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Comment by jlevers | Hacker News Reader

I've actually been thinking about the issue of energy storage a lot recently -- I've read a ton about how lithium-ion battery production is exploding (usually not literally), but it seems unreasonable to store enormous amounts of renewable energy in a device that itself has to be replaced after a certain number of cycles. The device used for pumped energy storage -- a tank (to simplify greatly) -- basically never needs to be replaced.

It's really cool to see a feasible alternative to batteries. I think climate change is the single most important problem anyone can be working on right now -- amazing that you've found such a massive lever to pull on this issue.

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> it seems unreasonable to store enormous amounts of renewable energy in a device that itself has to be replaced after a certain number of cycles. The device used for pumped energy storage -- a tank (to simplify greatly) -- basically never needs to be replaced.

It's a fair point but I think that you oversimplify the pumped hydro case. Pumped hydro also has quite some electronic components (turbines/motors), electronics, and other moving parts (valves, overflows). I can imagine some of these components require semi-regular maintenance (hence the maintenance shaft in the diagram on the website of OP).

You'd really have to run the numbers to see which costs more to maintain in the long term.

That's a good point. I think it seems likely that given the relatively larger storage capacity per unit[0] with pumped hydro vs. batteries, that the overall maintenance costs -- including the environmental costs of materials needed -- would be lower, but you're absolutely right that we'd have to do the math to know for sure.

[0]When I say "per unit," I just mean that a huge battery is made up of many cells that will need to be replaced individually, whereas large pumped hydro facilities are still only a small number of total reservoirs.

There IS the cost of maintaining pumps systems too, which isn't terrible, but it's not zero either.

Thanks! Yes, and our research agrees that the short-life of Li-ion is a problem, and it's one of the reasons why we believe our solution has so much more promise than Li-ion for grid scale storage.

The pump/turbine technology we use is the same that's been used for a hundred plus years for traditional pumped hydro dams, and the maintenance cost is very low. The life of a project is 40+ years. And in reality it can be 100 years with relatively low amount of maintenance. The San Diego County research posted on our website has good figures on this. thanks!

Hydro has more loss, which is worrisome. Maybe 5X the loss per cycle than battery?

Interesting, I didn't know that. Are there practical ways to decrease the amount of loss? Do you know what the actual loss percentages are for each?

Hmm, I think what you're alluding to is that pumped hydro is about 70-85% efficient and Li-ion is sometimes quoted at 100% efficient (in theory). But here are some more details.

In reality, when Li-ion batteries are installed in a large system, I believe the round-trip efficiency is quoted much closer to PHS. Sorry, i can't find the best research to cite right now, but here are a couple sources i found with a quick search.

"lithium-based ESS rated for two hours at rated power will have an AC round-trip efficiency of 75 to 85%." https://www.windpowerengineering.com/how-three-battery-types...

https://www.sciencedirect.com/science/article/abs/pii/S03062... "Conversion round-trip efficiency is in the range of 70–80%"

This one says 90-95% https://researchinterfaces.com/lithium-ion-batteries-grid-en...

I've heard that Li-ion installations can get up to 90-95% round trip, which is fantastic, and better than PHS for sure. But it's not the most important detail in the equation. Here's why:

One thing to remember is that power is lost all over the system in conversion and transmission. So raw efficiency can be less important than getting the right capacity to the right place on the grid. And that brings us to cost.

Even though PHS is a little less efficient than Li-ion, 85% for PHS is still really good. (see other my other comment below about 70% vs 85%) And the math shows that investing in PHS is simply cheaper -- even after assuming that Li-ion will drop in price by 3x in the coming decades. This is partly because Li-ion has a much shorter life span and needs to be replaced about every decade.

Li-ion is still great and super important! But it's not looking like the best contender right now for grid-scale storage.

I think the point you just made about the rate of replacement of lithium-ion batteries is very pertinent to solar panels(to the point that many consider solar a scam)

I've never heard that said before, but I'm interested to hear more. Do you have any sources on that? My understanding was that solar panels last quite a long time.

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