Ann: Redflow Quarterly Update Q2 FY2021 and Appendix 4C, page-29

I haven’t looked for a while, but a few years back I looked at compressed air, and the round trip efficiency wasn’t great - somewhere less than 70%. Response time and energy density not as good as batteries. Typically, the cost-effectiveness case for a more expensive product wins over price if there is a performance advantage; the less well spec’d competitor has to keep dropping price, which is not conducive to new product development. So at least for the foreseeable future, I think batteries will outcompete compressed air.