A mylar balloon that concentrates the sun's rays on the small collector attached to the middle of the transparent side. The shape of the balloon directs the light onto the collector to get 350 times the power than if the collector was naked and pointed at the sun.
So - cheap mylar, tiny collector, and no expensive mechanism to move the collector:
win win win.
The company that produces these collectors, Cool Earth, has just rounded up 21 million in funding.