You never know--Anyone could suddenly have an epiphany about how to make perpetual motion possible (Think Doc from "Back to the Future"
).
i want to know how the doc's plutonium reactor made 1.21 Gwatts without a steam turbine.
....
'perpetual motion' might well suffer a misnomer problem. 'over-unity'? not possible.
perpetual motion? well, we have it... it's called a water-fall. with one cubic metre of water weighing in at a metric tonne, there's a lot of push available.
china has a +18Gwatt waterwheel installation running at the moment... yes the environmental impact has been huge, but it doesn't always have to be... and one might be tempted to compare the fallout of dams with nuclear waste which stays bad for 1000's of years.
i know it's kind of site-specific, but tidal power is basically 'free energy' which uses the movement of celestial bodies. twice a day, more potential energy than you could ever hope to even use, let alone harness just ripples across the planet.
but back to more practical movements. i was just recently looking at some sealed - pumped-water-storage designs. excess power during off-peak times is diverted to the lifting of subterranean weights via a water-pump. during peak energy use hours, the pump becomes a generator as the weight is pulled by gravity and pushes water through the closed system.
there are losses of course, but much less than having to drop your powerplant below optimum efficiency during off-peak hours.
i've heard that all those windmills in the north-sea have their power diverted to lighting up the highway between amsterdam and paris.
actually, i've heard that the TGV train drivers had a competition going to sea who could nail the most efficient coast into paris. this would basically involve a train traveling at +300Km/hour letting go of the throttle and just coasting to a full stop at the station with the least amount of brake use possible. the train's 3phase motors seamlessly become generators when the train is coasting and essentially pump power back into the grid as they slow down.
also of note is the montréal metro system. many of the stations feature inclines so that the trains must go a bit uphill just before approaching the platform, and downhill as they leave. much of the braking performed by using the motors as generators. i've heard that the schedule is timed so that, ideally, the braking of trains coming into stations will provide a boost to the power of trains which are simultaneously leaving stations. . . with that, current-chopper traction motors, and the rubber-tires,,,, music to my ears : )
..oh. so yeah. you can use motors as generators. and headphones as microphones, and cameras as projectors... just depends on sink and source, which way the power is compelled to move (if there is any)