I Another 'Gravity Battery' Question

[Jon Richfield]

Sounds like you need to write a business plan!

Nice thought; I never thought much along these lines before, being enamoured more of submarine tents for storing gravitational energy, because they could scale up to national or international proportions, not just megawatthour orders of magnitude, but this one does have potential I suppose.

The idea is immature as yet and there could be many variations on the scheme.

Lead is good because it is modestly dense and comparatively cheap nowadays because its market has slumped rather, ever since its (justifiable) omission from high octane fuels, though depleted Uranium would have nearly twice the density. However, I suspect that even depleted U238 would be more expensive than lead.

If one did cast the lead as a slug (or an assembly of smaller slugs, which might be cheaper and more manageable and maintainable) one might sheath it with copper or something similar that could be machined more precisely, and coat it with a low-friction, wear-resistant gasketing such as ultra-high-molecular-weight polyethylene. With such a surface on the inside of the cylinder shaft, or perhaps in the form of piston rings,the seal could be really good. If for some reason we decided that a one-piece piston really would be desirable, then lead particles in a matrix of say, polyester might have advantages, with the shaft still lined with UHMWPE for friction.

Instead of putting any mechanisms inside the slug, let alone attaching it to dangerous, expensive ropes etc both the raising and power offtake could be performed by pumps and turbines both for raising it by pumping fluid below the slug for energy accumulation, and extracting the potential energy by letting the fluid out under pressure for driving the turbines.

An attraction of floating the lead in the shaft is that one could greatly improve the storage and cost of withdrawing the power by returning the power offtake fluid to on top of the free-floating piston instead of to a retention vessel. There would be many advantages to a closed system of that type; it might offer opportunities for using say, kerosene or nitrogen (though I doubt the properties of gases, but hmmmm... liquid SO2...) instead of water in cold regions, or to tune the working properties such as viscosity and corrosiveness.

But there are many aspects to investigate; for example how much of the shaft to have underground and how much in a tower. The working pressure for a given mass would be affected by many factors, such as the diameter of the shaft and slug.

It is a tricky subject, but attractive, I think. Haven't there been any industrial investigations of the type, does anyone know?

 

[sophiecentaur]

a couple of alternating 100 metre deep cylindrical holes

Boy, that sounds expensive! Above ground construction tends to be cheaper than underground - but I admit you'd need a pretty strong tower for that job.

 

[Jon Richfield]

Boy, that sounds expensive! Above ground construction tends to be cheaper than underground - but I admit you'd need a pretty strong tower for that job.

Agreed Sophie, on both points, but looking at what that rails scheme looked like, I reckon this piston idea probably is dirt cheap in comparison. Also I reckon it is more versatile and scaleable. IMO the best design, depending on the local geology, would be as far down as might be affordable, and a reasonable way up, partly for capacity and partly for maintenance. Underground or not, each tunnel would be an investment and there would be a calculable height and technology worth investing in above the tunnel.

It also is interesting to contemplate the types of scaleability applicability to a piston scheme. The pressure that a piston can yield is surprisingly small, as opposed to the amount of energy it could store, which is a different matter, but both are important. Roughly speaking one expects greater efficiency from higher pressures, but a column of lead roughly 0.9m tall would yield only about 1 bar of pressure. (Mind you, it could yield it all the way down! No tapering off as your stored power is depleted )

But anyway, a piston say 10m tall would yield only about 11 atmospheres. Usable, but not impressive. But I suggest that to make the piston 100m tall would be unpractical or even impracticable.

HOOOOWeverrrr... That is one of the ways in which we could exploit scaleability in ways analogous to electric sources. Piston cylinders could be arrayed and designed with sealed tops and bottoms in such ways as to permit their being joined each other either in parallel or in series.

Ten 11-atmosphere pistons in series could be equivalent to a single piston yielding 110 bar. And by now we are talking some serious usable pressures. Furthermore, if there were breaks or maintenance, or requirements for splitting the output, the system could continue to operate at reduced pressure with hot bypassing or insertion of modules.

If pistons make solar, wind, and wave power practicable for moderate-sized systems, well, why not? Hubris is one of humanity's virtues, no?

 

[jbriggs444]

Why use a piston at all? Just put more water on top of the water and call it pumped storage hydroelectricity.

 

[NTL2009]

Why use a piston at all? Just put more water on top of the water and call it pumped storage hydroelectricity.

Agreed. I don't see what the piston adds, other than added mass. But that adds so much more complexity. If I'm following, we are talking about a shaft and piston with fairly precise machining to hold back water under pressure, for hours?

Just dig a wider, deeper hole, and pump the water. Keep It Simple Stanley.

 

[Jon Richfield]

Why use a piston at all? Just put more water on top of the water and call it pumped storage hydroelectricity.

Oh COME ON!
You kidding or what?
Try a bit of arithmetic...
You also might like to consider the nature of the duty cycle.

 

[Jon Richfield]

Agreed. I don't see what the piston adds, other than added mass. But that adds so much more complexity. If I'm following, we are talking about a shaft and piston with fairly precise machining to hold back water under pressure, for hours?

Just dig a wider, deeper hole, and pump the water. Keep It Simple Stanley.

Keeping it simple is great, just great.
That is why the piston is floating on the water.
No shaft, no gearing, no exceptionally precise machining, and it should be good for a lot more than hours.
A few simple valves in the circuit should do for holding it back.
Glad you noticed the added mass, now see whether you can notice what has been subtracted.

 

[jbriggs444]

Oh COME ON!
You kidding or what?

Not kidding. The arrangement is not clearly described and appears to be nonsense.

Possible: you have air above the piston and brine below. Variable pressure head between 0 meters H₂O (piston halfway down, two tubes almost evenly filled) and 109 meters H₂O (piston at top of live shaft and twin shaft empty) due to the 1 meter lead piston and up to 99 meters of water. You store energy by pumping brine into the bottom of the live shaft, allowing air to fill in its twin shaft. This arrangement is improved by removing the piston and optionally digging another few meters down.

Possible: you have brine both above and below the piston in a closed loop. Constant pressure head of 10 meters H₂O due to the 1 meter lead piston alone. You store energy by pumping brine into the bottom of the live shaft, allowing the overflow brine to fill in the twin shaft. This arrangement is improved by replacing the shafts with two ponds, one 10 meters higher than the other.

Possible: something else, not yet described.

Edit: corrected arithmetic for first possibility.

 

[NTL2009]

Keeping it simple is great, just great.
That is why the piston is floating on the water.
No shaft, no gearing, no exceptionally precise machining, and it should be good for a lot more than hours.
A few simple valves in the circuit should do for holding it back.
Glad you noticed the added mass, now see whether you can notice what has been subtracted.

Maybe you need to sketch this out for us - in one case you were talking about sealing, low friction, or piston rings - so I thought you meant a sealed piston, holding back the water pressure. But then you also talked about floating the lead piston (so I picture it shaped like a squat drinking glass or bowl?).

Maybe I'm fuzzy on this, but if you float a lead bowl on water, isn't is only displacing as much mass as that volume of water? If I have a cylinder with 10 meters of water height in it, I have a certain pressure at the bottom of the cylinder. Isn't that pressure the same if I float a hollow lead cylinder in the water, and maintain the 10 meter water height?

Anyhow, it was your proposal, it seems it is you who should be providing the arithmetic?

 

[sophiecentaur]

Ten 11-atmosphere pistons in series could be equivalent to a single piston yielding 110 bar.

There will be an optimum way to use any particular combination of area and depth but I still reckon that 'holes' are actually quite expensive. You could do the old canal / railway trick and put the spoil from digging into a vertical cone (Cuttings and embankments) but that would increase the area of the site. Why not use existing vertical mine shafts? They can be very deep but I guess their volume is only as big as was absolutely necessary.

 

[Jon Richfield]

There will be an optimum way to use any particular combination of area and depth but I still reckon that 'holes' are actually quite expensive. You could do the old canal / railway trick and put the spoil from digging into a vertical cone (Cuttings and embankments) but that would increase the area of the site. Why not use existing vertical mine shafts? They can be very deep but I guess their volume is only as big as was absolutely necessary.

I certainly agree with your points Sophie. Where one is installing an array or battery of such shafts, I disagree that the idea of using the spoil need greatly increase the area; as a thumbsuck let's imagine a battery of 400 shafts, each 1 to 4 square metre shaft occupying 49 square metres of real estate on predominantly hard rock (yes, we COULD have larger area shafts, but that would decrease the height of the lead columns and thereby the working pressure, and we do need working space between them ) then the spoil could be set aside for concrete. Suppose the shafts were about 500 metres deep, that would give us material for roughly 10 to 50-m high upward concrete extension of the shafts plus working space above, and the whole lot occupying less than two football fields. As real estate application goes, that is startlin efficiency.

As for mine shafts, some worked-out mine shafts, including in particular some of the world's deepest in hard rock, occur in South Africa, and I often think they could be put to better use, possibly including one like this, but they are not always conveniently sited, nor portable...

 

[sophiecentaur]

I disagree that the idea of using the spoil need greatly increase the area;

I was working on the principle that the thickness of the support, above ground would need to be substantial (the alternative would be a very expensive fabricated cylinder. - leading to a necessary increase in space between the cylinders and overall area. And you have to dispose of the spoil, one way or another. My intuitive view of the cost just goes up and up. A simple railway track on a hillside sounds far cheaper and simpler but I haven't ventured to the back of an envelope yet.

nor portable...

 

[Jon Richfield]

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... )

 

[QPDO]

Concerning gravity batteries I'd suggest the following though I haven't made any calculations:
If I put a ball screw into a shaft, use a magnetic threaded rod as a stator and a ring shaped brushless DC on the ball screw as rotor. Would it actually be possible to run it up and down storing or releasing surplus energy? Like having the the thread turn the rotor by deflecting the vertical force to release the energy and allow the bearing to work itself up the rod with surplus energy fed into it.

 

[Jon Richfield]

Maybe you need to sketch this out for us - in one case you were talking about sealing, low friction, or piston rings - so I thought you meant a sealed piston, holding back the water pressure. But then you also talked about floating the lead piston (so I picture it shaped like a squat drinking glass or bowl?).

Maybe I'm fuzzy on this, but if you float a lead bowl on water, isn't is only displacing as much mass as that volume of water? If I have a cylinder with 10 meters of water height in it, I have a certain pressure at the bottom of the cylinder. Isn't that pressure the same if I float a hollow lead cylinder in the water, and maintain the 10 meter water height?

Anyhow, it was your proposal, it seems it is you who should be providing the arithmetic?

Fair enough, having so far just been remarking on an idea as it arose, I now am in the throes of writing an essay on the subject and even (unusually for me) have provided some (I hope helpful) illustrations. Will report back as soon as real life relaxes its stranglehold.
As for floating, that may have been a terminological inexactitude; I was referring to an example of displacement by mechanical impasse rather than by the excess of buoyancy over gravity. The lead would have stayed suspended equally uncompromisingly over liquid butane or carbon tetrachloride or even mercury.

 

[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.