The concept of using a direct current (DC) rather than an alternating current (AC) power source in datacentres as a way of reducing electricity costs is not new, but growing utility bills have recently pushed the idea back into the frame.

For most companies, the cost of converting an entire datacentre to DC power would outweigh any savings, but some believe that a smaller-scale approach that sees equipment racks modified to distribute DC power to individual servers can yield benefits.

Storage hosting firm Ultraspeed recently committed to converting 40 of the servers in its suite at the Telehouse Metro datacentre in London to using DC power. The AC/DC conversion is performed at the rack level, where AC current is taken in, converted using a rectifier and transformer, and then pumped out to servers and storage appliances within the rack.

Jordan Gross, Ultraspeed commercial director, believes he can cut per-server electricity bills by up to half by using DC power.

“DC offers 20 to 30 percent less power per server when you measure it on the feed. An equivalent number of servers would all have half the current than previously, so a customer that deployed 1A would now need 0.5A,” Gross said.

Some of that electricity saving, however, comes from other elements of the Ultraspeed diskless server design. Because the server does not include CD-ROM, floppy or hard drives, and the DC power board is entirely passive, less heat is produced, meaning less power is needed to cool the system.

Jordan conceded that installing rectifiers to confirm the AC/DC conversion is not cheap, but said that expense can be reduced when bigger rectifiers able to supply DC power to four or five racks at a time are installed.

“There is a cost associated with this type of infrastructure, but it does address efficiency and environmental concerns,” Gross said.

Others are not convinced that rack-level use of DC power is worth the effort. Shri Karve, director of business development for UPS specialist APC/MGE, acknowledged that a DC solution can be more reliable, more compact and easier to manage if centralised at the rack. But he pointed out that cost and space savings aside, switching to DC power also requires extra staff training, changes to cabling infrastructure and risks damage to the equipment within the rack because DC power has no zero crossover circuit, which can make safe shut-down tricky.

“A company may be able to save 1KW to 2KW of power, but use of rack-level DC is more of a fashion than a real solution. It offers some business benefits, but also some risks, and the benefits are not substantial enough to counter the risks,” Karve said.

Graham Titterington, principal analyst at research firm Ovum, believes some savings using DC at rack level are possible, but said it is difficult to quantify exactly how much they are worth.

“People have been talking about using DC power in datacentres for a long time but not many are actually doing it, which suggests that the sums are not as clear-cut as some would have us believe,” Titterington said.

Intel datacentre solutions architect, Ciaran Flanagan, conceded that the cost argument of converting datacentre racks to distribute DC power remains shaky based on current utility prices, but believes that this will change over the next two to three years.

“A lot of long-term energy contracts are up for renewal and companies are facing more expensive bills, so chief executives are looking to make infrastructure alterations even if they prove expensive in the short term,” Flanagan said.

But Titterington believes DC advocates face an uphill battle. Despite Thomas Edison’s first electrical circuit being DC-based, the world has adopted AC as its standard. This means off-the-shelf electrical components are rarely available for DC, and datacentre utility managers are relatively unfamiliar with DC engineering.
Gartner analyst Chris Ingle agreed. “Because of the volume of AC equipment and the effort put into standardising that equipment, it will take very strong pressure from the industry and major customers for this to change,” he said. Ingle added that this pressure could come from service providers and telcos, however.

The use of DC power is only one approach to cutting electricity costs. Businesses need to consider everything from server consolidation and storage virtualisation, to more efficient cooling solutions in order to reduce their datacentre power consumption going forwards.