Imagine walking through a grand public space --- say the Transbay Terminal --- abreast with thousands of other city-dwellers secure in the knowledge that the simple act of walking provides light for the space through which you move. That footsteps power the system announcing arrivals and departures five floors below. That one person's mad rush to catch a train heats tea for a dozen others less pressed for time.
This could become a reality with widespread adoption of piezoelectric power generation. Certain crystalline structures --- including readily-available materials such as quartz, cane sugar and rochelle salt --- create an electrical charge differential in response to an applied mechanical load. If provided conductive polar pathways and not allowed to short circuit, this charge can induce voltage usable as electricity.
At this point, the technology has been deployed only in fairly unsophisticated applications. The smokers amongst you are familiar with slick single-click lighters, the one-button dealies lacking a circular flint strike. Those utilize piezoelectricity, transferring your thumb's point load to ratchet mechanism which strikes a quartz crystal to induce current across a tiny gap, creating an arc to ignite the gas.
Fade to clubs powered by the revelry inside them,
roads that recapture the energy expended in locomotion, containers gleaning power from the pressure of the substance stored inside...The opportunities are endless, end endlessly exciting.