"Piezo crystals" are special minerals that have electromagnetic properties. When piezo crystals are compressed or stretched, they produce an electric field. This is called the "piezoelectric effect." Piezo cells have small, positively charged particles at their center. Whenever force is exerted on the crystal, this small particle is forced to move and create a charge. This electric field can be harnessed to produce voltage.
One common use of piezo crystals is as sensors, creating a signal whenever force is applied. Recently, however, scientists have begun developing ways to use this power as alternative "green" energy.
The electrical charge created by a piezo crystal is fairly low. To compensate for this, they are usually used in high-repetition applications. One method that uses piezo energy to create electricity harnesses personal human power.
People move thousands of times each day. Piezo crystals can be embedded in everyday apparel like shoes. Every time a person takes a step, the crystal generates a small charge. Over time and with thousands of steps, these small charges build up until the amount becomes significant. This energy can be used to keep personal electronics, such as cellphones and MP3 players, fully charged.
Another way that piezoelectric energy can be used is by linking many separate crystals. Because the small charges from each crystal combine, they can create one large source of power.
In high-traffic areas like subway stations and sidewalks, piezoelectric crystals are embedded in staircases and floor tiles. These individual generators are all linked. As crowds of people walk through the area and generate force, the system collects the energy. Individually, the small charges are insignificant, but together, they can power electronics or be stored for future use.
The advantage of piezoelectric energy is that it is completely clean and renewable. Innovations in coming years will create systems that generate power at many different levels, the individual sources working together toward energy independence.