Batteries can get up to 95%+ efficiency, pumped hydro up to 85% efficiency so batteries win out in pure energy efficiency. However, when you take into account the economic efficiency then pumped hydro usually comes out very favorable. Battery prices have been falling rapidly over the last few decades, but simply damming up a mid-sized valley can store such a ginormous amount of water that is it hard to compete.

I would imagine energy density to be a bigger shortcoming (gravitational acceleration times height difference) -- especially for typically accessible height differences. Eg for 500m this is something like an order of magnitude lower than chemical batteries (which are themselves presumably at least an order of magnitude lower than gasoline). This would mean that we need reservoirs to be much larger than equivalent chemical batteries.

> simply damming up a mid-sized valley can store such a ginormous amount of water that is it hard to compete.

I would love to see an analysis of whether it is feasible to build enough such large scale reservoirs (and how many we would need) to store an order one fraction of the daily energy needs. (at city/country/world levels)

> This would mean that we need reservoirs to be much larger than equivalent chemical batteries.

Yes we know. It is still cheaper on a cost-per-kwh basis than batteries, by a significant margin.

> I would love to see an analysis of whether it is feasible to build enough such large scale reservoirs (and how many we would need) to store an order one fraction of the daily energy needs. (at city/country/world levels)

No it is not, there are not enough suitable sites in most places in the world to make this work for world levels. That said, it is entirely up in the air if there would be enough mineable lithium to make batteries for similar amounts of storage.

Efficient electrical energy storage at scale is currently unsolved.

I quite like what this startup is thinking. https://www.energybank.nz/. For offshore wind energy storage.

This Energy Bank system could possibly be practical.

Unlike "Energy Vault" (NRGV), a purely fraudulent investment scam. Energy technologies seem to be favorites of frauds (fusion especially so). It seems like nothing is so obviously nonsensical as to attract the attention of regulators.

There are, in fact, far more than enough suitable sites to store as much energy as we could ever care to store. Hydro power generation needs a watershed, but storage really needs only a hill.

But there are lots of different storage technologies, and costs are falling fast, so pumped hydro may be undercut in places.

Lake volume is just that: it's volume, plus the dam, times two for the lower reservoir. Battery storage facilities however mostly consist of maintenance access, scaffolding, temperature control and fire suppression, we don't just dump cells on a big heap and call it an energy storage solution. Pumped hydro is doing fine in density, particularly since we rarely think in volume for large facilities, we think in acres, and nobody would build a battery facility vertically stacked.

Lets take a pumped hydro I know of, https://en.wikipedia.org/wiki/Taum_Sauk_Hydroelectric_Power_.... It's 5,370,000 m^3 of water, 3600 MW·h for 8 hours to empty, 28,800 MW total. Cubic meter of water weighs 1,000 kg aka a metric ton. So 5,370,000 Metric tons of water.

Looking at https://energetechsolar.com/1mwh-500v-800v-battery-energy-st..., they're around .88 LB / AH, so 880,000 LB/MWh. 399,161 KG / MWh. 11,179,036,800, or 11,179,036 Metric tons of batteries. I don't know what the breakdown is but global lithium production was 100,000 metric tons in 2021, which is off by 2 oom.

My math may be wrong, and you do have losses of water from evaporation, but with pumped you can go up and down. But that's 11 million metric tons of lithium and other metals. Water falls from the sky (in most places still). Lithium has to be refined. You can recharge the battery, you can reload the water or pump it the other way. Tomsauk is mostly concrete and water (by volume). You do need almost a mountain for that head, or a mine. Concrete breaks down over time and can be maintained,. Batteries wear out as well but can be rebuilt. Water seems the easiest of the things to replace currently. The only things I can think of that are easier are provably gasses and maybe salt or rock.

This is all napkin math, and I may have missed a decimal some places. But they seem within the same order of magnitude for efficiency. But is it even possible to build batteries that big?

Of course when they got greedy with Tomsauk they ruind a great natural area. I'd love to be corrected on my math and assumptions.