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Two separate developments — a mini refrigeration system and a method to network tiny water pipes, both small enough to fit inside computers — might change the way processors are cooled in the near future, which could lead to improved performance and reliability, as well as reduced energy costs for many businesses.
At Purdue University in West Lafayette, Ind., a team of researchers is developing a miniature refrigeration system intended for notebook computers. Researchers at IBM have invented a way to network tiny pipes of water that can be used to cool next-generation PC chips and that could significantly reduce the energy consumed by data centers.
Conventional computer cooling systems use fans that circulate air through finned devices called “heat sinks.” The heat sinks are attached to computer chips, which tend to release large amounts of heat. In contrast to the common fan-based cooling systems, Purdue’s system uses refrigeration technology. Because heat emission is one of the main considerations faced by computer engineers, the new system is expected to cool computers more efficiently than before.
The concept of refrigeration-based cooling systems is not new. The Purdue team is focusing on designing miniature components, called compressors and evaporators, that are critical for these systems. The researchers developed an analytical model for producing tiny compressors that pump refrigerants using penny-sized diaphragms. This model has been validated with experimental data. The elastic membranes are made of ultra-thin sheets of a plastic called polyimide and are coated with an electrically conductive metallic layer. The metal layer lets the diaphragm move back and forth, producing a pumping action using electrical charges. This is known as “electrostatic diaphragm compression.”
“We feel we have a very good handle on this technology now, but there are still difficulties in the implementation in practical applications,” says Suresh Garimella, professor of mechanical engineering at Purdue. “One challenge is that it’s difficult to make a compressor really small that runs efficiently and reliably.”
Garimella says that new types of cooling systems will be needed for future computer chips, which are expected to generate 10 times more heat than do today’s microprocessors.
“The best that all other cooling methods can achieve is to cool the chip down to ambient temperature, whereas refrigeration allows you to cool below surrounding temperatures,” says Eckhard Groll, the Purdue team’s second lead scientist.
Besides the obvious result of smaller, more powerful computers, this improved cooling ability may increase the computer’s reliability by reducing long-term heat-related damage to chips.
Although the new technology is promising, challenges remain. One complication is that many diaphragms must operate in parallel to pump a large enough volume of refrigerant for the cooling system.
“So you have an array of 50 or 100 tiny diaphragm compressors, and you can stack them,” says Groll.
The findings show that, by stacking the diaphragms, it is possible to design a prototype system small enough to fit inside a notebook. The design is optimized using a model that lets the computer engineers determine how many diaphragms to use and how to stack them, either in parallel to each other or in series.
Groll says stacking in one direction might increase the pressure, while stacking in the other direction allows the necessary volume to be pumped. Another challenge is learning how to manufacture the devices at a low cost. The industry currently requires a cost of about $30 each, but the researchers say it is not yet possible to achieve that.
The IBM research is under way in partnership with the Fraunhofer Institute in Berlin. The German researchers demonstrated a prototype device that incorporates a cooling system into the 3D chips by piping water directly between each layer in the stack. The 3D processor stacks chips and memory devices that conventionally sit side by side on a silicon wafer and stack them together on top of one another. This feat presents one of the most innovative methods for increasing microchip performance developed in recent years.
Placing chips vertically rather than side by side trims the distance data has to travel, thus improving efficiency and saving critical space.
“As we package chips on top of each other, we have found that conventional coolers attached to the back of a chip don’t scale," says Thomas Brunschwiler from IBM’s Zurich Research Laboratory in an IBM press release. “To exploit the potential of high-performance 3D chip stacking, we need interlayer cooling.”
One of the main obstacles of manufacturing smaller and faster chips is heat; it is the result of the transfer of electrons through the tiny wires linking millions of components on a modern processor. As more components are added onto chips, the problems become worse. To avoid this, the researchers piped water through sealed tubes just 50 microns (50 millionths of a meter) in diameter between individual layers. Water is much more effective than air at absorbing heat, and with a tiny measure of liquid flowing through the system the researchers saw impressive results. IBM says its new micro water-cooling technology could reach the market within five years.
Iddo Genuth is the editor of the electronic magazine The Future of Things at www.thefutureofthing.com.