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Deep lake water cooling
From Wikipedia, the free encyclopedia
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.
Unlike most materials, water has a temperature at which it is most dense: 3.98 °C at standard atmospheric pressure. Below this temperature, the density of water falls. As a result, the bottom of most deep bodies of water located well away from the equatorial regions is at a constant 3.98 °C.[citation needed]
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.
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Cornell University's Lake Source Cooling System uses Cayuga Lake as a heat sink to operate the central chilled water system for its campus and to also provide cooling to the Ithaca School District. The system has operated since the summer of 2000 and was built at a cost of $55-60 million. It cools a 14,500 ton load.
Lake water enters the system via a screened intake structure 10,400 feet away in 250 feet of water. The intake pipeline is 63 inch High Density Polyethylene (HDPE) that was deployed from the surface using a "controlled" sink process where water was pumped in at the shallow end and air was released at the other end. A series of stiffener rings and concrete collars keep the pipeline on the lake floor and protect it from mechanical forces. The outfall is 48 inch HDPE and is approximately 750 feet long. The last 100 feet of the outfall has 38, 6 inch nozzles about 1 foot above the bottom of the lake floor in 14 feet of water, pointed up at a 20 degree angle and pointed north only. This helps promote mixing of the return water into the receiving water. The water cools a heat-exchanger which is connected to a closed-loop campus chilled water distribution system which links many buildings on the main campus.
Since August 2004, a deep lake water cooling system has been operated by the Enwave Energy Corporation in Toronto, Ontario.[1] It draws water from Lake Ontario through tubes extending 5 km into the lake, 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 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 power of 59,000 tons (207 MW). The system currently has enough capacity to cool 3.2 million square meters of office space. [1]
The cold water drawn from Lake Ontario's deep layer in the Enwave system is not returned directly to the lake, once it has been run through the heat exchange system. The Enwave system only uses water that is destined to meet the city's domestic water needs. Therefore, the Enwave system does not pollute the lake with a plume of waste heat.
This water-cooling technology has some relationship to an older technology and a possible future technology.
Looking back to the past, water-cooling recalls well 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.
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 temperature 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.
- Enwave -- Corporate page on Enwave and Toronto's deep lake water cooling system.
- From Lake Depths, a Blast of Cool for Consumers
- Cornell University Lake Source Cooling Website
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