With lake-source cooling, the chillers in the basement and cooling tower on the roof are obsolete. After heat exchangers use the lake water to lower the temperature of the cooling water, a central pumping station sends the cooling water around the city loop, branching off to each building. The cooling water eventually returns to each building's basement, where other heat exchangers extract some of the heat before sending it back to the pumping station to repeat the cycle.
The project got the green light several years ago, after Toronto residents began complaining about foul drinking water. The city's water commission gave Enwave permission to tap the lake to fulfill Tamblyn's vision of a cooling system under a deal that would also bring clean drinking water to the city through the 3.5-mile-long intake pipes the company wanted to build.
Enwave's long pipes draw water from depths untouched by the sun and most contaminants, allowing the city to save on purification. But at 40 degrees, deep-lake water would give people a doozy of a headache if they drank it directly, and it would cost a small fortune to heat up for bathing.
The system's central heat exchangers solve that problem by extracting the coldness from the lake water to cool the air conditioning supply and then sending the warmed lake water to the city's drinking-water supply. The cooling-water loop is a closed system, and the two supplies never mix.
At Cornell, W.S. "Lanny" Joyce, founder of its Lake Source Cooling Project, said the university began looking for alternatives after the 1987 Montreal Protocol restricted the use of chlorofluorocarbons widely used in air conditioning systems at the time.
"An institutional body like us, we figured we'd be around for at least another 135 years. So we needed a long-term solution," said Joyce, Cornell's manager of engineering, planning and energy management.
The university system was designed to last 100 years -- typical air conditioners have a life span of 15 to 20 years -- to justify the initial investment of $58 million. Since it started in 2000, the project has reduced campus cooling costs by 87 percent.
Lake-source cooling could raise its own environmental issues if it returned large quantities of warmed water to the lake, potentially harming marine life or promoting the growth of organisms that could cause contamination. Enwave avoids that by sending the warmed water to the drinking supply.
"The temperature of deep water doesn't fluctuate over the year, so it can be a consistent, low-energy source of cooling anytime of year," says Rob Watson, a senior scientist at the Natural Resources Defense Council in New York. "But if an entire city did this, you might have thermal pollution issues."
The high cost of setting up such systems, however, has kept them on the fringes of mainstream energy management.
Toronto's initial investment of $148 million mostly went toward laying three miles of piping beneath city streets and installing three intake lines to feed deep-lake water to the system.
Several U.S. communities along the West Coast are researching deep-source cooling, and Hawaii has an Ocean Thermal Energy Conversion plan underway. It will rely on seawater, which, because of tides and currents, must be drawn from far greater depths -- about 3,000 feet -- to get consistently cold temperatures.
But it takes more than a dense population next to a big, cold body of water for deep-source cooling to work.
"The factor that helps make it cost effective is if your local electricity costs are high," said Joyce, citing the Pacific Northwest as an example of a poor candidate for lake-source cooling because of cheap hydroelectricity there.
"Honolulu's electricity prices are at least twice the national average," he noted. "And they run nearly a full load every day of the year. They're the ultimate customer."
