Deep water source cooling (DWSC) or deep water air cooling is a form of air cooling for process and comfort space cooling which uses a large body of naturally cold water as a heat sink. It uses water at 4 to 10 °C (39 to 50 °F) drawn from deep areas within lakes, oceans, aquifers or rivers, which is pumped through the one side of a heat exchanger, which cools warm water on the other side of the exchanger.

Fresh water is most dense at 3.98 °C (39.16 °F) at standard atmospheric pressure. Thus as water cools or heats to 3.98 °C, it increases in density and will settle below. As the water temperature climbs above or below 3.98 °C, water density decreases and causes the water to rise, which is why lakes are warmer on the surface during the summer. The combination of these 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, residential air conditioners use electricity to transfer heat from the cooler interior of a building to the warmer yet exterior.

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. 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, consuming less electricity. 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", reflecting the ability to take advantage of a natural heat sink, rather than create cold through the energy intense refrigeration cycle of compression and expansion of a contained refrigerant; pumps are still required to circulate the water, including raising cold water hundreds of feet against great head pressure, and fans to circulate 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.

Deep water source cooling is very energy efficient, requiring only 1/10 of the average energy required by conventional cooler systems. Consequently, its running costs can also be expected to be much lower.