As data centers take an ever larger bite of worldwide energy production, efficiency becomes an increasingly urgent topic. Much of the focus in recent years has been on areas like virtualization, improved cooling practices (like free cooling and bumping up the thermostat), consolidation of idle equipment and so on—and rightly so. In the background, however, has been a growing discussion about a fundamental topic: alternating-current (AC) versus direct-current (DC) power. AC power won the day when electrical infrastructure was first being deployed, but could DC be poised for a return from the dead?
Is DC More Efficient in the Data Center?
Proponents of DC power have claimed data center efficiency improvements of 30% or more compared with AC power; in particular, a 2008 report conducted by Lawrence Livermore National Laboratory (LLNL) found that an examined DC distribution system “will have the potential of using 28% less energy than the typical AC system found in today’s data centers.” This may be true in light of all the caveats of the study, but the average data center probably isn’t using the most efficient AC system available. A white paper by The Green Grid and APC by Schneider Electric (“Review of Four Studies Comparing Efficiency of AC and DC Distribution for Data Centers”) claims that the true benefit is only in the range of 0–1%—far less than other studies have indicated.
The debate on this topic continues, but there seems to be a reasonable consensus that DC power distribution is (or at least can be) more efficient than AC in the data center; the question, obviously, is how much more efficient.
Incumbent technologies almost invariably have an advantage over new technologies in that they generally have some surrounding infrastructure that enables them, and that infrastructure represents an investment. The same is true of AC power: switching to DC in the data center would require infrastructure changes that would consume capital and would represent something of a loss as existing infrastructure is discarded. This problem is less of a challenge for new data center construction, but it may be insurmountable for existing data centers, particularly if capital funding is tight.
The above-mentioned white paper correctly points out the need for “a compelling advantage to DC distribution in order to cause a move of the industry from the AC standard.” If DC only provides small, incremental improvements over AC, the capital barrier may simply be too high. Furthermore, most equipment is tailored to AC-power systems, which are by far the most common. This isn’t to say companies that want to implement a DC-based data center have no options, but those options are fewer.
Furthermore, another barrier to direct current is a lack of standards, although certain standards are emerging, such as 380V DC. Some companies may also hesitate to pursue DC owing to a lack of examples elsewhere in the industry; data centers are expensive, and the risks of deploying a relatively untried technology are too great for some businesses, particularly in uncertain economic times. Not many data centers have implemented DC, but many (i.e., most) use AC: the numbers point to AC as—at least seemingly—the safer bet.
Alternative Energy: Favoring DC?
The internal dynamics of the data center are not the only factors affecting DC adoption, or a lack thereof. As companies like Apple, with its facility in Maiden, North Carolina, increasingly turn to alternative energy sources like solar and fuel cells, questions arise regarding how to integrate this power both on a company campus and into the broader grid. Solar, for instance, naturally produces direct current, unlike turbines (used in wind, coal and hydroelectric power). It must thus be converted to AC to enable use in the grid—a step that has a cost, however small, in efficiency.
With a growing societal focus on alternative energy as a means of avoiding the pollution and safety concerns of coal, natural gas and nuclear, a greater reliance on solar and other alternative power sources could lead to a broader move toward DC. Although much of the existing grid uses AC, some already uses DC: particularly, for transmitting lots of power over large distances.
In addition, many common devices—both in the data center (servers, for instance) and elsewhere—operate on DC power and must convert AC from the socket before the energy is usable. Thus, pressure at the generation end in the form of greater reliance on certain alternative energy sources along with the basic power needs of equipment at the user end create an impetus for using DC distribution. Clearly, if DC becomes more widely used in the grid, companies may have sufficient motivation for a shift to DC distribution in their data centers.
AC vs. DC: One or the Other?
No technology does all things well. AC and DC have different characteristics that lend them to different applications, so expecting the entire power grid to be exclusively AC or exclusively DC is unreasonable. It is already a mixture of the two, and different energy sources and energy-consuming components will always have a certain affinity for one or the other. The task is finding the best use of each—AC and DC—in the right places to maximize both efficiency and economy. For example, if moving the data center from AC distribution to DC distribution yields just a tiny efficiency improvement, the peripheral costs may be insufficient to justify doing so, even though DC would in that case have a demonstrable superiority.
AC power distribution is the incumbent in data centers. Whether DC has a shot at edging out AC—at least in a good portion of facilities—depends on a number of factors, such as the actual efficiency gains. Although some studies indicate that using DC garners up to roughly a 15–30% improvement, others place that gain much closer to 1% or less. In addition to its momentum advantage, AC also benefits from greater maturity; thus, even if the current efficiency edge of DC is very low, it may grow over time as more design effort focuses on applying DC to the data center. But there are certainly no guarantees one way or the other.
Alternative energy—particularly solar—may drive greater interest in DC, both in the data center and in the larger power grid. Because solar produces DC electricity, why convert to AC and then back to DC for consumption by electrical or electronic devices? Of course, AC and DC each have their respective advantages, so neither one nor the other will become the sole approach. But as interest in DC increases, research and development should enable efficiency improvements through optimal deployment of both AC and DC.
Image courtesy of Evan-Amos