Deep lake water cooling uses cold water pumped from the bottom of a lake as a heat sink for climate control systems. Because heat pump efficiency improves as the heat sink gets colder, deep lake water cooling can reduce the electrical demands of large cooling systems where it is available. The warmed lake water is returned to the lake.

Unlike most materials, water has a temperature at which it is most dense: 4 ° C at standard atmospheric pressure. Below this temperature, the density of water falls. As a result, the bottom of most deep bodies of water is at a constant 4 ° C.

Air conditioners are heat pumps. During the summer, when outside air temperatures are higher than the thermostat set temperature inside a building, air conditioners use electricity to pump heat "uphill", from the cooler interior of the building to the warmer exterior ambient. This process is expensive because large buildings collect an enormous amount of solar thermal energy (at noon, about one kilowatt per square meter facing the sun), and require lots of electrical energy to pump out all that heat.

Unlike residential air conditioners, most modern commercial air conditioning systems do not pump heat directly into the exterior air. Instead, water is brought down to the wet bulb temperature by partial evaporation in a cooling tower. This cold water then acts as the heat sink for the heat pump. The improvement in heat pump efficiency saves so much energy that cooling towers have become ubiquitous on the rooftops and mechanical floors of skyscrapers.

Deep lake water cooling goes even farther. Except in the dryest of summer conditions, deep lake water will be cooler than the ambient wet bulb temperature. Because it is a colder heat sink it saves still more electricity. For many buildings, the sink should be sufficiently cold that the heat pumps can be shut down and the building can use "free cooling" -- allowing interior heat to conduct directly to the heat sink. "Free cooling" is not actually free, since pumps and fans still must be run to circulate the heat sink water and building air.

One added attraction of deep lake water cooling is that it saves energy during peak load times -- summer afternoons when a sizeable chunk of the total electrical grid load is air conditioning.

First system in use

The first deep lake water cooling system was installed by Enwave in Toronto and draws water from Lake Ontario through tubes extending 5km into Lake Ontario, reaching to a depth of 83 metres. The lake-bottom water is at 4°C year-round even at the height of summer, when the surface water is warm. The cooler, denser water near the bottom remains near the bottom. The deep lake water cooling system is part of an integrated district cooling system that covers Toronto's financial district, and has a cooling capacity of 59,000 tons.

Comparison to related technologies

This water-cooling technology has some relationship to an older technology and a possible future technology.

Icehouse cooling

Looking back to the past, water-cooling recalls will insulated icehouses which were used to store ice throughout the year prior to the invention of refrigeration. Icehouses stored frozen water during the winter whereas deep lake water cooling taps a permanent store of cold water.

OTEC power generation

Looking towards the future, water-cooling uses cold deep water just as ocean thermal energy conversion (OTEC) does. However, OTEC is intended to be used for generating energy by operating a heat engine on the energy difference between the ocean bottom and the ocean surface. Deep lake water cooling bypasses the need for electricity generation altogether and, so, is a simpler and more immediately practical technology than OTEC. Ambitious OTEC projects have yet to realize their full potential because they present far more demanding engineering challenges.

External sources

• Enwave -- Corporate page on Enwava and Toronto's deep lake water cooling system.