Free cooling uses cold outside air (air-side) or naturally cooled water (water-side) to remove heat from the halls, instead of running chillers continuously. Below a certain outdoor temperature the compressors stop: the energy saving is major.
It is the number-one lever on PUE: a free-cooled data center often drops below 1.3, versus 1.7–2.0 for an older site. Since power is billed at 150–260 €/kVA/month across our catalog and weighs heavily on TCO, a good PUE translates directly into euros — and into an ESG argument (fewer emissions).
What to check when choosing: is the quoted PUE annual and measured (not a marketing peak), and does the site combine free cooling with redundancy without compromising availability.