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The space-cooling savings that one might suppose have been most thoroughly mined out, because they're the basis of a sophisticated global industry, are in the conventional (refrigerative) air-conditioning systems that render conventional large buildings more or less habitable. About 10-20% of the total capital cost of a major commercial building is its HVAC (Heating, Cooling, and Air-Conditioning) systemespecially the cooling part. The central water-cooled chillers at the heart of almost every big building complex today are massive, precise, expensive machines that must be ordered years ahead. The building is constructed around them. They run for decades. They and their smaller, even less efficient cousins use one-sixth of all electricity in the United States. On hot summer days, air-conditioning uses the full output of about 200 thousand-megawatt (billion-dollar or more) power stationsabout 43% of the nation's entire peak electric load. Residents and businesses in the city of Houston in 1982 paid $3.3 billion for cold airmore, The Wall Street Journal remarked2, "than the gross national product of 42 African nations." In East Asia in the early 1990s, air-conditioning was adding about 25,000 to 50,000 megawatts of new peak demand per year. One might suppose that these air-conditioning systems' design has been pretty well perfected over the past 70-odd years. Well, not exactly. In fact, one of their most basic design assumptions turned out to be fallacious. In the 1980s, Professor Sam Luxton, an iconoclastic mechanical engineer at the University of Adelaide in South Australia, and his colleagues became curious about how air actually gets cooled and dried by being blown through a cooling coil. The traditional cooling coil consists of many closely spaced sheets of copper, penetrated by tubes that weave back and forth carrying chilled water. Textbooks following 1921 laboratory findings by air-conditioning's inventor, Willis Carrier, say that the airflow is turbulent, full of puffs and eddies, and that as the air cools, the water condenses in a thin film. Luxton's group built a special wind tunnel to check. Surprise! Actually the airflow was almost completely smooth (laminar), and the water beaded up into a dense sweat of little droplets covering the cold plates. Mr. Carrier, it seems, had misinterpreted his lab data, and every air-conditioning company in the world, including his distinguished namesake, had been misdesigning its coils ever since. Armed with actual observations, Luxton realized that it would make more sense to turn the coil around sideways: to make it shallow instead of deep, space the plates widely instead of closely, and above all, move the air through it slowly and the coolant quickly, rather than the other way around. Why? First, because it takes hundreds of times less energy to move water (per unit of heat transferred) than to move air. Second, because slow airflow leaves the little droplets to bulge out from the plates, extending the surface area over which heat transfer can occur. Blow too fast and you smear out the droplets into a film. Blow harder and you blow them away, eliminating their advantage. This redesign into a "low-face-velocity/high-coolant-velocity" coil sounds simple and obvious enough, but its effect is profound. The pressure drop from forcing air through the redesigned coil drops by 95%. This decreases fan size, capital cost, and noise. Dehumidification per unit of cooling improves by nearly one-third. The coil costs the same, but the smaller fan costs less, so total capital cost goes down. And the more you reduce the airflowto cool the room on days cooler than the very hottestthe better the coil dehumidifies. This matches cooling needs, maintaining excellent comfort year-round and far outperforming conventional coils. Furthermore, when combined with the very low-friction air-handling system and efficient fan already described, the smaller fan and lower airflow needed for the redesigned system often fall so low that the conventional "silencer" is no longer required: at most, residual noise can be electronically cancelled with inexpensive "antinoise" devices, such as are becoming common in high-end Japanese refrigerators and cars. But removing the silencer still further reduces the pressure drop against which the fan must fight, making it still smaller, cheaper, quieter, and less energy-consuming. Next step: Make the air filters bigger. Less friction. Smaller fans. Less capital cost. Less energy. Slower airflow. Less noise. Even less need for silencers and antinoise. Less friction. Smaller fans. Less energy. Less noise. Also better filtration. Filters last longertheir lifetime increases as the inverse square of air velocity. Less maintenance. (Same thing in cleanrooms, only more so.) The new cooling-coil design is only part of a whole family of interlinked innovations marshaled by Eng Lock Lee, the negawatt wizard of Singapore. Together, they let him air-condition big buildings and electronics factories in some of the world's muggiest climatesSingapore has 84% relative humidity year-round, and ranges from hot to broilingusing about one-third the normal amount of air-conditioning energy: 0.61 kilowatts per ton3 of cooling for the entire system, vs. a local norm of about 1.75. Among Lee's main innovations:
1 Cler, G., Shepard, H., Gregerson, J., Houghton, D.J., Fryer, L., Elleson, J., Pattinson, B., Hawthorne, W., Webster, L., Stein, J., Davis, D. & Parsons, S. 1997: Commercial Space Cooling and Air Handling Technology Atlas, E SOURCE, Boulder CO, www.esource.com. 2 Burrough, B., "In Houston, The Ubiquitous Air Conditioner Makes Tolerable an Otherwise Muggy Life," Wall Street Journal, p 31, 21 Sept 1983. 3 A refrigerative ton, or 12,000 BTU/hour, or 3.52 thermal kilowatts, is a rate of cooling, named for the coolth provided by melting one short ton (2,000 lb) of icethe traditional way of buying coolingin 24 hours. 4 His chillers use about one-third less toonot around 0.600.75 but originally around 0.50 and lately in the low 0.40sbut that's the source of only a fifth of his total savings; the other four-fifths are in the normally neglected "auxiliaries" that actually use two-thirds of the system's total energy.
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