Can we really build batteries at the scale needed for this? I’m a bit skeptical.
Compared to the size of the fossil fuel extraction industry, it's tiny.
The only question is how quickly we can build up to the scale needed, not if we can.
We're certainly not discussing things which are currently in the realm of unrealized dreams.
Op mentioned someone doing a simulation that claimed that scaled up solar+wind+hydro generation with only 5 hours of storage capacity would be sufficient to meet the needs of Australia. That seems odd given that cloudy, windless conditions can last for days in many cases. You would at least need sufficient storage+hydro capacity to cover several days worth of shortfalls from wind+solar.
Battery systems are great. Pumped hydro storage is great. But I'm unaware of such systems coming anywhere close to the capacities required in the near term.
At this point, the hydrogen infrastructure is years away. It looks like hydrogen storage underground is sufficiently proven to be feasible, but there are a host of other issues in the hydrogen energy chain that need to be addressed, most notably green generation at scale.
The equipment has not been built yet because the factories to build it are still under construction, and because money is still overwhelmingly better spent building out renewable generating capacity needed to charge it from. Storage you cannot fill up is wasted money.
People responsible for keeping the grid operating are not at all worried about their ability to continue, using renewables and storage that will be built.
In places and times where renewable generation falls short for longer than local storage can cover, energy will be imported from places with an excess, via transmission lines or shipments of synthetic fuel, most likely liquified anhydrous ammonia. Until that is available in sufficient quantity, they will burn NG during those shortfalls. The amount burnt will be much, much less than is today.
Producing and then storing hydrogen underground takes very little of "hydrogen infrastructure". H2 may be generated, stored, and drawn down all within a single facility built over or near the underground storage cavity. The power involved may be moved in or out by transmission lines.
The simulation over produced energy from green source. Looking at the chart - looks like the wind power alone was enough to satisfy the grid if available. Given that, I do think it's possible that a mere 5 hours of backup can keep the grid 98% renewable, turning to hydro when solar + wind is not enough. Also solar / wind may be somewhat inversely correlated and north / south Australia also may have different weather patterns.
Given a large enough sampling of the world, I'd imagine it would be easier to find the wind / solar power you want, although politics could potentially make that harder.