Size Matters: Making Big Smaller

Size Matters: Making Big Smaller

This piece was originally published in the July 2016 issue of electroindustry.

Jim Montgomery, Senior Product Manager, GE’s Critical Power business

Server cabinet. Courtesy of GE
Server cabinet. Courtesy of GE

Lowering the cost while increasing the efficiency of power conversion at data centers—that is, turning alternating current (AC) power into usable, efficient, and reliable direct current (DC) power for sensitive and power-hungry servers—is a major and often contradictory power design challenge facing the industry.

This challenge is contradictory because, while servers are producing higher and higher levels of processing capacity in the same blade-cabinet dimension, their power demands and number of DC rectifiers have to increase.

For example, the interior of most server cabinets has up to 42 vertical rack units, or 74 inches of usable space. Typically, these racks are not completely filled with servers because 10 to 15 percent of the space is occupied by AC-to-DC rectifiers and a power distribution unit (PDU), which is needed to power the entire rack. Think of the family car: while the engine is smaller, drivers want the performance of a powerful sports car. In both cases, more power is demanded, yet the power supply is not allowed to take up any more space.

Designing in Negative Space

There are two common approaches to reducing either the size or number of rectifiers required per server cabinet.

The first starts with the concept of having to convert and balance three-phase AC power coming from a utility or external generator (i.e., the power used by most data centers). A typical data center power-distribution system involves balancing three-phase power through the use of three separate rectifiers fed by a PDU. Newer three-phase AC-to-DC rectifiers can replace the three single-phase conversion units for each phase with a single unit that balances all three phases. This unit delivers the same amount of power in half the rack space of previous-generation three-phase power solutions, and the PDU can simply be eliminated.

The second approach, returning more cabinet space for greater server capacity, involves relocating the rectifiers away from the space taken by the server blades. Applying the philosophy of designing in the negative space, it is possible to put usable power in “unusable” space.

For example, the interior of most server cabinets is 24 inches wide, but installed servers only take up 19 inches, leaving five inches of unused space along the interior sides and back corners. By relocating the power supplies vertically along the side rails of the cabinet, data center designers can recapture interior rack space for more server capacity. This configuration allows for newer, three-phase-balancing, compact rectifiers to be plugged directly into the PDU, completely removing the power supplies from the space assigned to servers.

With this recovered server space, additional revenue-generating equipment can be installed, ultimately reducing the capital expenditure on each cabinet and the number of cabinets required. Reduced hardware, use of floor space, and installation costs—multiplied by hundreds of server bays per data center—equates to the potential for millions of dollars in savings per facility.

With the combination of a PDU and fewer, more compact rectifiers placed in previously unusable space, up to 10 percent more information technology processing capacity can be located in existing server bay footprints. For data center designers, adding capacity to a server is like adding an 11th floor inside a 10-story building.


Mr. Montgomery works with data center customers to provide advanced power solutions.


Read this month’s issue of electroindustry.

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