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$/kWh energy storage technologies

svetz

Works in theory! Practice? That's something else
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Ran across this DOE paper comparing different types of energy storage:

• lithium-ion (Li-ion) batteries
• lead-acid batteries
• redox flow batteries
• sodium-sulfur batteries
• sodium metal halide batteries
• zinc-hybrid cathode batteries
• pumped storage hydropower (PSH)
• flywheels
• compressed air energy storage (CAES)
• ultracapacitors

Here's a table of the costs, the 2025 projections are based on current rate of decline rather than any new technology breakthroughs:
Capture.PNG

1588937694803.png

None of them look like they're going to get the cost down to $10/kWh anytime soon, so probably some totally new technology is needed.
 
The CAES cost is surprisingly the lowest... If I read it correctly though, CAES doesn't seem to include "cave" costs and depends on being able to use cave systems.

So, is this something usable in West Virginia for a community utility, or could it scale to a residential system? Even so, historically large scale CAES are only around 50% efficient. This makes sense because in addition to normal conversion losses when you compress the air you're also squishing the latent heat into a smaller volume (e.g., the gas heats up) and that heat is lost as you store it, Since PV ~= nRT, as temperature decreases due to radiation from the storage process pressure must also decrease.

Let's say you excavated your own e-cave under your house. From this ref, it's about 22 cubic meters per kWh, higher pressures would take less volume, but let's just run with that. Keep in mind if you could double the pressure you'd halve the volume (e.g., PV~=nRT).

The average U.S. home is 2687 sqft and consumes 30kWh/day. so, if you restricted the case to just what was under your house and stored 2 days of power, how deep would the e-cave need to be? 22 cubic meters is about 775 cuft, so...

60 kWh x 775 cuft/kWh = 46500 cuft
46500 / 2687 = 17 feet high, so a volume twice the size of your house for two day reserve.

So, how much pressure would be in that e-cave under your house? Is it important?
From this we see increasing the pressure has diminishing returns: 1588942401626.png and...
1588941062442.png
Where 10 bar is 147 psig, typically higher than those steel tanks on the compressors you see at the big box stores.

Interestingly, in the reference listed above, they used the air to push a liquid through a turbine to increase efficiency.
1588941216238.png
 
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The Wikipedia entry has a calculation for a 70 bar system at generating 6.3 kWh/m3. So, assuming the pressure vessels weren't cost prohibitive, the fluid volume for a 60 kWh system is 9.48m3, 2,500 gallons, 335 cuft, or about 7x7x7', or an 8' diameter sphere.
 
One potential method not mentioned would be to use a very heavy suspended weight and a pulley.
This need not be expensive, if you just happen to have a disused vertical mine shaft or a suitable cliff edge nearby.
It might work something like a lift well. If you have a VERY long steep slope, something on rails might be possible ?

The weight could be winched upwards with surplus energy, and return that energy as the weight is allowed to fall back downwards.
This should be about the most efficient mechanical energy storage system possible, and its nothing new.

For centuries, clocks have been powered either by clockwork, or a suspended weight. To rewind the clock just quickly winch the weight back up by hand once a day. Falling weights were traditionally fitted to light houses to rotate the light beacon at the top of the tower, before the age of electricity.

Just looked up the weight of a single cubic metre of concrete, about 5,295 Lbs.
https://www.omnicalculator.com/construction/concrete-weight

One horsepower equals 33,000 pound feet per minute. So our concrete weight needs to rise or fall 33,000/5,295 = 6.23 feet per minute for 1Hp.
One Hp = 746 watts.

A single cubic metre is not very big, and 746 watts is not a lot of power.
It would need to fall roughly 37.38 feet per hour.
Or 444 feet for twelve hours for 8.952 Kwh of storage.
Should be possible to recover 75% to 80% of that in a practical system.
 
Or 444 feet for twelve hours for 8.952 Kwh of storage.
Or geared twice as low and only needing to fall half as far?
And twice as low gearing again halving the fall distance further?

Which obviously there's an upper end limit of what your gearing could be, at some point you would reach a limit where the weight wouldn't be enough to overcome the gearing friction.
 
Or geared twice as low and only needing to fall half as far?
And twice as low gearing again halving the fall distance further?
To fall half as far requires twice the weight, to end up with the same amount of stored energy (think of a lever).

The figures I suggested were just an indicator to give a rough basis of how large the whole thing might need to be, versus stored power.
 
Gravity based storage only makes sense at very large (grid) scale or in a grandfather clock.

One regular server rack battery stores as much energy as a 20 tonne mass raised up 100 metres.
True, but we are looking at alternatives here, especially relative to cost.
Someone here might even have an old abandoned mine shaft on their property.
Even if it was flooded, it might still work !!
 
It makes a whole lot of sense using old mine shafts.
Large power feed should already be near by.
It brings work back to the area.
It makes use of useless holes.
New mines can be utilized at end of life.

I need to do more homework on gravity batteries, lots of land I've came across is mostly sloped. Recently a great spot came up besides being on the N side of a mountain and on a steep slope...

 
I thought gravity storage was interesting until thunderf00t on youtube convincingly debunked it for me.

It doesn't work at very small scale, and as soon as you increase the scale a bit, pumped hydro becomes more attractive. Hundreds of tons requires very heavy rigging equipment, when you could just fill the mineshaft with hundreds of tons of water and pump it up and down.
 
Someone here might even have an old abandoned mine shaft on their property.
Even if it was flooded, it might still work !
I had the opportunity to get one, about 100m (330 feet) deep, but there were other things to spend money on.
Would have made a great storage area, hidden under an out building! Unless poison gas was leaking out of that hard rock !

I wonder about bladders in water, pump air in during peak power generation, recover later as needed. The bladders wouldn't need to be such heavy structures as a high pressure (70-bar/ 1,000psi) air tanks. Deeper water provides higher pressures, and salt water or fresh can be used since the bladder is just air in the water. Many high population areas are near water, meaning storage potential generally correlates with population centres.
 
I was not thinking large scale commercial size.
Just something family sized for off grid.
Like hydro, you need to be lucky in choosing a location.
 
Why is that ?

I would have thought a day or two's worth of stored energy might be quite useful.
Well look at the thought experiment, in the first place it seems reserved for those who can acquire a mineshaft, and then even if you do, is it really worth it to rig up 10 tons of concrete when you could just fill it with 200 tons of water?
 
That would be an interesting option if the mine is already flooded.
How do you stop your bladder from floating upwards ? The buoyancy forces would be immense.

If it's dry, the weight on a string idea would be simpler and more efficient as far as the machinery required goes.
 
I would have thought a day or two's worth of stored energy might be quite useful.
Of course two day's worth of stored energy is useful but the energy density of gravity storage is abysmal and a deep narrow mine shaft makes zero sense.

100m deep, 1.6m across ~= 200 m³ ~= 200 tonnes of water. The head varies significantly with a deep narrow shaft. Pumping water up 100m is not easy. The energy losses would be insane.

A shaft full of water has to have somewhere to go, you need both an upper and lower reservoir.

Let's say an average head of 50 metres. 200 tonnes lifted 50 metres is still only 27 kWh. And that's the full gravitational potential energy of the water mass of which you will only ever recover a small fraction after round trip system losses are taken into account.

Pumped hydro is uneconomic at anything other than very large scale using massive water reservoirs with a fairly consistent head.
 
But what about a weight on a string ?
10 tonnes x 100 m = 2.7 kWh of gravitational potential energy.

Of which the losses would be substantial. You'd be lucky to get 1 kWh of useable storage capacity out of it.

What a monumental pain in the arse for something which a bog standard 100Ah 12 V LiFePO₄ battery costing a few hundred bucks can do day in day out for thousands of cycles.
 
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