In Europe after the last autumn when for months there were little wind and cloudy sky there is a realization that energy based purely on renewables will require much bigger storage and much more solar or at least significantly more expensive solar that works under clouds. Doable, but cost is non-trivial.

So if any of fusion startups can make operating plant within the next 20 years, it can be used. But if that takes longer, then I agree that large-scale fusion will not happen.

Long term darkness/calm is solved with hydrogen. Europe has plenty of underground geology suitable for mass hydrogen storage (it's just like natural gas is stored.) Most of the energy still would go directly from solar/wind to the grid, or through efficient batteries, so the low round trip efficiency of power to gas to power doesn't matter much.

I suspect long-term, large-scale hydrogen storage is not "just like natural gas"... those pesky little H₂ molecules are awfully small, and may leak out rather easily compared to hydrocarbons.

Hydrogen storage is difficult only in metal tanks at high pressure and temperature. Store it underground or liquify it, and the problem dissipates.

Heating and lamp gas composed of of hydrogen and carbon monoxide (!) were once delivered throughout cities through cast-iron plumbing. That was displaced by "natural gas" as increasing numbers of people were found to have been asphyxiated after a lamp flame in their house died, typically from a pressure fluctuation.

Natural gas then had to have mercaptan odorants added after leaking gas accumulated undetected and exploded. Then, valves that would only stay open when hot came into regular use, for pilot lights. Now pilot lights are gone, replaced by spark generators. Hydrogen will displace natural gas in industrial settings, but synthetic methane or propane will probably be needed for domestic use.

There are already underground storage caverns for hydrogen, and more planned. It works fine.

If hydrogen is produced from electricity at scale, it won't be used for energy storage. It'll get sold to chemical plants first. They'll pay good prices.

Yes, surely that will be a major market. It just makes the case easier for renewables, as that's a major source of dispatchable demand that can smooth variation in renewable output without having to pay a cost for turning it back to electric power later.

And, anywhere it is used, it will also be stockpiled. And sold back whenever the spot price rises.

Or to steel smelters.

Electricity -> Hydrogen -> Electricity is insanely wasteful. If this is the only large scale long term storage we can come up with, we have a huge problem...

It would be a serious problem if hydrogen were used for diurnal storage. But for rare event backup, efficiency is really unimportant, since so little of the energy produced will go through that channel. What matters for use cases like that is minimizing capital cost, not maximizing efficiency.

I'll add that the alternative proposal, excess renewables with curtailment, has 0% efficiency on the curtailed power. Using that power for anything at all, including making hydrogen, is more efficient that just dropping it on the ground.

Hydrogen is so useful for so many other things that its (present) poor round-trip efficiency doesn't matter much. And, that efficiency will only ever get better, not worse, and will get better faster the more of it there is.

But hydrogen is very far from the only storage choice, it is just one that is attractive for numerous ancillary reasons: we need to make the hydrogen regardless, so might as well store it too.