Launch Slideshow

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Building a dual-purpose water system

Building a dual-purpose water system

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    Distribution process serves dual purpose39° F water is drawn through three new intake pipes, treated at the Island filtration plant, and directed through an existing cross-harbor tunnel to the John Street pumping station. Prior to entering the drinking water supply, the water is sent through heat exchangers and thermal energy is transferred from the Enwave system to the city system. Water enters the city side of the heat exchangers at 40° F and leaves at 55° F, with 15° F transferred from the water on Enwave's side of the heat exchanger. Physical separation between Enwave's system and the city's system is maintained via heat exchangers that are designed to facilitate the transfer of energy, not water. A single source of water provides coldness for Enwave's system and drinking water for the city. Source: Enwave

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    Three 63-inch outside diameter HDPE pipes stretch 3 miles out into Lake Ontario to draw 39° F water from 275 feet down. Source: Enwave

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    Toronto's water passes through many stages1. Three intake pipes draw 39° F water from Lake Ontario at a depth of 275 feet. The water is then filtered and treated for the city's potable water supply.2. At the energy transfer station (ETS), the icy cold water is used to cool Enwave's closed chilled water supply loop through pairs of heat exchangers. The ETS is adjacent to the city of Toronto's John Street pumping station.3. Chilled water can bypass the cooling plant and continue to the customer building. If necessary, water can be further chilled by two, 4700 ton, steam-driven, centrifugal chillers.4. Heat exchangers at the customer building cool the internal building loop, providing chilled water for the building cooling system. Source: Enwave

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    One of the 3-mile, 63-inch pipes is towed out into Lake Ontario. The pipe actually traveled more than 290 miles from the extruding plant to the assembly site to the Toronto lakefront. Photo: Enwave

Last year the city of Toronto began receiving a new supply of water from Lake Ontario for drinking and building cooling use. The intake system was installed by Enwave District Energy Ltd. of Toronto, in partnership with the city, to help eliminate odor and taste problems and create an alternative to conventional air conditioning for buildings located in the downtown area. Shareholders in Enwave include Toronto and the Ontario Municipal Employees Retirement System (OMERS).

The concept of a dual-purpose system using deep lake water, first proposed almost 25 years ago, was brought to the planning and design phase through the efforts of Michael Nobrega, president of Borelais Penco Fund—a wholly owned subsidiary of OMERS. Nobrega was instrumental in securing a $5 million advance from OMERS to cover pre-design and design of the system.

“Michael has been a driving force from day one,” said Enwave president and CEO, Dennis Fotinos. “From 1996, studies and preliminary work were carried on until system design was completed in 2001. Construction began on June 19,2002. Pipes were deployed in June, July, and August of 2003, and the system turned operational in July 2004. That's 25 years of dreaming and eight years of direct action.”

The resulting project has established a fresh drinking water system thermally interfaced with a closed-loop, 52,000 ton capacity district cooling system capable of serving 20 million square feet of downtown office space. The system consists of three, 3-mile-long, 63-inch outside diameter, high-density polyethylene (HDPE) Sclairpipe intake pipes that bring 39° F fresh water from nearly 275 feet down in Lake Ontario to an upgraded water filtration plant on Toronto Island.

After treatment, the water continues through a 97½-inch-diameter tunnel in bedrock under the Toronto harbor to the energy transfer facility in the John Street pumping station. There, heat exchangers are used to transfer the cooler lake temperatures to the closed water loop used for cooling. Water in the cooling system is returned to 40° F while water from the lake is heated to about 55° F and then pumped into the city's distribution system.

Each system remains completely separated throughout the energy transfer process. Separation is also ensured by creating a pressure differential of 25 psi between the drinking water and cooling water systems. “It's a real physical world and someday a leak will happen,” said Robert Shute, a principal of The Mitchell Partnership, a consulting engineering firm based in Toronto responsible for design of the heat exchangers/chiller at the energy transfer plant. “If a leak ever does develop it will be from the city water. No leakage from Enwave water is possible.”

All improvements were paid for through a joint effort of the city of Toronto and Enwave. The Toronto Island filtration plant was upgraded by the city from a standby summer plant to a year-round facility. As part of Enwave's obligation, the corporation paid the $50 million cost of the three pipe intake lines, manufactured by KWH Pipe based in Mississauga, Ontario, and the building addition at the pumping station housing 36 heat exchangers. Enwave anticipates the deep lake water cooling system to break even by 2007.

Improving the Systems

The Enwave deep lake water cooling system has benefited the city by providing a clear, cool, and clean source of water. In the past, the growth of algae became an occasional problem at existing plant intakes during the summer. Although algae and other impurities were filtered out leaving the water safe to drink, noticeable taste and odor problems would occur for short periods of time. With the new HDPE pipelines, “water quality is much better and consistent coldness will make it fresh, tasty, and pure,” said Shute.

The Enwave building cooling system saves at least 30 million kW hours of electricity per year—a 75% reduction in energy use compared with conventional chillers. This savings in electricity removes a load demand of 35 MW from Ontario's electrical grid and reduces annual carbon dioxide emissions by 36,400 tons—the equivalent of taking 5000 cars off the road. Sulfur dioxide emissions are also reduced by 176 tons a year.

This cooling system also replaced chillers in downtown buildings, eliminating the use of ozone-depleting coolants such as chlorofluorocarbons and hydrofluorocarbons and removing noise, visual pollution, and humidity otherwise generated by chillers and associated equipment. In addition nitrogenoxide levels, which contribute to respiratory ailments, were reduced by 70 tons per year.