In last week’s Kibbles and Bytes, I talked about thermoelectric power generation. Thermoelectrics are actually part of a larger family of devices and techniques used in something called energy harvesting.
I alluded to this in describing how thermoelectric devices are sometimes used in industrial processes to recover waste heat energy. Energy harvesting generally describes the process of taking energy (or “harvesting” it) from the ambient environment. Technically speaking, solar is a very efficient form of energy harvesting. Most types of energy harvesting yield far less power.
In this week’s article, I’ll discuss another form of energy harvesting: piezoelectrics. Unless you’re all caught up on your knowledge of Greek, that name won’t mean much to you. The root “piezo” comes from the Greek “piezein” which means “to squeeze.” That’s precisely what a piezoelectric device does; you apply pressure, or squeeze it, and it generates electricity. Many materials have piezoelectric properties including quartz, wood, Rochelle salt, and even DNA and table sugar. Quartz crystal is a common material used in piezoelectrics.
One use of piezoelectrics in generating electricity is in a barbecue lighter. One of the mechanisms on the lighter (usually a slide) is used to open a valve for the gas. Once the gas is flowing, you can pull the trigger and get a flame. Pulling the trigger drops a tiny hammer onto a piezoelectric device. This instantly generates hundreds or even thousands of volts which passes over a spark gap, creating a spark and lighting the gas. Despite the high voltage, very little charge is actually conducted, so it’s no more dangerous than a static shock.
Starting in the late 1990s, research has been conducted into other applications such as placing piezoelectrics into shoe soles or sidewalks to generate power while someone walks around. Could you walk around and charge your iPhone? That would be cool! Unfortunately, you’d need to be willing to walk hundreds (probably thousands) of miles for a single charge. The problem with piezoelectrics is that while each generating “event” or “squeeze” might generate dozens, if not hundreds of volts (like the lighter), the charge conducted is very low. Those charges are what you need to fill up your iPhone’s battery, and the smaller each one, the more you need.
The potential for piezoelectrics to generate real amounts of power still exists though. Research at Cornell University has focused on mimicking nature by using the concept of a leaf flapping in the wind to generate power from piezoelectrics. Maybe one day instead of plugging your phone into the wall, you’ll be able to plug it into an artificial tree.