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MEMSbased designs can do some strange and wonderful things. While thumbing through arecent issue of Wireless Design and Development (yes, it was print), I saw the story "MicroWindmills: From Lab to Market" on MEMS devices which could be used to generate power frompassing airflow, whether from the convection currents of circuit heat, or general exposure to theirsurroundings, Figure 1. I did some further digging online and found more details on the work atthe relevant pressrelease page of the University of Texas at Arlington, where much of the workis being done in conjunction with support and production from WinMEMS Technologies, aTaiwanbased company.

Figure 1: The nickelbased MEMS microwindmill can be bulkmanufactured on a waverusing processes similar to conventional siliconbased ICs (courtesy University of Texas).

Building a MEMS windmill does seem pretty impressive, in part because you need a rotating joint,Figure 2. While I have seen microgears and gear trains in experimental MEMS devices, and sowe know this sort of mechanical structure can be fabricated, is still a real challenge.

Tilting at MEMS Windmills for Energy Harvesting?Bill Schweber February 19, 2015

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Figure 2: A microphotograph of the windmill shows its apparent simplicity and also itscomplexity (from WinMEMS Technologies Co,. Ltd.)

Several points in the story caught my attention. First, I assumed they used silicon as a basematerial, as nearly all MEMS devices do. It turns out that silicon doesn't work well here, becauseit is brittle despite its strength; instead, the team used nickel alloys. Also, at the tiny dimensions ofthese moving blades, the usually ignorable effects of factors such as moisture become an issue;in this case, the water molecules start to become gluelike and create static friction (stiction), so aspecial lubricant may be needed at the rotary joint

Further, the article noted that there are very few development tools and established processtechniques for fabricating nickelbased MEMS devices, so a lot of their progress was "learn asyou go", based on their technical knowledge and dozenplus years of experience with nickelalloys, etching, and plating. They wanted to make as much use as possible of the generalprocess flow and waferscale manufacturing which siliconbased MEMS uses, to leverage thetechnology, production and cost curves.

The work they have done on MEMS microwindmills is absolutely impressive, no doubt of it. Theyenvision a "field" of such microwindmills implemented on a single die, which could produce "free"power the epitome of energy harvesting.

I wonder about a few issues, though. I suspect that in order to get the attention of the broaderaudience, the press coverage touted applications such as recharging of cell phones, which isclearly a "hot button" angle to use. Yet while press releases talks about this microwindmill as ifthe total energyharvesting function is available, my closer reading of the reports indicates thatthey have the windmill part working, but there is no explanation of how actually they use it togenerate electrical power from the turning motion. Is the next step placing micromagnets on theblades and microcoils right next to them?

Other questions: what level of output power could you expect from these microwindmills,assuming 100% conversion efficiency: is it microwatts, picowatts, femtowatts, or less? Since thepower output and efficiency of windmills are each roughly proportional to the square of the airvelocity, a microwindmill might be have very low output at best, even before conversion losses.With all these tiny windmills in close proximity, there's also the wellknown "shadowing" effect toconsider: will the air flow to each be obstructed or turbulent, rather than laminar?

However practical or realistic these microwindmills will be, this is another dramatic example of

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how MEMS technology is opening up radically new approaches to implementation of existingdevices as well as new concepts, from accelerometers to microphones, timing oscillators, andmore. With silicon and even nonsilicon MEMSbased components at one end of the designresource continuum, and additive manufacturing/rapid prototyping at the other, there's lots ofroom for creativity.

What new technologies do you see opening up radically new productdesign opportunities?

Also related:

http://www.edn.com/electronicsblogs/benchtalk/4438400/DesignersNotebookHotcoldrunningoscillators

http://edn.com/electronicsproducts/ces/4438245/ComboMEMSsensorsolutionwithintegratedgassensorlaunchesatCES

http://edn.com/electronicsblogs/nowhearthis/4438060/Sensorsneedtobesmartsmallandintegrated

http://edn.com/electronicsblogs/techedge/4437579/Jewelryharvestsenergyfromyourveins

http://edn.com/design/powermanagement/4432016/ProductHowtoAninflexionpointforenergyharvestingandtheInternetofThingsisnigh

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