AC/DC: Highway to Data Center Efficiency Hell?

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For some years now there have been people claiming DC power distribution systems in data centers were (“would be”?) more efficient than using AC power.  Efficiency gains of 25% and more have been claimed.  Unfortunately these misleading claims have been based on comparisons to old AC distribution system designs that included hugely inefficient transformers and UPSs.  There’s no doubt that such ancient systems still exist out there somewhere.  And, for sure, many data centers out there are still horribly inefficient.  From our experience, the main reasons for poor data center energy efficiency have been primarily due to inefficient IT power supplies, lack of IT power management policy use, operation at loads well below the design of the system, and, yes, also from the use of inefficient transformer-based PDUs and inefficient UPSs too.  But, for many of us, this comparison to old technologies and systems is irrelevant.  When researching new car choices, do you compare them to a 1980’s Chrysler Lebaron?  The question for project teams designing a new data center today should be how DC compares to today’s modern AC power distribution systems.  A quantitative analysis of a 380Vdc distribution system to the most efficient forms of AC distribution (i.e., 415Vac in North America or 400Vac in Europe and Asia) shows that there is, in fact, very little difference between the two.  The difference is typically on the order of 1% or less.  In fact, if UPSs are operating in “eco-mode”, then the data center’s infrastructure using AC power distribution is actually MORE efficient by about 1%.  You can read about this analysis here.


The bottom line from this analysis is that efficiency is not a basis upon which to choose one or the other.  There are also other considerations that people think about when making this decision.  Which is more reliable?  Which costs more?  Or which uses less copper?  Are there any safety differences?  Schneider Electric Chief Innovation Officer, Neil Rasmussen, speaks on many of these topics in this clip

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  • It’s Good

  • Hello. Several quick points about your assumptions about the 400Vac vs direct current power systems.
    1. Your own calculator “Trade off Tools” demonstrates that direct current is more efficient.
    2. Your comment about “eco-mode” while it can be more efficient. Please provide a case study of one mission critical user that has their UPS’s configured in “eco-mode.” “Eco-mode” only protects against voltage dips … there is NO protection against surges. And never mind about the high short circuit currents of these types of implementations.
    3. When using titanium grade power supplies in a 400Vac system to get to the 95+% efficiency range the total harmonic distortion reflected by the supplies becomes a critical factor in distribution system losses which is not included in the discussion.

    But hey I am biased about +/-190Vdc systems and the inherent advantages over AC.

    There seems to be a desire to discuss certain parts of an AC system being better or worse than a DC system. However in an end to end design a direct current system will be more efficient, and reliable than an AC system with UPS’s. (medium voltage to 12Vdc)

    • Jim Spitaels

      11 years ago

      Hi John,

      Very interesting questions you posed. Here are my thoughts on the subject:

      1. The AC vs. DC TradeOff Tool defaults to a situation where 380VDC is 1.1% more efficient than 415VAC and 2.6% more efficient than 208VAC, which is inline with what our white papers say (380VDC can be 1-3% more efficient that double conversion AC). However, the inputs can easily be adjusted so that either AC or DC is higher efficiency. The whole point of the tool is to allow users to put in actual efficiency values from known equipment and see what the comparison reveals.

      2. Eco-mode, along with TVSS protection that all data centers should have, provides protection against sags and surges. It doesn’t provide frequency regulation, nor does it provide precise, low distortion output voltage that double conversion would provide. We’re not saying that everyone should use Eco-mode, but there are applications where it’s appropriate. Regarding available short circuit current, that issue needs to be considered during the design phase of any data center because the UPS can operate in bypass even when Eco-mode is not enabled (e.g. during overload, maintenance bypass or failure bypass). BTW – Any DC system that has batteries directly connected to the DC bus also has high available short circuit current.

      3. High efficiency AC supplies don’t inherently generate high harmonic currents. If they did, DC rectifiers would also have this same problem as the input boost conversion stage in both systems is identical. Here’s an example with only 4-6% Ithd – >

      Any questions, please let me know

  • madge gafford

    8 years ago

    Useful article – I learned a lot from the details ! Does anyone know if my assistant would be able to locate a template CA NRCC-ELC-01-E example to fill out ?

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