Response of the Thermal Structure of Lake Ontario to Deep Cooling Water Withdrawals and to Global Warming

It has been proposed to cool buildings in downtown Toronto using cold, deep water withdrawn from Lake Ontario. 50 m 3 s −1 of sub-thermocline water would be withdrawn from a depth of 80 m at a temperature close to 4°C, distributed to chillers throughout the city, and discharged on the surface in the nearshore zone at a temperature of 12°C. Compared with electrically-powered chillers, the Deep Lake Water Cooling (DLWC) scheme is thermodynamically elegant and environmentally beneficial in many ways. Using a one-dimensional thermodynamic model of Lake Ontario, this paper assesses the physical impact of the DLWC scheme on the whole lake under present and future conditions. We conclude that the lake could presently absorb the heat from the proposed Toronto installation and 20 others like it without major lake-wide physical changes. At a conservative estimate of 1000 m 3 s −1 with an 8°C temperature differential, the DLWC scheme would reject approximately 33.5 GW of “waste heat” to Lake Ontario. Under our best estimate of a 2 x CO 2 climate scenario, the deep waters could be 2–3°C warmer than they are now. The DLWC system would be less efficient, but the lake would retain a similar overall cooling capacity. Neither biological consequences nor the local physical impact of the discharge of heated subsurface water from the DLWC system are addressed in this study.