you wouldnt be able to draw energy from the object falling through portals because the energy you draw will slow down the object eventually causing it to stop, like if you drop a magnet down a copper pipe drawing energy from the coil would slow it down,(reply>), Ah, but it is continually gaining kinetic energy from gravity via freefall. As long as you don't draw energy away from it at a faster rate than that, you could extract energy from the system forever.