Just a minor nit -- and some links for those who might not know where to find them. I of course agree with you.

> Pumped hydro works if you have mountains, valleys you can flood, and dams. It works great in the Alps; good luck doing that in the UK or in the Netherlands.

It works in Wales and Scotland -- but not in England where it is needed the most.

https://en.wikipedia.org/wiki/Cruachan_Power_Station https://en.wikipedia.org/wiki/Dinorwig_Power_Station

https://en.wikipedia.org/wiki/Hydroelectricity_in_the_United...

https://en.wikipedia.org/wiki/List_of_pumped-storage_hydroel...

Transmission lines work fine in England. Wales and Scotland are hard by.

(... as people actually say, in England. Or did.)

> Pumped hydro works if you have mountains, valleys you can flood, and dams.

This falsehood has been corrected numerous times on HN. It need not be repeated.

Pumped hydro is in no wise dependent on any of those. All it needs is a hill. Even England has hills. Even Australia has hills. The Netherlands are in Europe, which has hills.

Another falsehood is that "you need to vastly overbuild". This has also been frequently corrected. We use, today, NG for backup. We will have no need to demolish any of that infrastructure, even after storage is built. Overbuilding renewables to charge storage makes compelling economic sense, just because it is so cheap, but the NG backup remains. Eventually, synthetic fuels will be cheaper, for such occasions.

So, renewables have no such "single point of failure". This falsehood need not be repeated.

Finally, pumped hydro and batteries are just two of numerous practical storage technologies being deployed. Their round-trip efficiency is of minimal importance when they are charged from free surplus. Storage with NG backup provides fully ample 100% service with exactly zero "constant base production" needed or wanted.

My I-think-conservative WAG for the cost and land-area-use differences between England and Switzerland, for this kind of storage, would be about 50x and 5-10x, respectively, for similar amounts of stored energy. Both elevation and grade of the surrounding land matter a lot for how much energy you can reclaim per stored liter, and having two or three sides of your reservoir provided by nature in the form of steep-walled valleys has to provide serious cost savings.

Like of course any water you push up a hill is gonna gain potential energy, but I think a reasonable reading of "works" in that post was "works efficiently enough that it's actually worth doing", not "is technically possible".

Your "wild-ass guesses" for costs are worth as much as you say. Grade, less: it affects mainly how long the penstock must be, but pipe is cheap.

An earthen berm is a very cheap construction project. Classically those were achieved with hand labor worldwide, for millennia, eventually augmented with wheelbarrows starting in China, and oxcarts. They cost less today.

Nature providing one, two, three, or four walls (steepness optional) reduces construction cost, but the turbines and pumps are a substantial part of the total.

Opex is of course extremely low. Bonus points for solar farms floating on the reservoirs. Round-trip efficiency of pumped hydro is near the top of all alternatives, although efficiency is relatively unimportant when the power being stored was free.

Not seeing how the physics can possibly work out for the real costs and land-area-use being too much lower than my guesses to achieve the same total stored energy, but sure, maybe there's some engineering magic I'm not aware of. Could be. In particular, maybe I'm overestimating how much more energy one can efficiently extract from, say, a liter of water dropping 5,000 feet over three miles, versus a liter of water dropping 600 feet over three miles.

Anywhere pumped hydro would cost more than another of many storage alternatives, the other alternative will reliably be chosen. England is, e.g., preparing hydrogen storage in depleted NG cavities. Pumped hydro will be built in the places where it is cheapest, favoring high hills and variable topography.

Costs for many storage alternatives are falling fast, and it is unclear which choices will win. Adding pumps to existing dams has lately been cheapest, by far. Future people will choose according to local conditions without asking us.

Right, I get that there are alternatives that would be used instead, what I was surprised at was characterizing "hydro only works in mountainous areas" as flatly false, if we take "works" to mean "is worth doing", which is a common usage and I think a fair reading of it, and since it seemed to me like there would be large differences in the economics of it, depending on local geography.

Low hills, where the hilltops are not otherwise in use, may suffice. Some places have very deep gullies or underground cavities that would serve in place of or augment hills.

The point is that whether a storage medium is practical in an area depends more on the landscape of choices than on the physical topography. Even Nebraska might have deep cavities that would serve in place of hills. But the extrema are extreme.

Pumped hydro also works if you have water at all, which my country doesn’t. So, some kind of new molten-salt reactor which doesn’t need water for cooling and can be built in the desert could work very well for us.