Cut power costs with DC power

Using DC power in the datacenter can reduce wasteful conversions and yield space

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 datacenter. 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 datacenter 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 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 percent of the electricity that comes into the datacenter 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 datacenter over 1.5-inch cabling. Each rack in the datacenter then has a 575-to-48-VDC converter that is 95 percent efficient. The direct DC approach can save users 50 percent or more between cooling savings and elimination of conversion losses, according to Ron Croce, COO of Validus.

It might be tempting to place an AC-DC conversion unit outside the datacenter so that heat dissipation occurs outdoors and to run 48 VDC into the datacenter. 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 -- about 20 percent for every 100 feet of cabling.

One of the common arguments against using DC power in the datacenter 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 datacenters, a number of server vendors, including HP, 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.

Moreover, some large systems, such as the IBM P Series, are already designed to use 575 VDC. Although there is no current standard for high-voltage DC power in datacenters, Panduit and other companies are working on a standardized 400-VDC connector and cabling solution. General Electric is currently working on listing 600-VDC circuit breakers with the Underwriters Laboratories. These breakers already function at 600 VDC but were not previously rated because there was no demand.

In addition to providing cost savings through higher efficiency, DC systems may also provide an opportunity to expand datacenter capacity: Many existing datacenters are using only part of their available square footage because they can't get more power or cooling capacity. Some telecommunications centers are finding that newer rack systems require 100 watts per square foot rather than the old standard of 40 watts per square foot. Due to lack of space, their buildings can't support the 4-foot-thick bundles of cabling necessary for that much 48-VDC power. Moving to high-voltage DC could get around these limitations, because the required cabling would be just 1.5 inches thick.

In many areas, including New York, the San Francisco Bay Area, and Los Angeles, companies are unable to get additional power from the local utilities. Increasing the efficiency of existing systems could also allow companies to continue to use existing buildings and still expand datacenter capacity.

A high-voltage power system like that from Validus requires substantial installation and investment, including running large diameter cabling from the utility pad outside into the datacenter, installing the 575-to-48-VDC converters for each rack, and converting servers to 48 VDC. However, saving 50 percent or more on power over many years represents a big return. For companies that are unable to increase datacenter capacity by buying more power capacity, turning to DC may be the only solution.

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