That has a round trip efficiency of around 20-25 % I think (more if you can also use the process heat), so we would need to over-provision renewables by 400 % to make it work.

There is a ton of unused roof space that could be decked out with solar, though. There are few heat pumps deployed so far, lots of inefficient old houses that could be modernized, plus there is an array of concepts for other storage tech still at a pretty early stage. It's probably going to be a mix of different generation and storage tech coupled with gains in efficiency, and that seems like it could work out.

Sure, feasible in the long run but don't hold your breath on it to work in the next 5-10 years. China has just doubled their growth targets for solar so producers have trouble keeping up with demand already and we have almost no domestic production anymore, so there's just no way we'll be able to act on this in the near future.

I would love to see a reference about this being actively used, in a meaningful capacity (as in: enough to prevent a countrywide blackout).

We don't Electrolyse Hydrogen in Germany there is not one large-scale installation. You can't pump 100% hydrogen in the natural gas infrastructure, and the gas plants need to support hydrogen burning. You also need a surplus of energy, which Germany obviously not has.

There is no large scale hydrogen storage facility that could take even 1% of German's energy needs.

Yet.

Indeed. If the annual natural gas use of all of Germany was represented as liquid hydrogen, it would be a ~700m cube.

A year is probably more than is needed, but I don't know how low storage can go before it causes problems. If I guess a week is sufficient then every million German residents need a 43m cube.

This isn't unbuildable if there is sufficient political will.