Undergrad Another 'Gravity Battery' Question

[Nugatory]

One of the better ways to store energy at a domestic/farm level is as fuel . Tank of Diesel , LPG .

Yep - there's a reason why we so often store energy as flammable liquid in a fuel tank. It's tough to beat the combination of high J/Kg and J/m^3, efficient conversion to useful work, low cost, and fast refill time with minimal infrastructure.

 

[kcaldwel]

For grid level storage, there is a gravity system that has been tested:

http://www.aresnorthamerica.com/
http://www.scientificamerican.com/article/energy-storage-hits-the-rails-out-west/

It doesn't scale down that well, unless you live on a mountainside.

There are also grid level compressed air systems in use already since the '70s in geological formations. Compressed air in pipes might scale to home size OK:

http://spectrum.ieee.org/energywise...ompressed-air-energy-storage-makes-a-comeback

I suspect a home-sized system would be costly (buried pipes for less blow-out danger) and noisy. Perhaps a neighbourhood sized system would work. They have one that fits in a trailer:

http://lightsailenergy.com/

 

[dipole]

The problem with gravity storage is that the energy density of gravitational fields is just too low. That's why you need massive amounts of stuff in order to get a lot of energy. This is a fundamental limitation that will never change. You could perhaps engineer man-made reservoirs (even underground ones that minimize evaporation and ecological impact) that feed hydroelectric generators in a closed-loop system which are easily turned on and off - and on a large scale this might actually be a good solution, because I think water can be pumped quite efficiently. However this is probably best for storing megajoules of energy - you'd need something that requires less infrastructure for smaller storage requirements.

 

[Jon Richfield]

...
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.

...

Please approach this with an open mind. If we could somehow work our way through the practicalities just think of what this approach could mean for us, for developing nations, and for our energy future. I'm looking forward to some great minds and optimistic solutions here. Thanks.

Captain, as you will by now have seen in the responses so far, the theme has been around for quite a long time and some approaches have been discussed online in depth and breadth, including design principles, gas compression costs and benefits, energy distributions, and the major potential benefits. None of these is either as trivial or as intransigent as you might at first think. Whether you raise a mass of water or of concrete or lead through a given height is neither here nor there in terms of how much energy you store, whether for cities, homes, ships or launching spacecraft . If you google the string "Energy Storage compressed submarine tent farms" you should find adequate material on page one, without exploring thousands of extra pages.

 

[sophiecentaur]

It's easy (unless you're a grizzled old Professional) to overlook the fact that every Joule has to be sourced from somewhere. A 'storage scheme' generates nothing. Also "efficiency" is a word that is very underplayed by enthusiastic green amateurs. Conversion and storage always involves loss.
The approach that is not so popular is just to use LESS energy. That can involve a kind of efficiency that amounts to more than 100%. Allowing for the fact that not spending money is like a tax free earning. Worth 25% when tax rates are 20%.

 

[rumborak]

Dipole really caught the crux here, which is the energy density. Except in a few places where nature already put something very large in place that can be exploited (Mountains, caves), the overall displaced volume for the generated energy, and thus by E/V the energy density, is horrendously low.
Now, if you had an artificial black hole ...

In the end, any technology that doesn't store energy on a molecular or atomic level simply won't have the required density. One non-chemical idea could be to store energy by bending or stretching something.

 

[thegreenglen]

Coming to the discussion a few months late, we'll see if anyone responds.

Seems things ended on a question of density verses efficiency. For the original question density is an important factor. However, if efficiency is preferred over density, the discussion can change.

When discussing gravity batteries, storage of energy, seems efficiency is agreed to be good. So, the discussion needs to go towards the space required for an amount of desired storage space. During my research I found a few underground based proposals.

One aspect of gravity batteries that is significant in terms of space usage is the vertical nature of the setup. An argument against gravity batteries is building a structure to hold it. One way to overcome that objection is to go down instead of up. And as a bonus, you start with your battery charged. Shaft construction is a concern, however, in areas where there are vertical, or near vertical mine shafts, there are ready made locations for batteries. Increase the vertical size of the weight, go with high density materials such as lead or deleted uranium (though neither of those are appealing to use near underground water supplies) and you have a decent setup. Not perfect, but decent and likely relatively inexpensive and safe.

Anyone willing to poke holes in my thinking? Please?

 

[Drakkith]

Increase the vertical size of the weight, go with high density materials such as lead or deleted uranium (though neither of those are appealing to use near underground water supplies) and you have a decent setup.

You'd probably do just fine with concrete. It's around 2400 kg/m³ and can easily be poured into any simple shape or dimensions you could need for a setup like this.

 

[CWatters]

The storage capacity is proportional to area as well as height. With hydro you can easily fill up a valley giving you massive area. How do you achieve the same raising an lowering a weight underground?

 

[sophiecentaur]

You have to do the mgh sum in every case to assess whether it fits the bill. You should be able to improve on Water by a factor if 10X if you used a lead mass. Furthermore, there wouldn't be the losses associated with turbulence in water. But just look at the size of a modest Hydro Power Storage reservoir.
I was listening to a guy talking on the radio yesterday and he was suggesting 'local' storage, coupled with domestic solar or wind generation. You would need several kWh of storage and even that's a lot of m and a lot of h. 1kWh would correspond to 36tonnes and 10m, involving a lump of lead 3.6m³ - which is a big battery. 1kWh would not be enough for anything but the lightest overnight loads in the home. Heating and major appliances could be dealt with separately, of course.

 

[Khashishi]

Unless you already have some handy natural structure to work with, like a lake on a mountain, it seems totally impractical. If you want to go massive, you are better off with flywheel energy storage.

 

[sophiecentaur]

The storage capacity is proportional to area as well as height. With hydro you can easily fill up a valley giving you massive area. How do you achieve the same raising an lowering a weight underground?

That's one way of looking at it but what counts, in the end is Mass and distance raised. The high reservoir would, ideally, be situated way above the lower reservoir and be as large an area as possible so that the 'h' would not be affected too much as the level goes down.
I would not think that the expense of digging a deep well would necessarily be good value for achieving a height difference. True, there could be inherent strength in a well, compared with a frame but underground building can be expensive. A house could be built over such a hole, of course, but it could also be built around a massive 'lift shaft'. Perhaps a combination of below and under ground construction. No one seems to have found my estimated figures to be wrong so we could base the figures on 10m and 3.6tonnes for 1kWh of storage. In winter, the necessary storage time could be well over 12hrs, which would mean less than 100W of available supply. Far too stingy, I would say. Four times that would be more reasonable so we'd be talking in terms of say 15 tonnes and 10m. Quite a machine, for every house in the street

Unless you already have some handy natural structure to work with, like a lake on a mountain, it seems totally impractical. If you want to go massive, you are better off with flywheel energy storage.

OK. It's your turn to do the equivalent sums to help us compare the two approaches.

 

[Khashishi]

Instead of reinventing the flywheel, I can give you numbers from a presently existing system. The Alcator C-Mod power system uses a 120 ton alternator rotor which stores 500MJ at 1800RPM and an additional 75 ton flywheel which stores 1500MJ at 1800RPM.
https://www-internal.psfc.mit.edu/research/alcator/pubs/SOFE/SOFE2015/Terry_SOFE-15_poster.pdf

 

[sophiecentaur]

Instead of reinventing the flywheel, I can give you numbers from a presently existing system. The Alcator C-Mod power system uses a 120 ton alternator rotor which stores 500MJ at 1800RPM and an additional 75 ton flywheel which stores 1500MJ at 1800RPM.
https://www-internal.psfc.mit.edu/research/alcator/pubs/SOFE/SOFE2015/Terry_SOFE-15_poster.pdf

But then we would have to scale it to individual home storage. Why can't you do it for us? Do it do it do it go on go on go on. You know you want to.

 

[Jon Richfield]

Unless you already have some handy natural structure to work with, like a lake on a mountain, it seems totally impractical. If you want to go massive, you are better off with flywheel energy storage.

If you want to go massive, you are far better off with undersea compressed air storage. It is one of the few technically accessible, coherent, and versatile storage media, clean, efficient, scaleable and economical, that would be ecologically beneficial and would advance the use of wind, wave, solar, and space energy collection, and act as an effectively open-ended buffer for both spikes and dips in power production and demand. It could be rapidly expanded and easily maintained or replaced in the event of disaster.

And it is one of the very few that could be installed and applied to store energy, not only on a city or country scale, but on an international scale, though variations could be applied down to domestic scale.

It could be implemented on a large scale within a few years (not decades). On a massive scale (internationally being established as the primary storage facility) a couple of decades would be plenty.Let's see anyone top that with flywheels. So far the figures given here for flywheels are pathetic! And the flexibility and scaleability are worse. The only better medium than undersea storage on a massive scale is not at present technically accessible.

 

[heldfabrication]

hoist a car up a telephone or power pole w/solar drop to recover!

 

[Drakkith]

hoist a car up a telephone or power pole w/solar drop to recover!

Let's see... 1500 kg * 9.81 m/s² = 14,715 Newtons of force.
14,715 N * 10 meters = 147,150 joules of energy required to raise the car.

147,150 joules is 4.0875 Watt hours. Your average car battery holds more energy.

 

[jbriggs444]

147,150 joules is 4.0875 Watt hours. Your average car battery holds more energy.

You may have slipped a digit there, but the average car battery still holds way more.

 

[sophiecentaur]

Thing about trying to store energy in elevated lumps of mass is that the forces involved are very high. Any mechanism to recover the energy in a slow and controlled manner will very likely be low efficiency. (Friction is not our friend - think of a screw jack that is so inefficient that the car stays up there without the need for a 'stop'.)

 

[sophiecentaur]

Storing GPE with fluids is a much less lossy method until the release of the energy involves particularly high flow speeds.
Compressed air intuitively scares me but I could get over that. Deep sea storage eliminates the explosion problem, largely and the damage from a massive bubble (mini tsunami) could be no worse than from a burst dam.

 

[Jon Richfield]

Storing GPE with fluids is a much less lossy method until the release of the energy involves particularly high flow speeds.
Compressed air intuitively scares me but I could get over that. Deep sea storage eliminates the explosion problem, largely and the damage from a massive bubble (mini tsunami) could be no worse than from a burst dam.

I think you are too pessimistic in comparison with burst dams Sophie. I discuss aspects of safety and efficiency in some depth in an essay too long for here, but if you paste 'jonrichfield Full Duplex: Energy Storage & Renewable Energy Sources' into a suitable online search engine, it should be pretty near to the top.

 

[sophiecentaur]

I think you are too pessimistic in comparison with burst dams Sophie.

It would depend on the volume of air being stored as to whether the tsunami effect is significant. Possible collapse of cliffs in the Canary Islands (Madeira??) is thought to be a potential tsunami hazard for the US seaboard.
I am not too pessimistic tho', just wondering about a possible risk. It would need a very fast escape of air to be relevant.

 

[Jon Richfield]

It would depend on the volume of air being stored as to whether the tsunami effect is significant. Possible collapse of cliffs in the Canary Islands (Madeira??) is thought to be a potential tsunami hazard for the US seaboard.
I am not too pessimistic tho', just wondering about a possible risk. It would need a very fast escape of air to be relevant.

Well Sophie, in the scenario that I sketched in the essay I mentioned, I had in mind million-cubic metre tents possibly a couple of km below the surface. That sounds a lot, but even a complete rip notionally releasing a single bubble (impossible, but let's assume a spherical cow anyway) would only be about 160 metres in diameter. It would go roughly straight up, stirring say a 200 metre circle of seawater, which you would have to be in a very special position to see when it breaks surface. The splash of water imploding in on it as it breaks surface would be imperceptible 1 km away. The only people with a legitimate gripe would be those whose vessel happened to pass over exactly when and where it breaks surface, because they would sink for sure unless they were in a very big ship.

But admit it, that would be asking for a HUGE coincidence, and in any case no one should be passing over a farm of energy-storage airbags, any more than anyone should be flying his helicopter through a wind farm, unless he had special duties there.

 

[sophiecentaur]

A million cubic metres at 1km would become a pretty vast volume at the surface 2¹⁰⁰ increase? That's a lot of displaced water. (I don't think I have got that wrong)?

 

[sophiecentaur]

If the above is right then the energy stored would be embarrassingly high. Implies less volume or less depth required.

 

[Jon Richfield]

I think your guestimation slipped a cog there Sophie; 2^100 would be about 10^30, giving about 10^36 cubic metres of water, equivalent to a cube about 10^12 metres on a side, which would exceed the volume of the Earth. Assuming a perfect gas, and ignoring complications like sources of energy to warm it on the way up and to accelerate the water to make way for the expansion, the pressure decrease would be only about hmmm... 100 atmospheres. I think the figure you intended to type was 100. I still wouldn't like to be above the escaping bubble, even in a much larger ship (or even a not-too-high-flying aircraft) than I had hypothesised for my spherical cow, but it would be a far smaller disturbance than dropping the many cubic km of rock that caused the fairly recent collapse of the Atlantic island that you mentioned. (I remember reading of it, but am not inclined to research it just at present.)

Incidentally, your question does open a rather interesting field for speculation. Does anyone here have any firm information on the nature of the behaviour of really large air bubbles released abruptly far below tranquil liquid surfaces? Say a 10-metre sphere at 100 metres down? I never wondered about it before, but I bet it is nothing like as simple as releasing 1 litre or so. I also think that the resultant effect would be greatly mitigated by the complications, but as an experiment in catastrophic engineering...

 

[sophiecentaur]

Ha! I am on a train without my thinking head. Yes, it's 100 Ats at 1km. But even that would / could involve a big bubble. I heard of ships foundering in bubbles of released Methane (Horizon, BBC TV) but that's easily dealt with by having an exclusion zone. But wouldn't a bubble of 100 million cubic m still cause a significant wave? The actual Energy involved would be what was originally stored. You would have a surface wave with a wavelength of perhaps hundreds of m and the energy density follows only an inverse law (2d) spreading. That wave would have some significant effect all along the coast, I think. But, as you say, the air would hardly appear all at once. There would be only a small pressure differential across a tear on the sea bed.
I'm not knocking the idea. I made my original comparison with hydro power, in a positive way, aamof. One massive advantage of a sea bed system would be that it wouldn't be affected by weather like wind, tidal and wave systems. You would have a choice of any convenient site, too. Unlike oil drilling rigs. And no risk of pollution. Pipes along the sea bed to shore would allow most of the equipment to be ashore. Only valves would be needed out at sea.
I warming to the idea.

 

[Jon Richfield]

Ha! I am on a train without my thinking head. Yes, it's 100 Ats at 1km. But even that would / could involve a big bubble. I heard of ships foundering in bubbles of released Methane (Horizon, BBC TV) but that's easily dealt with by having an exclusion zone. But wouldn't a bubble of 100 million cubic m still cause a significant wave? The actual Energy involved would be what was originally stored. You would have a surface wave with a wavelength of perhaps hundreds of m and the energy density follows only an inverse law (2d) spreading. That wave would have some significant effect all along the coast, I think. But, as you say, the air would hardly appear all at once. There would be only a small pressure differential across a tear on the sea bed.
I'm not knocking the idea. I made my original comparison with hydro power, in a positive way, aamof. One massive advantage of a sea bed system would be that it wouldn't be affected by weather like wind, tidal and wave systems. You would have a choice of any convenient site, too. Unlike oil drilling rigs. And no risk of pollution. Pipes along the sea bed to shore would allow most of the equipment to be ashore. Only valves would be needed out at sea.
I warming to the idea.

As mental glitches go Sophie, well, I don't like to brag, but I repeatedly outdo that little one by orders of magnitude, but I will neither bore nor pain you with cases in point.
I also have been interested in the methane bubbles you mentioned; the topic has long intrigued me.
The field is full of emergent points of interest (like most exploratory engineering). I am pleased by your remarks, because I agree with your points. In fact, from the ecological point of view, fields of tents of the type I describe would act intrinsically as natural conservation areas, even on a scale of energy stores adequate to act as buffers for the entire industrial world.
When I wrote the essay I was quite startled by the potential, in spite of the complications of handling compressed gas, some of which turned out to be advantages, on closer inspection.

 

[Jon Richfield]

Oh, and I forgot. The design I had in mind would not be on the sea bed, except for tethers and ballast. I won't go into details, because it is all in the essay, but part of the merit is that the tents would be tethered in stacks at convenient heights above the sea floor, at depths down to say, 4 km, separated by a few hundred metres laterally and a few hundred vertically up to say a depth of say 200 metres (Thumbsucks of course!) Each litre of compressed gas would represent something like the potential energy of the column of water above itself. (That is why the deeper gas would be more compressed.)

No compressive force would be necessary for the stored gas (the water pressure would supply that), only the tensile strength to counter the buoyancy, which is considerable, but trivial in contrast to the pressure.

There are all sorts of goodies to the scheme, which, I admit, is nothing like as mature as it would be after a few pilot schemes had revealed the gotchas.

 

[sophiecentaur]

No compressive force would be necessary for the stored gas (the water pressure would supply that),

The description 'tents' is a good one because it tells you that they would need no 'floor'. You pump more and more air at ambient pressure until the tent is full. It looks as if the stress on the structure is no more than the weight of the displaced water (less the weight of the air within, which would be only 100 times that of that volume at AP) and there would be virtually no impulsive stresses. Tethers could be stainless steel wires.

after a few pilot schemes had revealed the gotchas.

Those would be very interesting. But there again, no one expects the unexpected.