What is a balloon that uses sunlight to heat its internal air called?

A balloon that uses sunlight to heat its internal air is called a solar balloon.

Solar balloons are usually made with a dark envelope that absorbs solar radiation. The absorbed energy warms the air inside, making it less dense than the surrounding atmosphere and producing lift without a burner or fuel tank.

The idea follows the same buoyancy principle as an ordinary hot-air balloon, but the heat source is sunlight rather than a propane flame. Solar balloons are often experimental, recreational, or educational because their lift depends strongly on sunshine and the envelope’s ability to absorb radiation.

They are different from solar-powered airships, which may use electric motors and photovoltaic panels for propulsion. A solar balloon’s defining feature is passive solar heating of the air inside its envelope.