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It’s remarkable that we continue to see what I consider pseudo-scientific ‘studies’ claiming that DC distribution is significantly more efficient than AC distribution in data centers. These ‘studies’ make the photographic evidence of Bigfoot and UFO’s look like incontrovertible truth by comparison.
A widely quoted study published in 2008 claims a 28% efficiency improvement of 380 VDC versus AC distribution. We have looked at this study in depth and have serious issues with its conclusions.
The paper basically compares an antiquated AC architecture against a prototype DC architecture and reaches the following conclusion:
“We were also able to conclusively demonstrate to the data center industry (via the 200+ open house attendees and the media coverage) that DC delivery systems are viable, can be 20% or more efficient than current AC delivery systems, be more reliable, and potentially cost less in the long run.”
I think the only thing conclusive out of this study was that the Green Grid wrote a white paper refuting many of the claims and we published our own white paper (#127) to debunk some of the aforementioned study’s conclusions.
Our research concludes that the key flawed assumptions made in this study are:
1) They used a 208V AC power distribution which no one would reasonably recommend be implemented today. Reality is that in Europe there is no need for a transformer so the distribution losses, or lack thereof, are the same.
2) They used older 208V UPSs that are much less efficient than today’s commercially available UPS. Their assumption: UPS runs at 85% efficiency. Reality: UPS runs at 96% efficiency. (As a side note: the study assumes the DC UPS is only 92% efficient – in our white paper we found a published DC UPS efficiency 96% which actually improves the DC efficiency).
3) They assumed an old AC power supply which is much less efficient than ones that ship in every piece of IT equipment today. Their assumption: AC 73% efficient and DC 92% efficient. Reality: AC are 90.25% efficient and DC 91.75% efficient.
4) The AC system was immensely oversized compared to the DC system so it wasn’t operating at its peak efficiency
5) Data was only taken at a single fixed load as opposed to publishing an efficiency curve.
The bulk of their 28% number simply comes from the false assumptions of the UPS and power supply efficiency. Using the numbers above shows the AC system to be 86.6% efficient (.96*.9025) and the DC system to be 88% efficient (.96*.9175). This 1.4% difference is a whole lot smaller than the 28% number that gets so widely and easily quoted by the press.
What’s even more disturbing is that there has been another Bigfoot/UFO sighting. This time it’s a study claiming a 15% improvement in DC versus AC. We are digging into this one but science is science – there is really very little difference (~2-4% at most) in the efficiency of DC versus a well designed AC system. It’ll be interesting to understand the details of this ‘study’ to see if they tipped the scales in favor of DC.
All this being said, there is clearly a significant interest in using DC distribution for data centers and it’s likely that you will see some data centers built this way somewhere in the world. But we should stop kidding ourselves as an industry that there is some mythical amount of energy savings to be had. It’s just not the case.
Bigfoot and UFOs? Now that’s worth looking into…
Also, you may want to check out this article on Mission Critical Magazine’s web site: Ian Bitterlin article
Please follow me on Twitter @KevinBrown77
About Kevin Brown:
Kevin Brown is Vice President, Data Center Global Offer for Schneider Electric. He leads a team of industry professionals to develop and bring to market solutions for the data center market. In this role, he has responsibility to articulate the vision for Schneider Electric’s data center offer and create comprehensive data center solutions that solve real customer problems today. Kevin is an experienced industry professional in both the IT and HVAC industry. He has over 20 years experience at Schneider Electric in a variety of senior management roles including product development, product management, marketing, and sales.