A powerful debate: AC vs. DC distribution

Dec. 1, 2008
Despite praise from an esteemed research group, some question the viability of DC power for improved energy efficiency in the data center and offer up alternatives.

by Betsy Ziobron

Despite praise from an esteemed research group, some question the viability of DC power for improved energy efficiency in the data center and offer up alternatives.

In March, a report from the Lawrence Berkeley National Laboratory's (LBNL; hightech.lbl.gov/datacenters.html) "High-Performance High-Tech Buildings" project demonstrated that a direct-current (DC) power distribution system has the potential of using 28% less energy than the typical alternating-current (AC) system found in today's data centers. With such significant results, the LBNL report received much attention throughout the industry and is prompting many to consider DC as one of the power-consumption-reduction methods for data center environments.

Not unlike the red-state/blue-state maps from last month's presidential election, this chart indicates the voltage frequencies in use around the world.
Click here to enlarge image

But others remain skeptical of its viability and potential market adoption, and instead are promoting new AC power-distribution methods that rival DC in efficiency.

Refueling a century-old debate

The AC vs. DC debate stems back to the so-called "current wars" of the late 1800s when Thomas Edison promoted DC as the most efficient and safest distribution method over AC, which was advocated by George Westinghouse. Due to the demand for electricity and the ability for AC to be more easily carried over long distances, AC won out and became the standard for power distribution and appliances.

"It's less expensive for utilities to transmit AC power at a high voltage and then step it down closer to the residential area using transformers," explains Victor Avelar, senior research analyst for APC by Schneider Electric (www.apc.com). "You can't do this with DC powerbecause it's a steady voltage with no ups and downs. To deliver DC over long distances, the conductor would have to be oversized due to voltage drop caused byresistance and could not be suspended between poles."

AC power is distributed to most of today's facilities at 600V AC or 480V AC, which has been stepped down via transformers from the high-voltage AC coming from the utility. Inside the data center, uninterruptiblepower supply (UPS) systems protect equipment from power interruptions by converting the incoming AC power to DC in the rectifier and then back to AC in theinverter. Transformer-based power distribution units (PDUs) then step the 480/277V AC power from theUPS down to 208/120V AC for distribution to IT equipment. Inside the IT equipment, internal power supplies again convert the AC power to DC needed for digital electronics.

"Traditional power-distribution designs use an approach in which the UPS feeds a required number of PDUs; this was adequate when the number of servers and racks was rela-tively low, but with today's equipment, it presents scala-bility and flexibility challenges," says Gary Anderson,marketing manager for Liebert AC power systems, Emerson Network Power (www.emerson.com).

The many power-conversion steps in the data center cause a significant loss of electricity, and the industry-wide quest for energy efficiency in the data center is what prompted LBNL to demonstrate a power-delivery system that does not contain as many conversion stages. The LBNL project implemented a power-delivery system that distributes DC to the server racks using a single rectificationstage, thereby removing the conventional UPS transformer.The LBNL demonstration report, which is available at theLab's web site, showed a potential of using 28% less energy with DC power distribution, refueling the century-old AC vs. DC debate.

"When we did our demonstration, we were just trying to show that a DC power-distribution system could be puttogether with existing equipment and provide some energy savings compared to a typical AC system of today," says Bill Tschudi, principal investigator for the applications team inthe environmental energy technologies division at LBNL. "We had some inkling that our report would create a lot of debate in the industry, and it was our intent to get people thinking about the potential use of DC power in the data center."

AC options

But some in the industry believe that the potential 28% savings stated in the LBNL report has been taken out of context and inappropriately interpreted in the industry. This belief is based on the fact that the LBNL demonstration did not compare a DC system against a higher-voltage AC distribution system with the highest possible efficiency.

"A DC system is efficient, but if both sides were equally matched and optimized, we believe the difference would be less than 2%," says Avelar. "A DC system just isn't realistic when you compare it to a higher-voltage AC distribution system."APC is a proponent of an AC distribution system in which a higher voltage is distributed directly to the equipment from the UPS, eliminating a whole set of transformer-based PDUs needed to step the voltage down to 208/120V AC. This type of system is common outside of North America where AC power is distributed directly to equipment at 400/230V AC.

"Because vendors don't want to create separate equipment for different parts of the world, today's IT equipment hasauto-sensing capabilities that allow it to operate between 100V and 240V," explains Avelar. "If we step 480V AC down just alittle bit to 415V AC, the line to neutral is 240V, and that's within the range of today's equipment. Stepping 480V down to 415V can be done very efficiently with auto transformers, and using this model with today's high-efficiency UPS is just as efficient as a DC model."

Avelar points out that even more efficiencies can be gained by delivering power directly to equipment at 480/277V AC, eliminating the need to step down to 415V. This would, however, require the maximum operating range of IT equipment to be bumped up from 240V to 277V.

Anderson of Emerson Network Power agrees with this concept: "By distributing the UPS output directly to the servers, valuable floor space in crowded data centers is saved and the efficiency of the power distribution system is increased." For example, Anderson says, "when our rack-sized Liebert FDC remote power distribution cabinet is used with servers that operate at 480/277V AC, data center mangers can achieve an additional 2% energy savings by eliminating the step-downlosses created when 480V is converted to 208/120V. Additionally, because the units can be placed in the rows, cables can run directly beneath the racks, shortening cable runs andensuring they do not impede air floor under the floor."

APC's Avelar is quick to point out that cooling systems are responsible for the most losses and represent the area wheredata centers can gain the most efficiency. "We need to look at solutions like row-based cooling, and that's where data center managers can get the biggest bang for their buck," he says.

Opponents of DC power distribution point to the absence of DC equipment, UL-approved connectors, or market demand. Additionally, transmitting DC power requires larger cables and upfront capital expenditure to switch over.

"The devil is in the details, and it's important to look at the ROI [return on investment]," comments Avelar. "If DC really were 28% more efficient, the ROI would come quickly. But that's not the case when you compare it to higher-voltage AC distribution and only see a 1 to 2% gain." Avelar adds, "Data centers are still going to have to power HVAC with AC power, and unless vendors build DC equipment, people are going to have a hard time building a DC data center. The market is just not there."

LBNL's Tschudi admits that it's not cost effective to swap out AC equipment for DC, but argues that since the equipment is available today, using DC power distribution makes sense for any new data center or complete retrofit. Some in the industry even say that the total cost of ownership could add up to a 50% savings over a five-year period.

"The equipment vendors have said they'll build whatever the market demands, and DC equipment already exists," says Tschudi. "The military uses DC equipment quite extensively, and there are entire ships that run on DC. It just hasn't hit the data center market yet."

Tschudi adds, "The biggest challenge we have to overcome is the mindset in the industry. Many early adopters in the U.S., Europe, and Japan agree that moving to DC power at some point in the future makes the most sense for improving the overall power-chain efficiency."

Supporting alternative energy sources

Tschudi points out that DC power also offers the benefit ofoptimizing power distribution from alternative energy sources, such as solar photovoltaics, wind turbines, and fuel cells, which are quickly becoming a global initiative. Solutions alreadyexist that switch between different power sources, allowinga data center to rely on DC wind and solar power whenviable, and switch over to the AC grid as needed.

DC pilot projects are already underway or approved at Clarkson University in New York, UC Santa Barbara, and a major telco on the west coast. "UC Santa Barbara has the idea of running a DC distribution system around part of their campus because they have photovoltaics and wind power that could be used as their own DC micro grid," says Tschudi.

No one knows what the outcome will be, but as The Green Grid (www.thegreengrid.org) and others release additional reports comparing AC and DC power distribution for the data center, the industry will have more information to evaluate. Tschudi concludes, "Almost everyone has had a positive attitude about our DC demonstration, except for the UPS vendors. They tend to zero in on power delivery up to theserver, but we're also considering the power conversion inside the server. The fact that our report is causing UPS vendors to come up with a more-efficient AC system is a good thing because we're trying to move the market toward efficiency."

BETSY ZIOBRON is a freelance writer. She can be reached at: [email protected]

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