Comment by cogman10

> A few tens of GW of battery capacity (i.e. a few dozen nuclear plants worth of capacity) that can switch on/off in milliseconds can do a lot for grid stability.

I think this is a misunderstanding of the problem.

Now, don't get me wrong, I believe flywheels are no longer involved but at one point they were for batteries and solar. Not to store energy, but rather to form the output voltage and to give the correct "inertia" of the waveform to maintain a correct phase with the grid. Prior, both batteries and solar were frequency followers. They'd look at the previous peaks and valleys to determine what their output voltage (or resistance) should be. If the input voltage fell too far, both solar and batteries would cut off to avoid damaging equipment on the grid. This is part of why the winter storm killed the texas grid (to my understanding) the voltage dipped too low which ultimately caused renewables to shut off completely to avoid damaging the grid. That all was somewhat of a cascading disaster.

Flywheels have been used as an inertia source to allow for both solar and batteries to act more like a hydro or fossil fuel generator. That's the grid forming technology. I believe (can someone verify?) that there are now all digital versions of this. But it's delicate software. Getting it wrong can do really bad things like destroying other generators or breaking expensive fuses.