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Quiet, low power cooling technology is only 1mm thick

Users don't like fans because they are noisy. Designers don't like them because they are greedy consumers of both power and space. But as more processing power is piled into all sorts of electronic equipment, particularly mobile devices, the choice becomes simple – have a noisy device with active cooling, like a typical laptop, or one with passive cooling that can get almost too hot to handle, like some tablets.

However, developments in the avionics world could have opened up a middle route between passive and active cooling that could impact on virtually every device that contains electronic circuits with a tendency to get too hot. The technology started out with GE Research, which was looking ways of cooling jet engines. It was picked up by GE Licensing, which effectively acts as an internal technology transfer agency, with the view to making it smaller and quieter for the electronics market. The result is DCJ – Dual Piezoelectric Cooling Jet.

Peter de Bock, the lead electronics cooling researcher on this project at GE Global Research, believes that DCJ is a 'fundamental and new technology' that offers a middle ground between active and passive cooling. "There are two trends I see in the tablet industry. Tablets are getting more powerful – Intel is trying to put its chips in tablets which are, by definition, more powerful. The other one is that consumer laptops are not just getting much faster, they are getting much thinner – the height requirement is becoming much more stringent. It becomes a real challenge when you have to fit a fan into something which is 4mm or even 3mm thick. If you only have 4mm of space available, your device can only be 3mm high and that means that your bearings and your mechanical components need to be so small that they are either very expensive or very unreliable. In order to make a bearing with a diameter of 1mm, you need to have a manufacturing tolerance that is much greater – and that is very expensive."

DCJ, claims de Bock, can provide an answer to many of the downsides associated with using fans in electronic designs and these include lower cost, power, space and noise.

The new device is very simple, which in itself is an advantage. It is fundamentally two small metal plates that are flexed in unison. De Bock explained: "The piezo material sits on top of the steel. The piezo crystal works such that if you apply a positive voltage, it expands; if you apply a negative voltage, it contracts. And so, if you do this by applying a sine wave, you can generate an oscillation and that is what we do. We typically drive this device at 100 to 170Hz and can generate a system in resonance that gets the plates to pump like a pair of bellows. It is so simple, yet it can provide enough cooling flow for a tablet and that is very exciting."

Bigger devices create more flow, while smaller devices tend to be noisier, so GE has settled initially on a device measuring 40 x 40mm. But what is key here is that the device thickness is just 1mm, which should allow it to be fitted into most devices, irrespective of how thin the product is required to be.

This earns it the tag of being an enabling technology – it is now possible to include a cooling component in ultra thin tablets or laptops where it would previously have been impossible, allowing their components to not only consume more power, but also to offer more processing power. De Bock said: "[tablet designers] need something that provides just enough airflow, yet which can fit in a thin space and, in that respect, this technology is unique."

When the two plates are at their widest, air is pulled in at a low velocity. When at their narrowest they expel air at a relatively high velocity – the same volume of air is being forced through a smaller gap. "If you look at the air surrounding the device," explained de Bock, "the air being expelled leaves at such a high velocity that the adjacent air volume gets moved along with it. It is the same idea as that of the Dyson fanless fan, where a high velocity ring of air pulls additional air through the centre. This is similar in that high velocity air leaving the device draws in the additional air. The device does generate a net air flow very similar to a fan, but it comes out of the device in a pulsating stream of air."

The only moving parts are the pulsing plates, so there are no bearings or other parts rubbing against each other, making wear minimal and increasing reliability, an important consideration in critical applications like avionics and automotive. Equally, to avoid the fatigue failures to which some metals, like aluminium, are prone, the DCJ employs a variety of steels and maintains their bending well within the elastic range.

GE's tests have shown that it can take as little as 250mW to power the DCJ – around half the power consumption of a fan producing an equivalent airflow. There are also differences in acoustic performance. "A fan has very different tonal behaviour than this device," said de Bock. "Currently, the DCJ device is low frequency – 100 to 170Hz – because that is where the human ear is very insensitive to noise. It sounds like a very low base tone, instead of the very high pitched whine you can get out of a fan. Typically, a project has a budget in terms of cooling performance versus acoustic signature versus thermal performance and your design space is where these three overlap. For the 40mm device, this is at about 170Hz, which is a very attractive spot as it is very quiet."

GE is being partnered by Texas Instruments, whose control ic from its Haptic Group converts a 3.3 or 5V dc supply into the necessary ac signal.

Entering the market
GE is in the development stage with some partners, but has also licensed the technology to a Japanese company, allowing those interested in exploring the benefits of the technology to get samples or quotes.

De Bock concluded: "We think we have a fundamental new way of moving air in a thinner device and in a more energy efficient way. We are looking to work with people with interesting ideas or partnerships in order to bring this device to market. We see a lot of applications, including displays, televisions, mobile devices, laptops and set top boxes – anything that is portable and thin. But there are also industrial applications, such as network basestations, power supplies – anything really. The application area is really broad."

Tim Fryer

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