Facebook’s engineers also created a novel electrical design that cuts the number of times that the electricity from the grid is run through a transformer to reduce its voltage en route to the servers inside. Most data centers use transformers to reduce the 480 volts from the nearest substation down to 208 volts, but Facebook’s design skips that step. “We run 480 volts right up to the server,” says Jay Park, Facebook’s director of data-center engineering. “That eliminates the need for a transformer that wastes energy.”
To make this possible, Park and colleagues created a new type of server power supply that takes 277 volts and which can be split off from the 408-volt supply without the need for a transformer. The 408 volts is delivered using a method known as “three phase power”: three wires carry three alternating currents with carefully different timings. Splitting off one of those wires extracts a 277-volt supply.
Park and colleagues also came up with a new design for the backup batteries that keep servers running during power outages before backup generators kick in—a period of about 90 seconds. Instead of building one huge battery store in a dedicated room, many cabinet-sized battery packs are spread among the servers. This is more efficient because the batteries share electrical connections with the computers around them, eliminating the dedicated connections and transformers needed for one large store. Park calculates that his new electrical design wastes about 7 percent of the power fed into it, compared to around 23 percent for a more conventional design.
According to the standard measure of data-center efficiency—the power usage efficiency (PUE) score—Facebook’s tweaks have created one of the most efficient data centers ever. A PUE is calculated by dividing a building’s total power use by the energy used by its computers - a perfect data center would score 1. “Our tests show that Prineville has a PUE of 1.07,” says Park. Google, which invests heavily in data-center efficiency, reported an average PUE of 1.13 across all its locations for the last quarter of 2010 (when winter temperatures make data centers most efficient), with the most efficient scoring 1.1.
Google and others will now be able to cherry pick elements from Facebook’s designs, but that poses no threat to Facebook’s real business, says Frank Frankovsky, the company’s director of hardware design. “Facebook is successful because of the great social product, not [because] we can build low-cost infrastructure,” he says. “There’s no reason we shouldn’t help others out with this.”