What is Free Cooling?
Free cooling uses outside air or water that is cooler than the return temperature to reduce or eliminate the need for mechanical refrigeration in data center cooling systems.
Free cooling is a method of rejecting heat from a data center by using ambient environmental conditions instead of running compressors or chillers. It works by drawing in outside air when the temperature and humidity are within acceptable ranges, or by using a water-side economizer that circulates cool water from a cooling tower or dry cooler directly to the computer room air handlers. The key principle is that the heat rejection medium (air or water) is naturally cooler than the data center return air temperature, so mechanical cooling can be bypassed or partially supplemented.
In practice, free cooling is implemented through air-side economizers, which use dampers and fans to bring outside air directly into the server halls, or water-side economizers, which use a heat exchanger to transfer heat from the building cooling loop to an external water loop without running the chiller compressor. Modern data centers often combine free cooling with evaporative pre-cooling or adiabatic systems to extend the operating range in warmer climates. The effectiveness of free cooling depends on the local climate; it is most efficient in temperate or cold regions where ambient temperatures fall below the data center supply setpoint for many hours per year.
Free cooling is a mature energy-saving strategy in the data center industry. It directly reduces the power usage effectiveness (PUE) by lowering the energy consumed by cooling infrastructure, which can account for 30-40% of total data center energy. Standards such as ASHRAE TC 9.9 provide environmental envelopes that define allowable temperature and humidity ranges for IT equipment, enabling operators to safely use free cooling for longer periods. While free cooling can reduce mechanical cooling load by 60-90% in suitable climates, it requires careful filtration, humidity control, and monitoring to protect sensitive electronics from particulates and condensation.
Key facts
- Free cooling can reduce data center cooling energy use by 60-90% in suitable climates.
- Air-side economizers bring outside air directly into the server room; water-side economizers use external water loops.
- ASHRAE TC 9.9 guidelines define allowable temperature and humidity ranges for safe free cooling operation.
- Free cooling is most effective in temperate or cold climates but can be extended with evaporative pre-cooling.
- Proper filtration and humidity control are required to prevent contamination and condensation in air-side systems.
How it works in practice
Related terms
References
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