Hidden Costs of Power Distribution

By Ed Spears, Product Marketing Manager, Eaton Critical Power Solutions Division, Raleigh, North Carolina, US


Changing voltage can dramatically reduce the cost

Figure 1: End-to-end efficiency in the traditional, 480V AC power distribution system

Data center energy costs as a percent of total revenue are at an all-time high - emerging as the second largest operating cost in the IT organization, behind labor. The five-year cost for powering and cooling the data center now exceeds the cost of the servers themselves. A million dollars' worth of server hardware will consume about $400,000 of power a year—four times what it was less than a decade ago. That figure is likely to reach 1.2 million dollars a year by 2011. You would expect energy costs to rise with increases in processing demand, but not this much. It doesn't have to be this way. Within the power distribution system are untapped opportunities to reduce energy consumption and heat output. Traditional power distribution systems rob a lot of the power that comes into the data center. Every element in the power chain takes a little bit of that power to do its job, and each element wastes a little bit of power too. By the time the power has passed through power protection systems, power distribution units (PDUs) with transformers and finally through server power supplies, more than half of the incoming power has been used up or wasted. The wasted power is dissipated as heat, which drives up cooling costs. Ultimately, only about 40 percent of the power brought into the data center is actually used for processing data. If you think that's unacceptable, you've got lots of company. There has been a lot of positive momentum to address this issue. For example, under legislation signed by President Bush, the Environmental Protection Agency (EPA) is studying energy use in data centers. EPA work will likely result in benchmarks to assess the energy efficiency of servers, processors and other data center equipment, just as the EnergyStar program has done for home appliances and office equipment. At Eaton's Innovation Center in Cleveland, Ohio, we have been conducting our own research along these lines—looking not just at individual components but also at end-to-end efficiency of the power delivery system. We found that by modifying the voltages at which power was distributed in the data center, we could dramatically reduce the cost and energy consumption of power equipment. Traditional powering in U.S. data centers—480VAC Today's 480VAC power distribution systems—the standard in most U.S. data centers—are not optimized for efficiency. In a legacy 480V data center, the uninterruptible power system (UPS) might be about 94 percent efficient. The PDUs are about 98 percent efficient, and the servers about 84 percent. That means a little bit of power is being skimmed off at each step along the way. Power gets converted between AC and DC five times along the way. End-to-end efficiency ends up being about 77 percent.

A compelling alternative—400VAC power The 400VAC power distribution model—common in Europe, Asia and South America—offers several advantages compared to 480VAC and 600VAC. For one, the neutral is distributed throughout the building, eliminating the need for PDU isolation transformers and delivering 230V phase-neutral power directly to the load. You reduce costs by omitting the isolation transformers and branch circuit conductors that are required in 480V and 600VAC systems. With only three points of power conversion, end-to-end efficiency is about 79 percent. That doesn't sound like much of an increase—two percentage points—but the dollars add up fast, especially in data centers with redundant, dual bus power systems.

Why not use DC power distribution to eliminate all those AC/DC conversions? That question has been asked for 20 years. Telecommunications equipment, for instance, is powered by -48V DC power. Ma Bell took this approach all those years ago because cables were run in corrosive and damp conditions, and low voltage was needed to protect the safety of line technicians. For data center applications though, -48 DC requires such large, expensive copper cabling to deliver a relatively small amount of power for any distance. Higher DC voltage shows a lot of promise though. In 2007 the California Public Energy Commission did a study on 380VDC and found it was 5-7 percent more efficient than best-in-class AC systems. They theorized that up to 28 percent improvement is possible over today's average AC systems. Trouble is, there are few standards for high-voltage DC power outside of industrial applications—none for IT systems. Not much IT equipment will run on this power. And there are safety issues to be addressed. So 380VDC is promising, but at the moment it is a future vision. The prevailing Canadian power system—600VAC A 600VAC power system offers certain advantages over 480V and 400VAC systems, but inherent inefficiencies make it impractical for most U.S. data centers. The 600VAC system offers a modest equipment cost savings, requiring less copper wiring into the UPS and from UPS to PDU. Lower currents also enable less heating of the wires, reducing energy cost. Certain UPS/switchboard configurations offer capacity gains at nominal cost and with no increase in switchgear footprint. The primary drawback to 600VAC power, compared to 400VAC, is that the distribution system requires multiple isolation transformer-based PDUs to step down the incoming voltage to 208/120VAC. These extra PDUs add significant cost and reduce overall efficiency to about 77 percent, the same as traditional 480VAC power distribution.

The business case for 400V In the Eaton study of distribution options for applications ranging from 300kVA to 10MW, the 400V alternative was the clear winner. Across every load range evaluated, the 400VAC system:

  • Offered an average of 10 percent lower total cost of ownership in the first year alone
  • Reduced capital expense by an average of 15 percent
  • Saved an average of four percent in operating expense, with savings increasing in direct proportion to system size
The 400VAC power distribution option—stepped down to 230V to support IT systems—has proven reliable in the field, conforms to current U.S. regulatory standards, can be easily deployed into existing 480VAC power systems, and doesn't require significant changes to IT systems. Servers run more efficiently, you eliminate the power distribution unit with its transformer, and now there are only three points of power conversion. End-to-end efficiency is about 79 percent. So it is no surprise that managers of many large data centers are planning to upgrade their existing 480VAC power infrastructure, or design their new data centers with more efficient 400VAC equipment in the next few years. For more details about the Eaton study, download the free white paper, "Alternative data center power—A quantitative TCO comparison of 400VAC and 600VAC power www.eaton.com