The technologies are expected to appear in commercial products and military gear as soon as 2018.
Today, chips are cooled by fans which push air through heatsinks that sit on top of the chips to carry away excess heat. Advanced water-cooling approaches, which are more effective than air-cooling approaches, replace the heatsink with a cold plate that is closer to the chip. But because of the electrical conductivity of water, this approach requires a barrier to protect the chip. ICECool uses a nonconductive fluid to take the next step of bringing the fluid into the chip (as shown in the image below). This does away with the need for a barrier between the chip and fluid. It not only delivers a lower device junction temperature (Tj), but also reduces system size, weight, and power consumption (SWaP). Our tests on IBM Power 7+ chips demonstrated junction temperature reduction by 25ᵒ C, and chip power usage reduction by 7 percent compared to traditional air cooling
The Defense Advanced Research Projects Agency’s Intrachip/Interchip Enhanced Cooling (ICECool) program, which teamed IBM and the Georgia Institute of Technology to solve the liquid cooling problem for 3-D chip stacks, has yielded an approach that uses an insulating dielectric refrigerant instead of water. Researchers who worked on the prototype say the approach will lower the cost of cooling supercomputer CPUs by pumping refrigerants through microfluidic on-chip channels and will cool the interior of even the thickest 3-D chip stacks by safely running refrigerants between each die.
The dielectric fluid used in ICECool can come into contact with electrical connections, so is not limited to one part of a chip or stack. This “go anywhere” ability benefits chip stacks in terms of materials and architecture, such as putting memory directly on the stack, which improves the speed of everything from graphics rendering to deep learning algorithms.
ICECool works much like coolant in a car’s air conditioning. It’s pumped into the chips, where it removes the heat from the chip by boiling from liquid-phase to vapor-phase. It then re-condenses, dumping the heat to the ambient environment where the process begins again. Cars, though, need a compressor to cool the air below the ambient temperature (because rolling down the window doesn’t help much in rush hour traffic). Chips, unlike humans, can operate at 85ᵒ C or 185ᵒ F. So the outdoor ambient temperatures are already cooler than the chips. Therefore, our ICECool process doesn’t need a compressor (one of many elements that contribute to lowering a datacenter’s energy expenditure).
Datacenters chill out with ICECool, too
Datacenters in the US – often non-descript buildings spanning millions of square feet – full of servers that, among many things, power the internet, use about 70 million megawatts of electricity, annually. Those MWs translate to about 2 percent of the country’s energy. Two percent may not sound like much, but that’s more electricity than 29 states, as well as the District of Columbia use individually in a year.
IBM Research teams have been hard at work reducing the heat produced by datacenters – which accounts for a third of those 70 million MWs. While most data centers today are air cooled, IBM has developed warm-water cooling with projects such as a Department of Energy project (Economizer Based Data Center Liquid Cooling) and the SuperMUC hot water-cooled data center in Munich. While water is an effective coolant and shown to provide significant cooling energy savings, it requires isolation from the electronics. As ICECool uses a non-conductive dielectric fluid it can come in direct contact with electronics and remove heat by converting from liquid to vapor-phase as it flows through the electronics package.
CRAC (Computer Room Air-Conditioning) and CRAH (Computer Room Air Handler) units, which are like “heatsinks” for today’s datacenters, blow chilled air across the rows and rows of servers. That chilled air is supported by a compressor-based chiller (like a car’s AC). The chiller removes the heat via a tower on top of the exterior of the datacenter. Think of the tower as a giant radiator that dumps heat into the atmosphere – as shown on the top-left of the diagram, below. This is the loop that accounts for one-third of a datacenter’s costs.
Experimental investigation of data center cooling and computational energy efficiency improvement through advanced thermal management was performed. A chiller-less data center liquid cooling system was developed that transfers the heat generated from computer systems to the outdoor ambient environment while eliminating the need for energy-intensive vapor-compression refrigeration. This liquid cooling system utilizes a direct-attach cold-plate approach that enables the use of warm water at temperature a few degrees above outdoor ambient to achieve lower chip junction temperatures than refrigerated air. Using this approach, we demonstrated a cooling energy reduction by over 90% and computational energy reduction of up to 14% compared to traditional refrigerated air cooled data centers. To enable future computational efficiency improvements through high-density 3-D-chip stacking, we developed a 3-D compatible chip-embedded two-phase liquid cooling technology where a dielectric coolant is pumped through microscale cavities to provide thermal management of chips within the stack. In two-phase cooling, liquid is converted to vapor, which increases the capacity to remove heat, while the dielectric fluid enables integration with chip electrical interconnects. A test vehicle simulating an eight-core microprocessor was fabricated with embedded cooling channels. Results demonstrate that this volumetrically efficient cooling solution compatible with 3-D chip stacks can manage three times the core power density of today’s high-power processor while maintaining the device temperature well within limits.
SOURCES- IBM, DARPA, IEEE, IEEE Transactions on Components, Packaging and Manufacturing Technology – Improving Data Center Energy Efficiency With Advanced Thermal Management