That's a really interesting read Docweemo. I'm not an electrical...

That's a really interesting read Docweemo. I'm not an electrical engineer so forgive my ignorance, but I would have thought that flow batteries on the grid would have equivalent response times to lithium ion batteries given the same switch gear? FCAS arbitrage certainly is a strong economic factor in early ROI on grid connected BESS so if there is a differential it would be an issue as you point out.
(...ahhhh, 'ol BESS. ...aaahhh, acronyms. PCMCIA Cards. People Can't Memorise Computer Industry Acronyms...).

That Yadlamalka Vanadium flow battery in SA (Invinity) is reportedly supposed to provide FCAS services... LINK. ...reportedly.

As an aside, there's also research into electrochemical flow capacitors (EFCs) which are a hybrid of supercapacitor and flow battery concepts that has been going on for a while but is still in the lab as far as I can see, but is progressing albeit slowly (trouble with adequate energy densities from the latest papers I glanced at).

I'd like to know for sure if there is a fundamental response time issue for flow batteries cf. lithium for VCAS vis a vis grid scale BESS.
Cheers.