Radical energy savings method 6: Use direct current in the data center Yes, direct current is back. This seemingly fickle energy source enjoys periodic resurgences as electrical technologies ebb and flow. The lure is a simple one: Servers use direct current internally, so feeding that power to them directly should reap savings by eliminating the AC-to-DC conversion performed by a server's internal power supply.
Direct current was popular in the early 2000s because the power supplies in servers of that era had data center conversion efficiencies as low as 75%. But then power supply efficiencies improved, and data centers shifted to also-more-efficient 208-volt AC. By 2007, direct current fell out of favor. InfoWorld even counted it among the myths in our 2008 article "10 power-saving myths debunked." Then in 2009 direct current bounced back, owing to the introduction of high-voltage data center products.
In the earliest data centers, utility-supplied 16,000 VAC (volts of alternating current) electricity was first converted to 440 VAC for routing within a building, then to 220 VAC, and finally to the 110 VAC used by the era's servers. Each conversion wasted power by dint of being less than 100% efficient, with the lost power being cast off as heat (which had to be removed by cooling, incurring yet more power expense). The switch to 208 VAC eliminated one conversion, and with in-server power supplies running at 95% efficiency, there wasn't any longer much to gain.
But 2009 brought a new line of data center equipment that could convert 13,000 VAC utility power directly to 575 VDC (volts of direct current), which can then be distributed directly to racks, where a final step-down converter takes it to 48 VDC for consumption by servers in the rack. Each conversion is about twice as efficient as older AC transformer technology and emits far less heat. Although vendors claim as much a 50% savings when electrical and cooling reductions are combined, most experts say that 25% is a more credible number.