“The Battle of the Currents, Take II” – AC versus DC in the data center

Talk of the “Battle of the Currents” is spilling into mainstream society

Recently, I caught up with a former colleague who told me that she was now on “Team Edison.”  Then today my wife asked me over lunch whether I had ever heard of a certain Nikola Tesla and about what a genius he was (note that neither my wife nor colleague is an electrical engineer nor has ever shown any interest in our fine profession).  And just the other day even my favorite online cycling magazine sported a link showing an old photo of Nikola Tesla’s not-so-cheerful face!

So I began to wonder why this old and long-forgotten “Battle of the Currents” with Nikola Tesla as proponent of alternating current and Thomas Edison advocating direct current all of a sudden not only caught the attention of us “geeks” but was spilling into the mainstream of society.  130 years and countless gigawatt hours of AC power later we have un-dusted the history books and once again engaged in fierce arguments that would have made Messrs. Tesla and Edison proud (thankfully without the electrocution of any circus elephants!).  And as the 21^st Century battleground for the revival of this centennial argument we chose one of the most advanced and power intensive fields of modern technology – data centers.

Alright, so let us too engage in this epic struggle that is playing out within data centers and hear each side’s arguments without picking any favorite just yet:

1. DC is more efficient than AC:  DC proponents claim 25-30% efficiency improvement, AC advocates counter with data that implies marginal if any differences in power supply efficiencies.
   The fact is, if you compare a state-of-the-art AC power supply with a similarly advanced DC power supply, the efficiency improvements for DC are indeed only in the range of 2%.  However, when comparing true efficiency “from grid to chip,” DC power architecture is typically 8-12% more efficient, depending on the IT power supply. Granted there is no disagreement that other factors such as server loading and cooling have a more profound impact on data center efficiency than the power architecture but every improvement matters.
2. DC is more reliable than AC:  Independently collected empirical data as well as laboratory tests give evidence that DC power systems increase reliability over comparable AC systems by a factor of up to 100.
   Flip that coin, and you could say that DC allows simplification of power systems architecture without jeopardizing availability.
3. DC is cheaper than AC:  This angle of the debate has seen a fair amount of cheating but our own honest analysis shows that the true cost of a DC power system (including switchgear, UPS, cabling etc.) is at least 20 % lower than that of a comparable AC architecture.
   However, as pointed out above, the biggest cost savings lies in the potential to simplify the architecture and achieve the same (or greater) savings with significantly less equipment.  And that’s where the discussion with our data center customers becomes really interesting, because as Leonardo put it “Simplicity is the ultimate sophistication”.

Speaking of “simplicity” – an indisputable advantage of DC is that it allows to connect multiple energy sources such as the utility grid and on-site generated power (e.g. from fuel cells, solar, wind etc.) onto a common bus without complicated controls and synchronization.

Being a global leader in AC as well as DC technologies, ABB is in a unique position to take a fact-based view – like a proper engineering company should do.  However there are some strong individual points of view out there – so come on off the fence, pick your side and engage in this never ending debate. I look forward to hearing from you.