- 2 MW of energy
MW is a unit of power not energy, maybe they mean 2 MWh, but if that's the case, it is a joke. That's like 30 Teslas cars, or half of a Tesla Megapack, which is a shipping container sized battery. 2 MW of power is not that big either, that's about what you get from a typical wind turbine.
- 70TWh of energy
At least, we have a proper unit, but I wish they tell us a bit more about how they got a number 7 orders of magnitude larger than the previous one.
- A study last year by the International Institute for Applied Systems Analysis (IIASA)
What study? Source please
But it's an interesting approach despite the bad writing: if you fill a 1400m shaft with a dense chain of buckets (my term, they seem to call them vessels) you'll need a surprising low amount of mass throughput for a given amount of power flowing in/out. Because there is just so much in transit at any given point in time.
A naive implementation would quickly run into limits in rope strength and the like, but the schematic drawing suggests that the system they suggest consists of a chain of shorter loops, each just carrying whatever low number of vessels is easiest to engineer, and handing them over bucket-brigade style. This sounds complicated, but a similar thing is routinely done by gondola lifts with intermediate stations between separate wire loops (but here, handover could presumably be done simpler and in less maintenance-demanding ways because latency is no goal at all). So you'd design a system that works for three loops in a shaft not much deeper than a deep cellar and then scale it out to a very deep mine.
The power in/out is simply the throughput of a single stage times how many of those stages can be stacked in the shaft (or rather: hung to the sides of the shaft on top of each other, they'd certainly not be stacked in the statical sense). And if your battery is ever considered full but you still have extra energy to store, you could always decide to dig more horizontal tunnel, like when the mine was still serving it's original purpose. All you need is room (and transportation) for a larger spoil heap in the sun.
Digression: it's hot down there. Could you, with sufficiently insulated pipes, spin up a geothermally powered loop of air where the hot air carries moisture up skyside? If the sand is drier on the way up than it was on the way down while discharging you could theoretically end up with efficiency > 100. (in reality, more energy will certainly be lost to uninvited water finding it's way down, but perhaps this geothermal harvest could help battle the losses to water ingress)
Still, the approach of stacking many copies of the same loop design, with a well designed handover mechanism remains the way to go, instead of one big elevator trying to achieve throughput by faster travel. The key core idea is that the structure that carries the upper parts of the whole thing is already there (all the ground that hasn't been dug into)
The 70TWh isn’t for this mine, but globally. FTA:
“A study last year by the International Institute for Applied Systems Analysis (IIASA) estimated that gravity batteries in abandoned underground mines could store up to 70TWh of energy – enough to meet global electricity demands.”
(See https://iiasa.ac.at/news/jan-2023/turning-abandoned-mines-in...)
Reminded me of the punchline to this water storage video:
https://m.youtube.com/watch?v=CMR9z9Xr8GM
That guy went mechanical with rocks:
https://www.marketscreener.com/quote/stock/ABB-LTD-9365000/n...
There are tons of equally bad articles about this story, which is sad to see. Quality journalism might not be dead but it sure is outnumbered!
However, reading about the proposed mechanism I might charitably interpret it as being able to deliver 2MW peak power, for however long it has available sand.
So an interesting aspect of this is how much area is available for sand, not just how tall the shaft is. If it has sand for 10 hours of operation it can store 20 MWh, if it has sand for 100 hours it stores 200MWh.
Many questions remain, but if it is more efficient than generating hydrogen, and takes less space than pumped hydro (or can be made in areas unsuitable for pumped hydro) there might be a point to it.
If you are into this thing and looking for an even more stupid idea to store energy, I present to you the StEnSEA [3]. Rolls right off the tongue, right? It is a hollow concrete sphere that is lowered to the bottom of the lake. Pumps then remove water from it, creating a vacuum. Letting the water back in and using the pumps as generators, the energy is reclaimed. Curiously absent from all documentation of this project is the amount of energy stored. I did some back of the envelope calculations a while back and it is 3.8kWh, for a multi-million-euro prototype!
[1] https://www.energyvault.com/ev1
[2] https://www.youtube.com/watch?v=iGGOjD_OtAM
[3] https://www.iee.fraunhofer.de/de/projekte/suche/2013/stensea...
That seems very low. Their website mentions 20 Mwh+.
Though my back of the envelope agrees with yours.
Most proposals are even worse, as they suggest concrete rather than sand. The monetary cost works out even then, but concrete production currently emits CO2, and the combination of that with the low energy density means they'd have to run for around a century to only be as bad as fossil fuels.
https://gravity-storage.com/ are trying to make compact hydroelectric plants by putting weight on the fluid. Wish someone would give them loads of money :/
I wonder if that is part of the impetus - make cleanup/decommissioning happen in distant future dollars helps the balance sheet today.
https://www.marketscreener.com/quote/stock/ABB-LTD-9365000/n...
Potential energy, U = mgh. So the energy required to raise 1m^3 of sand 1400m is 1600 x 1400 x 9.8 = 22MJ = 6.1kWh
I've no idea how large their mine galleries are, but lets say they're 3m wide x 2m high - in 500m of gallery, we can store 3000m^3 of sand, so that's 18MWh.
I'm sure they've got a lot more space than that, but it just gives some idea of how much sand you're talking about.
If you lowered 10 m^3 of sand (61kWh of potential energy), to generate the minimum 100kW power to participate in grid stabilization markets, you'd have to drop that 16000kg of sand for 61kWh/100kW = 0.61hr = 2196 sec. 1400 meters in 2196 seconds is 0.64 m/sec. That seems reasonable, but you'd need a lot of these (so a wide mineshaft) to generate a more meaningful amount of power (like at least 1 MW). Current grid scale batteries are capable of outputting hundreds of MW of power.
https://en.wikipedia.org/wiki/Battery_storage_power_station
> we can store 3000m^3 of sand, so that's 18MWh.
> I'm sure they've got a lot more space than that, but it just gives some idea of how much sand you're talking about.
They're going to need 3 orders of magnitude more space then because current generation grid scale batteries store GWh of energy, and generally speaking lower cost energy storage competes by offering much higher storage capacity.
It's always going to be easier to move water around in an automated fashion, though, so I'm immediately skeptical of any system that isn't some variant of two tanks and a pump/turbine.
If you really do want to use gravity as a power source and don't want to go the hydro route you're better off building narrow-gauge train lines up the sides of hills. The lower-impact and lower-output version of that would be aerial ropeways.
A machine for lifting/lowering loose material is more complicated than a pump, no doubt about that. But a deep shaft would mean that you don't build one machine per shaft, you build a few dozen smaller ones with a good handover mechanism and start getting small serial production benefits right from the first installation. Capacity would be virtually infinite, because with excess energy you could just mine more of whatever stuff is down there.
I guess if it's expensive to switch the dry mass mechanism between directions or speed states, it might be worthwhile to prepare some basin volume up and down and run a small capacity pumped storage in parallel at the same site for higher frequency load changes and short peaks. You might even find yourself discharging the wet battery while charging the dry one or the reverse if there is a sufficient delta in dispatchability.
> The Pyhäsalmi Mine, roughly 450 kilometres north of Helsinki, is Europe’s deepest zinc and copper mine and holds the potential to store up to 2 MW of energy within its 1,400-metre-deep shafts.
But I think 2MWh might be correct - in press elsewhere Gravitricity says 500t over 800m produces 1MWh. This is a deeper depth but not that much, and this is very much an experimental project so I think they might be sticking to 500-750t mass. They anticipate larger systems using multiple masses rather than a single larger one. Still, they've also announced a 4MWh project in the UK, so it's not like they see 2MWh as a limit.
A slightly better article: https://eepower.com/news/gravity-energy-storage-systems-tran...
The company: https://gravitricity.com/
The basic problem is that your heaviest, cheapest weight is concrete or stone which is only about 2.5 times as dense as water. But the cost you pay for that is you can no longer pump or flow it so by definition you're limited.
Gravity batteries are cool. They don't make a lot of economic sense by themselves, but they do if the vertical height already exists and doesn't need to be constructed.
The problem is that you could solve this by say, lowering a small pallet of whatever to the bottom of the shaft and moving it sideways out of the way - trading "power" for "energy storage".
But if you extend that idea you wind up at "pump a liquid" as the obvious way to do it, since that has essentially no limit on flow-ability.
The other problem of course it lead in the first place: 1000kg of lead at 1500m high is about 4kWh of potential energy. 1000kg of Lead-Acid batteries is about 25 kWh of potential energy. I suppose you could put the batteries on a cable for the extra 4kWh but I suspect the complexity isn't worth it.
1. The shaft is already dug, so no need to build a tower.
2. As the article points out, many/most of these sites are already well-connected to the grid.
3. Also as pointed out in the article, this could be a boon to depressed areas and help broaden economic value generation.
Am I missing something?
Not that your wholly unsupported naysaying isn't compelling.
(admittedly on that one I'm not really onboard with "oh lol why are we using wood for a bridge when we have steel" as an explanation, but conversely there were serious problems with engineering design of how wood was used on this bridge and it did collapse in the end - complexity of design versus known elements is an important consideration. A casual observation of "does this really make sense?" might've concluded that stepping so far out of bounds of normal design should have been more carefully treated or had exceptional requirements in the first place).
Cliff's Notes: any currently-considered form of gravity storage that isn't pumped hydro is orders of magnitude more expensive and more stupid than a Tesla Megapack.
Michael Barnard's abrasive tone aside, is he wrong?
It so happens that pumped hydro was considered for the same site but abandoned last year due to the high cost estimate.
https://re100.eng.anu.edu.au/global/
"ANU has identified 616,000 potential sites around the world." (note that not all countries are included in this because of lack of geographical elevation data)
A place like Nevada has an enormous surfeit of opportunities for pumped hydro, due to the Basin and Range geography. Here's a project going forward right now. Look how tiny the basins are for the energy stored:
I actually wondered if it might be possible to reapply the main hoist at a mine for this kind of application, many already have some energy recovery. But I don't think it would be practical in that many situations because these deep mines often have other "tenants" (this one has a cosmic ray observatory for example) that will want to continue use of the main hoist---and of course it is appealing to install this type of system in operational mines when there's a disused shaft.
The problem was the cost of excavation or building the cylinder. Then this came along, not even promoted as the future..
https://www.newcivilengineer.com/the-future-of/future-of-tun...
And it's ideal.as in they could turn testsites into batteries.
You could have such a battery in an any abandoned quary today. Chip the walls smoth, freeze an ice plug, pump water beneath and use the stored pressure.
(An interesting tidbit of trivia I learned from miners is that deep underground mines only became feasible after reliable pumping was put in place.[1] Prior to that, they flooded out from groundwater intrusion.)
[1] https://en.wikipedia.org/wiki/Flooded_mine
Edited to add the wikipedia link.
If the bottom is well above sea level and not in line of any underground rivers | streams | lakes it'll stay relatively dry.
For interest:
https://en.wikipedia.org/wiki/Pyh%C3%A4salmi_Mine
The "main level" of the mine is at 1400 meters depth and can be accessed with either the mine hoist or via the access tunnel. The main level houses a cafeteria, washrooms, showers, workshops, storage facilities, as well as a safety area.
It is also home to the world's deepest sauna, at 1,410 metres (4,626 ft) underground.
Pyhäsalmi Mine has hosted numerous events due to its attraction as a unique location.
It has hosted the deepest concert in the world (by Agonizer at 1271 m ) as well as dance performances.
The 11 km long spiral-shaped main tunnel has also seen several uphill running and cycling competitions.
Pyhäsalmi Mine can be recognized as a filming location for the new sci-fi television series White Wall, premiered in 2020.
Steep bits? Are you thinking of Norway :-) there’s no steep bits in Finland, highest point above sea level is 1324M!
There was even a Norwegian campaign to gift a mountain for its birthday https://www.bbc.co.uk/news/world-europe-37662811
A typo ?
They probably meant 2TWh if it is to "to power the planet"