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Theres nothing more refreshing on a hot summers day than a cool stream of water—and that goes for chips, too.
On a balmy day at the companys headquarters in Palo Alto, Hewlett-Packard research engineers disclosed new methods of keeping both semiconductors and datacenters cool. Whats novel is the way they are doing it: adapting an inkjet printer head to spray a fine mist of liquid on an overheating semiconductor, and using a mobile robot to look for “hot spots” in a data center.
“We believe we have to take a holistic approach to cooling,” said Chandrakant Patel, principal scientist and principal researcher on HPs thermomechanical research team at HP Laboratories. “We can not say we are going to cool the chip and not the data center.”
Patel is expected to make a presentation on the importance of data center cooling at Intels Developer Forum the week of Sept. 9, labs researchers said.
Mist for microprocessors
HP, a company that makes billions on its inkjet printers, is exploring adapting an inkjet-like cartridge to deliver a fine mist of fluid onto the chip. Engineers are exploring whether it makes the most sense to spray the mist onto the chip package, or the surface of the chip die itself.
“What were proposing to do is take the cartridge right over the silicon chip,” said Cullen Bash, a member of the technical staff at HPs System Technology Department. Fluid will be sprayed from the cartridge to the chip…then there will be a pump which will pump this fluid back into the cartridge.”
HP believes that in the near future, chips could dissipate 200 watts of thermal energy – heat – in a space an eighth of an inch on a side. Thats because CPUs dont dissipate heat uniformly. In a microprocessor, most of the available space is actually cache memory, where instructions are stored. The computations take place in the arithmetic logical units (ALUs), where heat is concentrated.
“And those heat sources will move around,” Patel said. “The difficulty is targeted cooling.”
At the chip level, the problem isnt the overall heat produced by a chip, but the heat density. Although Intel CTO Pat Gelsinger first noted publicly that upcoming chips could produce more heat per square millimeter than a nuclear reactor, HP engineers said they first began thinking about the problem in the mid-1990s.
Like a car, when chips overheat, they malfunction. Commercial-grade chips can generally tolerate temperatures up to 100 degrees centigrade. But heat can be a problem both in tiny spots on a chip as well as the ambient air swirling around banks of processors.
Today, computer and silicon makers use several different ways to cool microprocessors. Most common is a heat sink or heat spreader, a metal attachment to a microprocessor package that passively dissipates heat across a wide surface area. “Finned” heat sinks use vertical plates to add even more surface area, and fans can be clipped onto these as well. In notebook PCs where heat is even more critical, “heat pipes”—thin tubes containing liquid—passively ferry heat away from chips through evaporation and recondensation. Some firms also sell water pumps that expedite this process, cooling the chip further.
Bash said that HP found that a finned heat sink (also called a “thin fin”) dissipates between 30 to 35 watts of heat. Evaporative cooling can dissipate up to 100 watts, or heat densities of 200 watts per square centimeter. HP eliminated heat pipes as an option because the water can boil inside of them, destroying the convection process.
“The only reason we would consider this is the limit of the heat pipe itself,” Patel said. “The (thermal) interface is the problem. We would try to change that interface.”
Problems to solve
HP engineers still face a number of tricky engineering hurdles. Inkjet printers deposit ink on paper. What HP is proposing is to reuse the fluid, which will likely mean building a tiny reservoir into the processor die itself and then engineering a circulation system of some sort.
A far more serious problem is one of fluid thermodynamics, far away from HPs traditional expertise in electrical engineering. Inkjet printers use the finest mist possible to increase resolution. But if the fluid is too fine, the particles wont have enough momentum to actually overcome the rising heated air from the chip, said Ratnesh Sharma, also a member of HPs technical staff.
Worse still, heavier droplets will actually pool on the chips surface. While the drops will initially cool the chip, the pool actually serves as an insulator, again allowing the heat levels to rise. Researchers said that HP will likely use some sort of inductive fluid, not water, which wont short out any electrical connections.
In addition, if the thermal density pattern on a chip varies, then the cooling must change as well. HPs Sharma demonstrated a system by which different micronozzles in the cartridge pulsed a fine mist over different areas of the chip. The individual droplets are picoliters in size.
“Its like taking a shower in the morning and saying let me get more water on my nose and ears instead of a constant stream of water,” Patel said.
Finding a chip to apply the technology to is another problem. While HP co-designed the Itanium processor and its own PA-RISC architecture, the company is phasing out its proprietary lines and buying Intels chips instead. HP might not be able to convince Intel to ship it bare processor dice, allowing it to add its own cooling package, executives admitted. Instead, the technology might be more practical in laser diodes, used in optical communications, they said.
Sensing the patterns of heat is difficult as well. HPs software engineers can only predict so much. Processors today are typically monitored by a single thermal diode embedded in the chip. HPs system could require several, although they would also need their own I/O pins in the chips package.
“What we need is real data,” Bash said. However, design engineers havent traditionally made thermal issues a priority. In an HP chip, “we had to fight to just get one (diode),” he said. “They screamed.”
RoboRunner to the rescue
Sensing heat is also a priority in a datacenter, where racks and racks of computers are stored. Although data centers are built on raised floors which circulate cool air, each rack vents its waste heat out the back of each rack, creating a “hot spot”. If those hot spots arent constantly monitored, the heated air could be sucked into the cooling vent of another rack—overheating the processor, shutting down the rack, and costing the datacenter and customer money.
Datacenters already spend 10 percent of their power budget—about $1 million a year — on cooling alone. “Today, the state of the art in datacenter cooling is an engineer walking around and feeling the heat,” Patel said. “If it feels too hot, he turns up the air conditioner. Thats state of the art.
“Now what were proposing is to replace this technician with a robot that rolls around the aisles, into tight spaces and maps the temperature in the X, Y, and Z [height] direction,” Patel added.
The robot is relatively simple. HP bought a robot from a third-party manufacturer, hooked up a laptop computer, and attached a pole with several temperature sensors. The “RoboRunner” wanders around the datacenter on a preprogrammed course, taking temperature readings at predetermined points. The robot did have to be manually driven around the lap to map the area, but engineers said the process only took about an hour.
A separate server receives the data and plots hot spots, which HP is developing and testing. In the future, HP hopes that it or another firm can design louvered floor vents that can detect cool air to where its needed. “Controlled cooling is a smart cooling proposition,” Patel said.
HPs cooling labs already examine other ways off cooling rack-mounted systems, Sharma said, who noted that its budget hasnt suffered even through the HP-Compaq merger and subsequent cost-cutting. “Our CEO has said that datacenters are a priority,” he said.
HP has looked at three technologies to cool its Vectra line of servers: a thermal actuator, or liquid heat exchanger; a plane roll bond panel containing liquid; and a thermosyphon, an advanced heat pipe. The thermal actuator pushes a “cold plate” against the processor with thermal grease or some other substance. A plane roll bond is actually a flat radiator-like plate mounted on the wall of the server, which circulates a 60% glycol/40% water solution through buoyancy-induced convection, eliminating the need for fans.
Thermosyphons funnel fluid from the outside condenser to the evaporator on top of the microprocessor. But HP discarded thermosyphons, and heat pipes, because of “pulse boiling”, a rapid cycle of heating and cooling that can damage the microprocessor, according to an internal study conducted in May of 2000. HP developed its evaporative cooling to eliminate “pulse boiling,” Patel said.
HP holds two patents on the “inkjet” evaporative cooling methods, and ten on the robotic monitoring system. HP might not mind licensing the technology, although it was developed for internal purposes, Patel said. The inkjet technology could be available in three years, executives said, while the smart datacenter could be only two years out. The robot might just need a year of polishing.
HPs corporate envisions datacenters scattered all over the world. If thermal concerns were the only issue, Patel said, datacenter managers could shift their workloads to, say, Alaska, where cooling isnt usually a problem. “We need to think of energy management and shift workloads around globally,” he said.
Note: the following corrections were made to the previous version of the story:
- At IDF on Sept. 9th, Patel is expected to give a presentation on the importance of data center cooling. (Changed from “multiprocessor cooling”.)
- Evaporative cooling can dissipate up to 100 watts, or heat densities of 200 watts per square centimeter. (Changed from “20 watts”.)
- In an HP chip, “we had to fight to just get one (diode),” he said. “They screamed.” (Changed from “Itanium chip”.)