One of the things I've always been fascinated with, every since a...well, let's just say a long time, has been perpetual motion. The question is, can it really exist in a usable form.
After thinking about such a question forever, I'm inclined to think so and I'm sure my prior answer would have been, "no." I say prior because only after really exploring solar energy did my mind change. You'd have to pay much more attention to Earth's physical attributes than you might think for Photovoltaic energy to work properly.