On the last leg of an around-the-world journey in a hot air balloon, Bertrand Piccard almost ran out of propane on the desert of Egypt. He had barely enough to finish his trip in Switzerland. So he decided to circumnavigate the Earth with no fuel at all. A plane called Solar Impulse is ready for its most ambitious flight yet, across the country, powered by photovoltaic cells. Ucilia Wang, who covers clean tech for Forbes, GigaOm and Renewable Energy World, says the plane sits in a hangar at Moffett Field, south of San Francisco near San Jose.