Energy Harvesting With Piezos

by Raphael Zollinger & Evrim Sahin

Indoor Micro-Energy Harvesting By Piezoelectric Transducers

• WHAT: Testing the feasibility of indoor harvesting from routine motion in the environment.
• HOW: A comparison of three energy harvesters gathering from linear motion produced by a sliding door. Looking at piezo elements, stepper motor generator and faraday induction.
• WHY:Converting waste mechanical energy into electrical energy to either power small devises/sensor networks or via a capacitor to battery storage for small devices:cell phones,mp3,etc.

A Little Bit About Piezos

• It is the inherent property of piezo electric material to produce an electrical potential, high voltage - low current, when put under strain, either deflection or compression.

Used both micro-fiber composite (mfc) and traditional ceramic types.

Piezoelectricity is the ability of some materials(notably crystals and certain ceramics) to generate an electric potential(current or voltage) in response to applied mechanical stress-and vice versa. This strain can come from many different sources: human motion, low-frequency seismic vibrations, and acoustic noise are everyday examples. The word is derived from the Greek 'piezin',which means to squeeze or press.

History
Discovered in the mid-18 Century, the first practical application for piezoelectric devices was sonar, first developed during World War 1 for submarines.
Direct piezoelectricity of some substances like quartz, as mentioned above can generate potential differences of thousands of volts, or just power a wrist watch.
Probably the best known application is the electric cigarette lighter: pressing the button causes a spring-loaded hammer to hit a piezoelectric crystal and the high voltage produced ignites the gas as the current jumps over a small spark gap.

Stepper motor generators

• Produce power at low rotation rates
• Produces twice as much power as a single pole dc motor
• common and easily available

Electro-Magnetic Induction (Farady's Law)

• Passing a magnetic field through a conductive coil produces an electrical potential.
• Utilized a commonly available “shake flash light” and circuit.

Past Efforts

Research by DARPA has attempted to power battlefield equipment by piezoelectric generators embedded in soldiers' botts. However, these energy harvesting sources by association have an impact on the body. DARPA's effort to harness 1-2 Watts from continuous shoe impact while walking were abandoned due to impracticality and the discomfort from the additional energy expended by a person wearing the shoes.
Similar efforts were made by MIT Media Lab as well with the limited success.
Rick Dickson has developed a passive wind harvesting technology that uses *PVDF and piezoelectric ceramic materials woven into textile-like material to form artificial leaves for a bio-mimicking "tree".
Piezoelectric Backpack: The straps, made from polyvinylidene fluoride(PVDF)- a strong, flexible material that feels similar to nylon-- generate an electrical charge from applied stress, sort of like when you rub your feet on the carpet and shock your little brother, in this case, though, the energy can be used to keep rechargable gadgets like iPods and cell phones charged up.

We propose placing small piezo harvesting systems around our environment, on such appliances as hvac units, printers, refrigerators, industrial shop tools, etc…that run continuously and measure over time their output. Each module would be equip with a radio transceiver to data log via a base station its total energy collected over time. This converted electricity can self power the network, LED lamps, as well as recharge batteries for needed devices on the ITP floor later on.\\\

Making Piezo Crystals AT HOME

We found this recipe online in order to make piezo crystals from things you can find at a grocery store like baking soda and cream of tartar. The picture below shows the first batch of crystalizing rochelle salt. According to the recipe it has to wait out for 20 days.

First Testing

Until we get our own piezoelectric transducers that are more effective for energy harvesting, we started by playing with a simple piezo film we got from Tom Igoe. This film is actually a vibration sensor from MSI- you can find at Images SI.

Then we hooked it up directly to an oscilloscope to see how much voltage we are getting. Here you can see that the voltage division is set to 5 V, so the graph shows that we were getting more than 20 V. Watch the video
After seeing the raw output from piezo, we put it through a rectifying circuit shown below.




Raphael wrote a Data logging app in Processing that graphs the analog data measured as the storage capacitor charges as voltage with a 5 volt reference from an Arduino. It graphs it as it receives values form the arduino every 20milli-seconds. The voltage values are printed to text file as well as the graph is saved as a QuickTime movie file.

• The code for the processing app is HERE
  
  
  
  Then on the oscilloscope, we changed the voltage division to 2V, so this video shows 12+V as Raphael is flicking the piezo.
  Next step was to charge a capacitor and measure the current it releases. Although we flicked the piezo until it reached 1V and tried to discharge it in a LED circuit, it didn't give us any current. So in order to flick it a lot more without tiring your arm, we decided to flick it with a rotating motor to see what results we get and if we can even get an LED light up.
  
  Watch the the video to see how fast the capacitor charges up.
  
  
  
  

• Each Harvester has an xbee radio checking the charge of the capacitor from its analog i/o pin and sends the byte value and address in an api packet to a base station every 60 sec.
• The base station, running processing, pulls apart the api packet and logs the data by address into a mySQL database running on a server.
• A php script grabs the analog data, converts them into volts and then into joules and graphs them online.
• Link to graph - cursor over graph to see values
• Processing code HERE for Parsing Xbee api packets, posting them to mySQL and parsing an Asterisk server.
• Php graphing code HERE

Final Graph




Part 2: monitoring and user feed back.
FlashLite mobile phone app:

• Pulls the last value from the mySQL database via a php scipt
• graphs the three values as volts as related to the xbee ref
• if a cap reaches the xbee max:1023==3.3, the app allows the user to press a button that places a call to the asterisk server.
• acts as a testing management tool
  'Flashlite -> Asterisk -> Processing'* FlashLite places a call to the asterisk server asking the user to enter the extension.
• User enters the dial plan where text to speech introduces them and explains the interface.
• A shell scipt then launches a Java file: JEAGIClient, which grabs the call data.
• JAEAGIServer.java runs on the server handing data back and forth between asterisk and processing.
• Processing parses the key values from the server that thentrigger an api at command to the local xbee.
• click HERE for the .fla flashlite file
  
  

System Diagram

Recommendations

Parts and Purchasing: First of all a list of Piezo film manufacturers can be found here
Some companies like Mide have their all-in-one harvester products that offer piezo and the harvesting circuitry together. These modules are pricy but offer the best performance.We got ours from Smart Materials.

Advance Linear Devices sell a separate circuit called EPAD for an efficient harvesting.

If you decide to make your own circuit like we did:
* Don't use regular diodes to make a rectifier, either use Schottky or "Super Barrier" diodes(SBs we couldn't find easily), or bridge rectifiers from a Radioshack. These rectifier will allow less leakage. Get a high voltage rectifier, since the AC voltage these piezos produce are much higher than you might think.

* To see immediate voltage charge on your capacitor to start your testing, start with a 3300uF capacitor. The smaller the capacitor, the faster you will see a voltage change on your multimeter, although it is not going to have much energy stored. ( Energy=1/2*C(Vsquared) )For your final prototype, you should use Super Capacitor. The one we have from the shake flash light is a 1.5 Farad.

Circuit

* Solder on a circuit board instead of working off from a breadboard, to get the maximum and accurate reading. Don't forget you are working on a micro scale.
* You can put multiple piezos in parallel before the rectifier or after. \\\

Piezo setup

* You will get the most energy out of a piezo with the right combination of circuitry and how it is strained. It requires more frequency as well as displacement.