There has been some pretty interesting research in to storing energy in molten salts. This is a bit different than the photovoltaic cells which I believe this map is referencing; It involves capturing heat and turning it in to electricity rather than light. This is mainly just research right now, but if it pans out it could potentially be much more practical than photovoltaic cells because it doesn't require as many precious materials to manufacture and stores energy at night. See: http://en.wikipedia.org/wiki/Solar_power_tower and http://en.wikipedia.org/wiki/Thermal_energy_storage