Free cooling (economizer cooling)

Free cooling is a technique that uses outside air or water temperature directly, or via a heat exchanger, to remove heat from a data hall's cooling loop without running mechanical refrigeration compressors, cutting cooling energy whenever ambient conditions are favorable. Air-side economizers pull filtered outdoor air straight into the data hall in cool climates; water-side economizers reject heat through cooling towers or dry coolers into the chilled-water loop before compressors engage. Facilities in temperate regions such as the US Pacific Northwest, Northern Europe or Scandinavia can run on free cooling for 8,000+ hours a year, pushing annual PUE toward 1.1-1.2, while tropical sites need adiabatic assistance or mechanical backup most of the year. Hyperscalers popularized the technique in the early 2010s specifically to cut compressor-driven energy use.