'Microchannel' cooling could make for better electric cars

Understanding how fluid boils in tiny “microchannels” could help engineers design more efficiently cooled electric and hybrid cars, aircraft, computers and other devices, according to research from Purdue University.

Allowing a liquid to boil in cooling systems dramatically increases how much heat can be removed, compared to simply heating a liquid to below its boiling point, said Suresh Garimella, a professor of mechanical engineering at Purdue. However, boiling occurs differently in tiny channels than it does in ordinary-size tubing used in conventional cooling systems.

“One big question has always been, where is the transition from macroscale boiling to microscale boiling?” said doctoral student Tannaz Harirchian. “How do you define a microchannel versus a macrochannel, and at what point do we need to apply different models to design systems? Now we have an answer.”

Indiana’s 21st Century Research and Technology Fund has provided $1.9 million to Purdue and Delphi Corp. to help commercialise an advanced cooling system that uses microchannels for electronic components in hybrid and electric cars. The research also is funded by the National Science Foundation Cooling Technologies Research Centre, a consortium of corporations, university and government laboratories working to overcome heat-transfer obstacles in developing new compact cooling technologies.

The new type of cooling system can prevent overheating of devices called “insulated gate bipolar transistors,” which are high-power switching transistors used in hybrid and electric vehicles. The chips are required to drive electric motors, switching large amounts of power from the battery pack to electrical coils needed to accelerate a vehicle from zero to 60 mph in 10 seconds or less.

The devices also are needed for “regenerative braking,” in which the electric motors serve as generators to brake the vehicle, generating power to recharge the battery pack. Regenerative braking also helps to convert electrical current to run accessories in the vehicle, and to convert alternating current to direct current to charge the battery from a plug-in line.

The high-power devices produce about four times as much heat as a conventional computer chip. Too much heat can hinder the performance of electronic chips or damage the tiny circuitry, especially in small “hot spots.”

“In order to design these systems properly you need to be able to predict the heat-transfer rate and how much cooling you will get,” Garimella said.

Conventional chip-cooling methods use a small fan and finned metal plates called heat sinks, which are attached to computer chips to dissipate heat. Such air-cooled methods, however, do not remove enough heat for the advanced automotive electronics, especially because of hot air under a car’s hood, Garimella said.

The microchannels are etched directly on top of the silicon chips. Because both the channels and the chip are made of silicon, there is no dramatic difference in expansion from heating, which allows chips to be stacked on top of each other with the cooling channels between each chip.

This stacking makes it possible to create more compact systems, because the chips do not have to be laid out horizontally on a circuit board as they ordinarily would.

“We can fit a lot more chips in much less real estate using this approach,” Garimella said.

The researchers have created a database of movies accessible on the NSF center’s website to demonstrate the boiling behavior in microchannels. They also have created a “complete test matrix” that enables engineers to determine how a particular system would perform given a range of channel dimensions, amount of heating and fluid flow.

“You can basically mix and match different design specifics and see the result,” Garimella said.

The cooling systems also are being developed to cool the electronic controls in aircraft, military systems and for other applications.