The AC of Tomorrow? Tapping Deep Water for Cooling
for National Geographic News
|September 10, 2004|
To anyone who's taken a dip in Lake Ontario, it seems like a no-brainer: Use the lake's icy waters to keep nearby cities cool. Last month Toronto did just that, announcing that its 170-million-dollar (U.S.) deep-lake water cooling system, the largest of its kind, was up and running.
Also known as "lake-source cooling" or "deep-source cooling," the process uses water pumped from the frigid depths of adjacent lakes or oceans to cool municipal buildings.
The technology is described as a clean, renewable, and sustainable alternative to conventional air conditioning and is used around the world. But such systems can have their critics.
Toronto's Hollywood-style unveiling featured music, dancers, and film star Alec Baldwin. But most of the action was below the surface in nearby Lake Ontario.
The Canadian city's new cooling project draws water from three miles (five kilometers) offshore and 270 feet (83 meters) down, where the temperature of Lake Ontario stays near 39.2 degrees Fahrenheit (4 degrees Celsius) throughout the year. Water is pumped to an island-based filtration plant and then sent through heat exchangers at an onshore pumping station.
The heat exchangers allow the lake water to cool a separate, self-contained water circulation system, which flows through buildings in downtown Toronto. The lake water, meanwhile, ends up as drinking water for the city.
Enwave, a Toronto energy company partly owned by the city, developed the system. According to the company, the new cooling system will eventually be able to cool 30 million square feet (2.8 million square meters) of office space, while using 75 percent less energy than conventional air-conditioning.
Chris Asimakis, Enwave's chief operating officer, says that when Toronto's system reaches full capacity, the city will conserve 59 megawatts of energythe amount of power required to cool 12,000 homes by traditional means.
The project will keep 44,100 tons (40,000 metric tons) of carbon dioxide out of the air, according to the company's Web site. The greenhouse-gas savings is equivalent to keeping 8,000 cars off the road.
Enwave says the system will not only trim use of ozone-depleting refrigerants such as CFCs but also keep buildings cool even during blackouts when fully commissioned.
"People were skeptical at first," Asimakis said. "But eventually the local environmental community gave us their unanimous support."
The project currently cools ten Toronto buildings, including the tiny Steamwhistle Brewery and three skyscrapers in the Toronto Dominion Center, a financial-and-business office center.
"People on the system are truly proud to be on it," Asimakis said.
For over a century engineers have experimented with different kinds of lake-based cooling systems. But modern technology has finally made it possible to apply what's known as district cooling (using chilled liquids to cool groups of buildings) on a larger scale with more benefits to the environment.
While Toronto has the largest deep-source cooling project yet, it's not the first city to plumb the depths of North America's glacial lakes.
Four years ago Cornell University inaugurated a 57-million-dollar (U.S.) lake-source cooling plant. The system cools university buildings and a nearby high school in Ithaca, New York.
The plant draws 39-degree Fahrenheit (3.9-degree Celsius) water from 250 feet (70 meters) below the surface of Cayuga Lake, a glacially carved lake that is 435 feet (132.6 meters) deep at its lowest point.
W.S. "Lanny" Joyce is an engineering manager at Cornell's department of utilities and energy management. "The system has definitely exceeded our expectations," Joyce said, speaking by cell phone from a boat on Cayuga Lake as he checked the plant's intake system.
"We're saving 25 million kilowatt-hours per year now," he said, adding, "That's an 86 percent energy savings"6 percent more than estimated.
Cornell's lake-source cooling plant has proven more reliable and easier to operate than predicted, according to Joyce, who said it has also won numerous engineering and environmental awards, including the New York State Governor's Award for Pollution Prevention.
The Cornell plant differs from its larger Toronto counterpart in at least one key aspect, however: Warmed water from the university system is pumped back into a shallower part of Cayuga Lake. This has some environmentalists worried.
The Cayuga Lake Defense Fund, a local environmental group, has warned that the warmed water will promote blooms of algae, bacteria, and aquatic weeds in a part of the lake already impacted by human activity.
"It's a good idea, but bad siting and implementation," said Rich DePaolo, a group spokesman. "The discharge should be returned to the same strata of the lake [from which it's drawn]."
"We're not against the technology," DePaolo added. "If it was like Toronto"with no discharge"we'd stand up and applaud it."
DePaolo said "the jury is still out" on the project's ecological effects. But university officials contend that the effect of the warmed water is negligiblethe equivalent of four or five extra hours of sunlight per year.
"Four years of data show that in summer, the returned lake water is colder, clearer, and lower in total nutrients than the part of the lake to which it is returned," Cornell's Joyce said.
Engineers have also turned the deep ocean as a cooling source. Because of the churning action of wind, waves, and currents, ocean water must be drawn from greater depths to get consistently cold temperatures.
The Natural Energy Laboratory of Hawaii Authority (NELHA), a state research facility located on the Big Island of Hawaii, runs its own deep-source cooling plant. The system cools buildings on the agency's campus, which overlooks the Pacific Ocean. The plant draws 42.8-degree Fahrenheit (6-degree Celsius) seawater from a depth of 2,000 feet (610 meters).
"NELHA saves about [U.S.] $3,000 a month in electrical costs by using the cold seawater air-conditioning process," said Jan War, an operations manager. "We still use a freshwater loop to cool our buildings, since seawater is so corrosive."
Makai Ocean Engineering, a private company based in Honolulu, is also developing plans to cool all of the city's downtown using a similar system.
Elsewhere, Stockholm has used its unique location on the shore of the Baltic Sea and at the mouth of Lake Malaren (the largest lake in Sweden) to build a deep-source cooling system for its downtown buildings. Another large project is planned for Dubai in the United Arab Emirates.
So far deep-source cooling is only practical for communities with numerous buildings located near large bodies of water. But many of the world's major cities, settled during the golden age of sailing ships, are close to shoresomething to think about the next time a dip in the ocean takes your breath away.
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