At the end of the article: >> The problem remains that fully solar balloons need sunlight, and it is rare for the sky to remain unclouded for long journeys. A

At the end of the article: >> The problem remains that fully solar balloons need sunlight, and it is rare for the sky to remain unclouded for long journeys. Above 6,000 metres, it is rather rare that any cloud blocks the sun in daylight hours. This method of transport has potential for freight. Rather than black, you'd want a transparent skin or layers of skins. Focus sunlight with internal reflectors onto an engine. Usefully, waste heat is not a problem here, but valuable, when captured in the envelope. The engine should use more than just heat energy. Visible light for PV power generation. Maybe ultraviolet can be used to elecrolyse environmental water vapour to hydrogen. To provide both lift and energy storage for night time.