Free Cooling: the Server Side of the Story

Data centers are the massive engines under the hood of the mobile internet economy. And it is no secret that they demand a lot of energy: with energy capacities ranging from 10MW to 100MW, they require up to 80,000 times more than what a typical US home needs.

And yet, you do not have to be a genius to figure out how the enormous energy bills could be reduced. The main energy gobblers are the CRACs, Computer Room Air Conditioners or the alternative, the CRAHs, the Computer Room Air Handlers. Most data centers still rely on some form of mechanical cooling. And to the outsider, it looks pretty wasteful, even stupid, that a data center is consuming energy to cool servers down while the outside air in a mild climate is more than cold enough most of the time (less than 20°C/68 °F).

Free cooling

There are quite a few data centers that have embraced "free cooling" totally, i.e. using the cold air outside. The data center of Microsoft in Dublin uses large air-side economizers and make good use of the lower temperature of the outside air.

Microsoft's data center in Dublin: free cooling with air economizers (source: Microsoft)

The air side economizers bring outside air into the building and distribute it via a series of dampers and fans. Hot air is simply flushed outside. As mechanical cooling is typically good for 40-50% of the traditional data center's energy consumption, it is clear that enormous energy savings can be possible with "free cooling".

Air economizers in the data center

This is easy to illustrate with the most important - although far from perfect - benchmark for data centers, PUE or Power Usage Effectiveness. PUE is simply the ratio of the total amount of energy consumed by the data center as a whole to the energy consumed by the IT equipment. Ideally it is 1, which means that all energy goes to the IT equipment. Most data centers that host third party IT equipment are in the range of 1.4 to 2. In other words, for each watt consumed by the servers/storage/network equipment, 0.4 to 1 Watt is necessary for cooling, ventilation, UPS, power conversion and so on.

The "single-tenant" data centers of Facebook, Google, Microsoft and Yahoo that use "free cooling" to its full potential are able to achieve an astonishing PUE of 1.15-1.2. You can imagine that the internet giants save massive amounts of energy this way. But as you have guessed, most enterprises and "multi-tenant" data centers cannot simply copy the data center technologies of the internet giants. According to a survey of more than 500 data centers conducted by The Uptime Institute, the average Power Usage Effectiveness (PUE) rating for data centers is 1.8. There is still a lot of room for improvement.

Let's see what the hurdles are and how buying the right servers could lead to much more efficient data centers and ultimately an Internet that requires much less energy.