DC Power Distribution for ICT
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 often DC to begin with, and super-efficient LED lighting is also DC. All this seems to make DC power distribution an attractive option for ICT facilities, but there have been vigorous arguments for and against. Recent events, beginning a 380-V DC standard for ICT power distribution in 2011, suggest the tide is turning in favor of DC distribution.
Image courtesy IEEE Spectrum
Google, which has joined Facebook's Open Compute Project (OCP), proposed a new design for server racks in March 2016. PC World reports, "Google's first contribution [to Open Compute] will be a new rack design that distributes power to servers at 48 volts, compared with the 12 volts that's common in most data centers. The increase will help to accommodate more powerful computing equipment. Google says the new design is more efficient than its old 12-volt system because it reduces electrical conversion losses by 30 percent. Google says it's deployed thousands of the racks in its own data centers so the technology is ready for widespread use. 'The key thing that we figured out was, to get the efficiency in cost and power, you have to directly feed the 48 volts to the motherboard and convert it only in one step,' Urs Holzle, the senior vice president in charge of Google's infrastructure, said at the OCP Summit. 'So these workloads have only one AC-to-DC transformation step, and you step down the 48 volts -- for example, at the CPU -- to around 1 volt'...Google and Facebook apparently worked on the design together, and Facebook may use the 48-volt racks in its own data centers, Holzle said. That would be an unusual level of cooperation between two companies that compete in other areas.'
Click the "VDC" tag above for more about DC in ICT
A new factor in the debate is 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."
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."
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.
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."
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.