Cut power costs with DC power

InfoWorld - Our recent article "10 power-saving myths debunked" generated a lot of interest and controversy. One topic that sparked plenty of discussion was the use of DC power in the data center. Because all computers use DC power internally, the basic concept is to limit the number of energy-wasting AC-to-DC conversions between the utility pad and the servers, and to make those conversions as efficient as possible.

[For more on how companies can battle soaring energy costs, please read " IT vs. the permanent energy crisis," as well as Ted Samson's Sustainable IT blog.]

In a typical data center environment, power conversions abound along the path from the outside utility pad to the servers. With each conversion, some power is lost. The power starts at the utility pad at 16,000 VAC (volts alternating current), then is converted to 440 VAC, to 220 VAC, then to 110 VAC before it reaches the UPSes feeding each server rack. Each UPS converts the incoming AC power to DC power, then back to AC. The UPSes then distribute that AC power to their respective servers -- where it's converted back to DC.

As much as 50% to 70% of the electricity that comes into the data center is wasted throughout this long and winding conversion process.

There's a more efficient approach, one promoted by Validus DC Systems: taking the utility-supplied 13,000 VAC and converting it directly to 575 VDC (volts direct current) using an outdoor-rated conversion unit, then running power into the data center over 1.5-in. cabling. Each rack in the data center then has a 575-to-48-VDC converter that is 95% efficient. The direct DC approach can save users 50% or more between cooling savings and elimination of conversion losses, according to Ron Croce, chief operating officer at Validus.

It might be tempting to place an AC-DC conversion unit outside the data center so that heat dissipation occurs outdoors and to run 48 VDC into the data center. However, long runs at 48 VDC suffer from voltage drop, which means that a good deal of power is lost before it gets to the servers -- there's about a 20% drop for every 100 feet of cabling.

One of the common arguments against using DC power in the data center is that machines don't support it: Most servers run on 110 VAC, which is then converted internally into 5 VDC and 12 VDC. However, with the use of DC power gaining some traction in data centers, a number of server vendors, including Hewlett-Packard, IBM, and Sun, are making DC power supplies available on some or all of their server lines, such that the machines can run on 48 VDC. HP's next generation of server chassis will be the same for all AC- and DC-powered systems, with modular power supplies.

Reprinted with permission from

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