Basic Concept
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 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.
Deep lake water cooling allows higher thermodynamic efficiency by using cold deep lake water, which is colder 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.
Read more about this topic: Deep Water Source Cooling
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