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The air inside the balloon is heated by greenhouse effect and the black screens pick up the energy that has gone through the transparent polyester film.

from TreeHugger

CoolEarth Raises $21 Million for Solar Balloons

Solar Balloons attached to cables

Solar balloon. A gas burner is fitted to ease the inflation, or to cope with the possibility of cloud cover during the flight. The altitude control is done by the opening or closing of horizontal panels located at the balloon equator.