Industrial Designers + Engineers

Hey everyone. I'm a New York City based Industrial Designer (product designer) and I'm looking into developing products, specifically electronics, in the realm of piezoelectricity and other forms of alternative energy. I'm curious if it is practical to power small electronics like cell phones, and ipods with piezoelectricity through human power. I know that piezoelectricity emits a charge of electricity, but is it sufficient? How does the size of the piezo directly relate to the power it emits? In other words, does it have to be something cumbersome to take advantage of higher power supplies? Any input is greatly appreciated. Thanks!