You're comparing using an existing reservoir for hydrogen to building a new reservoir for PHES. There similarly exist dry lake beds that could be used for water storage. But generally they're not in suitable locations, which is the same problem that salt mines will have.
You're also comparing hypothetical costs to historical costs. Hypothetical costs put out by industry are usually out by about an order of magnitude.
There's a reason that PHES is the only one with historical costs.
No, I was describing the cost of constructing a new hydrogen storage reservoir in a salt formation by solution mining. Of course existing natural gas storage caverns could be repurposed; that would be even cheaper.
These are not hypothetical costs. Construction of these caverns is state of the practice for natural gas storage. Vast volumes of gas are stored in these things, allowing steady production of natural gas and constrained pipeline capacity to serve seasonally unsteady consumption patterns.
The reason PHES is the only one with historical costs is that, historically, PHES has been used for diurnal storage, from the days when baseload plants were cheaper. There was never a market for long term storage via hydrogen (although some hydrogen storage has been constructed and used to help steady the hydrogen input to ammonia plants); why bother for the grid when just varying the use of fossil fuels would serve that function just as well?
Here's a presentation of a comprehensive NREL study from 2018, but I don't know the source of the numbers. It finds hydrogen and flexible generation (that is, natural gas turbines) are best for long duration storage. Notice the slides on page 13. PHS is way out of the running for the storage case being discussed here; it's not close.
Thanks. I mean it’s from 2018 and that is ancient history as far as storage costs are concerned. But yeah those $/kWh numbers for PHS are orders of magnitude higher. Thanks for the link, I’ll try to find the final study tomorrow
You're also comparing hypothetical costs to historical costs. Hypothetical costs put out by industry are usually out by about an order of magnitude.
There's a reason that PHES is the only one with historical costs.