Staring at a lake on a hot day, many people might think the most straightforward way to cool off would be to jump into it. But the city of Toronto recently unveiled a much more intricate way to use Lake Ontario to cool some of its citizens. Beginning on July 15, an elaborate set of pipes began dredging cold water from the bottom of the lake to cool downtown buildings. According to Enwave, the company responsible for the $170-million project, it will reduce overall annual power usage by more than 40 megawatts and greenhouse gas emissions by nearly 40,000 metric tons--the equivalent of taking 8,000 cars off the road--once it is fully operational.
The new system is an example of Deep Lake Water Cooling (DLWC), an alternative to airconditioning systems dependent on fossil fuels and electricity. The idea is not novel. As Lanny Joyce, program manager of the Lake Source Cooling Project at Cornell University in Ithaca, N.Y., points out, cities have had to rid themselves of excess heat since the industrial revolution. Indeed, a number of Scandinavian cities, including Stockholm, use bodies of water as heat sinks and have been doing so for more than a decade. Seattle recently commissioned a study that investigated the potential of DLWC for a new development area; small systems in San Francisco and Vancouver are in place, and a site in Hawaii is also under construction.
The process relies on a simple premise: heat flows from hot to cold. A closed loop of chilled water flows through buildings, taking with it heat removed by air conditioning. Often, expensive chillers use refrigeration to remove this excess energy. But with DLWC, the cold lake water whisks the heat away. For example, in Cornell's set up, water drawn from Cayuga Lake is between 39 and 41 degrees Fahrenheit, but when it returns it is slightly warmer, averaging about 47 degrees F during the winter and 56 degrees F in the summer.
The Toronto project is more ambitious in scope. In the planning stages for more than 20 years, the development includes a new intake pipe for the city's drinking water supply. Water removed from 80 meters below the surface of Lake Ontario is pumped through three pipes to the Toronto Island Filtration Plant and treated to meet drinking water standards. Next, it travels to an energy transfer station and moves through heat exchangers to chill Enwave's closed water-supply loop that is distributed to customers. The two water supplies don't mix, and the lake water, now warmer and potable, moves to the John Street Pumping Station to be distributed to consumers. Meanwhile, the chilly water in Enwave's closed loop is distributed to their network of customers, with the cold water doing the work that used to be the domain of old-fashioned mechanical chillers.
Since its July 15 start-up, the system has been cooling nearly a dozen downtown buildings, including banks and office towers. ¿Everybody's thrilled because the water's colder than we had expected, slightly, and the heat exchange is working well,¿ Enwave's COO Chris Asimakis reports. ¿It's nothing but good news so far; the customers seem happy.¿ To date, however, Enwave has only signed up enough businesses to utilize about 34 percent of the system's total capacity.
Part of the resistance, Asimakis surmises, stems from the unknown nature of the project. ¿Everyone's familiar with chillers,¿ he says, referring to the devices that use chemical refrigerants, such as chlorofluorocarbons (CFC) until they were banned in 1993 by the Montreal Protocol, to control temperature and humidity. He points out that businesses signing up with Enwave can view the move as insurance against rising electricity costs, because the dependency on ever-present lake water leads to price stability. In addition, the heat exchangers used by the buildings on the Enwave loop last 50 years, on average, whereas a chiller's lifetime is estimated to be 20 to 25 years at most. Citing the example of the CFC ban, Asimakis further notes that many companies had to replace or retrofit their chillers to comply with the new regulations. For Enwave customers, the company manages the infrastructure and would be responsible for any such concerns in the future.