The main problem with photovoltaic energy is that sunlight can be obscured by clouds, which makes electrical production intermittent and uncertain. But above the cloud cover, the sun shines all day, every day. Anywhere above the planet, there are very few clouds at an altitude of 6 km—and none at all at 20 km.
Solar energy is five times more abundant at 20 kilometers than on the ground
Today, one of the obstacles to the deployment of renewable energies like solar electric power is storage. Batteries—lithium-based especially—are effective but prohibitively expensive for application on the scale envisioned here, and could pose their own environmental problems.
Resorting to hydrogen as an energy vector could provide an elegant solution. Hydrogen can be obtained by water electrolysis, using the excess electricity generated during the day, and then recombining it with oxygen in a fuel cell to release its power as needed, with pure water as the only by-product. In addition, that same hydrogen could also be used to fill the balloons and keep them in the air at low cost, while providing a low-cost solution for hydrogen storage.
A moored high-altitude balloon of reasonable size could store about 10 days equivalent of its own solar electricity production, which is more than enough to meet energy needs overnight, until production resumes in the morning.
Reducing structural and installation costs, ensuring high conversion efficiency and providing nearly round-the-clock access to an abundant power supply would help prove that clean energy can cost less to produce than coal-fired electricity.
The most common photovoltaic modules today have a yield of 15 to 20%, but CPV modules in intense sunlight achieve a conversion efficiency of nearly 40% (Fraunhofer-ISE), and solar cells under concentrated sunlight in the laboratory recently exceeded 45% (SOITEC).