlpool will soon have company in the superefficient market. The SERP runner-up, Frigidaire CO, Cleveland, Ohio, is reportedly readying its design for market. Other super- efficient designs are in the whgs

The Whirlpool team spent a fair amount

IEEE SPECTRUM AUGUST 1994 28

I

of time considering alternative refrigerants before selecting HFC-l34a, according to Whirlpool’s manager of refrigeration tech- nology during the SERP effort, Vicent P. Anderson. He is now the director of envi- ronmental and regulatory programs. For the blowing agent, the group rejected cy- clopentane because of the problems it would cause in energy efficiency, decided against HCFC-123 because it caused benign tumors during rat tests, and chose HCFC-141b instead [see “A chemistry lesson,” p. 261.

As for refrigerator “smarts,” Anderson said, “Adaptive defrost control is the only mi- croprocessor-based control we’ve utilized. It has some impact, but not a huge one because the entire defrost system does not impact energy consumption much.”

Mostly, he said, the SERP contest “accel- erated things we were already doing, al- lowing us to package in one product several technologies and design options we were working on anyway.”

And that point irks some. Stinfrost’s Schlwler criticized SEW as an inefficient use of funds. It would have been better, he said, if the contest had been opened up to smaller companies that were not major re- frigerator manufacturers and that might have had innovative designs.

But SERP president Ray Farhang argued that the reason for the contest was to en- courage manufacturers to bring an efficient product to market quickly at a reasonable price. That is why, he said, it was critical to ensure that anybody entering the contest had a history of selling large volumes of products. And, happy with the program’s results, Farhang told Spectrum: “Now there is an opportunity for manufacturers to compete for green customers, a new niche created in superefficient environmentally friendly refrigerators. I feel that we had a major role in creating this market niche.”

It does appear, though, that the SEW re- Erigerator might have been more energy ef- ficient, even using conventional tech- nologies. Indeed, such a refrigerator already :xisted, at least for people unconcerned about paying premium prices for what is today a niche product.

Sun Frost began building its energy-ef- Ficient, manual-defrost model in the early ‘80% for people who get their electricity from solar cells on their house. Its 16-ft3 :0.4 m3) refrigerator uses 22 kwh of elec- tricity per month, whereas a conventional automatic defrost refrigerator might use 40 cWh per month. (Of course, at $2350 the Sun Frost costs about $1500 more than a :omparable conventional model.)

The unit is built on top of a cabinet, not )n top of the compressor. So heat can :scape more easily, the compressor is on :op of the refrigerator compartment. hderson at Whirlpool said that tests have shown that compressor location has no :ffect on efficiency.) The unit also has more nsulation than a standard refrigerator.

Perry-‘Green’ refrigerators

But perhaps the key to the Sun Frost system is its use of two compressors, one to cool the refrigerator compartment and another for the freezer. A similar US. system is being marketed by Sub-zero Freezer Co., Madison, Wis., and other types of two-compressor systems are being man- ufactured in Europe and Japan.

Having comparable advantages is the single-compressor/two-evaporator Lorenz cycle, which uses a blend of refrigerants that have varying evaporating temperatures, a s d e d “glide.”

Such systems preserve cooling power because the cooling for the refrigerator compartment occurs at a higher tem- perature, and consequently under more ef- ficient conditions. Only the cooling that is needed for the freezer compartment is done at the less efficient conditions of the lower freezer temperature. S i c e air is not exchanged between the refrigerator and the freezer, the freezer 40 longer saps hu- midity from the refrigerator compartment, and fruit and vegetables last longer.

In return, since the freezer is kept dry, little condensation and frost builds up, elim- inating the need for heaters (and, as a bonus, preventing ice cream from crystal- lizing). An advanced Lorenz cycle beiig de- veloped at the University of Maryland with funding from the EPA will consume ap- proximately 20 percent less energy, said John Hoffman, director of the EPA’s Global Change Division. ON THE HORIZON. It appears that Sun Frost has gone about as far as possible in product efficiency, given today’s technologies. Because of the extra insulation required, the unit pays a slight penalty in having below-normal storage space for its size. Researchers around the world hope to take energy efficiency several steps further with less conventional cooling technologies, which, combined with vacuum insulation, could have dramatic energy savings.

Today’s refrigerators typically cool using the vapor-compression, or Rankine, cycle. This system requires an evaporator, in which a liquid coolant boils at a low tem- perature for cooling and a compressor, or a piston in a cylinder, which connects to a motor drive by a crankshaft and a con- necting rod. The compressor raises the pressure and the temperature of the now- gaseous coolant, and a condenser is used to discharge the heat and return the re- frigerant to its liquid state. Also part of the system is a capillary tube through which the liquid coolant moves from a high- to a low- pressure state to begin evaporating again.

Among the new refrigeration method- ologies under development that use the same Rankine cycle is the linear com- pressor. In a research project funded by the EPA, Sunpower Inc., Athens, Ohio, built a linear motor that moves back and forth, instead of around, and connects directly to the compressor piston, with no crankshaft

or connecting rod. The piston, mounted on a spring and positioned within a magnetic coil, moves up and down in response to changes in the strength of the magnetic field.

This method has two great advantages, said Sunpower’s chief engineer, David Berchowitz. First, the design has few moving parts, and does not need oil as a lubricant. Second, it is easier to modulate the cooling capacity. Simply changing the driving voltage changes the amplitude of the piston and, hence, the amount of heat removed from the coolant. Such a system would be amenable to fuzzy logic control.

“The physics for a h e a r cycle was always there,” Berchowitz said, “and people have tried it in the past. They failed in the control of the piston. By using a smart control, we are able to bring the piston very close to the top of the cylinder, and we patented this technique.”

“This technology is still eight to 10 years off,” said AHAM’s Swatkowski, “but it shows great promise for increased energy efficiency.” (Sunpower estimates that com- mercial units could be marketed in two to four years.) EPA calculations indicate that such a compressor in universal use in US. refrigerators would cut carbon dioxide emissions from utility power plants by 9.1 billion kilograms a year. (Some question this number as overly optimistic.) Also under development are cooling units

that use the so-called Stirling cycle, in which the refrigerant remains a gas. Hot when compressed and cold when expanded, the gas never liquefies. This type of cooling unit uses a motor like the one in a linear compressor, and is currently considered most appropriate for very low-temperature applications.

It could be used, however, for residential refrigerators in combination with vacuum insulation, which would reduce the load on the cooling unit. According to Berchowitz of Sunpower, a prototype Stirling cooler in- stalled in a super-insulated European re- frigerator could operate on less than 8 W. “Energy usage over a whole year would be less than 70 kilowatthours,” he said. “It therefore could be fully driven by solar power, without batteries, even in northern European climates.”

Berchowitz indicated that a Stirling re- frigerator is about four years from the market, as optimization continues and reli- ability issues are worked out. The tech- nology is currently fairly expensive, because it is not in mass production. COOLIN6 BY SOUND. Also under development is thermoacoustic refrigeration. A pro- totype thermoacoustic home refrigerator has been built at the Naval Postgraduate School in Monterey, Calif. According to Steven L. Garrett, a professor of physics in- volved in its development, the technology uses stereo speakers to create sound waves. The sound waves are used to compress and expand a gas-a mixture of helium and argon-inside a stack of plastic plates.

29

An expen’mental version of a thennoacoustic cooling engine employs two electrodynamic loudspeakers, each capable of delivering 60 W of acoustical power to the resonator. The two speakers maintain a half-wavelength standing waye at a resonant frequency of 320 Hz in a 20-atmosphere (2-megapascall mixture of helium and argon. The two stacks are 11-em-diameter spirals of 52-pm plastic with a spacing of 204 pm. At either end of each stack are finned heat ex- changers attached to tubing, which allows the heat transport fluids to deliver useful cooling to the refrigerated enclosure and to exhaust the waste heat.

, I ) I

Acting like heat exchangers, the plates draw heat from the compressed gas and pass it along the stack, much like a bucket brigade, and eventually pass it outside the refrig- erator. In its expanded phase, the gas cools and then absorbs more heat from the re- frigerator. The system is environmentally preferable, as helium and argon are neither omnedepleting nor globally warming gases. rt is a forgiving technology,” Garrett told

Spectrum. “Currently, our stacks are made out of plastic film with fishing line glued to it, and our loudspeakers are of a type in- vented 50 years ago.”

Garrett indicated that as yet the thermo- acoustic technology is at a fairly early de- velopment stage, and energy efficiency is not yet as high as a traditional compressor using CFC refrigerant. “But we are close, and we are on a steep learning curve, av- eraging a breakthrough a year,” he said.

Acoustic compressors are nowhere near as efficient as today’s reciprocating piston compressors, AHAM’s Swatkowski said. “I” not saying they don’t have a chance,” he told Spectrum, ”but they need a quantum leap before they can compete. For example, they have to double the efficiency of the

30

,

speaker. And they have to tangle with reli- ability. Refrigerators are expected to remain sealed for 20 years without anybody having to do anything to them.”

Currently, a prototype acoustic refrig- erator is being tested by a European man- ufacturer, and a number of other com- panies, including Ford Motor Co. and Harmon JBL, have disclosed research efforts in the technology.

Garrett is proud that his low-budget re- search effort has prompted larger manufac- turers to investigate an alternative refrig- eration technology. “I heard someone at a large manufacturer

once say that alternative refrigeration tech- niques have been 10 years away from com- mercialization for decades,” he said. “That is the attitude large manufacturers have to al- ternatives. People who are successful in an area have no interest in changing their tech- nology.’’ (The NRDC‘s Goldstein said the problem historically has not been a lack of new technology but rather a lack of incen- tives for manufacturers to implement it.)

Garrett said his group’s recent prototype ended the question of whether thermo- acoustics was or was not a viable technology,

U-tube assembl

and now the focus is on who can do it better. “I changed the debate,” he said, “and that is what I wanted to do.” TO PROBE FURTHER. For a detailed expla- nation of refrigeration technology, see the McGraw-Hill Encyclopedia of Science & Technology, Vol. 15,7th edition, 1992.

A discussion of the pros and cons of the many alternatives for chemicals used in re- frigeration and a host of other industries appears in “Climbmg Out of the Ozone Hole,” from Greenpeace U.S.A., Public Information Office, Washington, D.C.; 202-319-2444.

For details on the thermoacoustic ap- proach, see ”Thermoacoustic Refrigerator for Space Applications,” by S. L. Garrett, J.A. Adeff, and T. J. Hofler, Journal of Thenno- physics and Heat Transfer, published by the American Institute of Aeronautics and Astre MU~~CS, Washington, D.C., OctoberDecem- ber 1993, pp. 595-99; 202-646-7400.

More information is supplied in “Thermo- acoustic Refrigeration,” by Steven L. Garrett and Thomas J. Hofler, ASHRAE Journal, published by the American Society of Heating, Refrigeration, and Air-conditioning Engineers Inc., Atlanta, Ga., December 1992, pp. 88-96; 404-636-8400. +

IEEE SPECTRUM AUGUST 1994