I first noticed the Gravia on Ecogeek in February. I immediately ruled it out as unrealizable and moved on. I later found that the concept was created by a fellow Virginia Tech graduate & became interested. Virginia Tech even recognized this design on their website. While the Gravia has a fun kinematic element to the design, the forces don't add up.
Let's add a little math to the Gravia.
The falling weight is 50 lbs (22.68kg) and is designed to fall 58" (1.47m).
gravitational potential energy = mass*acceleration of gravity*height
326.72J = 22.68kg X 9.8m/(s^2) X 1.47m
Now if we consider that 1J = 1 watt-second, and ignore real world losses & LED inefficiencies, the Gravia will support a one watt LED for 326.72s (5.44m).
The Gravia claims:
...light output will be 600-800 lumens - roughly equal to a 40-watt incandescent bulb over a period of 4 hours.
Obviously this claim is impossible even with an ideal unrealizable LED. I commend the graduate on developing an entertaining kinematic device that I am sure he can find other uses for.
Assuming the Gravia generates 326.72 watt seconds (ideal) he could power a small circuit with an average consumption of 22.688mW for 4 hours.