The fanless spinning heatsink: more efficient, and immune to dust and detritus
extremetech.com
extremetech.com
They claim a 0.2°C/W, which would be really something. You can get down to 0.37°C/W[1] with air cooling, using heroic measures, and blisteringly awful efficiency. Doing 0.2°C/W would be a real step up.
I don't really think they've done it, here. Their experimental setup used six 1"x1" 10 watt heating elements. This is because their heatsink needs a very large cross-section to overcome the lousy thermal conductivity of the air gap between the impeller and the base plate.[2] Total area, 38.7cm^2, total power, 60W.
The Intel Core i7's heatspreader has a surface area of ~20.25cm^2, and a thermal design power of 130W. 52% smaller, 216% times the heat output. That's about four times more heat per square centimetre.
The smaller the heat source, the longer the average thermal path between the source and the air/heatsink boundry, the worse the C/W, and the less effective the heatsink will be. If they had actually used a computer processor, rather than a bunch of heating elements, then my WAG is that they would have done 0.35°C/W.
It's a beautiful idea, but their setup looks nothing like reality.
1: http://www.dansdata.com/quickshot012.htm
2: They say that, due to the high sheer speed, there's no boundary layer, which "increases thermal conductivity several-fold". Well that's a cool story, bro, but air (0.025 W/(mxk)) is still sixteen thousand times less thermally conductive than copper! (401.0 W/mxk)) If you increased the thermal conductivity of air by 6.4 times, then it would be as conductive as... rubber, something which is not world renowned as a good conductor of heat!
So lets say their device can handle 60W when it is 38.7cm^2 (or a bit more than 1.5W per square cm. Now you want to cool a 150W processor. That means 100cm^2 of cooling surface. If I did the math right 100cm^2 is sqrt(100/pi) * 2 diameter circle. Or 12cm diameter circle. So lets call it a 4 3/4" diameter circle with one of these gizmos on it.
That isn't a huge stretch to imagine. I'm thinking a copper slug which sits on the processor and makes this 4.75" diameter 'landing pad' for their cooler. Doesn't work well if the processor isn't oriented horizontally though.
In data centers this might up reliability and density if the net solution was 'shorter' than the current passive sinks.
EDIT: Apparently, DLC is more conductive than copper (http://en.wikipedia.org/wiki/List_of_thermal_conductivities), and by a lot. Copper is around 400 W/mk, DLC is 1100-3500 W/mk ...
[edited to reflect acquired info, pre-comments]
A conductive fluid bearing might work better than an air gap. DLC vs copper makes little difference since that's not the problem.
I see you just edited it to say that DLC is very slick - that doesn't help as much as you think. It's a fact of heatsinks that two solid surfaces only touch in three places. The rest is filled with a tiny air gap. You typically fill in this gap with a fluid.
Is it?
You raise a very good point. The philosophy thus far is to see how far we can get with conventional, inexpensive materials, and periodically reevaluate where problems remain. Low-cost manufacturability is critically important. For example, in the case of the rotating heat-sink-impeller we are converging towards cold-forging as the fabrication process. Having said that, in a manufacturing setting many seemingly exotic coating materials are used on a routine basis in small quantities without incurring significant additional cost (e.g. CVD coatings for tungsten carbide end mills). With regard to CVD diamond coating, the main questions I have concern the current state of affairs in this technology area. How expensive are such coatings, and what are typical values for coefficient of friction?
With regard to the thermal resistance of the air gap, unfortunately it is not possible to reduce the air gap much below ~10 microns without incurring large frictional losses; the power dissipation associated with shearing of the “fluid” layer in the air gap regions scales as 1/h, were h is the air gap distance. The approach you suggest would be tractable at very low pressure, but at very low pressure convection cooling is ineffective.
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Thanks for your interest. With regard to filling the air gap region with conductive fluid, the problem is that frictional shearing losses become prohibitively large even at low rotation speeds. It’s not that it wouldn’t work at all, but that it wouldn’t work very well; thermal conductivity is important, but so is viscosity.
There a number of other issues that apparently have caused confusion as well. Maybe we can use this forum to provide clarification. Please feel free to post what follows.
It appears that many people were unclear about what I was attempting to convey with regard to the subject of dust fouling. I did not mean to imply that there is literally no dust fouling; some dust accumulation eventually becomes visible to the naked eye on the very leading edge of the blades. The point is that dust fouling is reduced to such a large extent that we are unable to detect any degradation of cooling performance operating the device in a relatively dirty environment over an extended period of time. Thus for all intents and purposes the dust fouling problem has been taken of the table. In contrast, with conventional CPU coolers, eventually the entire heat exchanger surface becomes entombed in dust.
Some people have expressed concern that such a rotating heat-sink-impeller would constitute a safety hazard. Any real world device would include a screen, grill, or other form of protective enclosure. Such protective measures are widely used on conventional fans. We photographed our device without an enclosure so that it would be easier for people to see what it looks like.
There seems to be confusion about where the potential for significant electricity savings resides. The vast majority of it is associated with applications such as air conditioning and refrigeration, not electronics cooling. But such energy sector applications will only materialize if air bearing heat exchanger technology proves amenable to size scaling. We are in the process of evaluating this question.
Many have expressed skepticism about the practicality of a 0.001” air gap. That’s certainly understandable, and it was one of the first things we investigated. After all, if the requirement for a small air gap precludes the possibility of low-cost manufacturing, reliability, etc. then I would be the first to agree that all of this is a pointless exercise. The end-of-project report has a discussion of why this is not the case. Another counter-intuitive point is that air bearings are extremely mechanically stiff, rugged and reliable. This is also discussed the report.
On a related subject, many were concerned about the manufacturability of the heat-sink-impeller. We are converging on cold forging as the best route to low cost fabrication. We understand that if we can’t drive the cost down that air bearing heat exchanger technology will have little impact.
Others have pointed out that intuitively it would seem like the last thing you’d want to do is intentionally introduce an air gap (rather than something like thermal grease) in the thermal conduction path between the CPU and heat exchanger. Qualitatively this sounds like a persuasive argument. But as discussed in the report, quantitatively, the numbers (gap distance, gap area, thermal conductivity of air, enhancement of conductivity by convection) work out quite well. For a 10 cm diameter device, an air gap resistance of 0.02 C/W is certainly feasible.
Another important point is that the version 1 prototype device discussed in this report is badly unoptimized. The main objective for version 1 was to test our hypotheses regarding the advantages of such a device architecture. If things continue to go well in lab, I suspect eventually we’ll end up at about 0.05 C/W for a 10 cm diameter device that’s of order 3 cm high, operates at three to four thousand rpm, and consumes about 5 watts of electrical power. But believe it when you see it. There’s always risk involved in try to solve tough problems. We’re giving it our best shot. We’re also working on alternative device geometries that may be capable of providing considerably better performance.
What else? A couple people stated that the thermal brick wall is at 4 GHz, not 3 GHz. Fair enough. My point is that if you introduce a drastic improvement in thermal management technology, whatever the number for thermal brick wall may be, it gets pushed a lot higher.
Other people had questions about why such a heat-sink-impeller is so quiet. What it boils down to is that you’ve got a lot more flexibility with regard to blade geometry than you do with a fan. That means you are free to design the blade geometry to smoothly split and smoothly rejoin the flow field at the impeller entrance and exit; the device architecture allows you to decouple the engineering constraints of adequate air flow and low noise.
A couple of people surmised that the air bearing heat exchanger requires a source of compressed air because we used a hydrostatic air bearing in the version 1 device. As described in the report, in a real-world device you’d use a hydrodynamic (or self pressurizing) air bearing. That’s what we’re using now in versions 2 and 3. The use of a hydrostatic bearing in version 1 was an experimental convenience.
I have to go to a 5:30 meeting, so I will leave it at that. Hopefully people will find portions of the above material informative.
Sincerely, Jeff Koplow Sandia National Labs Livermore, CA
Readers of this kind of news often seem to underestimate the kind of thought that went into an idea/invention/prototype. They go "well, that won't work, because ..." and they come up with some reason that may sound reasonable (or simply is reasonable), but just isn't important enough. I think it would be good if more scientists replied in this kind of matter to comments to news of their ideas/inventions. It would, I hope, increase the respect people have for the work of scientists.
These days you can find huge coolers with (or at least claimed[1]) less than 0.2°C/W. Most of the lower power CPUs (40-50W) can run fanless at decent temperatures with half a kilo of metal fins + some heatpipes, and you can mount that kind of heat spreader easily with bolt-through screws.
Also, that metal fan is dangerous! If you ever touched even a slow-spinning plastic fan's blades, you know how painful (or bloody) it can be. And that was only a very light plastic blade, probably less than 30 grams. Now think about the momentum of a 200 gram metal thingie spinning with more than 1000rpm (as I understand, they tested it to 7-8000 rpm). You really want to be sure you will not touch it while spinning. But if you enclose it, even in a wire cage, the performance will decrease.
1: http://www.scythe-usa.com/support/cpu/006/scmn1000_scnj1000p...
But yeah, I've been out of the overclocking scene for a couple years, now. I have no idea how well heatpipe heatsinks perform, other than "better".
As for the danger of a large piece of metal spinning at kRPM, I agree. This is bad enough that there probably would have to be an safety interlock on computer case access panels that reverses the impeller to a halt when the enclosure is opened.
Page 6 states °C/W values of 0.29 down to 0.14 depending on the amount of air pushed with the fans. SPCR's test fans are pretty low-flow to begin with (47 CFM/1080 RPM at 12 V), so you could probably go slightly lower by using real screamers.
(page 18 of PDF) "The prototype device is configured as a static (externally pressurized) thrust bearing. In realworld thermal management applications such an externally pressurized air bearing would be replaced by a hydrodynamic (self-pressurizing) air bearing, which uses a minute fraction of the mechanical power supplied by the brushless motor to generate the required lifting force."
So the external pump at the moment keeps the big rotor above the surface? They haven't demonstrated how the setup would otherwise work, but maybe for "self-pressurizing" variant the RPM would have to be really high, which would again mean not quiet at all?
Can anyone help me overcome my ignorance and understand these points?
It'll probably fall apart in heavy dust environments, but most things do anyway.
To quote the pdf:
"""Rotation of the heat sink at several thousand rpm also provides a potent remedy to the longstanding problem of heat exchanger fouling. Consider for example the CPU cooler shown in Figure 1. The finned, metal heat sink cannot be seen because it’s covered in dust. But the fan blade, which operates in the same environment, is for all intents and purposes perfectly clean. This contrast in dust accumulation is at first startling, but in hindsight entirely expected. The air bearing heat exchanger therefore provides a complete solution to the problem of performance degradation due to heat sink fouling. In specialized applications involving extremely high particle loading, a straight-radial rather than backward-swept fin design would likely be used [Bleier, 1997]."""
So like current tech = chip -> heatsink -> fan whereas this = chip -> magic spinning heatsink
I'm not exactly sure how transmitting heat across the gap would work. I imagine it would just be by convection, but I don't know how efficient that would be.
That said, I imagine that this type of design would be problematic in laptops. With the spacing so tight, it seems to me that any shocks to the fan could result in the vanes touching the baseplate.
As the metal blades spin, centrifugal force kicks up the air and throws it up and outwards, much like an impeller, creating a cooling effect.
I had to read that part twice too, but it sounds like the air cushion is only there initially, but gets sucked out when the fan spins. (That said, if it leaves a vacuum, that doesn't sound great for conductivity either...)
Wikipedia says 11% of US electricity goes to air conditioning and 5% to all electronics combined. That about half of that is cooling fans does not survive the sniff test.
http://en.wikipedia.org/wiki/Energy_in_the_United_States#Cur...
Which says, for Commercial energy use: 25% lighting 13% heating 11% cooling 6% refrigeration 6% water heating 6% ventilation 6% electronics
You could find gains in ventilation, electronics, refrigeration, cooling, and probably even heating if this technology was widely used and performs well over all uses, so the 7% isn't out of the question, even if it might be very optimistic.
It looks like the benefit of this over other high-end coolers (i.e., water cooling) is that it can be a drop-in replacement for them. It doesn't seem to require special parts, knowledge, or tools to install. It could be installed as easily as a normal CPU cooler. The reduction in electricity costs is exciting for businesses (consumers don't care about a 7% electricity reduction). That it doesn't get clogged with dust is also very useful in the long run: less maintenance.
I'll be more excited when this is in production, even at a high cost. Let's hope it gets there.
OK, there's only 0.001-inch between them, but still a heatsink + a fan, no?
Edit: Of course, in a computer case there would be other fans which would keep replacing the air. Silly me!
The good old heatpipe could extend from the stationary baseplate, as an axle/shaft of the impeller, well into the spinning part and here flange out internally. The seal/bearing would have to be quite gas-tight, but if that's achieved, heat transfer could be great.
It must also not leak at all over the course of several years at elevated temperature, while spinning at several thousand RPM, or else the heat pipe will stop working.
It also must be cheap enough to compete commercially with a solid piece of metal.
The design constraints are... loosely possible. But it sure as hell won't be cheap.
http://www.dyson.co.uk/technology/airmultiplier.asp#HowItWor...
1: http://www.engineeringtoolbox.com/specific-heat-fluids-d_151... (search for "mineral")
2: http://www.engineeringtoolbox.com/air-specific-heat-capacity...
3: http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/spht.html
(However, mineral oil has much higher viscosity than water, which I'm not sure is what you want as it suppresses convection. I guess it's a question of whether you take that over the non-conductivity.)
Don't try this using water: http://youtu.be/PtufuXLvOok?t=2m10s
There are chips closer to 4Ghz than 3, though. The author should have aimed higher. http://www.intel.com/products/processor/corei7EE/index.htm