Solar energy. Diagram showing the principles behind the solar updraft power station. A chimney stands at the centre of a large, circular roof-like structure made of glass. This allows sunlight through, heating the ground and the air immediately above it. Because the roof is slightly angled upward toward the centre, the hot air moves toward the chimney. As it rushes upward by thermal convection, it passes turbine blades which turn electricity generators. The collecting area of such systems under design cover several square kilometres with central towers several hundreds of metres tall.