By Virginia Heffernan
TORONTO WATERFRONT: Toronto Cools Off Naturally – A Deep Lake Water Cooling SystemEngineering
In an underground command centre near Toronto's waterfront, engineers are quietly working toward a goal environmentalists have been touting for years: deep water cooling of the city's downtown buildin...
In an underground command centre near Toronto’s waterfront, engineers are quietly working toward a goal environmentalists have been touting for years: deep water cooling of the city’s downtown buildings.
The expansion under way at the Enwave district cooling plant, buried beneath a grassy park adjacent to the Metro Toronto Convention Centre, is one segment of a $165 million civil and mechanical engineering project designed to use cold water from the depths of Lake Ontario to cool buildings such as the convention centre, the Air Canada Centre and several office towers.
While other municipalities and university campuses in Canada have district energy heating systems, Toronto is ideally suited for deep lake water cooling because of its building density and geography. The highrise towers clustered downtown need year round cooling (which makes the economics of such a system worthwhile), and they are beside a body of Lake Ontario water that reaches a depth of 85 metres less than five kilometres offshore. The water at this depth is permanently just above freezing (4C), kept at that temperature by the natural tendency of cold water to sink.
By using this natural energy source in a cold-energy transfer loop the project can replace building air-conditioning technologies that rely on electricity and fossil fuels. The undertaking is capable of servicing 20 million square feet of office space, is expected to reduce annual carbon dioxide emissions by 36,400 tonnes and save 30 million kilowatt-hours of electricity per year.
While the idea of using lake water to air-condition downtown buildings was first conceived in 1981, it wasn’t considered viable until five years ago when the Toronto District Heating Corporation (TDHC) proposed integrating a lake cooling system with infrastructure from the city’s lake water supply. Under this sharing arrangement, the cold water drawn from far out in Lake Ontario will serve a dual purpose — as chilling for the closed-loop district energy system, and as a fresh supply of raw water for the Toronto Island filtration plant. Not only does this approach bring the efficiencies of using existing infrastructure, but it also has the advantage of lowering the temperature of Toronto’s summer drinking water. The city has suffered taste and odour problems in the hot summer months with its present supply, which is being drawn from warmer waters, closer inshore.
The Deep Lake Water Cooling (DLWC) project is now a joint venture between Enwave District Energy, the successor to TDHC, and the City of Toronto. The project jumped a major hurdle in 1998 when it passed a schedule B class environmental assessment, but the integration has proved to be more contentious.
“The biggest roadblock to breaking ground has been the energy transfer agreement — the rule book for the deep lake water cooling project — that dictates how the arrangement is going to work between the two parties (Enwave and the city of Toronto),” says Steven Zucchet, chief operating officer of Enwave District Energy. “Now that there’s an agreement, you’ll see the project accelerate.”
First cold water will enter three parallel new intake pipes at a depth of 83 metres, five kilometres south of the city in Lake Ontario. The water will be pumped to the Toronto Island water filtration plant, which currently operates in standby mode only occasionally during the summer. The treated water will then flow by gravity through a 2,438-mm diameter existing tunnel to the city’s John Street pumping station.
The concept relies on a bank of stainless steel heat exchangers (about 700 huge plates sandwiched together) that are being constructed at the John Street station. Consulting engineering firm, The Mitchell Partnership, with president Robert Shute, P.Eng. in charge, is lead designer on this part of the project.
The treated deep lake water will enter the exchanger at a temperature of about 4.7C. Having captured heat from the district energy loop, it leaves at 12.5C to continue on its way through the city’s municipal water distribution system.
Return water from the district energy system’s chilled water closed loop enters the exchange at 13.1C and with the borrowed chill from the deep lake water is cooled down to 5C. From there it is pumped through 1,200 mm pipes to centrifugal polishing chillers at Enwave’s plant below the convention centre, where it will be further cooled to 3.3C and distributed to commercial customers The 40,600 TR (tons of refrigeration) heat exchange assembly in the John Street plant works together with up to 16,500 TR of bottoming chillers to provide 52,200 deliverable tons of refrigeration capacity. The chillers have reserve cooling capacity to compensate for natural temperature excursions in the lake water.
An overriding concern was the potential contamination at the heat exchanger of potable city water with the Enwave return water, which may contain corrosion inhibitors. The simple solution, says Shute, is to ensure that the city water is always at a higher pressure than the return water. That way, if there is a breach in the physical separation of the circuits, the city water will flow into the Enwave chilled water, not vice versa. City water pressure at the pumping station will be 810-635 KPa, with occasional surges that can be compensated for by design.1
Another concern for designers was that the lake water would gain heat on its journey through the delivery network, especially in the Island filtration plant and the uninsulated John Street pumping station. Assuming that the filtration plant equipment will be insulated and that there will be 50 mm fibreglass insulation on pipes, calculations show that at the proposed flow rate of 69,400 US gallons per minute (gpm), the heat gain is only 0.5F. Even at the lowest conceivable flow rate of 20,000 US gpm during warm periods, the heat gain is an acceptable 1.035F.
The design of the intake system also proved challenging. Initially, engineers envisioned tunneling a channel below the lake bottom but it was expected to take more than two years to complete and cost $26 million. Instead, Enwave looked south to a similar project at Cornell University in Ithaca, New York. There, engineers chose a high-density polyethylene (HDPE) to build a 3.2-kilometre long intake pipe at a cost of US$7.5 million. Cornell is one of a handful of locales around the world, including Stockholm, Keahole Point in Hawaii, and Halifax2 that use deep water cooling systems.
Once Enwave determined that HDPE was the most cost-effective material for Toronto’s intake pipes, Gryphon International Engineering Services of St. Catharines, Ontario designed the system. The mechanical properties of HDPE, more than those of steel or reinforced concrete, are dependent on stress, strain, time and temperature. Considering these variables, Gryphon determined an apparent modulus of elasticity (E) for the material, then applied traditional buckling and stress/strain formulae for truly elastic materials to design the HDPE pipelines.
Using the pseudo-elastic approach was inherently conservative, says Gryphon’s Michael Steadman, P.Eng. In fact, HDPE tends to relax and “rejuvenate” its material properties during periods of reduced loading.
The intake system will consist of three parallel 600-mm-diameter HDPE pipelines each approximately 5.6 kilometres long. To the 10-metre water depth level the pipelines will be buried to prevent damage from large wave and current loads, potential ice scour, marine traffic and anchors. Beyond this point, they will rest on the lake bottom. At the actual intake is a grille to keep out fish and other large organisms and debris. The slightly buoyant pipes will be held in place by concrete collars spaced to provide resistance to wave and current loads. The collars will provide additional stiffening, increasing the pipes’ ability to resist in-plane buckling. Chlorine lines will be attached to the exterior of the intake pipes to eliminate fouling with zebra mussels etc.
The HDPE pipes will be laid by floating a section of the pipeline into position, then submerg
ing the near-shore end of the pipe. The rate of deployment will be controlled by venting air from the floating end and monitoring the volume of water entering the submerged end.
The construction of the intake system will be one of the last milestones in a green project that has taken years to implement. With the introduction of deep lake water cooling technology, energy use in Toronto’s lower downtown buildings is expected to decline by up to 75% compared with conventional air conditioning. Also the use of ozone-depleting coolants such as CFCs and HCFCs will be eliminated along with the noise, visual pollution and humidity generated by chillers, fans and cooling towers.CCE
Virginia Heffernan is a Toronto-based science and business writer.
1 Consulting engineers Earth Tech of Toronto did the piping network analysis for distributing the chilled water from the plant to customers.
2 In 1986 Purdy’s Wharf Development constructed a system to use ocean water to cool two commercial buildings along the waterfront in Halifax, Nova Scotia. The system works in conjunction with motor driven centrifugal chillers.