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Alternative refrigeration systems: much greater efficiency is needed.

Alternative Refrigeration Systems: Much Greater Efficiency is Needed

There might be a future without both CFCs and compressors -- if such alternative systems as the Stirling engine can be made to work on a commercial scale.

Is there a Stirling engine in your future? Not yet, it seems -- but there are people working on that and other alternatives to the now-standard refrigeration systems using compressors and ammonia or chlorofluorocarbons (CFCs).

With production of CFCs being banned by the turn of the century under international treaty, and hydrochlorofluorocarbons (HCFCs) not likely to remain legal much longer, the search is on to find other, environmentally-safe refrigerants and refrigeration systems.

Most of the attention is on replacements for CFCs and HCFCs: hydrofluorocarbons (HFCs) being developed by DuPont. There is also a campaign on to increase the use of ammonia systems, which already dominate the refrigerated warehouse industry and could be made safer even for applications like air conditioning at hospitals.

With the recent breakthrough in Europe (see related story on page 176) on use of polyol esters to replace mineral oils in single-chamber systems also using HCFCs and HFCs to replace CFCs, it no longer seems as urgent to find alternatives for the entire systems now in use by the commercial refrigeration industry.

But research is underway on a number of systems that would do away with compressors altogether, along with the need for CFCs, HCFCs or HFCs. There are also suggestions for replacing complex chemicals with propane or hydrogen, which have traditionally been taboo for the obvious reason that they are explosive. But are any of these ideas viable alternatives?

Apparently closest to commercial feasibility, although only for the refrigerated transport industry, is the Stirling engine. This is actually an idea that goes back to 1816, when Robert Stirling, a Scottish clergyman,

invented a system that could be used as either an engine with external heat energy, or as a heat pump driven by mechanical energy. Its potential for refrigeration was first realized in 1873, but it wasn't efficient enough to compete with compressor systems.

More than three years ago, the Refrigeration Research Foundation sent a team to investigate the potential of a modified Stirling engine produced by Cryodynamics, Inc., Mountainside, New Jersey, USA. A demonstration of the device impressed the team, headed by David Frackelton, vice president of the Foundation and co-chairman of the Refrigeration Engineering Committee, that it "indeed got cold in short order." But the team concluded that, with a brake horsepower per to (bhp/tr) ratio of 4.1 at ambient temperature of 0 [degree] to 100 [degrees] F, it just wasn't energy efficient enough.

Even a small industrial reciprocating compressor has a rating of 1.85 bhp/tr, Frackelton noted in his report, while an economized screw compressor or a large reciprocating compressor rates 1.7. "This means that the Cryodynamics unit would require 2.35 times as much energy as conventional industrial units today. This would be unbearable for the public refrigerated warehouse industry."

Well, that was three years ago, but the chief operating officer for Cryodynamics, Irv Gerb, admits that modified Stirling engines still can't be scaled up for frozen food plants or refrigerated warehouses. They can, however, be used in refrigerated transport, he told Quick Frozen Foods International.

Gerb said he questions the validity of the bhp/tr ratio as a true measure of efficiency. But on Cryodynamics' test stands right now, he said, is a Stirling unit that can produce 10,000 BTU at a COP (cooling effect/input power) of 1.2. Conventional truck refrigeration units, he said, claim an efficiency of 1.5, but he disagrees with that figure because it doesn't reflect the power consumption of the fans and compressor. At any rate, he told QFFI, Cryodynamics has proved it can create efficiencies "very much akin" to those of compression cycle refrigeration system.

A basic Stirling engine uses two opposing pistons, a compression piston and an expansion piston, in the same chamber. The refrigeration cycle begins with the compression piston compressing the working fluid (Cryodynamics uses helium, and so does Sunpower, another concern based in Athens, Ohio) in the space between that piston and the regenerator matrix -- a porous material that acts as a heat exchanger. The fluid heats up as it contracts, and heats up further when the expansion piston comes into play. But in a series of exchanges between the fluid and the matrix, the heat is ejected from the system.

Frackelton said that one problem is that a Stirling engine soon cools down to - 100 [degrees] F, and therefore becomes less efficient. What the industry needs is a system that cools down to -30 [degrees] to -40 [degrees] F, in order to maintain a refrigerated warehouse temperature of -10 [degrees] to -20 [degrees]. Maybe a medium other than helium would work, he suggested; as it is, "You'd need 10 or 12 Stirling engines to replace three or four compressors," in a million cubic foot facility. Gerb said the only medium more efficient than helium would be hydrogen, which has obvious problems, but added that he hasn't really done any work in "scaling up" the Stirling system for warehouse use.

"Scaling these units up gets you into very significant mechanical problems," he conceded. That's why he isn't working on anything larger than a three-ton system (Frackleton is thinking in terms of a 100-ton system). "Nothing about refrigeration is simple," he said; still, with enough research and development money, somebody might be able to make the Stirling engine viable on a larger scale. The same may be true of other dark horse candidates for the refrigeration system of the future, which have been described in technical and trade literature.

One of these, the thermoacoustic engine, was covered in the Aug. 11, 1990, New Scientist (the Sept. 1 issue detailed Sunpower's work with the Stirling engine). Developed by a research team at the U.S. Naval Postgraduate School in Monterey, California, it was originally conceived for use in space, but could have earthbound applications. The core of the system is a highly modified loudspeaker called an acoustic driver. The driver generates loud noise (160 decibels) in a tube filled with inert gases and layers of plastic strip. Oscillating gases transfer heat to the plastic, and the bottom of the tube is cooled.

In Japan, Sanyo Electric Co. is trying to perfect a refrigeration system for domestic appliances that uses a hydrogen absorbing alloy and does away with the compressor and CFC fluid. With hydrogen under heavy pressure, the alloy absorbs it and gives off heat; when the pressure is lowered or heat is added to the alloy, it releases hydrogen and absorbs heat. Applying heat to the alloy would thus make it possible for a refrigerator operating under the Sanyo system to keep food cold.

Environmentalists are trying to mount pressure or alternative refrigeration technologies that don't rely on either CFCs or any of the substitutes proposed by the chemical industry. Tracey Heslop, an atmospheric activist with the Greenpeace movement in Britain, accuses the chemical industry of trying to "blackmail" the world into accepting its products. "The trouble is that many user businesses such as in refrigeration get most of their advice from the chemical industry," she complained last summer to the London Financial Times.

Heslop thinks more research and development funding should be put into new technologies using helium or propane as coolants. Frederik Pohl, a writer now collaborating with Isaac Asimov on a book about the environment, also argues for the revival of propane for refrigerators, because modern designs would require only a fraction of the amount of gas as the versions that were abandoned decades ago in favor of Freon models. "The quantity of propane that would be necessary is so much smaller that the risk of explosion would be negligible," he told QFFI.

But Michael Shaw of the International Association of Refrigerated Warehouses (IARW) isn't keen on that idea, at least for commercial-scale installations. "You'd have all kinds of problems with OSHA (The U.S. Occupational Safety and Health Administration)," he said, adding that a refrigerant has to meet a number of criteria, such as being efficient in a given temperature range, and being compatible with materials used in the system (copper, rubber, plastics, etc.). "For that reason, I don't think propane is going to fly," he said. "The same goes for hydrogen."

"Let's hope there is a winner somewhere out there," wrote Hugh W. Symons, senior vice president for research and technical services at the American Frozen Food Institute (AFFI), in commenting on the Stirling engine, thermoacoustic engine, and propane. Frozen Food Report, AFFI's house organ, devoted a lenghty article, "A World Without CFCs: Can the Industry Survive?" to the subject in its July-August 1990 issue.

"Any |winner' has to have zero ODP, a low GWP, be thermally as much as or more efficient than CFCs/ammonia in use today, and be well tested for reliability," Symons told QFFI. "None meet all these criteria yet." Thermal efficiency is often overlooked, but it is very important in the United States, where most electricity is generated by coal-fired plants that add more carbon dioxide to the atmosphere per kilowatt hour than any other kind of fossil fuel-based plants. Switching to an inefficient refrigeration system would thus save the ozone only at the price of aggravating global warming.
COPYRIGHT 1991 E.W. Williams Publications, Inc.
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Title Annotation:Warehousing World
Publication:Quick Frozen Foods International
Date:Apr 1, 1991
Previous Article:Is the cold war really over? Europe makes a breakthrough.
Next Article:Int'l Assoc. of Refrigerated Warehouses marks 100 years of service to industry.

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