The focus on efficiency can be short sighted though. The whole point of storage is that the energy you sell is more expensive than the energy you buy. If the profit margin is high enough then you can afford to waste energy through inefficiency. And grids with lots of renewables will naturally have times where energy is cheap and times when it is expensive.

Also, I doubt this will exist in isolation. It will probably be built in places that already have demand for hydrogen, available land, have accessible energy, a good grid connection, and existing iron-ore infrastructure. It will be built in such a way to minimise costs and maximise available customers.

If it has 33% net efficiency that sets it back 3 times compared to batteries right from the outset. With stainless steel pressure vessels needed here and hydrogen precautions, not holding my breath.

But there comes a point where battery storage has all been used and prices go even higher. You sell at a x6 profit margin. And are able to sell at x3 profit margin for longer than the battery operators. And that assumes that 100% of the fuel would need to come from storage.

You are betting against batteries? That's becoming risky, Na-ion has the potential to be literally as cheap as dirt. And they are not flammable.

I was not betting on anything. Just explaining why poor efficiency may still be economically viable.

I absolutely agree that we should bet on the battery industry. But there is a difference. With battery technology the cost of storing 1GWh is fully coupled to the cost of the equipment. That is not the case when you can store something. It could run for longer durations with the same cost in equipment. You just store more fuel.

Another factor here is industrial capacity. Battery production has amazing capacity for growth. But that is not a zero-sum game. Other parts of industry could also be scaled up.

Its interesting how similar this thread is to debates I have had with people about solar/wind/battery storage. We take one measure and then focus on that exclusively. Whilst ignoring the wider commercial, industrial and technical factors.

But "just store more fuel" means adding more containers with larger quantities of flammable material, how can it be done without increasing the cost of equipment? That's the issue here. Maaybe if they directly used the iron ore seam underground, but this isn't the case.

This is for seasonal energy storage with only 1 charge and discharge per year. It competes with water reservoirs not with batteries.

That means it has only 1 chance per year to recoup the investment. And that requires exorbitant energy spot selling price, which too rarely happens to rely on. The economics just isn't there.