I read 'duration' as the charge/discharge cycle time rather than the battery's lifetime in number of cycles. Clearly the more cycles a battery can handle the more cost effective it will be (more cycles over which to amortize the capital cost) but you still have to pay interest so even if the battery can handle infinite cycles if you only cycle it once a year (to balance seasonal differences in generation capacity) it will probably not be cost effective.
I think stationary storage batteries are great, but only for short duration storage (1 day) rather than long duration storage (months).
Yes, that's precisely what I meant for the term; storage that is economical even with few discharges cycles per month or maybe even per year.
I am somewhat skeptical that it can work, but flow batteries are still in their industrial infancy, and with the right sort of cheap stuff to add to water this could potentially scale very well for some sources.
I also strongly agree that we will likely build renewable sources to match their output to the seasonal minimum needs, rather than have many TWh of storage, it I have been so wrong in the past that I no longer want to cut off potentially promising avenues.
I see our future as being one of electrical energy abundance at nearly all times, and only a few times a year will there be a crunch to limit consumption; however as we get used to that abundance, conservation could get more difficult, and also people will get far more clever with time arbitrage of energy demand. And once there's a huge market for arbitrage, people will get more clever to serve that economic need, in ways that we have not yet anticipated.
I think stationary storage batteries are great, but only for short duration storage (1 day) rather than long duration storage (months).