re: To harvest meaningfull amounts, your collectors would have to be huge
Maybe that is why there are so many KBOs scattered all over the place. They've all got alien solar cells on them.
Recall that Freeman Dyson's initial design for his Dyson Sphere (or Shell) was as a large collection of separate bodies surrounding a star, not one solid ball.
http://www.aeiveos.com/~bradbury/MatrioshkaBrains/DSSatDLFiG.html
