[ephbit]

> There are just way too many cheaper and more efficient storage methods for electricity like pumped hydro and battery storage.

Pumped hydro is cheaper and more efficient. But AFAIK there isn't much potential for new capacity in areas where it'd be needed. Batteries are more efficient. But cheaper per kWh stored for 6 months than power-to-gas-to-turbine? I don't think so. At say USD 500 per kWh battery, 1 GWh would cost USD 500 MM. But seasonal storage is likelier in the TWh magnitude. 500 billion then.

550 MW electrolyzers (* 0.5 a * gas turbine efficiency 0.4 = ~ 1 TWh electricity), methanation plants, 2.5 TWh salt caverns and say 5 GW gas turbine plants to store 1 TWh for the 6 months that have less solar+wind than the other 6 would cost a lot less than 500 billion I assume.

> .. ammonia is more useful as an agrochemical than as an energy storage medium.

I think this will change. You don't transport/consume the petawatts of the world's deserts via HVDC to somewhere to 100 % immediately use them. You'll ship them in the form of some molecule. Which could well be ammonia.

> With regards to seasonal storage, it isn't as big of an issue as you would think.

The combination of PV/wind isn't perfect though. The more of it there is, the larger the storage needed to flatten the seasonal fluctuations.

It comes down to whether one assumes that either (A) electricity demand will adapt to some seasonal pattern (meaning that economic activity might fluctuate in synchrony) or (B) that economic activity will drive deployment of technology such that electricity consumption can be mostly even throughout the year.

I'd guess B is more likely.