Chilling Out With DC Power

Computerworld - As vendors continue to pack more servers into a smaller footprint, keeping a lid on power requirements -- and keeping server racks cool -- has become a huge challenge. And the lowly AC power supply remains the toughest part of the problem to solve.
A typical power supply, which converts AC power into the various DC voltages required by individual server components, has an efficiency range of just 65% to 85%, vendors say. Just one 1-kilowatt power supply may generate 300 watts of waste heat, and today's blade servers can consume more than 14 kilowatts per rack.
"That's bad," says Scott Tease, product marketing manager for eServer BladeCenter at IBM. "One, I paid for that electricity, and two, I've released the heat into the environment and I have to pay to air-condition it."
To make matters worse, AC power-supply efficiency drops with the utilization level. In servers with redundant power supplies, where the load is shared, best-case utilization levels are below 50%. As a result, power supplies in most servers tend to operate at the low end of the efficiency range, says Ken Baker, data center infrastructure technologist at Hewlett-Packard Co.
Some data center managers have responded by using DC-based power distribution systems, eliminating the need for AC power supplies for server racks. IBM and HP both offer servers that can accept bulk DC power from a centralized, telecommunications-grade -48-volt DC power distribution unit (PDU) and then step it down to the voltages required at the server level.
Rackable Systems Inc.'s products support both bulk power and an option that moves the AC/DC converter away from individual servers to the top of each rack, where heat can be vented into the air-handling system.
Milpitas, Calif.-based Rackable claims that its DC-powered servers reduce heat by up to 30%. HP makes more modest claims of 15% reduction, which can add up across many racks of servers, Baker says.
Data393 Holdings LLC has made the leap to DC-powered servers. The company, which operates a collocation center in Englewood, Colo., uses a DC power distribution system inherited from a previous tenant to power 140 servers from Rackable. Data393's DC power plant includes rectifiers that convert incoming AC power to DC and charge a bank of uninterruptible power supply batteries as well as its servers and network equipment.
Chris Leebelt, senior vice president at Data393, says the IT services provider chose DC-powered equipment because it needed to make the most of its available square footage and its ability to cool that space. While the power