Most ICT gear - core facilities, communications infrastructures, and edge devices - runs on DC power. Converting AC to DC within a building is inefficient, on-site renewable power generation is usually DC to begin with, and super-efficient LED lighting is also DC. So DC power distribution has been a attractive option, but there have been vigorous arguments for and against. Recent events suggest the tide is turning in favor of DC distribution, although skeptics continue to press their case. The growing use of solar-generated electricity might be providing the latest boost DC.

Image courtesy IEEE Spectrum

The emergence of the 380-V DC standard for ICT power distribution in early 2011 and the testing around it were the first signs of a DC resurgence. (Details below.)

Universal Electrical Corporation (UEC) is betting the the viability of 380-V DC distribution with its STARLINE DC Solutions division. UEC claims, "STARLINE DC Solutions is a 380 VDC Computer Infrastructure that yields 200% increase in reliability, energy savings up to 15%, capital expenditures reduction up to 16%, floor space reduction up to 33%, and operating expense decrease up to 35% as compared to conventional AC powering solutions." ZBB Energy Corporation, a provider of advanced electrical power management platforms which is partnering with UEC, says, "We…believe that this type of 'smart' DC micro-grid application with telecom and data centers will become a standardized construction approach."

IBM India is showcasing what it says in "the first solar-power array designed specifically to run high-voltage data centers, integrating AC- and DC-based servers, water-cooled computing systems and related electronics…By employing unique high-voltage DC power conditioning methods – and reducing AC-DC conversion losses – the new IBM solution can cut energy consumption of data centers by about 10 percent and tailors solar technology for wider use in industrial IT and electronics installations."

IBM clearly sees an opportunity to make ICT more globally accessible. "In many emerging markets, electrical grids are undependable or non-existent. Companies are forced to rely on expensive diesel generators. That makes it difficult and expensive to deploy a lot of computers, especially in the concentrated way they're used in data centers. Using IBM’s solution, a bank, a telecommunications company or a government agency could contemplate setting up a data center that doesn't need the grid. The solution, in effect, creates its own DC mini-grid inside the data center."

The data center at Syracuse University (NY-USA) ties an electricity generation strategy to the facility's DC power needs. "The data center’s electrical tri-generation system uses 12 patented, natural-gas-fueled microturbines to provide electricity for the Green Data Center. Ten of the microturbines can generate all of the power needed, enabling the data center to operate completely off-grid. Where a typical data center converts alternating current (AC) power from the utility’s electrical grid to direct current (DC) to power the servers, these microturbines can generate any combination of AC and DC power on site, avoiding the loss of power that typically occurs during transmission and conversion."

DC skeptics have not been silent. Schneider Electric, which published the 2011 A Quantitative Comparison… paper cited below, as added Review of Four Studies Comparing Efficiency of AC and DC Distribution for Data Centers by the same author. "…the latest AC and DC systems provide effectively the same efficiency…The highest efficiency power system for data centers today is an AC 415/240V system with a UPS in Eco-mode. This is actually more efficient than any known DC system."

Now there is a new factor in the debate - the growing use of solar-generated electricity for ICT facilities and infrastructures. The DVL Group observes, "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...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...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."

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Two and a half years ago we wrote, "Much data center energy inefficiency comes from the conversion of a center's AC source power to the DC required to run its equipment. A lot of work is going on in this area. Good resources include the DCDataCenters site [now defunct] and LBNL's DC Power for Data Centers of the Future. A recent overview article by InfoWorld is a good place to get started if you're new to the topic."

Then, in 2010, we noted two white papers published by power supply manufacturer APC that cast doubt on DC's future.

In AC vs. DC Power Distribution for Data Centers, we read "…from time to time beginning in the early 1990’s various manufacturers and engineers have suggested that a change to DC distribution was advantageous, and predicted a widespread adoption of a DC standard for data center power. In fact, the opposite has occurred, and the usage of DC relative to AC has declined."

Additional arguments against the DC's advantages are presented in A Quantitative Comparison of High Efficiency AC vs. DC Power Distribution for Data Centers: "The data in this paper demonstrates that the best AC power distribution systems today already achieve essentially the same efficiency as hypothetical future DC systems, and that most of the quoted efficiency gains in the popular press are misleading, inaccurate, or false."

Despite these uncertaities, interest in DC distribution for ICT is moving forward. IEEE Spectrum reported in February 2011,

…by this month EMerge [Alliance] and the European Telecommunications Standards Institute (ETSI) both expect to issue draft standards for 380-V DC wiring for building-wide power distribution. DC supporters hope to merge the standards in the months ahead…

Losses in the lines limit 24-V DC distribution to 10 meters, so manufacturers are developing 380-V DC wiring to extend similar benefits to entire buildings. Telecommunications firms and data centers are the likely early adopters of 380-V DC.

Today's data centers generally take 480-V AC power from the grid and convert that to DC to charge up a battery-based uninterruptible power supply, or UPS. The secure DC stream is then converted back to AC and transformed to 208-V AC for distribution, only to be rectified back to 380-V DC by the first stage of each server's power supply to charge up power-smoothing capacitors.

DC distribution offers a comparatively simple scheme, whereby a single rectifier turns 480-V AC into 380-V DC that can both charge the UPS and supply the servers. The University of California, San Diego, began testing a 380-V DC data center last year, and in November, the Electric Power Research Institute (EPRI) and Duke Energy Corp. measured a 15 percent reduction in power consumption in a test of 380-V DC distribution at the utility's Charlotte, N.C., data center. Net energy savings could be twice that, they claim, once the cooler-running equipment's reduced air-conditioning burden is factored in.