Dec. 17, 2001 WEST LAFAYETTE, Ind. — Research engineers at Purdue University are developing tiny, quiet fans that wiggle back and forth to help cool future laptop computers and other portable electronic gear. The devices remove heat by waving a small blade in alternate directions, like the motion of a classic hand-held Chinese fan. They consume only about 1/150th as much electricity as conventional fans, and they have no gears or bearings, which produce friction and heat.
Because the new fans work without motors that contain magnets, they do not produce electromagnetic "noise" that can interfere with electronic signals in computer circuits, said Suresh Garimella, an associate professor of mechanical engineering at Purdue.
New findings about the fans will be detailed in a paper to be presented Jan. 15, during the Thermal Challenges in Next Generation Electronic Systems: THERMES 2002 conference, in Santa Fe, N.M. The paper was written by Philipp Bürmann, a former Purdue graduate student who is now a student at the Dresden University of Technology in Germany, Arvind Raman, an assistant professor of mechanical engineering at Purdue, and Garimella.
As future computer chips become increasingly compact, more circuitry will be crammed into a smaller area, producing additional heat. Because excess heat reduces the performance of computer chips and can ultimately destroy the delicate circuits, it will be important to develop new cooling technologies, Garimella said.
"Even if it's just a bit overheated, its performance and reliability goes down," Garimella said. "Another reason for cooling is to improve performance as you go to smaller and smaller devices."
The cramped interiors of laptop computers and cell phones contain empty spaces that are too small to house conventional fans but large enough to accommodate the new fans, some of which have blades about an inch long. Placing the fans in these previously empty spaces has been shown to dramatically reduce the interior temperatures of laptop computers.
The innovative fans will not replace conventional fans. Instead, they will be used to enhance the cooling now provided by conventional fans and passive design features, such as heat-dissipating fins.
In experiments on laptop computers, the Purdue researchers reduced the interior temperatures by as much as 8 degrees Celsius, Garimella said.
"For a very small power expenditure, we are able to get a huge benefit," he said.
The fans, which could be in use commercially in about two years, run on 2 milliwatts of electricity, or 2 1/1,000ths of a watt, compared to 300 milliwatts for conventional fans, the researchers said.
The fans are moved back and forth by a "piezoelectric" ceramic material that is attached to the blade. As electricity is applied to the ceramic, it expands, causing the blade to move in one direction. Then, electricity is applied in the alternate direction, causing the ceramic material to contract and moving the blade back in the opposite direction. The alternating current causes the fan to move back and forth continuously.
The operating efficiency of a fan can be optimized by carefully adjusting the frequency of alternating current until it is just right for that particular fan.
The piezoelectric fans can be made in a wide range of sizes. The Purdue engineers will be developing fans small enough to fit on a computer chip: their blades will be only about 100 microns long, which is roughly the width of a human hair, Garimella said.
Such fans might be used to cool future chips that produce more heat than their conventional counterparts. The concentrated circuits in a semiconductor computer chip can generate more heat per square centimeter of chip area than an area of equal size on the sun's surface, he said.
Piezoelectric fans were developed during the 1970s and are available as novelty items. These older versions were considered noisy, but the Purdue group has developed fans that are almost inaudible, the researchers said.
It is difficult to design the fans so they perform properly for a given application. The fans are made by attaching a tiny "patch" of piezoelectric ceramic to a metal or Mylar blade. Two factors affecting the performance of the fans are how much the ceramic patch overlaps the blade and how thick the patch is compared to the blade's thickness.
Another critical factor is precisely where to attach the blade to the patch.
Those factors dictate performance characteristics such as how far the blade moves, how much airflow it produces and how that flow produces complicated circulation patterns. An improperly designed fan could actually make matters worse by recirculating hot air back onto electronic components, Raman said.
The Purdue researchers, in findings being reported at the upcoming conference, have developed mathematical techniques that take these factors into consideration when designing fans for specific purposes.
"These fans typically have been novelty items," Raman said. "If you want to really be serious about putting them into any practical use, there are so many things you need to understand about how they work and how to optimize them."
Mathematical models developed by Purdue researchers can be used to provide design guidelines for engineers.
"What we bring to the table is a knowledge of the modeling of these fans," Garimella said. "How to analyze the design, to figure out how large a patch should be for how long a blade, how thick the patch should be and what happens if you modify all these quantities.
"In short, it's how to optimize the performance of these fans."
Raman and his students developed relatively simple mathematical formulas that make it easier for engineers to begin designing fans for specific jobs. Engineers can use the formulas to do a quick, "back-of-the-envelope" design.
"And then you might want to do some fine tuning and tweaking with more detailed analysis," Garimella said.
The research is funded through Purdue's Compact High-Performance Cooling Technologies Research Consortium, led by Garimella, which is now becoming a National Science Foundation Industry/University Cooperative Research Center. The center is being formed to help industry develop miniature cooling technologies for a wide range of applications, from electronics and computers to telecommunications and advanced aircraft.
The consortium, founded by Garimella while he was a faculty member at the University of Wisconsin, has nine corporate members, including Apple Computer and electronics and telecommunications giants Nokia Research Center, General Electric and Delphi Delco Electronics Systems.
"Industry comes to us with a technical problem, and we conduct research to help solve those problems," Garimella said.
The NSF will provide $70,000 annually for the center. Corporate members pay $30,000 a year, and additional funding will be provided by Purdue.
Center researchers are working on technologies that could have a variety of applications, including fuel cells for electric cars, automotive and military electronics, cellular base stations for mobile phones and high-performance electronic equipment used for distributing electricity.
Future work on the piezoelectric fans will aim to better understand the complex airflows created by the fans, Garimella and Raman said.
Related Web sites:
Suresh Garimella: http://ME.www.ecn.purdue.edu/ME/Fac_Staff/garimella.whtml
Cooling Technologies Research Consortium: http://widget.ecn.purdue.edu/~CTRC/THERMES 2002; http://www.engfnd.org/2at.html
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