John Moody did something similar with the Solar Riser. IIRC, it was powered by a Hughes helicopter battery and a 3 hp electric motor. He also had a panel on the top wing to charge the battery. He could get a 3-4 minute flight after leaving it in the sun for 4 hours or so, though the thing had a climb rate of approximately .00000569 fpm.

I've considered doing something similar. I know that there is at least one electric motorglider available now, from what I've heard it will climb to 8500' on a single charge. With a lithium-ion polymer battery and a brushless motor, it should be possible to rig something up for under a fortune - and the components are getting cheaper every day. Electric power has pretty much surpassed gas for small r/c planes already.