> Also pumped hydro and other gravity batteries

pumped hydro is not a winner. the locations that could be used are few and far between and require massive amounts of water and wreak ecological nightmare on a wide area.

The locations were pumped hydro could be used are extremely abundant. Remember, it doesn't have to be on a river. It can be out in a desert! Here's an example of a project being built in Nevada. Basin and Range geography, more vertical relief than you can shake a stick at. Look how small this thing for the capacity!

https://www.cityofelynv.gov/pdf/CityCouncil2021/cc1-28-21/Wh...

https://www.whitepinepumpedstorage.com/

(the whole thing could be sped up; that's a general problem in the US)

The problem with pumped-hydro is - and all storage based solutions is - how low can you let the storage get before it's an emergency?

Electricity is a vital service: completely vital. Without it, modern civilization halts. It might be annoying being unable to make a cup of coffee, but municipal water and sewage need electricity to work. You go without power for a week, and the entire wastewater infrastructure will start shutting down. Refrigeration and food storage fails. Even backup fuel storage becomes a liability because you need electricity to pump it around.

So the question is, how low can you let the reservoir get? Because it's not about how long you could run going from 100% to 0% - it's how much of it can you use. And we have a model for this, in the form of another service: city townwater supplies.

In Australia, water restrictions go into effect when we hit <50% water capacity in the dams. That's the level at which usage cuts are applied to try and ensure we don't run out. At <40% we increase the severity. But this sort of resource exhaustion is also slow - we lose storage capacity over the course of months, not days.

And this is a resource which is dependent on electricity to supply (we also have a desalination plant, so we have some guaranteed capacity).

So within that context then - i.e. imagine you're planning a nation-state electricity supply, what are your risks? - how good does pumped hydro - or any storage-based solution - look, when your requirement is "the power cannot go off - ever". Put on your systems engineering hat, treat it like a software deployment - what level of redundancy and overbuild would you want when you're told "this is a mission critical, safety-critical system consuming an intermittently available resource". How much capacity and overbuild would you believe is necessary to have confidence, or even decision-making capability, when pressured?

You seem to be assuming pumped hydro is the only storage technology used.

This is a common problem in the anti-renewable arguments. You pick a particular design for an energy system, argue it doesn't work, then (wrongly) claim no renewable energy system can work. But to reach that conclusion, you have to show that no combination of elements can make a system that works.

It makes sense to have multiple storage technologies with different performance characteristics. You want efficient, if somewhat expensive, technologies for short term storage with large numbers of charge/discharge cycles. You want low capital cost systems for ultimate backup, even if those systems are not as efficient.

For example, one could back up the entire grid with combustion turbines burning an e-fuel like hydrogen. These are massively cheaper per unit of power output than nuclear. Because we are not using them very often, the low round trip efficiency doesn't matter much. You want guarantees this won't run out? Make the storage caverns larger. This is already what we do with natural gas -- we store a good chunk of seasonal demand and count this being sized large enough to not run out.

You are proposing build 4 times the power generation: Renewables for the daytime load, Renewables for charging the night time storage, the night time storage, and then a whole extra set of power plants for emergency standby.

Why would hydrogen combustion plants - which don't exist at the moment, don't have turbines on the market, don't have a fuel supply pipeline - be cheaper then current coal fired powerplants?

So your cost of generation already is - at minimum - at least as expensive as a coal fired powerplant, in terms of fixed costs for maintenance (and investment - who's building these when they can't sell the power from them?)

Hydrogen combustion CC plants will certainly have a capital cost less than coal fired plants. They will be almost identical to natural gas fired CC plants (just the combustor will be different). These are known to be quite inexpensive, costing maybe $1/W.

There need not be any fuel pipeline, since the plant can be built at the hydrogen storage site. The electrolysers will be there also.

There have been industrial turbines that burn hydrogen for decades. It's not some sort of exotic technology.

Yes, there are many parts here. And it's still cheaper than nuclear. Nuclear is pathetic in that way.

Reading through the linked article, I don't think that's the way it's designed to be used. It can completely empty itself in 8 hours. That sounds like it's specifically designed to augment renewables at night and charge during the day, cycling every 24 hours.

That's absolutely how it's going to be used because it's cost efficient for the owner.

But if you're electrical grid doesn't mostly have always on sources to backstop it, if you were all renewables and storage, then how much storage would you need to guarantee supply - 24/7/365 days a year.

So, you're saying it would be stupid to use 8 hours capacity pumped hydro for long term storage.

Thanks for the observation, Mr. Obvious.

You also seem to be implying longer term storage of some form isn't feasible. If so, you are incorrect.

Note that I pointed to this project to debunk the falsehood that the locations for PHES are scarce, not to claim that PHES is good for long term storage.

Pumped hydro can work anywhere that has hills. It's just a lot cheaper in some locations than others, due to favorable geography.

Water use just has to keep up with evaporation on average.

I'm not sure why building a reservoir needs to be an "ecological nightnmare" except in the sense that it's a sudden change to an environment.

I was lumping dam infrastructure into the "other gravity batteries" which does cause ecological nightmare situations. if they only meant the fringe gravity batteries like block elevators and the like then it's not as bad a thing but you still require a large amount of fresh water which is an issue in a lot of places too.

Pumped storage, even in a desert, consumes at least an order of magnitude less water than a nuclear power plant with the same energy output.

All the coal and gas storage currently exists. We need to stop using that for non-storage uses, replacing with solar and wind, before we need to replace its storage function once we get down to the last 15% or so of grid electricity being fossil based.