Deep lake water cooling
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. It is similar in concept to modern geothermal sinks, but generally simpler to construct given a suitable water source.
Water is most dense at 3.98 °C (39.16 °F) at standard atmospheric pressure. Thus as water cools below 3.98 °C it decreases in density and will rise. As the temperature climbs above 3.98 °C, water density also decreases and causes the water to rise, which is why lakes are warmer on the surface during the summer. The combination of these two effects means that the bottom of most deep bodies of water located well away from the equatorial regions is at a constant 3.98 °C.
Air conditioners are heat pumps. During the summer, when outside air temperatures are higher than the temperature inside a building, air conditioners use electricity to transfer heat from the cooler interior of the building to the warmer exterior ambient. This process uses electrical energy.
Unlike residential air conditioners, most modern commercial air conditioning systems do not transfer heat directly into the exterior air. The thermodynamic efficiency of the overall system can be improved by utilizing evaporative cooling, where the temperature of the cooling water is lowered close to the wet-bulb temperature by evaporation in a cooling tower. This cooled water then acts as the heat sink for the heat pump.
Deep lake water cooling allows an even higher thermodynamic efficiency by utilizing the deep lake water, which is at a lower heat rejection temperature than the ambient wet bulb temperature. The higher efficiency results in less electricity used. For many buildings, the lake water is sufficiently cold that the refrigeration portion of the air conditioning systems can be shut down during some environmental conditions and the building interior heat can be transferred directly to the lake water heat sink. This is referred to as "free cooling", but is not actually free, since pumps and fans run to circulate the lake water and building air.
One added attraction of deep lake water cooling is that it saves energy during peak load times, such as summer afternoons, when a sizable amount of the total electrical grid load is air conditioning.
First major system in the United States
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 City 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 (51 megawatt) load.
Lake water enters the system via a screened intake structure 10,400 feet (3,200 m) away in 250 feet (76 m) of water. The intake pipeline is 63-inch (1.6 m) 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 (1,200 mm) HDPE and is approximately 750 feet (230 m) long. The last 100 feet (30 m) of the outfall has 38 six-inch (152 mm) nozzles, about 1 foot (0.30 m) above the bottom of the lake floor in 14 feet (4.3 m) 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 linked to many buildings on the main campus.
First system in Canada
Since August 2004, a deep lake water cooling system has been operated by the Enwave Energy Corporation in Toronto, Ontario. It draws water from Lake Ontario through tubes extending 5 kilometres (3.1 mi) into the lake, reaching to a depth of 83 metres (272 ft). 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 3,200,000 square metres (34,000,000 sq ft) of office space.
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.
Ocean water cooling
The InterContinental Resort and Thalasso-Spa on the island of Bora Bora uses an OTEC system to air-condition its buildings. The system accomplishes this by passing cold seawater through a heat exchanger where it cools freshwater in a closed loop system. This cool freshwater is then pumped to buildings and is used for cooling directly (no conversion to electricity takes place). Similar systems are also in place in The Excelsior hotel and The Hongkong and Shanghai Banking Corporation main building in Victoria City, Hong Kong.
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 were filled with fresh ice collected from lake surfaces 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 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.
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