Economizer (free cooling)
Beginner
Using cool outside air or water to cool the building, instead of big air conditioners, whenever the weather allows.
Novice
A cooling mode that rejects heat without a refrigeration compressor: either by bringing in filtered outside air (airside) or by cooling water in outdoor towers or dry coolers (waterside). The warmer the IT equipment can run, the more hours a year it works.
Expert
Airside and waterside economizers, often with evaporative (adiabatic) assist, cut chiller hours and PUE. Warm-water liquid cooling extends economizer hours, but evaporative assist trades energy for water use (higher WUE).
Explained in Power and cooling (Systems).
See also: Chiller, WUE (water usage effectiveness).