content: I would guess that they are running a massive surplus of energy during the daytime. Hydrogen fuel cells are actually a great storage method in that exact situation. In the 'real world' they're pretty awful for now since we don't have a surplus of energy, let alone clean energy, and so they just end up wasting otherwise very usable energy. But say 50 years from now (or perhaps much sooner) when we're similarly massively overproducing energy on a global scale, they could come into play again as a beneficial device. When you have that massive surplus, they don't have the capacity decay of batteries and have a much better energy density.