Raise the Temperature, Fight the Fans

Raising the temperature in the data center can save big money on power costs. But nudge the thermostat too high, and the energy savings can evaporate in a flurry of fan activity.

That was the takeaway from several presentations at last week's Data Center Energy Efficiency Summit (DCEE) in Sunnyvale, Calif. The case studies documented the benefits of raising the temperature in a data center environment, which can help save on energy used for air handlers and the chiller plant. But they also offered data on increased activity by server fans, which kick on as the temperature rises, nullifying gains from a warmer server room.

No More 'Meat Lockers'?
The presentations at DCEE, which was sponsored by the Silicon Valley Leadership Group, provide guidance for data center operators as the industry moves away from "meat locker" server environments. Companies like Google and Sun Microsystems have advocated raising the temperature to reduce the power required for cooling server-packed racks. The trend has also received a boost from ASHRAE, the industry group for heating and air conditioning professionals, which increased the top end of its recommended temperature range from 77 to 80 degrees.

In one case study, Cisco Systems (CSCO) said it expects to realize savings of $2 million a year by raising the temperature in its research labs. Cisco's Chris Noland and Vipha Kanakakorn oversaw the proof-of-concept project, in which they raised the temperature in three research labs on Cisco's San Jose campus. Most of the increases were implemented gradually, but in one lab the team hiked the temperature by two degrees per day for four consecutive days.

Raising Chiller Set Point
As the server room neared 80 degrees F (27 C), the Cisco researchers raised the chiller water set point from 44 to 46 degrees F (6 tp 7 degrees C). "Optimizing the room opened the door to raising the room temperature, which opened the door to raising the chiller temperatures," said Noland.

Because of the number of research labs at Cisco, optimizing the server rooms in lab environments offers substantial savings. But some of the variables change in data centers filled with high-dfensity racks, as seen in two case studies examined higher temperatures as part of broader testing on data center efficiency.

The Chill-Off 2 team, which included technologists from Data Center Pulse and Lawrence Berkeley National Labs, found that energy use declined as the temperature in the cold aisle increased - until it hit 80 degrees. At that point, the trend reversed and power usage soared as server fans kicked on. “If the fans start running at higher temperatures, we lose all those savings,” said Bill Tschudi of LBNL.

Vali Sorell of Syska Hennessy Group presented a similar case study in which he evaluated cooling options for a financial client, testing five different configurations at a power density of 20 kilowatts per rack. Once the supply air exceeded 75 degrees, there was a six-fold surge in fan energy. "You’ve got to be really careful about that," said Sorrel. "I think there’s a happy medium (between higher temperatures and fan energy)."