Is direct current power the silver bullet for data center efficiency?

Direct current power shows promise for better data center energy efficiency, but IT shops may balk at upfront costs and hardware lock-in.

The vast majority of electrical components in a data center run on low-voltage direct current power --chips, resistors,...

capacitors and so on. Yet we insist on bringing in high-voltage three- and single-phase alternating current energy, which has to run through step-down transformers until the desired voltage is reached. Each transformation involves a loss of energy; even the most modern transformers will generally be no better than 98% efficient.

Consider the power distribution path to your data center and then within it. First, the power generated at the power station must be boosted to a high voltage suitable for transmission. Next is the substation, where it will be brought back down to a distribution voltage. There may be two such steps involved, after which it will reach the data center itself, at a 450 V three-phase or a 110/220/240 V single-phase supply. From here, it needs to be stepped down to the main 12 V, 5 V, 3.3 V and 1.5 V direct current power voltages that are used by the IT components. This shows a straight path of around four transformations -- or an overall efficiency of around 92%.

Eight percent of the generated energy has been lost purely through transformers, and this is only where the transformers are 98% efficient. And, in reality, it is far worse than this.

The power distribution within the data center tends to be single-phase alternating current (AC). Each item of IT equipment has a main transformer inside it with multivoltage outputs; many more onboard transformers provide the voltages required by specific components. However, larger transformers tend to be more efficient than smaller ones, so these mini-transformers may only be 95% efficient. If this is the case and six steps of transformation are involved, then more than a quarter of the generated energy is lost in transformation.

Therefore, it is surely good practice to use direct current (DC) power across as many areas as possible, avoiding the need for additional transformers in data center equipment. But a number of issues need to be taken into account to see whether this is really possible.

Hurdles to clear before using DC power

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Balancing three-phase power

First, at a power transmission level, DC is highly inefficient. Through the laws of physics, to get a set amount of power (in watts) down a line is voltage (in volts) times current (in amps). High transmission voltages in the hundreds to thousands of kilovolts (kVA) are used to lower the current. This is important, as resistive losses are linked to the current. Direct current or alternating current can be used at high voltages for power transmission, but AC has been the main choice to date. Trying to eliminate the transformation losses for the transmission stage is unlikely to be workable. It's pretty much the same at the distribution level; changing the existing infrastructure from AC to DC is not going to be easy, and there would still be a need to use high-voltage DC, so transformation is still required.

Those who advocate for direct current data centers point to the telecommunications industry, whose facilities have run using DC for a long time. There are reasons behind this, however. A full industry was built up around the provision of DC equipment, and there was less need for multiple DC voltages within an old telecoms facility. Therefore, power distribution within such a facility could be carried out using transformers built into the fabric of the facility, and then large copper busbars would enable distribution of high-current DC power. Also, in the 1980s and 1990s, although variations in oil prices caused some uncertainty in energy pricing, the main generation capability was still through relatively cheap and available coal. Energy efficiency was not the focus it is today.

The main issue for a direct-current powered data center is that the equipment is still not widely available. Although the components are DC, the vendors have to focus on the mass market and build equipment that uses AC as its main input. Some vendors do DC versions of their equipment, but they are premium priced.

With the additional costs of equipping a facility with DC power transformers, DC power distribution and management systems, using servers, storage and network systems that perform the same as the AC variants but at a higher price just does not make sense. Better to follow the crowd and use the lower cost standardized, AC-based systems.

Making a DC data center possible

There are two big hopes for the DC data center, however. One is modularization. As systems such as Cisco's UCS, Dell's vStart, IBM's PureSystems and others come out, these preconfigured "blocks" can be wired internally in any way the vendor wishes. It makes sense for the vendor to cut out unneeded components, and multiple transformation stages can be removed during the design and build phase.

In the same way that cooling systems are moving from the facility to the module with in-rack cooling systems, it becomes far more likely that power management will move from the facility to the module as well, with in-facility power distribution based around the provision of single-phase AC cabling to where it's needed.

The second hope is the growing prevalence of cloud computing. For a cloud provider with tens to hundreds of thousands of servers, massive scale-out storage infrastructure and a complex network structure, buying in to DC from the start could create a viable payback period.

A strategic decision to move to a direct current infrastructure is likely to end up being expensive and forcing the organization to be dependent on certain types of hardware. However, the vendors will drive a better energy efficient direction through optimizing DC use within their own systems.

ABOUT THE AUTHOR: Clive Longbottom is the cofounder and service director at Quocirca and has been an ITC industry analyst for more than 15 years. Trained as a chemical engineer, he worked on anti-cancer drugs, car catalysts and fuel cells before moving to IT. He has worked on many office automation projects, as well as Control of Substances Hazardous to Health, document management and knowledge management projects.

This was last published in September 2012

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Do you prefer AC power or DC power for your data center? Why?
DC power and oil cooling!
used to work in the Teclo industry and DC does save money when the infrastructure is designed for it form the ground up
Most Data Centers out there are equipped to use both AC and DC. For efficient and performance related to development and high current requirements. I would have to go with high voltage AC. (208 or 240).
should not start the "war of currents" again
The problem with DC will be the voltage drop over a large datacentre. This will be reflected in much higher costs in larger cables.
AC Power allows one to sit and wallow and experience burn outs of infrastructure ! DC when governed correctly can be used with Green Energy so easily. So taking into consideration that all E energy enters into a data centre through a controlled interface why the need for regulators. If we were smart enough to realise that DC is a by product of PV cells we would know that feeding them into a battery grid which is actually illegal in the UK due £'s not being shared ! So understanding that a greener way is being actively rejected ! Houston we have a problem !
It makes sense to use DC power distribution downstream of UPS for data racks. DC power MV or low voltage distribution is more expensive. Moreover, cooling uses AC induction motors.
House Hold DC generation would help us become independent of greedy AC generator businesses in Africa! These guys rip us of the biggest part of the very little we can get hold of!
Although not yet implemented in the mainstream, many will look back on 2012 as a watershed year for new power distribution architectures for data centers. Just weeks ago, the EMerge Alliance, an industry funded, non-profit, open standards group, issued the world's first applications standard for using higher voltage dc power in the core of data center power distribution. While some debate the potential energy savings dc power can yield, early adopters like Green-ch. will help many to realize the magnatude of TCO savings and overall operations improvements that can be genrated by a move to higher voltage dc. The simplification of the power systems can yeild significant lowering of both CAPEX and OPEX (15% or more) while boosting overall reliability in power distribution and back-up along with modest (10% or more) energy savings. Note that lower CAPEX cost comes as a result of using less, not lower priced, equipment. In the future, at equivilent volumes (and yes, that day will come) simplier dc equipment should also be less expensive.

2013 will see the development of a robust eco-system of OEMs bringing to market a variety of competitively priced dc devices and equipment that will fuel a sustained trend to dc power in data centers, from small to mega size. It will likely effect colo's the quickest as that segment sees the fastest growth and can implement change in a modular fashion, something that dc power architecture is best suited to do.

By the end of 2013, many will be asking the question, "why didn't this happen sooner, it just makes sense," or dollars as the case may be. Learn more at
Edsion had in right all along
AC power is more suited as it is the common way of generating distributing and consuming Electricity
DC is not a solution and in fact it doesn't give savings in electricity costs. DC bus won't give already proper voltages to IT equipment. We need sevaral voltages DC inside IT equipment. It means that conversion DC/DC will be proceeded inside the IT equipment. Totally the result can be oposite - more losses. Sending power in low DC voltages (lets say 48VDC) will cause more losses on its supply way.
I really don't understand this enthusiasm about DC voltage i DATA CENTER.
So, if you can't do a full swap / new data centre, like a new cloud data centre mentioned in the article, then how do you phase this in? Plus the costs are prohibitive at this point for the smaller guys. The decision will have to be made at the larger corporate level so as to reduce cost long term... but if that is the case, what about the thousands of distributed desktops? DC=DC, PC=AC? Ooops, I plugged in the AC workstation on the DC server repair bench when I went to do a repair!!
When power that is DC and is cheap and very local, look out AC