You keep hand waving and say "build storage". You can't build stuff when you don't know how to.
Without too much feasibility study, I still think that the best renewable energy storage is liquid hydrogen. The safety has to be addressed of course. It can be mass produced offshore, the existing tankers might be retrofitted for transportation. The losses because of the fuel cell efficiency might be comparable by the energy grid losses. A lot of development and advancement is required in this field of course.
Take the hydrogen and add a little carbon, to make methanol, ethanol and other organic acids. Advantages are that you do not need cryogenic storage, insane pressures and it does not diffuse through apparently solid metal with ease. As well you can handle it with existing infrastructure and methods, and there is not that annoying 5/95% range in which hydrogen is an explosive.
However much you want the exhaust to be H2O, the only place hydrogen use is viable in in spacecraft where you can use the water, and the cost of the platinum group catalysts are worth it for the usage. Hydrogen fuel cells will use the entire world's platinum reserves in the first half decade of mass production, even if you use a near monatomic film on a substrate. Will a fuel cell still be viable if the platinum cost is at the $15k per troy ounce mark.