Another 'Gravity Battery' Question

[kuruman]

According to OP

A 'gravity battery' could be created by using an energy source to slowly lift a large weight (using gearing or pulleys), then discharged by lowering the weight to run a generator when needed. Kind of like a giant grandfather clock.

Don't we already have these? The energy source is the Sun and instead of "gravity battery" we use the name "hydroelectric plant." No gearing or pulleys needed; evaporation and condensation do the trick.

 

[sophiecentaur]

But Sophie, are we on the same wavelength? I was suggesting that the spoil fill the space between the towers as part of the concrete or cement, depending on the rock type. In the model I proposed the walls would be about 7m thick. Together with a bit of strategically placed rebar, they could form a pretty solid block. If OTOH, the shaft drilling process produced powdered rock (as I have seen with drilling for water) then disposing of the spoil might best be done by selling it to the horticultural industry as a compost component (tell them it is natural rock, and as such fully organic; they'll never know the difference... )

The geometry of the cylinders would need to be chosen to suit the strength of the above ground structure. I guess some extra 'strapping' could help to reduce the area occupied.

 

[Jon Richfield]

The geometry of the cylinders would need to be chosen to suit the strength of the above ground structure. I guess some extra 'strapping' could help to reduce the area occupied.

In suitable quarries, abandoned open pit mines and the like, the options for building could be very interesting

 

[Jon Richfield]

The geometry of the cylinders would need to be chosen to suit the strength of the above ground structure. I guess some extra 'strapping' could help to reduce the area occupied.

OK Sophie, I have done a description of my first thoughts on the subject, and unusually for me, I have included illustration. It is too long for here, but you can see it at http://fullduplexjonrichfield.blogspot.co.za/
If that is not an accessible place, please advise where I should post it.
Feel welcome to hoot; I am no engineer!

 

[NTL2009]

OK Sophie, I have done a description of my first thoughts on the subject, and unusually for me, I have included illustration. It is too long for here, but you can see it at http://fullduplexjonrichfield.blogspot.co.za/
If that is not an accessible place, please advise where I should post it.
Feel welcome to hoot; I am no engineer!

I was able to access that just fine. Thanks.

If I read it right, it matches one of my assumptions - you need that lead piston to be sealed, like the piston in a hydraulic cylinder. I am not a mechanical engineer, but I'm pretty good with practical applications, and that seems to be a very difficult task. A large diameter, very long stroke cylinder that needs to hold pressure for hours or days, and be low friction? I think that is asking a lot.

A lead piston would provide the same energy storage as water in ~ 1/10th the volume. But what is the cost of extra volume of water (with no mechanical tolerance issues - any shape will do), versus a machined piston, cylinder, seals, maintenance, and frictional losses? Hopefully, a mechanical engineer can provide a more solid analysis, but I feel pretty confident they will have the same concerns I present.

 

[Jon Richfield]

I was able to access that just fine. Thanks.

If I read it right, it matches one of my assumptions - you need that lead piston to be sealed, like the piston in a hydraulic cylinder. I am not a mechanical engineer, but I'm pretty good with practical applications, and that seems to be a very difficult task. A large diameter, very long stroke cylinder that needs to hold pressure for hours or days, and be low friction? I think that is asking a lot.

A lead piston would provide the same energy storage as water in ~ 1/10th the volume. But what is the cost of extra volume of water (with no mechanical tolerance issues - any shape will do), versus a machined piston, cylinder, seals, maintenance, and frictional losses? Hopefully, a mechanical engineer can provide a more solid analysis, but I feel pretty confident they will have the same concerns I present.

Thanks Sophie, I am glad you got it.
Think of an internal combustion piston. In one respect it would resemble our steel-jacketed lead piston as described, in that it would be only modestly precise, and the final fitting would be made precise by the piston rings.

In other respects there would be discrepancies.
The piston rings would be many cm wide, and lack a gap in their circumference.They would be polymer and stiffly elastic. Think nylon or teflon or UHMWPE (name your poison!) they would self-lubricate in contact with the cylinder wall. Instead of the gap, they would be elastically fitted into their grooves, and would be (comparatively!) gently squashed to fit into the cylinder. There would be a couple of them per metre at a guess, so that a ten-metre piston would have a couple of dozen acting as seals in series. They would be bridging a gap of perhaps 1 mm, possibly 2 mm (ask the plastics engineers) and MIGHT contain reinforcing fibres in a matrix, though I doubt the need.
Note that the cylinder, if steel-lined, would indeed have to be polished, but not very precise; it would be maintaining contact with flexible plastic, so needs to be smooth, but can have some give and tolerate temperature variations.
And it is not clear that the lining need be steel; I'd prefer a suitable polymer, but am unsure what the polymer engineers would say. If that were acceptable, the permissible tolerances would be ridiculously large for a metal-working engineer.
Frictional losses? I hardly think so! Especially if the choice of fluid is suitable. But I reckon that even running dry, the problem could hardly arise with self-lubricating material.

Maintenance? Of what? how many million passes of such a piston would it take to make a detectable difference to a solitary plastic ring, let alone the lead or steel?

The cost of the water as such is not a crucial parameter, but the cost of the material and equipment for handling the water is altogether a different matter. I gave a thumbsuck for the area occupied by battery of cylinders capable of raising 1000000 tonnes of lead 100 or even 200 metres. Note that you would need, not 1000000 tonnes of water, but about three times as much mass and thirty times the volume, with far poorer consistency of pressure and efficiency. You would pretty soon be asking yourself whatever happened to pumped storage dams. And where we would get conveniently sited, unused dams of suitable sizes in or near cities. For this type of lead-weight scheme a few disused quarries or oil-tank farms would suffice.

But sure, anyone with doubts on those points, please, shoot!

 

[Ephemarel]

So, I've been interested in this idea every since... probably a year ago, when I saw this:
(The part you're interested in is 7 1/2 minutes in)



I googled it yesterday, and found this forum thread, and also something that it seems you guys haven't heard of yet. Check this out, it's the same type of thing you're discussing: http://www.gravitypower.net/

And they're building a demo plant in Germany. This was only posted two weeks ago, so I think this is all pretty recent:

 

[Jon Richfield]

So, I've been interested in this idea every since... probably a year ago, when I saw this:
(The part you're interested in is 7 1/2 minutes in)



I googled it yesterday, and found this forum thread, and also something that it seems you guys haven't heard of yet. Check this out, it's the same type of thing you're discussing: http://www.gravitypower.net/

And they're building a demo plant in Germany. This was only posted two weeks ago, so I think this is all pretty recent:

Gosh! Well thanks for that; I had wondered why No-one had come up with the idea before; simply because Some-one had scooped him.

Interestingly they seem to be satisfied with a far less dense and compact slug than I had considered, and they have been pretty cagey with technical specs like sealing and size parameters, but if their trucks in the installation are anything to go by, they are pretty cheerful about excavation costs and compactness!

Well, I wish them luck, since I have no intention of building any of my own versions, and though I prefer some of my own ideas, at least I now can link to this site without folks thinking I am nuts. Maybe their next generation will use lead mushrooms over open tops

 

[CWatters]

This also in Germany..
http://www.ecowatch.com/coal-mine-hydroelectric-2321724350.html

Basically it's a conventional pumped storage system but the lower reservoir is the bottom of a mine and the upper reservoir on the surface.

 

[sophiecentaur]

This also in Germany..
http://www.ecowatch.com/coal-mine-hydroelectric-2321724350.html

Basically it's a conventional pumped storage system but the lower reservoir is the bottom of a mine and the upper reservoir on the surface.

I wonder how stable the lower reservoir would be. Without some serious extra bracing, I could imagine that the constant filling and emptying with water could be stressful and cause roof collapse. Would the lower reservoir be lined, I wonder.

 

[CWatters]

+1

At the very least they would have to mitigate the scouring effect of fast moving water?

 

[nitsuj]

I wonder how stable the lower reservoir would be. Without some serious extra bracing, I could imagine that the constant filling and emptying with water could be stressful and cause roof collapse. Would the lower reservoir be lined, I wonder.

True!

it might even stir up stuff not wanted in water, maybe itll be a closed system.

hydro electric from tides is also a neat use of the gravitational potential of water, and has less impact than traditional hydro dams

 

[Jon Richfield]

True!

it might even stir up stuff not wanted in water, maybe itll be a closed system.

hydro electric from tides is also a neat use of the gravitational potential of water, and has less impact than traditional hydro dams

Yes,one thing to bear in mind is that for any such scheme to be of any use, it needs to be BIIIG. Big implies emergent problems of intensity, such as indeed stress, strain, scouring, creep, cracking, clogging, you name it.

However, I am not too nervous in principle; all those are problems familiar to structural, chemical, and mechanical engineers.

Personally I am not keen on water as the working fluid, though it has undoubted advantages and properly managed can be used in sealed or semi-open systems, and in suitable sites such as by rivers or seaside, might be used in fully open systems, but I really like the idea of oils better. But one cannot decide on such points out of context.

 

[Jon Richfield]

So, I've been interested in this idea every since... probably a year ago, when I saw this:
...

I followed up your links with more searching, and it seems that there are several people with similar ideas, and quite a few companies either promising or working on such schemes. I must say, some of them I would never trust with my money, no matter HOW well-meaning. Some are too complex, some too simple, some simply too confident... And a lot of the ideas looked worryingly one-dimensional. Still, that might be helpful in getting a commercial idea off the ground...

But some might have a point.

One question for all the physicists and engineers out there. One presenter stated repeatedly that in a column of water the amount of energy increases with the square of the depth. Now I in my physical naïvite was under the impression that the energy would be a function of mass X height, the height through which the mass was raised or dropped. And that would increase linearly with height, not quadratically.

OK, so you could argue that in an open system the water column could be regarded as a the sum of a stack of masses times their successive heights 1+2+3+4... which is roughly quadratic, though, as I pointed out in my essay, it means that the water column is pretty useless near the bottom, but this same guy was talking about a closed system with massive pistons. On drawing off energy the output water was returned to the top, so that the net energy output was purely that of the pistons' rate of sinking -- strictly linear mh.

Does it seem likely that he should have stuck to running the company and left the explanations to the engineers? (Think twice before playing it; the reference is pretty far in!)

Or have I missed something?

 

[sophiecentaur]

Now I in my physical naïvite was under the impression that the energy would be a function of mass X height, the height through which the mass was raised or dropped.

The h² protionality assumes that you are getting energy out of the whole of a vertical column of water which could be emptied to the bottom. So the Energy available per m³ will depend on what height the water is coming from - you get more from the top bits than the bottom bits.
The result of integrating all the contributions the total energy will be proportional to h²/2 in the same way that the energy stored in a capacitor is qV²/2 and the energy stored in a stretched spring is kx²/2. Sneaky, huh?
The bottom few metres of the column are not much use to you but they would only be used when the stored energy was getting desperate, anyway.

 

[Jon Richfield]

The h² protionality assumes that you are getting energy out of the whole of a vertical column of water which could be emptied to the bottom. So the Energy available per m³ will depend on what height the water is coming from - you get more from the top bits than the bottom bits.
The result of integrating all the contributions the total energy will be proportional to h²/2 in the same way that the energy stored in a capacitor is qV²/2 and the energy stored in a stretched spring is kx²/2. Sneaky, huh?
The bottom few metres of the column are not much use to you but they would only be used when the stored energy was getting desperate, anyway.

Thanks Sophie. That is in line with what I had been thinking must be his line of reasoning. But then he was trying to have his cake and eat it; in a piston-driven closed system in which the water that emerged from the turbine went back into the top of the cyinder, the net output of the water's potential energy is notionally zero, and only the rise and fall of the piston is significant. To recover potential energy from the water, you would need water flowing in from the top and being discarded from the bottom. In a closed system it is NOT discarded.

I suspect he was talking like a salesman, not burdening the prospect with facts he didn't want to know
But then I was a bit shook that no one took him up on it.

 

[OmCheeto]

Here's an interesting article I found just now:

A brief review of underground coal mine energy storage

Posted on March 20, 2017 by Roger Andrews

As reported in last week’s Blowout the latest solution to the problem of storing intermittent renewable energy for re-use is to convert an underground coal mine into a pumped hydro facility. The mine in question is Prosper-Haniel in the Ruhr region of Germany, which is scheduled to shut down next year. ​

A lot more information about the mine than I've seen anywhere else.
And there's lots of VERY interesting commentary in the commentary section. Website host: About Euan Mearns
Author of the article: About Roger Andrews

 

[Jon Richfield]

Here's an interesting article I found just now:

A brief review of underground coal mine energy storage

Posted on March 20, 2017 by Roger Andrews

As reported in last week’s Blowout the latest solution to the problem of storing intermittent renewable energy for re-use is to convert an underground coal mine into a pumped hydro facility. The mine in question is Prosper-Haniel in the Ruhr region of Germany, which is scheduled to shut down next year. ​

A lot more information about the mine than I've seen anywhere else.
And there's lots of VERY interesting commentary in the commentary section.Website host: About Euan Mearns
Author of the article: About Roger Andrews

Not surprisingly, most of the inside of the mine is a very messy place, not having been designed for energy storage, and acocrdingly not very well suited. It could work, but naive figures based on the gross volume would be misleading. And a lot of the fluid pumped is likely to be slurry, unsuited for power generation. Going down to refurbish the mine for water storage would be tricky and expensive. I reckon that a deep mine would be particularly attractive as a place to build the sort of power cylinders discussed in the foregoing discussion, and I have elsewhere remarked on them as places for storing compressed air, but I have my doubts about the simple water storage. Anyway, deep mines need constant maintenence, which would not be easy if they were intermittently flooded.

 

[sophiecentaur]

deep mines need constant maintenence, which would not be easy if they were intermittently flooded.

As far as I am aware, the coal mining process works in a very unstable situation with an advancing coal face and the spent volume behind the mining equipment is just left to collapse. Flooding (to the roof) of a volume like that would probably lead to more and more roof collapse unless the original mine had been established differently.
I can see why a 'wasted hole' would be crying out to be used but whether or not it's worth while would depend on a lot of annoying practical details.
The first mine-full of water would represent free energy, too.

 

[Jon Richfield]

...Flooding (to the roof) of a volume like that would probably lead to more and more roof collapse unless the original mine had been established differently.

Right. Individual mines do in fact differ, and a lot of the side channels would not be flood-able, but would trap air, and that is the sort of reason why I reckoned that the main vertical shafts (or indeed abandoned open-pits) would best be occupied with piston cylinders.

...The first mine-full of water would represent free energy, too.

True, if it started empty .
I sometimes wonder whether some of our horrible underground coal fires (we have some in South Africa) might not invite some sort of related treatment... I haven't been able to come up with any material connection.

 

[NTL2009]

I think this was posted earlier -

www.youtube.com/watch?v=CujxJFXwOns

Some interesting points, obvious after a bit of thought:

The system is optimized (mass times travel) with the pistons taking up 1/2 of the depth of the shaft.

If you double the depth of the shaft, you get 4x the energy storage (2x the travel, and 2x the mass since you add piston mass as well). So keep drilling down until the cost is > 4x per volume going wider.​

Not sure if they mentioned what the pistons are made out of, my rough calcs say they are more dense than concrete. He mentioned that they add brakes to the pistons, so when the system is at rest, the pistons are held in place to reduce seal leakage. He also mentioned ( ~ 35:00) that each piston would move up/down sequentially, with bypass valves in each piston. It wasn't clear to me what the advantage would be - maybe keeping the brakes on all but one when active? He mentions losses due to friction and seal leaks.

He shows a 10M diameter by 2000M deep shaft providing 150Mw x 4 hours = 600 MWhr, and ~ 20% losses. How do those costs compare to peaking with NG?

 

[Jon Richfield]

I think this was posted earlier -

www.youtube.com/watch?v=CujxJFXwOns

... brakes to the pistons, so when the system is at rest, the pistons are held in place to reduce seal leakage. He also mentioned ( ~ 35:00) that each piston would move up/down sequentially, ... wasn't clear to me what the advantage would be - maybe keeping the brakes on all but one when active?
...

Your reactions in general are similar to mine. I neither like their version of the closed loop, nor the design of the brakes and the complications of the pistons, nor of multiple pistons. The whole thing looks like a messy design. And neither very scaleable nor very compact, in spite of his claims.
Sure it is a superficial judgement on a superficial presentation, but I was shaken when I saw how many presentations of rival systems there were on Youtube, and the few I looked at mostly looked better to me without any fish of my own to fry.

 

[NTL2009]

Your reactions in general are similar to mine. I neither like their version of the closed loop, nor the design of the brakes and the complications of the pistons, nor of multiple pistons. The whole thing looks like a messy design. And neither very scaleable nor very compact, in spite of his claims. ...

Actually, I didn't mean for any of my comments to be judgmental. I don't like nor dislike any aspect of it, I'm just trying to better understand it.

Brakes make sense to me - that eliminates the pressure on the seals when it is in idle storage mode.
Multiple pistons make sense to me - less of an alignment issue. Probably easier to construct, move to the site, and install. We are talking about ~ 1000 Meters of piston in the 150 MW / 600 MW-h design.

But I didn't understand why he'd move one piston at a time, rather then letting them all rise/fall together. Though I suppose with differences in leakage, they would end up with gaps after several cycles, so they might need bypass valves anyhow to correct for that, so just run it that way?

Why do you see scaleability as any more or less of an issue compared to similar designs?

 

[Jon Richfield]

...Brakes make sense to me - that eliminates the pressure on the seals when it is in idle storage mode.

The objective of the brakes is fine, whether one piston or many; but brakes as hinted at (plus the internal mechanisms and requirements for power inside the pistons plus comms for controlling them) raise ugly problems for ugly designs, and they raise ugly problems of wear and stress on the walls. It is better to put control and maintenance where it is accessible and base the static state on a locking mechanism rather than a stress/friction mechanism.

Even his weeny baby piston slices are huge and getting at any but the top one after the installation has been brought into operation is barely conceivable. We are talking about umpteen thousand tonnes each! More like tens of thousands of tonnes each for serious designs like the bigger ones.

Rather have a single dumb piston that can be assembled or disassembled from the top down and (notionally at any rate) left in place till we have forgotten how it works. Put the controls outside the cylinder. The only communication with the slug would be pumping in or letting out the fluid or stopping it when it is neither accumulating nor delivering power.

Instead of brakes, let static detents in the form of say, rectangular steel beams recessed radially into the cylinder walls at appropriate heights, so spaced that the piston's seal rings would be sufficiently wider than the recesses are high, to avoid leakage as the pistons pass up and down (there are alternatives, but this should be a viable approach). The detents would be controlled and powered from outside the cylinder and in the event of any need for access for maintenance, could be removed and replaced from outside as well. Unlike brakes they would work statically (perhaps with some sort of shock-absorbing mechanism; even at the immensely slow speeds in question, one does not simply say "whoa!" to a 10000-tonne mass. ) But then, valving the fluid flow would offer very fine control indeed, so it might be simpler than manoeuvring a 10000-tonne ship in harbour ) And the detents could be designed to remain passive either when engaged or retracted.

Whether they would be designed to engage automatically when there is an emergency, I'll not guess at this point. I suspect that for really large pistons multiple levels of engagement of multiple detents would be sensible -- but details, details...

...Multiple pistons make sense to me - less of an alignment issue. Probably easier to construct, move to the site, and install. We are talking about ~ 1000 Meters of piston in the 150 MW / 600 MW-h design.

There we diverge. A far better approach is a single piston, probably in the form of a steel jacket filled with modules from the top down, effectively a solid mass, but easily assembled and maintained with a computer-controlled gantry. If the cylinder is an open or semi-open system, then it would be possible to design the floating piston with an extension out of the top, either linear or "mushroom-shaped", making nonsense of the 50/50 piston/fluid space "optimum". In fact, we then could have a 100%/100% cylinder utilisation, or nearly.

...But I didn't understand why he'd move one piston at a time, rather then letting them all rise/fall together. Though I suppose with differences in leakage, they would end up with gaps after several cycles, so they might need bypass valves anyhow to correct for that, so just run it that way?

Well... I suspect that it is the consequence of a basically bad patch for a basically bad design.

Why do you see scaleability as any more or less of an issue compared to similar designs?

Depends on what you mean by "similar".
For one thing an open or semi-open design starts its scaleability with the assembly of the piston,but.
And by permitting combinations of digging (or existing pit utilisation) with cylinders that project from the ground, and not getting hung up with rosettes of pistons, but arranging them in arrays, possibly daisy-chained, you could achieve far higher density of energy accumulation and delivery than the compactness he suggested in his talk.

 

[NTL2009]

... but brakes as hinted at (plus the internal mechanisms and requirements for power inside the pistons plus comms for controlling them) raise ugly problems for ugly designs, and they raise ugly problems of wear and stress on the walls.
...
Instead of brakes, let static detents in the form of say, rectangular steel beams recessed radially into the cylinder walls at appropriate heights, ... And the detents could be designed to remain passive either when engaged or retracted. ...

I feel you are reading far too much into that "hinting". I see nothing to indicate that they are not considering a detent type system, and perhaps just used the term "brakes/braking" generically. These are design details, and yes the devil is in the details, but they aren't getting down to that detail level in anything I've seen. I would not assume they are making poor design decisions. The guy does have some credentials.

There we diverge. A far better approach is a single piston, probably in the form of a steel jacket filled with modules from the top down, effectively a solid mass,

I think that separate pistons versus 'modular pistons' is just semantics. The only real difference is whether they move together or individually as described in the video. I didn't catch any details on why he wanted them to move independently, but I will assume this has been thought through and he has reasons for it.

If the cylinder is an open or semi-open system, then it would be possible to design the floating piston with an extension out of the top, either linear or "mushroom-shaped", making nonsense of the 50/50 piston/fluid space "optimum". In fact, we then could have a 100%/100% cylinder utilisation, or nearly.

That depends on what you use for your denominator! If I'm following you, with an 'open system', you need some other place to pump the water. That should count in the utilization number.

For one thing an open or semi-open design starts its scaleability with the assembly of the piston,but.
And by permitting combinations of digging (or existing pit utilisation) with cylinders that project from the ground, and not getting hung up with rosettes of pistons, but arranging them in arrays, possibly daisy-chained, you could achieve far higher density of energy accumulation and delivery than the compactness he suggested in his talk.

I would imagine some very real difficulties with increasing mass above/outside the well. They are talking a 2000 meter deep well, extending that by just 10% would be a 200 meter tall projection. That is quite a structure to support! And to be mushroom shaped, it would either need to be higher then the full depth of the well, or detach as the piston sinks. Either way, a mushroom shape is going to make that projection top-heavy, making the support issues even more challenging. Is it worth it for 10%? And if you go very wide for that mass, you are talking about a large mass being supported at the center, that's is challenging as well.

 

[Jon Richfield]

... I see nothing to indicate that they are not considering a detent type system..

True, if one is being charitable, though it certainly does not jibe with the impression I got.
"...nothing to indicate..." Hm? Use of the word "brake might not be much, but it isn't nothing...

I think that separate pistons versus 'modular pistons' is just semantics. The only real difference is whether they move together or individually as described in the video. I didn't catch any details on why he wanted them to move independently, but I will assume this has been thought through and he has reasons for it..

It is a heck of a lot more than semantics; it is straightforward terminology. And a matter of design. Separate pistons are pistons plural. A modular piston is a piston that can be put together in manageable chunks, transported in chunks, and used as a unit. You want to use two modular pistons, fine, but then each of them has its own modules. But that is not the point of the criticism. I regard the intent to separate the pistons with reserve, but if they insist, let them have fun. What is more important is that they seem to think that makes it smart to have smart pistons with comms and moving parts. Big mistake!

I suppose that the reason they don't like a single piston might be because a piston with maximal height would be technologically challenging to design and maintain. Fair enough, say I, but to insist that the pistons therefore must have a passage through its inside plus internal controls and coms just leaves me breathless. Whatever happened to KISS? Dumb turtles all the way down, say I!

That depends on what you use for your denominator! If I'm following you, with an 'open system', you need some other place to pump the water. That should count in the utilization number.

Unlike Sophie, you seem not to have read the essay I wrote. It includes diagrams to illustrate such options. Of course the water (or other fluid) goes elsewhere; it must go elsewhere in any design, either open or closed, one piston or more, unless you work electromagnetically in a vacuum (which of course would have its own charms, but... ), including in a closed system, in particular the one I am criticising. The 100% simply means that 100% of the fluid gets usefully displaced in discharging the accumulated energy.

And puristically, also that the mass of the fluid itself contributes to the output. In the closed system it does not, because it has to be pumped up again during offtake. In at least some types of open systems, ie where the fluid output can go somewhere no higher than the bottom of the column, or out to sea, perhaps. You can in principle do that irrespective of the piston size in such a system. However, it takes a huge mass moving through a considerable height to deliver a gratifying amount of power, so a small piston would be unrewarding, instead one wants a piston that can fill the cylinder in the discharged state, and you want to lift it out as far as possible when charging it.
All proper engineering and costing considerations being favourable of course.
And thereby hangs a piston. For instance:

I would imagine some very real difficulties with increasing mass above/outside the well. They are talking a 2000 meter deep well, extending that by just 10% would be a 200 meter tall projection. That is quite a structure to support!

Quite right, except that the 10% applies to the height. If we can dig our 2km shaft for free, it would be a bit silly even by my standards to build any superstructure at all, especially as a cylinder in sound bedrock would have all sorts of attractive features such as assistance in resisting internal pressure. But one thing that all the chaps I have listened to on Youtube (not only this guy) insisted on was that building, well, digging, such shafts was cheap, easy, and standard technology.

OK, no doubt they know their stuff, but take it from a layman, it isn't free, you get only one chance to build it right, and the deeper you go, the more it costs. Does this mean it is wrong, or even wrong-headed?
Not necessarily, but it does mean that there is a trade-off between building high and digging deep.
What the trade-off curve looks like, presumably any structural civil engineer could tell me, but it is neither vertical not horizontal (nor, I am inclined to bet, linear! )
But there is more to it than that, as I am sure you have noticed. Let's have no superstructure; just fill your 100m or 200m or 2000m hole with solid piston and pump it up with water so that it gets to a point where supporting the free-standing column above ground starts becoming expensive. OK?
Does that sound attractive?
Raising a 10m piston 100m doesn't seem challenging. It is stiff and has a modest aspect ratio. 100m is only about the height of a 30-story building.
200m? Still not too bad. 2000m? mmmwelll... maybe, but...
But at what depth and height will the digging and building independently begin to become too expensive? Notice that the bottom line for energy storage is at all times a function of mXh. For an open or half-open system it makes no difference whether we have 1000m down and 100m up or the other way round. The rational question is how much of up and how much of down in combination is profitable for the pistons envisaged.
PERIOD.
Never mind whether you add 10% at the top or 200% at the top; each % up or down must remain profitable.
And one thing that matters to maximise profit, is to maximise the height that the piston moves through, subject to costs.
And that you can increase by not limiting the sweep to the length of the piston.
Right?

And to be mushroom shaped, it would either need to be higher then the full depth of the well, or detach as the piston sinks. Either way, a mushroom shape is going to make that projection top-heavy, making the support issues even more challenging. Is it worth it for 10%? And if you go very wide for that mass, you are talking about a large mass being supported at the center, that's is challenging as well.

Here I think you are getting your percentages out of proportion. I suspect that the most profitable depth would indeed be such that it is practical to sink the piston right to the bottom (though I do not insist).
I strongly suspect that the minimal profitable length of the piston would be the depth of the hole, with due allowance for how far it might be practical to raise the shaft. The cylinder might extend only to the surface, or might extend for a a significant height further (in fact, it might not be buried at all) but in any case, if the piston rises well above the cylinder, we might need some sort of stays, struts, or other anti-toppling structure, especially if the piston is not effectively monolithic.
But how much to increase the projection of the piston out of the hole? As far as you can do so safely and practically, because every metre is that much more yield, whether that part of the length ever enters the cylinder or not.
But if it does NOT enter the hole, and bearing in mind that making it go too high makes some folks nervous, then why should it be hole-shaped?
Suppose the cylinder is 10m in diameter. Suppose that the piston, say 2m above where the hole starts, changes its diameter from 10m to 50m and extends upwards for a modest 40m. (choose your own figures of course! I am just illustrating) that mushroom head would be equivalent to an extra hole depth of 1000m no? Full of piston too.
And with that aspect ratio it would resist toppling quite strongly, right?
And if it were to have a conical hollow beneath like the punt of a wine bottle, still with the same mass, you could increase the stability even more.

Over!

 

[NTL2009]

I'll formulate a fuller reply later when I have more time, but for now:

I'm just thinking that adding a 'top' to that piston is going to put a lot of demand on the seals. What kind of PSI would that be getting to? And maybe that is why he is running each of his pistons independently - the seals 'only' need to work against the weight of that piston?

I did read your blog, but forgot about that over the course of looking at the other links here. I will review it before I get back to you - thanks!

 

[Jon Richfield]

I'll formulate a fuller reply later when I have more time, but for now:

I'm just thinking that adding a 'top' to that piston is going to put a lot of demand on the seals. What kind of PSI would that be getting to? And maybe that is why he is running each of his pistons independently - the seals 'only' need to work against the weight of that piston?

I did read your blog, but forgot about that over the course of looking at the other links here. I will review it before I get back to you - thanks!

Mmmm... yes. I hadn't thought of the seals on the sub-pistons' through-flow. I still detest the idea of smart pistons, but I reckon that you are right and that the idea behind it might be that the cost of adequate valves in the sub-piston could be a limiting factor to how high a sub-piston should be.

As I do not know what the factors are that limit economically workable pressures in such a system (such as the pressures beneath say a 1-km-long piston of lead, which would exert a pressure of some 1000 bars, which sounds nice, but challenging) one approach certainly would be to segment the piston,say into 100m segments, raised one at a time, (and constructed and installed one at a time) which would be much more manageable at 100 bars, while nominally yielding the same output.
However, one doesn't need all that internal bypassing kerfuffle in the segments. Make each segment of a hollow steel shell filled with lead slices as described elsewhere. (This means that it becomes feasible (if expensive) to access each segment from the top if some disaster affecting the bottom segment demands their removal.)
Instead however, given that the length of each steel-jacketed lead segment would be standard, a recess round the bottom edge of the piston, sufficient to accommodate detents from the cylinder wall, plus inlet/outlets into the power fluid ducting to the control turbines, would permit full usage of any desired (permissible) combination of segment pistons without any fancy internal valves or coms. It also would permit dealing with local sealing failures, though not with jamming, though no events of that type should be possible.
Note though, that though stacking of segments would not forbid the use of an open-topped system (though a mushroom would be a lot trickier!) stacking might well limit how far out of the cylinder one could extrude the stack. OTOH, it could permit certain kinds of maintenance work on extruded segments, not that I would like to undertake or underwrite such work myself

As for the pressures required for really large cylinders, they are functions of the effective height of the storage medium and could be varied within wide ranges by suitable design. And the amount of work output would vary with mXh. This leads to very simple calculations and constraints. For example, we could in principle replace a 1000m high by 10m diameter cylinder with a 10m high 100m diameter cylinder yielding the same potential energy per metre raised, at 10 bars instead of 1000 bars, but the requirements for shaft sinking and fluid handling would be sobering, not to mention the stability of a piston with such an aspect ratio
But intermediate values might be more realistically of interest.

 

[Handy Andy]

Tides are powered by gravity, and are more reliable, than damming valleys, or waiting for the wind to blow.

 

[Jon Richfield]

I'm just thinking that adding a 'top' to that piston is going to put a lot of demand on the seals. What kind of PSI would that be getting to? And maybe that is why he is running each of his pistons independently - the seals 'only' need to work against the weight of that piston?

Well, I have just added another blog item, in which I deal with multiple pistons. There are some quite interesting aspects to that. The guy I was slagging off just seems to have missed them, or he wouldn't have made his smart. They are good for when your pistons are really, really long, but they make it harder to use a large percentage of your cylinder. Hmmm... I might add a section on some of the trade-offs.

 

[NTL2009]

Well, I have just added another blog item, in which I deal with multiple pistons. There are some quite interesting aspects to that. The guy I was slagging off just seems to have missed them, or he wouldn't have made his smart. They are good for when your pistons are really, really long, but they make it harder to use a large percentage of your cylinder. Hmmm... I might add a section on some of the trade-offs.

Interesting. At this point, I feel like we are discussing details of the engineering decisions involved in whatever specific approach is used. I guess that's a bit beyond where I care to go with it, I was more interested in the general concept, and can this actually be practical for renewable energy storage. Like roughly how large and involved for X MWh of storage and Y MW capacity.

It will be interesting to watch these initial installations develop and what they learn from them.

 

[Jon Richfield]

... I was more interested in the general concept, and can this actually be practical for renewable energy storage. Like roughly how large and involved for X MWh of storage and Y MW capacity.

I covered that in the "Principles" section at
http://fullduplexjonrichfield.blogspot.co.za/2017/02/heavy-duty-energy-banking.html
I give fairly rough figures, but they are close enough for convenient feasibility calculations and discussions of materials, configurations etc. Until we have some idea of the most practical approaches, troubling ourselves with greater precision could not be very profitable.

 

[adapterant]

Glad to see that the discussion of mechanical energy storage is still alive on this forum. I have made some changes to my Lifter design and replaced the solid material, that was being raised for energy storage, with a liquid, water. It is now a Hydro Lifter. The raised water is used to provide the energy needed to run a Gravity Light. The Gravity Light normally operates for 20 minutes with a 20-25 pound weight raised around eight feet. I just ran it through some test and was able to achieve easily three hours of output from around 30-33 gallons of water. A fully loaded Hydro Lifter, like the one I have, holding 70 gallons, could provide light for up to six hours. Just thought I'd drop a line and show that gravity storage on a small scale can be useful and easy to implement. You can see this latest implementation here: http://www.bclifters.com/

 

[CWatters]

Interesting but... Even hilly countries like Scotland struggle to build enough hydroelectric storage capacity without having to build the hills as well.

Might be worth you calculating the cost of construction per MWH of storage. See how it compares with other technologies.