A few years ago, direct air economization quickly became the trend in data center cooling design. The shift coincided with the expansion of the ASHRAE TC9.9 operating envelope for data centers, which widened the acceptable dry bulb temperature range for server inlets up to 80.6°F. Companies like Facebook, Google, and a number of large wholesale and colocation providers embraced the idea of introducing outside air into their data center, each developing their unique strategy for doing so.
The majority of direct economization designs followed the same concept. In Facebook’s design above, louvers on the exterior of the building introduce outside air into the supply plenum. The air first passes through a mixing chamber, which uses warm return air from the data center to increase the temperature of the outside air if it is too cool for the whitespace. If the air is above set point, an adiabatic or ‘misting’ system mists cool air into the air stream to reduce the temperature to an acceptable level for the whitespace. From there, the air passes through filters (not shown above) and a fan wall delivers the supply air into the whitespace.
The introduction of direct economization designs began an exciting time in our industry to minimize the amount of cooling energy used for a data center and deliver PUEs around 1.1. However, after these designs were commissioned and in service for several years, issues began to arise that impacted the operation of the cooling system. The first was the impact of regional environmental events on the air quality available to the data center. A large fire at a nearby facility could introduce smoke into the air stream; a local farm spraying their crops could do the same with pesticides. To minimize air pollutants inside the data center, filters with high MERV ratings were required. The increased the pressure drop across the system (i.e. more fan energy was needed).
Secondly, the introduction of water mist into the air stream created complications with humidity control. Although there was typically a means to control this before compromising IT equipment, it was a component that required more maintenance and oversight than expected.
It would be expected that issues would arise for any new and innovative cooling design, but many industry leaders are moving away from direct economization completely. In our recent roundtable discussion at Critical Facilities Summit, Joe Reele (who formerly ran Bank of America’s global data center portfolio) stated he would not recommend direct economization for any data center. Microsoft recently announced their new Generation 5 data center design, which moves away from direct economization and uses cooling towers and water-side economization (note: Microsoft uses a closed loop design to minimize potable water requirements). Oracle and Equinix are other examples of large users moving away from direct air strategies.
Many of these organizations are shifting to indirect air economization strategies. Although similar to the direct air economization design described above, in an indirect air system no outside air is introduced into the data center. Instead, the air stream for the data center and the direct outside air are separated into different tubes or chambers, but the outside air is still used to cool the hot return air from the data center. Instead of introducing water directly into the supply air stream with a misting system, the water is misted onto the ‘tube,’ which still provides the overall impact of reducing the air temperature.
Indirect economization strategies may not be right for every data center, but the adoption of these designs by large users like Microsoft may signal an end to direct air economization designs. As newer indirect systems are designed and commissioned, we will have to watch to determine if similar or new operational issues are experienced that drives the next innovation in data center design.