Operating a data centre without operating the chillers is often referred to as “free cooling” because, although the system may still require some energy to drive the system (fans or pumps), switching off the chillers eliminates so much of the cooling overhead as to render it almost negligible.
There are two types of free cooling:
- Indirect free cooling maintains a closed circuit for the air inside the data centre. This air is typically still cooled by the “chiller” water circuit, even when the chiller is not operating. Instead, the chilled water circuit is cooled by straightforward heat exchange with the ambient air (using a dry cooler as an “air-side economiser”). Thus there is some increase in thermal resistance.
- Direct free cooling, or “fresh air cooling”, can use an open loop air circuit – bringing ambient temperature air into the data centre (opening the windows in the simplest case). Whilst this does not exhibit the thermal losses of indirect free cooling, it is problematic in a number of environments due to moisture content (the CRAC units can no longer control this) and pollution in the air damaging sensitive electronics.
The range of ambient temperatures that a cooling technology supports is the “free cooling zone”. The greater the free cooling zone, the more effective the technology at reducing energy overhead costs. If the free cooling zone extends safely beyond the maximum temperature expected at a given site, based on site survey and historical data chiller plant can be entirely avoided.
If not, then depending on the frequency and duration expected for ambient temperatures to be above the free cooling zone, various alternatives can be explored. These range from installing a lower capacity chiller (lower cost) that is occasionally automatically switched on, through to changing from a standard dry cooler to an adiabatic dry cooler than sprays water to reduce the wet bulb temperature of the cooler.