Two-Phase Cooling
Saves Computer Power


Cooling data centers in the U.S. consume a huge amount of energy—up to 50 percent of total operating costs. But cooling is also absolutely essential. If the racks of computers overheat, they get sluggish or, even worse, shut down.

The standard cooling approach is to use basic air conditioning to cool the space that houses the servers. Overall, this is an inefficient process because air has a low density and low specific heat. Another alternative is using a liquid cooling system. This requires customized servers (more expensive) or the introduction of water into the server chassis, which makes data-center managers very uneasy, because even a drop or two of leaked water can seriously damage the electronics.

Now, however, Tim Shedd, an associate mechanical engineering professor at the University of Wisconsin-Madison, has invented a cooling system that achieves a 90 percent cooling energy reduction at the chip by circulating a nontoxic, nonconductive, and nonflammable fluid that removes heat in the vapor phase. Shedd founded Ebullient Cooling in 2013 to further develop and market this technology.

Tim Shedd examines a computer equipped with his novel cooling system. Image: David Tenenbaum / University of Wisconsin

Bubbles to the Rescue

Shedd’s technology is based on two-phase cooling. The fluid is pumped through a tube and collides with a copper plate that is mounted to the computer chip that is enclosed within a plastic chamber. The fluid absorbs heat from the chip and begins to evaporate. The bubbles, which have captured the heat, are transported through a tube to the roof, where it is released, and the liquid phase is cooled down and recirculated to repeat the process.

“The phase change gives us a big advantage because it can take a lot more heat away when we let the liquid boil, compared to the liquid just flowing through,” says Shedd. “The boiling behavior also improves the cooling efficiency, allowing us to cool even the most intense heat sources with outside air exceeding 120 degrees Fahrenheit. Phase change also allows us to cool 2,000 watts or more per server.”

Shedd selected a dielectric liquid refrigerant that would not damage the electronic equipment if it leaked. “You can literally dunk your phone or remote control in a cup of this working fluid and it has no negative effect,” he says.

Ebullient Cooling has also developed a way to 3D print the cooling modules that attach to the processors. Because they have superior thermal performance and low pressure loss, up to 10 devices can be cooled in series using a single cooling circuit connected with one-quarter-inch tubing. 

Looking Ahead

Shedd is somewhat surprised by how well this laboratory idea has proven itself in the field.

“Typically it is difficult to translate what works in the laboratory to the real world because laboratory results depend on the tightly controlled operating conditions,” says Shedd. “This technology, however, has proven to be very robust to environmental and operating variations due to inherent self-stabilizing behaviors and the patented flow-distribution system.”

This technology has a wide range of applications. The greatest energy savings is in the data center cooling space, but other opportunities include cooling desktop computers, high heat flux laser diodes, LED lighting, and medical equipment such as MRIs and CT-scanners. Modified forms of the technology could also improve the efficiency of battery cooling and exhaust waste heat recovery.

“At the university, we are still working on the fundamentals of nucleation and heat transfer to ‘sliding’ bubbles, which we believe give our technology its high performance,” says Shedd. “We are also uncovering methods for enhancing boiling without needing an enhanced surface. Being able to increase heat-transfer performance without the added cost of micro-machining a surface could be a real boon for heat-transfer devices.”

Mark Crawford is an independent writer.

Learn more about energy solutions for a sustainable future at ASME Power & Energy 2015.

The boiling behavior also improves the cooling efficiency, allowing us to cool even the most intense heat sources with outside air exceeding 120 degrees Fahrenheit. Phase change also allows us to cool 2,000 watts or more per server.

Prof. Tim Shedd,
University of Wisconsin-Madison


August 2015

by Mark Crawford,