A football field sized reservoir isn't going to hold much water unless it's deep. Realistic pumped hydro sites are much larger than 500 square meters. And those two reservoirs can't be too far apart, because excess lateral distance reduces output and increases the energy required to refill it.
It also needs to be close to a source of water in order to fill the reservoir and refill any loss due to evaporation.
It also needs to be close to a major highway otherwise the costs of construction will be immense because of the long distance that materials and equipment must be shipped. This is particularly problematic because most pumped hydro potential is in mountainous terrain where transportation is expensive.
And lastly, this site has to be close to places that have energy demand, otherwise you'll need to build miles and miles of transmission lines. This is also a problem because the places that have pumped hydro potential also have hydroelectric potential, and usually already generate electricity from dams. Norway and Brazil have huge pumped hydro potential, but already generate most electricity from dams.
The conditions for a viable pumped hydro sites are much more narrow than "two football field sized reservoir a few hundred meters apart in elevation".
The paper you cited just ran an algorithm over a terrain height map. It did not attempt to account for accessibility. Good luck developing the immense pumped hydro potential in Tibet. Pumped hydro would be great if we could teleport concrete, excavators, and turbo machinery into some of the most remote parts of the world. But unfortunately this is not in our capabilities.
China is building several "gravity towers" (different design to [1] but same concept), Australia is designing third generation (after a 1MW test pilot, and current 288MWh plant under construction) high temperature molten salt thermal energy storage tanks to night time buffer solar plants used to produce green methanol for use as a sustainable shipping fuel.
[1] https://www.wired.com/story/energy-vault-gravity-storage/
[2] https://reneweconomy.com.au/australias-biggest-solar-thermal...
Why are people exploring all these alternative energy storage methods? Because grid storage is not feasible with current storage options. We're still looking for the silver bullet to solve grid storage. Will these new ideas yield storage at lower cost than the existing systems? Maybe, maybe not. A grid powered by primarily intermittent sources is viable if such a silver bullet is found, but that's a pretty big if to bet the future of the electric grid on. A very realistic outlook for wind and solar is that we saturate peak production, waste a ton of excess electricity, and keep burning fossil fuels because the silver bullet never materializes.
So what? As is clearly implicit to the meanest intellect in my comment above these are alternatives that are being built right now after having already passed pilot testing and now being scaled up for grid storage.
> A very realistic outlook for wind and solar is that we
overproduce during daytime here in Australia, have sufficient wind through much of the night to tick over, and use the excess to produce green hydrogen, ammonia, methonal, etc to buffer nighttime demand and for use in mining, transport and agriculture.
No, they are not. They are still in the prototype stages. Gravity storage is still being prototyped. Heated sand is only being used for district heating. If you've got news about a real production deployment of grid storage using these systems, I'd be eager to read it.
> and use the excess to produce green hydrogen, ammonia, methonal [sic], etc to buffer nighttime demand
If we find a feasible way of doing these things. These storage schemes are theoretically possible, but as of yet they are not demonstrated to be viable at grid scale. The reason why most of the talk is around batteries is because they're the most viable storage option at the moment. Will one of the systems you listed deliver much better grid scale storage? Maybe, maybe not.
If your point is that intermittent sources are viable contingent on an energy storage breakthrough, then yeah I'm in agreement. But it's not exactly wise to bet the future of your electrical grid on a technological breakthrough that may or may not happen.