How Free is Evaporative Cooling in the Data Center

You’re probably familiar with “swamp cooling” at home, especially if you live in the dry West like we do. Swamp cooling is evaporative cooling, a more efficient method of air conditioning than vapor compression or absorption refrigeration, the latter relying on refrigerant that contributes to ozone depletion, in addition to consuming more energy. Free cooling has made inroads in the data center and is common in many new builds and retrofits as a method of saving energy and water alike, leading to its nickname of “free cooling”.

In free cooling, water evaporation is the cooling element. Air is drawn across wet pads or filters, with excess heat dissipating in the water and resulting cool air distributed back through the data center floor. It works best in environments with low humidity and of course requires a steady water supply. It is also better suited for white space at a higher operating temperature, as it does not result in air that is as cold as traditional methods. However, higher operating temps are a best practice for a modern, efficient data center with newer servers and equipment.

Free cooling only uses power for a fan and water pump, lacking the compressor that accounts for most of the energy use of other systems. The cost of operating a free cooling unit is as little as 25% of traditional HVAC, but it does have the drawback of introducing more humidity into white space. Water is not good for computing equipment, so humidity controls are necessary (many data center floors already include humidifiers/dehumidifiers to adjust as needed). In the dry climate of Cheyenne, this isn’t much of a problem.

Types of Evaporative Cooling

The concept and implementation of free cooling dates back millennia, with porous clay vessels full of water placed in rooms, or Egyptians and Persians using windcatchers, passing the air through subterranean water stores and back into buildings.

There are a few types of evaporative cooling. Direct evaporative cooling involves contact between the moist, cooled air and the conditioned air. Warm dry air is changed to cool moist air, with the heat used to evaporate water. Moist air must be released outside to avoid oversaturation, which halts the cooling process.

Indirect evaporative cooling uses a heat exchanger to avoid this contact. Moist air is released outside, while cooled, conditioned air is pushed into the building. Finally, two stage evaporative cooling, or “indirect-direct”, uses both methods, starting with an indirect heat exchange, so the exiting air is less humid. This is followed up with a direct stage that ultimately leads to less relative humidity because the first stage transferred less vapor. This method can be 10-20% less humid.

Considerations for Data Centers

Evaporative cooling is a great way to dramatically reduce PUE, but it does have its own considerations. Direct evaporative is possible and less expensive (Facebook has used straight up misters), but leads to much more humidity. Indirect or two-stage is more expensive and may require a custom installation.

Humidity should be carefully monitored, even though ASHRAE standards recommend anywhere from 20%-80% relative humidity, a wide and forgiving range. Water filtration systems might be necessary to avoid contaminants in the airstream. And finally, drought is an ever-present threat that can quickly increase the cost of free cooling, should water supplies be limited. This is a real concern in the very same dry climates where free cooling is most effective.