Very interesting read and I can't wait until a device like this hits mass production.
How much energy does it produce and how would it compare to a portable solar charger? I know it's not easy to answer this, since there are many variables at play with both kinds of chargers.
Appreciated @daan. Actually, it depends on the number and size of piezoelectric transducer you have installed. At the very least, it could charge mobile phones. Back in my college days, i've done together with my collegues a mechanical type of this system. We have produce average to 5W.