Ann: Redflow Quarterly Update Q4 FY2021 and Appendix 4C, page-26

I don't think the round trip efficiency is as important as you think it is.

The big advantage of a flow battery is that it scales in a much nicer way than lithium does. If you need more capacity (MWh), just add a bigger tank of electrolyte. If you need more power (MW), you need more (and/or bigger) anodes and cathodes. Lithium bundles both of these together - as the capacity goes up, so too does the power.

There are two things that you want for grid scale storage. The first, and this is the primary purpose of Hornsdale, is fast response, for frequency control. You need a short, sharp burst of power to stop the power grid sagging and browning/blacking out, while a power plant that can provide the necessary energy spins up to deliver it. The power plant spins up, starts delivering power, and the battery stops injecting the power to stabilise the grid. Lithium is pretty much perfect for this; it provides a high amount of power for its capacity. Think of a sprinter - runs very quickly, but not for very long.

The second thing you want is capacity. This is much more akin to your traditional power plant - providing a level of steady power for hours at a time. You don't need high power, you just want to be able to deliver a level of power for an extended period of time. Much like an endurance runner (marathon or similar). That's where flow batteries have a huge edge: if you need more capacity, you can make the tank bigger, whereas for lithium, you need to scale up power along with capacity (hence adding to the expense). And since they're not suited for mobile applications, the tank size really doesn't matter in the way it would for a car.

And if capacity is your big concern, an 80% round trip efficiency isn't going to be a deal breaker.

Long term, I see a market for both types of battery (flow and lithium) in grid stabilisation. Their strengths and weaknesses complement each other very nicely indeed.