The latest advance in piezoelectric technology sees an 80% conversion efficiency for converting pressure and momentum energy into electricity. Researchers at Princeton University have created rubber sheets that have the capacity to sustainably run small electronic devices (portable gadgets...pacemakers) with no external electrical input, but just from the movement that is inflicted on the device in normal use. Not sure how green the manufacture of these gadgets are.
This is experimental technology at this stage, but I can see the application to energy generating artwork on a massive scale that would run off of the vibrations of running people or the play of children. There are already more utilitarian applications being put forward for use on highways, bridges and speed bumps. Even the fast food drive-through is getting into the idea.