Energy harvesting: update

29Nov07

Piezo’s, we have concluded, both MFC or ACI types, are the least effective forms with which to scavenge usable energy. I decided, rather then test only piezo’s in varying environments, to compare three different forms of harvesting in the same location. At the moment I have restricted the harvesting to the two sliding doors that lead into the “Japanese Room” on the ITP floor at NYU. These are steel framed glass sliding door that have a linear travel of approximately 5 feet or 1.5 meters. On the top edge of one door I have installed a series of 6 “paddles” that hit or flick 3 msi 90 volt piezo’s wired together in parallel to boost the current output. Since so little current is produced, they are fed through a bridge rectifier into a 470 microFarad capacitor. Thus far – over the course of a day, about 5 – 8 volts are produced. On the other door, Evrim and I installed a 12volt stepper motor as generator that turns as the door is opened and closed. This works amazingly well as it charged a 4700 micorfarad capacitor to 8 volts in just a few slides. We are still working on moving a rare earth magnet through a series of coils.

piezo4door.jpg

QT movie or piezo harvester in action

steppergentop.jpg

steppergenside.jpg

QT movie of the stepper generator in action – watch the meter as the .33F cap charges.

Lastly, the third harvester using magnetic induction, ie, passing a magnet through a coil. QT movie here: can see the magnet being pulled through the coil by the attraction/repulsion of other magnets. Def needs greater frequency to generate enough to store a charge.

These values – when converted (j=0.5CV2) to joules(watt-sec’s) will be graphed via a php script every 30 seconds, where C is the value of the capactor in farads and V is the voltage produces squared. Rory and I will install the xbee series 1 radios tomorrow for data-logging via their i/0 analog pin from the capacitors. I’ll post some documentation of that soon.

door1.jpgdoor2.jpg

On the graph you can clearly see that there is significant leakage from the capacitor over time, which does appear more dramatic here since we are also playing with the threshold in the posting code. The threshold will only send values to the database if there is at least a difference of 10 from the last value sent, but in the case of the piezo which does not produce much – values are not sent, or in the case of the generator, which does not leak that much, but if, over a few hours at night where there is no use, a 10 point drop is posted.

graph13-1.png

I have also been working on a mobile phone app in flash lite using loadVars toparse the sensor value variables and graph their charging rates from anywhere in the world. Here’s a preview of the first attempt where im just pulling a sample value from the mySQL database.

phonelogger.png

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5 Responses to “Energy harvesting: update”

  1. That’s technically not “energy harvesting” as you don’t really take excess energy and store it — rather you are increasing the amount of friction for the users of those doors and then harvest a small portion of that added margin. On the other hand, if you could take the heat produced by the doors’ friction into energy, that would be true energy harvesting… keep it up, this is good work nevertheless.

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