A Cool Resource!

As the impact of global warming continues to be felt, we will have to find more and more innovative ways to stay cool in the summer.  Cooling is typically energy and cost intensive, demanding large amounts of electricity to drive compressors in mechanical refrigeration cycles.  You can read more about mechanical refrigeration on Chad’s previous blog post here.

Peak electricity is largely driven by demand for cooling, meaning that on those hot summer days Ontario’s utilities are forced to ramp up the use of coal and gas “peaker” plants.  Lower emissions hydro and nuclear power cannot ramp up and down very quickly to meet such variable loads.  Naturally, this means higher air pollution levels and higher greenhouse gas emissions during peak electricity times.  Today, on a hot August 28th afternoon, gas and coal are supplying 32% of the electricity on the grid (see screen grab taken from IESO website).  Coal and gas only made up 17.4% of Ontario’s electricity supply on average in 2012, so you can see how much of an environmental concern peak cooling really is!  If we could reduce the energy required for cooling, benefits would be both financial and environmental.

A ‘Made in Toronto’ Solution

Toronto is lucky to be situated right next to an enormous source of cold water – Lake Ontario.  Water is most dense at 4 degrees Celsius, so the water at the bottom of the lake consistently stays at this low temperature all year round.  A district cooling system was constructed in the early 2000s to bring cold water from a depth of 83m in Lake Ontario to cooling loads in the city.  The diagram below shows a simplified schematic of how the system operates.

Water at 4 degrees C is drawn from Lake Ontario and processed at a water treatment plant.  Heat exchangers cool the district chilled water loop by transferring heat to the treated lake water.  After passing through the heat exchangers, the treated water drawn from the lake feeds Toronto’s potable water supply, meaning that a high temperature discharge stream is not polluting the Lake.  Supplemental chillers are on standby to provide additional cooling when the lake water can’t meet the demand.  The company that owns and operates the system is called Enwave Energy Corporation.

Much less energy is required to run the deep lake water cooling plant than a traditional plant because of the temperature at which the heat is being rejected.  A cooling tower system rejects heat at the ambient wet bulb temperature (for Toronto, the design condition is 22.3 degrees Celsius), whereas deep lake cooling rejects heat at 4 degrees Celsius, achieving a much higher thermodynamic efficiency.  Electricity use can be reduced by up to 90% over standard building chiller/cooling tower systems.

Deep Lake Cooling at UHN

UHN is connected to the Enwave system at Toronto Rehab Institute University Centre.  Some of the benefits of this system include:

• Reduction in the amount of HVAC equipment installed on site (no chillers/cooling towers) – more space dedicated to patients
• Elimination of CFCs on site
• Elimination potential issues involving cooling tower contamination
• Lower electricity consumption and demand
• Environmental benefits of reduced electricity demand, such as less air pollution from generating stations and fewer greenhouse gas emissions attributable to the building/UHN

Contributing to economic, performance, and environmental goals, deep lake water cooling is a perfect example of the triple bottom line philosophy which guides projects at UHN’s Energy and Environment department.