Cooling Computers with Tiny Jet Engines

Hewlett-Packard is adapting fans from radio-controlled jets to relieve heat-stressed computer servers.

The computer servers that fill huge data centers are producing more heat with every new generation of processors. It’s a problem that’s sending engineers on a search for cooling fans that are both small enough to fit inside ever-smaller server chassis and powerful enough to dispel increasing amounts of heat. At Hewlett-Packard, they’ve found one answer in an unexpected place: model jet airplanes.

A cutaway view of Hewlett-Packard’s new electric-ducted server cooling fan, which was adapted from model jet airplane engines. (Image courtesy of Hewlett-Packard.)

To cool its next generation of commercial servers, the company is using electric-ducted fans (EDFs), originally developed by model airplane hobbyists to power radio-controlled jets. Essentially propellers in a box, the fans run so fast and produce so much air pressure that they should be able to provide the cooling needs for the next several generations of HP servers, according to Wade Vinson, an engineer in the company’s Industry Standard Server Group.

In an electric-ducted fan, which is the most popular form of radio-controlled jet motor, the fan’s blades are placed inside a tube, or “duct.” Because the blades are shorter than typical propeller blades, they spin faster, thereby creating more thrust. Furthermore, the duct reduces noise and prevents air vortices from forming around the tips of the blades – which saps the thrust produced by traditional propellers.

Of course computer servers don’t need thrust, since they generally don’t go anywhere. Instead, Vinson and his team showed that EDF blades can be redesigned to produce pressure. The fan blades on their prototypes force air into a server’s chassis, so that a certain volume of air per minute flows past the heat sinks (aluminum or copper fins attached to most CPUs) and carries away heat through convection.

The end product is HP’s Active Cool Fan, scheduled to debut in its next generation of BladeSystem servers. At their most efficient setting, according to Vinson, the fans consume just one-third the power of traditional computer fans; and they’re smaller than regular fans, which means engineers can make the servers thinner and pack more electronics into them. “If you have 10 traditional servers today, we could put 16 servers in the same space,” says Vinson.

The prototype HP fans are built from sturdier, more reliable parts than today’s computer fans, according to Vinson, and they deliver air with enough force to cool the smaller, denser, and hotter servers on HP’s drawing boards. “They literally blow you away,” he says; “it’s like picking up a leaf blower.”

The time is ripe for better computer cooling technology. In essence, CPUs are tiny radiators, which happen to do computational work as they busily convert electricity into heat. Every watt of energy used by a data center’s servers in the form of electricity has to be expelled as heated air. But, as computer manufacturers make processors smaller and faster and pack them more closely together, it’s become harder and harder to push enough air through a server to keep the electronics running smoothly.

This situation can translate into huge problems for corporate data centers with hundreds or thousands of servers – such as the ones that keep our online economy running at facilities managed by Google, Yahoo, eTrade, and the like. Servers that overheat can shut down, slowing processing and increasing the load on other servers; and they force companies to spend hundreds of thousands of dollars on new air-conditioning systems and the electricity to run them.

The problem is getting worse, too, since smaller processors mean more are squeezed into the same volume. “As recently as the year 2000, the average server consumed on the order of 100 to 150 watts,” says Vinson. “It’s now exceeding 400 to 500 watts. And it would not be unusual to have a rack of servers that pulls 15,000 or 20,000 watts” – enough to light more than 100 homes.

While U.S. computer manufacturers for years have been pushing mostly Chinese makers of computer fans to rethink their products’ motor designs and aerodynamics, the amount of air these traditional fans can push through has been increasing by no more than 5 percent per year. This lack of real progress has loomed as a potential roadblock in the development of server technology. “By 2003, we had an idea of what we wanted our future servers to look like – but when we started doing the calculations about the type of fan that would be needed to go there, it was three times what anybody in the industry would be able to deliver,” says Vinson.

Eventually Vinson and Ron Noblett, vice president of shared engineering services at the HP Industry Standard Server group, stumbled on the unorthodox solution. Noblett, a model airplane enthusiast, suggested that his colleague examine the electric-ducted fans used in radio-controlled jets.

Most propeller-driven model airplanes carry small gasoline engines (which produce that familiar high-pitched whine). But gas engines aren’t ideal for model jets. These $4,000 toys can buzz about at more than 200 kilometers per hour, requiring lightning reflexes; if a pilot loses control, the plane’s fuel tank can turn into a flying bomb. So in the mid-1980s, hobbyists started developing high-speed, battery-powered electric motors that could be placed in the fuselage or under the wings of a model plane.

“Over the last 20 years, this electric flying industry has kept pushing battery power, motor technology, and fan blade technology under the premise that even if they go too far and the motor dies in flight, the plane still has its control surfaces and just becomes a glider,” Vinson says. Adapting this new model-jet technology for cooling servers turned out to be a simple matter of changing the blade shape.

HP and rivals are also working on other ways to solve the problem of excessive heat. There’s water cooling, in which heat sinks are replaced by “water blocks” with channels for flowing water; phase-change cooling, which is similar to traditional refrigeration; and Peltier cooling, based on the Peltier effect, in which a current passing through two types of metal causes one to heat up and the other to cool. But a flow of air is still the simplest way to disperse heat.

“Because we’re able to take these high-power EDFs and re-engineer them, the guy with the data center, knowing he’s got this technology, doesn’t have to worry about doing an upgrade of his cooling systems two years from now, and a rip-and-replace five years from now,” says Vinson. He adds: “We don’t think we’re even close to running out of room to improve this technology.”

Home page image: The HP electric-ducted fan in its finished housing. (Courtesy of Hewlett-Packard.)

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