Most sizable IT networks have for years provided DC power to devices using Power over Ethernet (PoE). PoE is an IEEE standard where data communication and DC power are provided to a device over a four-pair Ethernet cable. Most buildings probably already have an IT network or infrastructure for DC distribution. The PoE devices typically include IP telephones, Wi-Fi access points, video surveillance cameras, access control card readers, and remote Ethernet hubs and switches. The use of PoE has gone beyond typical IT devices and been used in devices such as clocks, gas detection, and AV room controllers.
Figure 12.1 A rectifier is an electrical device that converts alternating currents (AC), which periodically reverse direction to direct current (DC), which flows in only one direction. This process is known as rectification.
Much of the equipment in a data center such as servers, motors, and batteries use DC power. The power conversions that takes place in a data center could include incoming AC power converting to DC at the UPS, converting back to AC, then finally converting back to DC within each server's power supply.
A few data centers have gone to a DC architecture where the main electrical AC feed into the data center is converted to DC and DC power is distributed. The benefits of this approach are threefold: (1) less power loss in multiple conversions of AC to DC and DC to AC, resulting in reduced energy consumption, (2) less space required by the DC infrastructure for equipment and the IT equipment can connect directly to backup batteries, and (3) a decrease in cooling requirements. Companies such as Facebook and SAP have piloted DC power in their data centers. Japanese telecom giant NTT has four data centers in the Tokyo region operating on DC; it completed a DC-based server center in Atsugi City, which is its first to serve external clients.
A Lawrence Berkley Laboratory (LBL) study indicated a 7% reduction of energy consumption and a 28% efficiency gain, comparing DC data centers to AC based data centers. The LBL study goes on to say "We were also able to conclusively demonstrate to the data center industry that DC delivery systems are viable, can be 20% or more efficient than current AC delivery systems, be more reliable, and potentially cost less in the long run.”
Efficiency isn't the only benefit that DC power offers data centers. EMerge Alliance representative BJ Sonnenberg, manager of business development at Emerson Network Power, says that "DC data center power distribution equipment is generally more compact, takes up to 33 percent less floor space, and can be between 200 to 1000 percent more reliable” than its AC counterparts. A study conducted by Validus (ABB) and GE revealed that DC data centers should have up to a 36 percent lower lifetime cost.
The Electric Power Research Institute (EPRI) identified at least a dozen new data centers built in the last two years that operate on higher voltage DC, a figure that doesn't include the many central telecom offices currently operating on lower voltage DC power. The most powerful DC data center built to-date was recently announced by leading power and automation technology group ABB and Switzerland-based Green, an information and communications technology service provider. The data center is housed in Green's new Zurich-West data center expansion and is based on The Emerge Alliance proposed standard 380VDC technology. Up to a 20 percent reduction in power consumption is expected from grid-to-chip and in cooling.