Microfluidic Cooling Turns Down the Heat On High-Tech Equipment

An anonymous reader writes with a snippet from HelpNet Security about a technology that sounds promising down the road for consumer equipment, but may land a lot sooner than that in high-end applications where cooling is critical: Thousands of electrical components make up today's most sophisticated systems – and without innovative cooling techniques, those systems get hot. Lockheed Martin is working with DARPA on its ICECool-Applications research program that could ultimately lead to a lighter, faster and cheaper way to cool high-powered microchips – by cooling the chips with microscopic drops of water. This technology has applications in electronic warfare, radars, high-performance computers and data servers. The micro-cooler is only 250 microns thick, and 5 millimeters long by 2.5 millimeters wide.

Comments

Really?

[ledow]

It might allow you to spread the heat or avoid direct water transfer but you still have to move that heat somewhere. You're not REDUCING the heat, are you? It's still producing the same amount of heat and you're still needing to get rid of it.

At the end of the day, whatever fancy technique you use, there's still going to be a large bit of aluminium somewhere, and probably a cheap fan blowing over it. If not, then you're into things more complicated, fragile or liquid than you want them to be.

The only other cooling technology I've seen was a heat-pipe cooled PSU that I still have. No moving parts at all, just clever design, and natural air-flow. But things like that aren't scaling and can't be used on more heat-generating parts (do PSU's really generate that much heat?).

No matter how you look at it, whether you water-cool or whatever, you still need a big piece of metal with huge surface area being cooled somehow to actually "get rid" of the heat. Everything else is just a matter of the efficiency or difficulty of how you get the heat to that point.

With consumer items like laptops and desktop PC's, you're not going to change anything. And bigger things like cars, planes, etc. don't really have a problem - localised heating might be problematic but space isn't at such a premium that you can't solve it with "normal" techniques and a huge heatsink (i.e. the bodywork).

However, I still don't get why all laptops / tablets don't just have a large metal base inside their plastics to just spread the heat over everything, so you don't get one burned thigh and one cold thigh.

Re:Really?

[occasional_dabbler]

You have missed the point of the device, even though you identified the problem it is solving:

Everything else is just a matter of the efficiency or difficulty of how you get the heat to that point..

It reduces the thermal resistance between the chip and the heat sink, so for a given installation and heat rejection rate the chip itself will be cooler.

...bigger things like cars, planes, etc. don't really have a problem... ...a large bit of aluminium somewhere, and probably a cheap fan blowing over it.

You do not add any weight to an aircraft that isn't absolutely necessary and you do not add any kind of active device where a passive one could work because of reliability. Keeping electronics cool in an aircraft is a very complex and expensive problem. Keeping a chip even two or three degrees cooler will have a measurable effect on the reliability over the aircraft life.

Re:Really?

[fuzzyfuzzyfungus]

To the best of my understanding the improvement here is in lowering the thermal resistance right at the point of contact between the IC and the cooling system, which doesn't change the fact that the waste heat still has to go somewhere; but does allow heat to be removed from that very small contact area faster, which increases the safe maximum wattage for a given die size.

It requires good engineering to do robustly, quietly, in a relatively compact unit, etc; but dumping large amounts of waste heat once you've gotten it into the cooling system is the relatively easy part. We'll always wish we could get away with something lighter or smaller or quieter; but we can build you a heat exchanger large enough to throw at pretty much any thermal load(chips are very much on the low end: engine cooling can easily bring you into the multiple kilowatts and power plant heat exchangers into the multiple megawatts); but what we cannot so easily do is move heat faster from a small, fixed, dissipation area. If your IC is only 1 square cm, you have to be able to extract all the waste heat generated by the power level you are operating it at through that single square cm, with a low enough delta T to keep the IC from cooking itself.

The relatively simple method is just a solid heat spreader; but if you want to hit power densities higher than the conductivity of copper will allow, you have limited options(diamond would be nice, if you can get in the size you need). Direct liquid cooling of the die is tricky, even if you have access to cryogens, because the Leidenfrost effect tends to produce an insulating layer of vapor right where you don't want it, surrounding the item to be cooled, which ruins the efficiency of heat transfer. My understanding is that this microfluidic apparatus is aimed at solving this problem; by forcing the liquid coolant onto the chip in a way that disrupts the vapor layer in a way that naive 'just pour it on/pump it across' approaches don't.

It might prove to be the case, especially if somebody comes up with a clever way of producing the stuff in bulk, that these microfluidic interfaces will end up being used in larger cooling applications as well; but at present they are aimed at the problem of cooling small, relatively fixed size(since die area is expensive; and sometimes design constraints related to clocking and signal propagation time don't even allow you to spread the same circuit over a larger die, even if you are willing to pay for the extra area), heat sources that are bumping up against the limits of how fast we can get heat away from them. Whatever cooler ends up dumping the heat into the atmosphere or chilled water supply or whatever will be purely conventional; it's the point of contact between the IC and the conventional cooling system that is being improved.

Re:Really?

[DigiShaman]

...a power supply rated at 500 W can deliver 500 W of power to the system regardless of its efficiency, the efficiency tells you how much power it must draw from the wall to deliver those 500 W it does not affect its output capacity, a 500 W power supply that is 90% efficient can deliever the same amount of power as one that is only 80% efficient. If the power supply is 80% efficient it needs 625 W(500/0.8) from the wall, those extra 125 W are turned into heat by the power supply, while a 90% efficient unit would only be drawing 555 W from the wall, meaning it is dissipating only 55 W as heat, or only 44% of the heat the 80% efficient unit was creating.

- Toms Hardware

Re:Really?

[Anonymous Coward]

However, I still don't get why all laptops / tablets don't just have a large metal base inside their plastics to just spread the heat over everything, so you don't get one burned thigh and one cold thigh.

1) weight 2) cost 3) it won't really have that effect because while it will spread heat it won't spread it equally, it'll still be localised to the heat source. It'd help, but not much.

You need to get the heat away instead of trying to spread it out and that needs airflow. This article boosts that through evaporative cooling, exactly the same as when you sweat.

Re:Really?

[Anonymous Coward]

Long term, just like with engines, computers will wind up migrating to liquid cooled systems. Closed loop systems are starting to become more common, with even Intel offering cooling systems.

It would be nice to see some sort of double valve standard, similar to how there are quick connect hose valves which don't allow water to flow when opened, but two valves so one's computer isn't ruined by a faulty valve causing a coolant leak.

As for phones and other devices which can't really be liquid cooled, this becomes a tougher problem. However, there is always something called "platform optimization" which was used by games and such in the past, to be able to run more stuff without requiring GPU/CPU advances as a must have.

Re:Really?

[Anonymous Coward]

It might allow you to spread the heat or avoid direct water transfer but you still have to move that heat somewhere. You're not REDUCING the heat, are you? It's still producing the same amount of heat and you're still needing to get rid of it.

Indireclty, they are also going to reduce the heat output and increase the efiiciency, simply by dropping the temperature of the part and thereby preventing increased temperature from increasing the electrical resistance.

Re:Really?

[jeffb (2.718)]

It might prove to be the case, especially if somebody comes up with a clever way of producing the stuff in bulk, that these microfluidic interfaces will end up being used in larger cooling applications as well;

Never mind cooling applications -- make this cheap enough, and I want to see it in cookware.

Not to be flip, but there are quite a few applications where keeping a uniform temperature across a surface with widely varying heat loads would be a big win. The mind boggles.

Re:Really?

[Solandri]

Evaporative cooling removes heat (energy) without a corresponding temperature increase - no heat to "get rid" of. The energy goes into the phase change of the material. For water, the phase change from liquid to gas results in (without an external heat source) a corresponding temperature drop. So if matched up properly you end up with cooling with no temperature increase. (It works for solid to liquid phase change as well, which is why we put ice in our drinks instead of hold them up next to the air conditioner.)

Course you do end up with humid air which needs to be vented out and replaced with dry air so it can continue to function. And occasionally you need to refill the water reservoir.

Re:Really?

[occasional_dabbler]

These devices will work more like a heat pipe as mentioned by the parent. The water evaporates from the chip surface and then condenses on the heatsink surface. You get the benefit of the high heat transfer rate without the temperature increase as you rightly say but the water remains inside the unit in a closed loop. They are very clever devices.

Re:Really?

[fuzzyfuzzyfungus]

I'm a bit surprised that I've never seen some sort of variation on the heatpipe concept(presumably with a different working fluid) in some sort of fancy cookware, now that you mention it. Plenty of designs that include a copper layer for its thermal properties, or use thicker-than-mechanically-necessary iron for the thermal mass; but PC cooling moved to heatpipes because those offer substantially better conductivity than even fairly alarming chunks of solid copper. Just too costly to be worth the marginal gains?

Re:Really?

[jeffb (2.718)]

It may be because fluid-based heat-piping systems rely on vaporizing a working fluid to absorb heat and condensing it to dispose of that heat. Evaporation and condensation tend to happen at a fixed temperature (varying with pressure) for a given working fluid. I don't know how well such a system would let you keep a wide surface at a uniform arbitrary temperature. In other words, it might be easy to build a plate that would keep your food warm for serving at a perfect 60 C, or one to fry things at 180 C, but not one that could be adjusted to both.

Re:Really?

[fuzzyfuzzyfungus]

I suspect that you could get some wiggle room from the 'varying with pressure' aspect(eg. if you can find a working fluid that vaporizes at relatively low temperature, when at modest pressure, so that the system works for merely warm food; but its own vapor rapidly raises the pressure, and the boiling point, so that distinct evaporation and condensation zones still exist, rather than just a bunch of probably-insulative pockets of hot vapor, at higher temperatures); but yes, a heatpipe-type arrangement would probably have to be targeted at at least a rough temperature band.

Given the success and availability of phase-change-material based thermal storage(usually modified paraffins or salt hydrates for relatively low temperature applications), I suspect that it'd be hard to beat those for plates, pots, etc. designed to keep food warm for extended periods(one area that these actually do show up a lot is in pizza delivery bags, as they don't require charging or anything unlike active heaters; but outperform mere insulated bags considerably in terms of keeping the pizza nice and warm during delivery); but a heatpipe-style arrangement might be more compelling in frying applications where the ability to smooth out any hot or cold spots quickly as the user moves or flips the object being fried would be an advantage. The added cost would likely make it a niche product in any case, so you'd have to hope that people who buy expensive niche kitchenware are willing to have multiple special-purpose devices.

You fools!

[fuzzyfuzzyfungus]

Surely DARPA has enough nerds on hand to know that adding a 'small thermal exhaust port' to expensive military hardware is going to end in disaster, no?

Re:You fools!

[mike1086]

Wrong type of nerds. They've got to many trekkies.

Re:You fools!

[Dutch Gun]

Fine, you worrywart. We'll make sure it's ray-shielded and and tucked away in a protective trench somewhere.

Easy hack

[scorp1us]

All you have to do is hook a stepper motor to a cam and position the cam against the trigger of a spray bottle filled with water, and viola, and point it at your CPU and voila, you've got microfluidic cooling. Your CPU fan will naturally direct the droplets to the heat sink. You should also tie the stepper motor to your cpu temperature sensor so that it can alter the squirt rate for higher demand times.

Re:Easy hack

[lhowaf]

Your CPU fan may be installed upside-down.

Re:Easy hack

[Shatrat]

CPU fans always blow down onto the heat sink. Mounting them the other way allows a hot spot to form in the center as the fan tends to just pull air from around the heat sink instead of through it.

Re:Easy hack

[Solandri]

In a strap-on water cooler or your spray-bottle suggestion, the heat transfer area between the heat source and water is limited to a flat surface (the minimum possible).

Microfluidic cooling increases the surface area for heat transfer by running the water through lots of tiny tubes in the heat spreader (metal-to-metal heat transfer generally is a lot faster than metal-to-water heat transfer, so the small surface area of the CPU-to-heat spreader interface isn't limiting). The surface area for heat transfer is then the sum total of the sides of all those tubes, which can be much larger than the exterior physical dimensions of the device. Same reason your lungs have lots of tiny alveoli, instead of presenting just a smooth flat surface for the air to flow over.

Re:Easy hack

[TeknoHog]

CPU fans always blow down onto the heat sink. Mounting them the other way allows a hot spot to form in the center as the fan tends to just pull air from around the heat sink instead of through it.

As opposed to the hot spot in the center of the blowing side? There's nothing blowing from the hub of the fan, and of course the highest speeds are on the outer edge. This doesn't yet account for idiotic heatsink designs that don't let air straight through, but instead divert it to the sides, thus leaving a high pressure center.

I agree with the general point though, it's almost always better to blow onto the heatsink. Try sucking out a candle, as our fluid mechanics professor used to challenge us.