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

Providing Capacity Redundancy

Additional benefits are realized from a system designed for capacity redundancy under various lake conditions.

“Actually, full drinking water capacity is available from two intakes at normal lake levels,” said Kevin Loughborough, P.E., vice president of Major Projects, Enwave District Energy Limited. “At the minimum lake level of the last 40 years, capacity of two intakes together would be slightly short of full capacity.

“Recent tests of the intakes found they had more capacity than anticipated. This is because the inside of the Sclairpipe is smoother than was assumed,” said Loughborough. “As a result of higher assumed roughness, the design called for three intakes. As we build out the cooling network, we can contemplate firm customers and interruptible customers for the last 2000 tons of cooling. Bottom line is that we have full redundancy under normal conditions and almost full redundancy under extreme conditions.”

Providing System Consistency

To assure a consistent supply of water at 39° F, three years were spent conducting studies of lake temperatures. The studies included an analysis of spring and fall temperature inversions plus consideration of summertime south and southwest winds that move water onto the shore. This water flows back out warmer— usually along the bottom of the lakebed. The studies showed that if the intake was located 3 miles offshore, these shoreline effects could be avoided.

“Through our investigations we were assured that the pipe intakes are far enough out into Lake Ontario and deep enough to avoid those effects,” said Shute.

“On July 21, 2004, we logged a temperature of 39.5° F in the city water in the energy transfer facility. This is the very best we expected,” said Loughborough. “At this temperature we may not need to run the polishing chillers nearly as much as assumed when we prepared the business case.”

Choosing the Intake System

The nature of the intake system was one of the design challenges. The idea of tunneling a channel below the lake bottom was advanced by RV Anderson Associates Ltd. of Toronto. However, this method was expected to take more than two years to complete at a cost of $90 million—primarily due to the deep shaft construction required for the tunnel to negotiate under valleys carved by glaciers into the shale bedrock under the lakebed.

As an example, Enwave studied a deep lake water cooling project at Cornell University in Ithaca, N.Y., supplying 20,000 tons of cooling compared to Enwave's 40,000 tons of lake cooling. The engineers for the Cornell project, Gryphon International Engineering of St. Catherines, Ontario, conducted a year-long study of various pipe line materials and chose a 63-inch outside diameter Sclairpipe for the 1.9-mile intake from Cayuga Lake.

“Based on that: study we went with HDPE Sclairpipe, and it's been a very positive experience,” said lead civil engineer Michael Steadman. “The design phase plus deployment was very successful and the pipes are performing very well.

“For the Enwave project we were also restricted by the 63-inch maximum outside diameter of the HDPE pipe, so we needed three lines to assure the required water volumes, instead of one large tunnel,” said Steadman. The KWH Pipe is thickest near the shore—with a 3.15-inch-thick wall. “As it goes deeper into the water the thickness is reduced.”

Concrete collars are used to weigh the Sclairpipe down and hold the pipe in place despite the lake bottom currents and wave-induced forces. “Near the shore the collars are spaced every 9 or 10 feet. We need more collars at the shore because of the higher wave and water current loads,” said Steadman. “In the deeper water we tend to go with the thinnest pipe suitable, and as under-pressure loads are less, we can spread the concrete collars out 45 or 50 feet. For both Enwave and Cornell, we also went with ductile iron stiffening rings every 15 feet at the deep end.”