The difference is because hydroelectric generation depends on currently existing rivers, damming them and turning them into electricity sources has to change the natural ecosystem in the river.

For pumped storage, we can essentially build it anywhere there is an elevation change. So we can avoid harming river ecosystems where they naturally exist and create pipes and tanks where it would be the least damaging to the ecosystem

Pumped Hydro doesn't have to be built on a river - you just need a pair of reservoirs that you can fill and transfer water between. This obviously destroys the ecosystem of the valley you use, but at least you're not impacting river water temperatures/sedimentation/fish spawning/etc.

It does not, in fact, need to destroy any ecosystem.

No matter where you put the few cubic kilometers of water that pumped hydro requires, you need to put it somewhere. The ecosystem of that place will then be displaced or destroyed.

It does not need more than the tiniest fraction of a cubic km of water.

A reservoir can increase scarce habitat for migratory birds, which is commonly much more valuable than your typical bald hilltop.

> It does not need more than the tiniest fraction of a cubic km of water.

One cubic km is one billion tons. Each ton is ~10kJ per meter lifted, and let's assume you can separate your reservoirs by 100m vertically.

That would give a total system capacity of 1e9 tons x 1e5 J/(ton x meter) x 1e3 meters, or 1e17 Joules.

A kilowatt hour is 3.6e6 Joules, so this gives us ~3e10 KWh. If we're imperfect in reclaiming that energy, we'll actually end up with 1-2e10 KWh.

US annual electricity consumption was ~4e12 KWh/year in 2018 (or ~1e10 KWh/day), so a 1km^3 * 100m installation is probably larger than required to provide a backup for replacing our entire electrical generation infrastructure with intermittent sources 99.9% of the time. Not by a huge amount though! Less than an order of magnitude, IMO.

> A reservoir can increase scarce habitat for migratory birds, which is commonly much more valuable than your typical bald hilltop.

How much is the level going to fluctuate on a daily basis, though? Does it really form useful habitat?

There are plenty of hills much more than 100m high. People will favor using the best places for it.

Efficiency of pumped hydro is always better than 70%, round-trip, and commonly better than 85%.

There is no reason to put all your storage in one place, and great reasons not to. A site moving 0.0001 km³ is useful for utility-scale storage. Of course, pumped hydro will be only one storage method among many.

Birds mostly use the top of the water. Probably, the lower pond is easiest to tailor for multiple use, but "floatovoltaics" are probably better in the upper one.

Check the one near Cape Town, Steenbras Dam. It is a set of two small dams near Cpt that uses a pumped storage scheme. It is currently used to provide load-shedding relieve during the day (cause the state owned entity Eskom is a failure).

So basically, during the daytime, they generate power from it and then at night the pump the water back to the upper dam. As far as I know its been working without any problems. And since the dams are fairly natural and small, and right by the coast (so very little downstream effects), it seems a bit more sustainable. The only wish would be for these mountain ranges to have multiple geographies like this so we can have multiple small pumped storage schemes with as little as possible negatives.