Again, the source article talks about this, but this is also not just a simple problem that can be wished away, and the only "fallacy" is pretending this problem doesn't exist.
Firstly, you are correct that on most days, demand peaks around 7pm when solar output is tiny.
Source: http://www.caiso.com/TodaysOutlook/Pages/index.html
However, on most years, demand often peaks around 3pm-5pm, not 7pm (and note that in late summer in California days are longer and solar noon is roughly 1pm, so output is not bad at all). The reason for this is that the days with the most electricity consumption tend to be hot with large AC loads, and those loads tend to be higher during midday.
Source: https://www.caiso.com/documents/californiaisopeakloadhistory...
So when you say solar output is 0 at "peak demand", it's worth mentioning whether you mean the daily peak or the annual peak or a peak measured over another timeframe.
For utility capacity planning, we use the annual peak, not the daily peak, because we want to avoid blackouts most years, not most days. Therefore, adding solar - at today's margin - still does reduce our capex and need for gas peakers.
The math gets a bit more multidimensional when you do this for real and consider aspects beyond peak demand (like ramping, fuel, current generation fleet, tradeoffs in characteristics of new gas plants, geographic spread, congestion, etc.), but the main point remains true and important: the daily peak is not the annual peak, and it's a clear mistake to use the daily peak to conclude that solar doesn't help with peak demand.
In fact, at today's margins, solar both helps with peak power and with energy generation. It's fantastic. (And of course it gets less and less fantastic the more you add.)
Anyway, this is a common misconception I've seen a lot, so I've added a new section to the Duck Curve Wikipedia article you cited above to help explain it.
You're saying there's a need for storage right now because without it peak demand can't be met by solar.
They're saying you don't need it right now because if you're ok wasting some of the solar power you can still reduce non-renewable use further without adding storage.
It's just two different ways of looking at it.
No matter how many panels you add, you won't add any electricity at 7pm / 8pm (depending on time of year) because the sun has already set. There's still substantial demand for electricity.
This means to solve the 7pm / 8pm duck curve, you need to build natural gas peaker plants, or figure out a storage methodology.
That's it. Otherwise you get brownouts at 7pm every night and need to manage the lack of electricity. Maybe rolling brownouts is the solution people are proposing?
The other solutions are: figure out storage, or build a gas peaker plant to temporarily provide power in those few hours.
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We focus at the "Duck Curve", because that's clearly where the next brownout will occur without intervention. Secondly: environmentalists are going to block the build of any natural gas plant (even if its necessary), so getting "ahead of the argument" only makes sense.
Also, the duck curve problem isn't the need for gas peakers after the sun goes down, it's the speed at which you need to ramp them up from being switched off.
If they can't ramp as fast as the solar drops, then you need to start them earlier and end up producing extra electricity (and carbon) that you don't want.
The problem you are talking about is called "nightime" and I'll refer you to the footnote in the article that covers this.
Wind is certainly a thing. Its not perfectly reliable, even over large areas (ex: a hurricane on the East Coast would shut down turbines due to too much wind... over a large area). But wind is at least part of the answer to the duck curve.
And Hydro is great, but has a peculiar weakness, especially in the context of California's drought conditions. California has the problem of water-shortages as well, which means that hydro-plants are in "water management mode" rather than energy-management.
That is to say: Hydro serves two masters: energy management AND water management. If you're using the dam as water-management, you're not necessarily getting the electricity you want at the right times.
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I'm not convinced that anything aside from peaker plants really solve the duck curve frankly.
Storage will get there eventually. But not today: there needs to be more research and development into storage technologies before we have something large enough. (Either that, or we need to solve the West's water issues, so that hydro plants can be more into energy-management mode rather than the more pressing water-management issues)
The storage needs that OP mentioned...the kind that gets us from 80% up to 100% renewables is a lot of storage. We're talking multiple days, possibly weeks of storage.
The supply/demand graphs for the California grid are public for all to discuss. We can look at 9/05/2021, and see that 25GW was demanded at 12-noon, while 36GW was demanded at 7pm at night (hour 19 on the graph).
The energy required between 5pm and 10pm on 9/5/2021 (from 17 to 22) is ~37GW per hour.
That's 185GW-hrs of energy. Exactly what technology can both store-and-output 185GW-hrs each day, and every day? The "Tesla Megabattery pack" in Australia is 0.3 GW-hrs FYI, to give you what a "very very large" battery can do.
California already has 4500 GWh of storage. They don't have those peak power levels (currently, 4.5GW) but that's a matter of capital investment into pumps and generators.
Not that that is even needed. California has a very large coastline with extremely reliable wind power. For example, there isn't a single spot anywhere along the coast of California that is forecast to have <8kts of wind for more than a day this month. Overprovisioning of wind power is a much bigger part of the solution to the duck curve than storage is.
https://www.energy.gov/sites/prod/files/2020/12/f81/SETO%20P...
This scheme would also allow backstop generation by using a fuel to reheat the storage material, potentially useful for "black swan" prolonged outages.
The fluidized bed heat exchangers seem particularly interesting to me, if sufficiently low escape of particulate matter can be achieved.