Fans: The new power hogs in the data center

IBM fellow Roger Schmidt thinks that the energy efficiency benefits of using water based cooling could win over data center managers as increasing energy densities in server racks push the room-based cooling systems to the limit.

While energy efficiency gains in processors have slowed the growth in energy density, the trend is still moving upwards. As energy densities rise, so does the volume of chilled air required to keep the equipment cool. More fans may be added and the fans may need to run faster to get sufficient air flow to keep equipment cool.

Schmidt says data centers with computer room air conditioning systems - especially those that have moved to using larger air handlers to cope with higher heat densities --- could save considerable energy by moving to water. "Fans are power pigs," he says, noting that energy use increases as a cubed function of fan speed.

With a well designed closely coupled liquid cooling system, he says, computer room air handling systems can be vastly scaled back - or turned off altogether. To demonstrate IBM is outfitting every server rack in its Syracuse data center with liquid cooled rear doors. Piping under the floor feeds chilled water to the units, which use a door-mounted fan and air-to-water heat exchanger system to draw heat away from the equipment and out of the data center. The systems do use small fans to pull air through the heat exchangers, but they don't use anywhere near the electricity that those big air handlers consume.

"We can almost get rid of CRAC units. We might not need them at all," he says.

While some organizations are already using water-based cooling systems to target a few high density server racks, most users would consider wide-scale use of water a last resort, says Peter Gross, vice president and general manager of HP Critical Facilities Services.

It's a cultural issue: The current generation of data center managers, who grew up in the age of distributed computing, has never seen water - and it makes them nervous. It also adds a network of plumbing infrastructure within the raised floor space that must be managed.

That wasn't as big of an issue in the mainframe days, when water was piped to a centralized mainframe system. But in a distributed environment, that piping must feed hundreds - or thousands - of racks across thousands of square feet of floor space. While water may reduce the load on the computer room air conditioning system, the idea that you could turn off the room air conditioning system entirely seems unlikely. Even if IBM can do that for a data center consisting entirely of server racks, that probably wouldn't work in real-world enterprise data centers, which tend to have a mix of different types of equipment, of different ages, each with different power and cooling requirements.

Schmidt also envisions a return to direct liquid cooling, which replaces the heat exchanger with a water-fed heat transfer plate that conducts heat away from hot components. But that option, if it comes to pass, faces its own hurdles. For more on the subject, see Data center density hits the wall.

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