Agreed that pumped hydro is the most efficient storage solution but it's limited by geography: You can only do it where there's an existing significant elevation change. Building a sufficiently elevated lake artifically by heaping thousands of cubic kilometers of dirt is not cost-effective. That is, the energy that would be required to build it would vastly exceed the total energy it could store integrated over its lifetime.

> That is, the energy that would be required to build it would vastly exceed the total energy it could store integrated over its lifetime.

Storage ‘buys’ energy when it’s cheap and ‘sells’ it when it’s expensive, where ‘cheap’ can even mean ‘free’ at times.

You would build it using averagely priced energy (using only cheap energy is impractical, as it would keep machines and personnel tied up waiting for prices to drop)

Also, having a large buffer can mean having to spend a lot less money on excess solar and wind farms. They wouldn’t have to meet all peaks in demand.

Those still could make it economically viable.

Yeah building mountains is not a realistic solution. Thermal storage is probably the best replacement where pumped hydro isn't feasible.

Inventing absurd versions of sound ideas just to have something to criticize is such a standard dishonest argument mode as to have been named.

We all know the name, but its main feature is its dishonesty.

What's the sound idea for build pumped hydro storage on flat land?

There is such a thing as a water tower, but that also is probably not practical utility-scale energy storage.

The sound idea is water stored up an existing hill. On flat land you store your energy some other way: underground hydrogen or compressed air, or iron-air batteries, or liquified air, or any of various other choices that are more expensive and less efficient than pumped hydro. Or just run a wire over to someplace that does have hills.

It is a good thing, then, that pumped hydro is not limited to existing dams, or even to places where a hydroelectric power station would be practical. Pumped hydro does not need the large watershed and elevated river valley a hydroelectric dam needs. All pumped hydro needs is an elevated depression somewhere not too porous. There are myriads of those, and building a penstock up to one is very cheap.

Some places have no hills, so are not candidates for pumped hydro storage, and would need to rely on something else, such as long-distance transmission lines, or liquified air, or synthesized hydrogen, or synthesized ammonia, or underground compressed air, or iron-air batteries, or numerous others, or (most usually) some combination of them.