It may seem strange to use energy density in this context but in many ways those are even worse than batteries due to the massive amount of work they require and poor scalability because of all the physical infrastructure and space.
In fact, this exact scheme looks to have been posted to HN in 2014. From my understanding they are looking to re-purpose holes that have already been dug, for the most part, which is a good idea, but if the planned prototypes are only doing 250 kW nameplate with maybe 100 KWH of stored power, there is almost no way it's going to beat a battery plant you can have more or less just built to order already commercially. And said battery plants aren't even remotely good enough at purpose for meaningful energy storage, they're mostly used for what are considered "ancillary services" in the US power markets, like eating or producing reactive power, voltage support, etc, because the modern grid scale inverters can react to grid conditions faster than grid frequencies (50-60 Hz). More rarely they might be scaled big enough to shave the peak off a demand curve for 5-10 minutes to save transmission capacity or congestion.
Keep in mind all this in happening when LNG is basically free except for the cost of moving it, so you can also put in 1-60 MW combined cycle gas plants in short amounts of time with very well understood technology and proven manufacturers.
The McIntosh CAES plant began in 1991 and from my understanding, has been widely viewed as successful. https://www.smithsonianmag.com/innovation/salt-power-plant-m...
That's 110 MWs for 26 hours. Probably not 2.8 GW-hrs of energy storage (they can't sustain 110MWs for all 26 hours), but this plant is probably GW-hr range. I'm pretty sure all of Li-Ion right now is less than 2GW-hrs for the entirety of the USA.
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The 290 MW plant in Huntorf Germany (1978) also is successful.
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There's a 2nd, 300MW+ plant (that's 300MW power for multiple hours. So near GW-hr scale as well). This is the more recent Apex Bethel Energy Center you were badmouthing earlier. Its not fully built yet, but given the history of CAES, I'm optimistic.
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There aren't many CAES plants in the world. But from what I can tell, they are all safe and successful. The one issue is that you need to reheat the air as it leaves the caverns: compressing it underground reduces its temperature. A bit of natural gas is used in this heating process, but not nearly as much as an actual natural-gas plant.
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The "Rail up a hill" project is 50MWs with 12.5 MWhr storage capacity: https://s3.amazonaws.com/siteninja/multitenant/assets/21126/...
50MWs is somewhat small, but there's something to be said about the simplicity and ease-of-deployment of rail energy storage. Those 50MWs were spec'd out with only 7 trains. It isn't too hard to imagine scaling the system up to support more than 7 trains.
I'd love to see people running more of these setups. Especially in places where DC locomotives are cheap (former CIS countries), this could be even more cost effective.
Pumped Hydro energy storage is the #1 energy storage in America, and probably the world. A single plant provides 24GW-hrs of energy storage, dozens of more energy storage than all American utility scale Li-Ion batteries combined.
Bath County is the biggest pumped-hydro battery in the USA (and probably the world), but there are dozens of pumped-hydro plants across the country.
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Turns out that all the water in a lake can hold a huge amount of potential energy through gravity: just pump it up a mountain.
Actual mechanical pumping.
Free as in money, costly in consequences. The whole point of energy storage schemes is to allow us to migrate away from fossil fueled energy.
Efficiency would be low, at about 70% * 60% ~= 40% but capacity would be huge. 1 kg of hydrogen has specific energy of over 140 MW.
We could even capture CO2 as well with that energy and make methan and use normal gas powerplants for retrieving the power :)
Just build double the number of solar panels to account for efficiency loses. Should be cheaper than grid-scale energy storage with 90%+ efficiency.
Pumped is great when you have the geography for it but most countries don't. And building dams is pretty harsh on the environment too, just in different ways than CO2 emmisions.
Germany has enough gas well storage to survive several months. This infrastructure is currently used for natural gas, but it can be repurposed for hydrogen.
I still think that commoditized battery storage will win in the end, but storage cost is not an argument against hydrogen.
For small and medium scale storage you are right. Which is why hydrogen for cars and even trucks is a bad idea.
Hydrogen at 0°C and 100 Bar (~10MPa) has a density of 8.3447 kg/m^3. It has an energy density of 120 MJ/kg. So about 33 kWh/kg. So you end up with 278 kWh/m^3.
Now of course you have to multiply this by the H2->Electricity efficiency. Let's be very pessimistic and take 0.6 or 60%, which is what a gas turbine plant can achieve today.
You end up with 166 kWh/m^3 of usable electricity per cubic meter.
One of many german gas storage facilities https://www.nafta-speicher.de/en/company has a volume of 1.8e9 m^3. That translates into 300 Terawatt-Hours of storage capacity for just this one facility.
It is not a limitation of the large storage facilities, which are just exhausted natural gas wells and work just fine with hydrogen.
The cost of outfitting one such facility with steel pipes that are not subject to hydrogen embrittlement is trivial compared to the cost for the electrolysis units etc.
Note that I am not a big fan of hydrogen at all. But large scale storage is one of its few redeeming qualities.
IIRC there was some study on doing something like that in Germany in the 2000s but I'm not finding references to it now. If you don't have to build the dam (since it already exists), pumping water back up into the reservoir is going to be better in the limited locations you can do that, and given the relatively tiny amount of energy storage vs power production I suspect the economies of scale on that won't change without a preceding sea change in global commitment to fossil fuel retirement.
There is also a better transition path to that as we already are building amazingly good gas turbines that could just as well be burning synthesized or biogas on demand from solar or nuclear over-generation. I sort of suspect that may end up being politically poisonous to the people that would normally be pushing for carbon-neutral storage options.
It would be way more than double the solar panels though. Solar generation is generally maxing at around 25% capacity factor, so replacing an existing fossil fuel generator requires 4x or more the nominal capacity in solar plus whatever efficiency loss in storage, so going with a pure solar + storage replacement of a 1500 MW combustion plant could easily require 12,0000 MW of equivalent generation.
We're comparing energy storage so no matter how much fossil fuel powerplants we replace - it would be x solar panels for energy storage with 80-90% efficiency (for example this gravicity stuff or hydro or batteries) and 2x for methane synthesis.
Reusing abandoned mine shafts could have an advantage. But there is also ample researching showing that the cost of digging from scratch is still worth it. The US Dept of energy even studied it back in the 1980s when they were evaluating underground pumped hydro energy storage. I wrote a white paper on it that's linked from here. see here https://www.terramenthq.com/uphs/
The key thing here is that adding 10x height gives you 10x more PE per weight without extra digging costs (or tower costs assuming the cost per unit of height is constant)
Here's how to think of it: by digging one deep hole instead of many shallow holes (with the same amount of digging) you get (n^2 / 2) instead of (n/2) Potential energy. The average height of all the modules is half the height of the shaft. With one deep shaft, the modules pass through the same excavated volume so they can all go deeper on average.
Edit: reviewed the Terraent slides they indeed plan to literally drive a mile long train vertically into the ground. In which case the quadratic argument stands.
At the cost of significant added complexity. It comes down to drilling cost vs installation/ maintenance cost.
But also, getting your quadratic gain on a mile deep hole requires a mile of weights to lower into it. That's a lot more above ground infrastructure than just the single shaft!
The way our design works, the modular weights are autonomous on a track. So we don't have huge cranes above ground. We just have a track above ground that also runs a mile. That track can be partly or fully buried if desired. It will be enclosed protecting it from weather, and solar can be installed on top to save real-estate.
But yes, these installations are enormous. They might cost around $150M and could provide about 200MW.
- Keeping water out of a hole is a solved problem.
- as discussed in other comments here, adding more height gives you more PE for the same amount of weight so it's a very valuable investment that pays profits over 20-100 years for your one time investment.
https://docs.google.com/presentation/d/17FI-jrI9RWS3q7Ng44Yh...
"We plan to roll out our technology in disused mine shafts worldwide."