“Free cooling,” refers to any technique used to reduce the energy consumed by cooling systems or the time that the cooling units run by using the outside temperature of air or water to cool the data center or other facilities. Generally, it comes from the use of air-side and water-side economizers.Air-side economizers work in two ways. Mainly, they use direct fresh-air cooling by filtering (and possibly adjusting the humidity of) outside air, which is piped in to cool the data center. In cold climates, it may even be warmed by the hot air being expelled, so as not to be too cold. Air-side economizers can also work without bringing direct air into the data center, but by heat transference from warmer inside air to cooler outside air.