I Another 'Gravity Battery' Question

[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.

 

[Jon Richfield]

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.

Exactly. One would of course choose areas little prone to strong currents.

There are however complications of scale when one enters the realm of million-tonne buoyancies. Tethers for example need to be anchored, so I designed weighted tether cables and anchors rather than trying to engineer full-strength anchors set into bedrock. Whether this is a good idea or not, let alone good in all circumstances, I cannot yet say.

I have a suspicion that the solubility of air at 100 atmospheres might be excessive, and if it is enough to be troublesome, then a light, flexible "floor" layer of tough plastic, or a suitable fluid, might be a good idea. I like the idea of polycarbonate cables myself, because I am nervous of metal corrosion. But those are details.

Then there are questions of growths and borers attacking materials, but we already have a lot of information on such factors, so I am sure we can manage something reliable.

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

Nor the Spanish inquisition!

 

[adapterant]

One of the most difficult problems to overcome, is the lifting and positioning of the weight. I started working on an improved lifting system and finally received a patent for a gravity storage system. Gravitational energy storage is possible and viable if you are willing to work with large weights. Take a look at the results I've had at http://www.bclifters.com. I have several examples showing the possible implementation of what I refer to as a "Lifter". A Lifter may be used for hydro storage, solar energy to gravity storage, gravity to mechanical energy, etc....

 

[sophiecentaur]

An old thread but a good one.
It's essential that the input and extraction of the energy (work) is achieved with optimum efficiency. That calls for the right choice of magnitudes of the Forces and fluid flow speeds. At least this type of system is more flexible than many other ideas.

 

[OldYat47]

If you have hills and old railroad equipment you can use surplus daytime solar energy (just one example) to move that train up a hill. Then you can use generators on the train to generate electricity as the train rolls down the hill. Lots of cheap mass, predictable output, mechanically simple. A couple of these systems are already in use in the U.S., and a few are in the planning stages. In the flatlands gravity storage isn't a good option. Too expensive to construct artificial elevation.

 

[sophiecentaur]

If you have hills and old railroad equipment you can use surplus daytime solar energy (just one example) to move that train up a hill. Then you can use generators on the train to generate electricity as the train rolls down the hill. Lots of cheap mass, predictable output, mechanically simple. A couple of these systems are already in use in the U.S., and a few are in the planning stages. In the flatlands gravity storage isn't a good option. Too expensive to construct artificial elevation.

The efficiency of a storage cycle using old rolling stuff would be very low.You would need loads of surplus source energy for a system like that to produce a useful amount of energy. Low capital cost, perhaps, which could be a big advantage but 12V accumulators are still pretty good value as storage devices. As far as I know, hydro storage beats them all if you have a handy valley / lake.

 

[OldYat47]

Actually, when properly sited and designed they are quite efficient. That's why they are being built. And if you don't have a lot of surplus energy then storing it in any fashion is probably not cost effective.

 

[sophiecentaur]

Actually, when properly sited and designed they are quite efficient. That's why they are being built. And if you don't have a lot of surplus energy then storing it in any fashion is probably not cost effective.

People try a lot of schemes for a lot of things that don't always make sense.

Installing a good rail track is a major expense and old rolling stock would have had plain bearings, I should imagine. For efficiency you would need balls or roller bearings etc. Then an electric supply to on board motor generators or a winch (more losses). What sort of scale are we talking? Many kW or tens of kW? And how many kWhr storage?
Is there any estimate of efficiency? Tesla does a good job at high end cost but that's more of a lovely toy, I think.

 

[OldYat47]

Note that, in hilly areas, you could use existing track. And plain bearings properly lubricated can be very efficient indeed.

One that I know of is 50 megawatts total capacity, 12.5 megawatts per hour peak delivery rate. Systems like this are useful (just one example) in areas that have a lot of solar energy on the grid. In the case of solar, fast moving clouds can wreak havoc with the rate of power delivery into the grid. Demand on generation fluctuates quite a bit. In that situation the gravity train works very well. When demand increases the train rolls. When demand falls the train gets pushed up the incline again. This (more or less) stabilizes demand on big generators. Wind farms are subject to fluctuating winds at times. Same situation.

 

[jbriggs444]

One that I know of is 50 megawatts total capacity, 12.5 megawatts per hour peak delivery rate.

The former is not a unit of capacity (energy). The latter is not a unit of power.

 

[sophiecentaur]

The former is not a unit of capacity (energy). The latter is not a unit of power.

That's a very common bit of sloppiness that we find in descriptions of 'energy' systems. I think they mean 12.5 MWh per hour - i.e. the average power output you can expect.

Demand on generation fluctuates quite a bit. In that situation the gravity train works very well. When demand increases the train rolls. When demand falls the train gets pushed up the incline again.

Yes - we get the basic principle, which is Energy Storage, rather than just Energy Conversion. From the figure you quote, it suggests that the efficiency could be 25% (?)
This wiki link suggests that the efficiency ("round trip") of the much higher tech flywheel storage s systems can be around 85%. That figure could be very speculative, of course. When the demand for storage is 'instant' (as with an uninterruptible power supply) the flywheel idea is good but the energy losses after a long delay could be total. Your system would have the advantage of power being available for days after it was stored. If efficiency is a major issue then I still say that plain bearings could be improved on. Steel wheels on (clean, shiny and very rigid) steel rails could be pretty good, I reckon.

 

[OldYat47]

jbriggs444, yes, those 50 megawatts are a statement of capacity. From the highest "parking" point to the bottom of its run the system can deliver a total of 50 megawatts of power. It stores those 50 megawatts as potential energy (capacity, elevation) and can deliver those 50 megawatts at a rate of 12.5 megawatts per hour.

The systems are simple and both easy and cheap to maintain, unlike massive flywheels. They are building and using them today so they must have some cost vs. benefit incentive.

sophiecentaur, why would you put the efficiency at 25%?

 

[rumborak]

@OldYat47 , you keep using a unit of power as a measure of energy. Capacity, I.e. energy content, just like in batteries, is measured in Joules or Watthours, not Watt. Even worse, you say "Megawatt per hour", which makes no sense at all.

 

[OldYat47]

Let me try again. Suppose the train is at the top of the hill. How many watts can it deliver in its entire run down the hill? 50 megawatts. The train stores this as potential energy. This energy storage is one type of capacity.

What is the maximum power delivery rate? 12.5 megawatts per hour. In one hour the system could deliver as much as 12.5 megawatts.

Watt hours and kilowatt hours are derived and not "standard" terms, but they are commonly used in electrical supply and generation systems. Joules are units of work, watts are units of power = work/time. Work / time X time = work, so a watt hour should be a unit of work. It is not, it is a measure of power delivered or used.

 

[Drakkith]

Let me try again. Suppose the train is at the top of the hill. How many watts can it deliver in its entire run down the hill? 50 megawatts. The train stores this as potential energy. This energy storage is one type of capacity.

What is the maximum power delivery rate? 12.5 megawatts per hour. In one hour the system could deliver as much as 12.5 megawatts.

Watt hours and kilowatt hours are derived and not "standard" terms, but they are commonly used in electrical supply and generation systems. Joules are units of work, watts are units of power = work/time. Work / time X time = work, so a watt hour should be a unit of work.

The watt is a unit of power, not of energy. Power is the rate that energy is produced/used or the rate that work is performed. You can store energy, but you cannot store power. Watt-hours is joules/time x time, which turns out to be just joules. Hence a battery can store 10 watt-hours, but it cannot store 10 watts.

Given a train with a mass of 10 tons (10,000 kg) on a hill with a height of 100 meters then the potential energy stored is 9,810,000 joules. If the hill is very steep the all of this potential energy can be converted to work at a high rate and the maximum power will be very high. If the hill is very shallow, then the power is lower since it takes longer to move the train down the hill. If the train takes 100 seconds to move down the hill and stop, then the average power was 98,100 watts. If the train took only 10 seconds the the average power was 981,000 watts.

Work / time X time = work, so a watt hour should be a unit of work. It is not, it is a measure of power delivered or used.

That is incorrect. Joules/hour* hours = joules, which is a unit of energy/work, not power.

 

[sophiecentaur]

What is the maximum power delivery rate? 12.5 megawatts per hour.

That sentence makes no sense at all, I'm afraid. If I told you my motor car can do 50mph per hour you would not know what I meant. You are doing the same equivalent thing with your "12.5MW per hour". If you want to be taken seriously, you really need to sort out your units. I appreciate that the articles you have read may well be guilty of the same sort of mis-use of terms but that only makes them dodgy as sources of other information.

sophiecentaur, why would you put the efficiency at 25%?

I made assumptions about what you 'really meant' by your original figures. What do you 'really mean"?

 

[CWatters]

Google found..
http://www.vox.com/2016/4/28/11524958/energy-storage-rail

The company claims the process, end to end, is 86 percent efficient

ARES already has a test track in the Tehachapi, California, region, but earlier this month, it got approval from the Bureau of Land Management for its first commercial-scale project.

That project, called ARES Nevada, will consist in a 5.5-mile track traveling up an 8-degree slope, covering 106 acres of public land near the delightfully named town of Pahrump, Nevada. It will boast 50 MW of power capacity and be capable of producing 12.5 MWh of energy. The company expects to start construction early next year and finish by 2019.

More in the full article.

 

[sophiecentaur]

Right. So that means 12.5 MWh from one full downhill run. A useful backup for a community. The efficiency figure is pretty fair, too.

 

[rumborak]

Interesting concept for sure. Unless I am mistaken, a single unit's failure takes your whole power station offline though, unless you have multiple tracks.
Given how each unit seems to have its own propulsion system, I could see how that could create many possible points of failure.

 

[jbriggs444]

If capacity is 12.5 MWh and peak power is 50MW then that's 15 minutes at peak power. On a 5.5 mile track, it would be 22 mph downgrade. On an 8% grade, that's about 700 meters of vertical distance. 12.5 MWh is 45 gigajoules. Dividing by 700 meters that means about 64 million Newtons. Or about 6.5 million kilograms/ 6500 metric tons. A hefty train, but the numbers are all plausible.

 

[CWatters]

Their first project is aimed at helping smooth the grid over relatively short timescales, and it seems ok for that. However the two pumped hydro systems in Scotland (for example) have a capacity of 6-7GWh so their storage capacity is two or three orders of magnitude larger.

 

[sophiecentaur]

unless you have multiple tracks.

The picture on the given link shows a number of tracks, side by side. I guess the reason for the initial use (power balancing) is the relatively low energy capacity. The various advantages over hydro, that are quoted make it a good proposition in some locations.
Suitable sites for hydro are a lot less common than a trip in 'the mountains' might suggest and the same could be true if you were looking for a suitable escarpment to build the railway system on, too.

 

[Jon Richfield]

@OldYat47 , you keep using a unit of power as a measure of energy. Capacity, I.e. energy content, just like in batteries, is measured in Joules or Watthours, not Watt. Even worse, you say "Megawatt per hour", which makes no sense at all.

Megawatt per hour is the rate at which power increases or decreases per hour.
Megawatt per hour per hour is the rate at which the rate at which power increases or decreases per hour increases or decreases per hour.
Megawatt per hour per hour per hour is ...

 

[sophiecentaur]

Megawatt per hour is the rate at which power increases or decreases per hour.
Megawatt per hour per hour is the rate at which the rate at which power increases or decreases per hour increases or decreases per hour.
Megawatt per hour per hour per hour is ...

I can see that you are cross about this but it's one of those slips that are often made by purveyors of snake oil or just people who are not Engineers but who think they can see an opening to make money. I would always take one step backwards an put my cheque book away if I was approached for some investment.

 

[Jon Richfield]

I can see that you are cross about this but it's one of those slips that are often made by purveyors of snake oil or just people who are not Engineers but who think they can see an opening to make money. I would always take one step backwards an put my cheque book away if I was approached for some investment.

Sophie, I apologise for not including a smiley at the end of my string of emoticons, but in fact I was not cross. rumborak had already made the response to the operative problem of irritating confusion of concepts. I just added a corollary to suggest a point that is usually neglected.

Mind you, I also do agree with you about "purveyors of snake oil or just people who are not Engineers but who think they can see an opening to make money". slips can be very very revealing.
Otoh, I did once have the startling experience of an engineer who not only confused kW with kWH, but also rejected the suggestion that he had it wrong.

 

[sophiecentaur]

but also rejected the suggestion that he had it wrong

I imagine he had a successful career in management!

PS What's wrong with being cross, anyway? I spend most of my times at the wheel or at the keyboard being 'cross' about something or other.

 

[Cutter Ketch]

You need to build your house next to a large hill so you can lift your weight further. Drag 73 tons 50m up or 7.3 tons 500m up. Which is how this company is trying to make a 12.5 MWh gravitational energy storage system power load leveling.

http://www.aresnorthamerica.com/grid-scale-energy-storage

This has had a lot of press in the last 3 years, but so far it seems to be a couple of demos and a lot of grand plans, so I have no idea if their claim of 80% energy recovery is real. But at least this shows that others have been putting some effort into gravity energy storage.

 

[RonL]

Google found..
http://www.vox.com/2016/4/28/11524958/energy-storage-rail

More in the full article.

Hope I'm not being too picky, but I can't comprehend the load weight twisting 90 degrees for compact storage and the carrier frame and wheels seem to disappear other than that I like the concept.

 

[Cutter Ketch]

Problem is that it isn't 'gravity powered', is it? It's powered by the guy who provides the movement with his muscles. He could be pedalling or turning a handle and achieving the same power output.

I don't think anyone is suggesting you can get free energy from such a gravity system. In all cases it is an energy storage device and is being compared to other energy storage devices like the power wall or flywheels. Despite the fact that this example stores muscle power rather than electricity, it is not so different from the others which have been mentioned.

 

[Jon Richfield]

You need to build your house next to a large hill so you can lift your weight further. Drag 73 tons 50m up or 7.3 tons 500m up. Which is how this company is trying to make a 12.5 MWh gravitational energy storage system power load leveling.

http://www.aresnorthamerica.com/grid-scale-energy-storage

This has had a lot of press in the last 3 years, but so far it seems to be a couple of demos and a lot of grand plans, so I have no idea if their claim of 80% energy recovery is real. But at least this shows that others have been putting some effort into gravity energy storage.

Assuming that your source of energy is renewable, I admit that it has attractions, but really, the infrastructure is horrendous with lots of moving parts.

I can't say I like it, so they will have to stop.

Frankly, if they can afford all that much rail and real estate and mechanism, then I reckon they could afford, and do better, digging a couple of alternating 100 metre deep cylindrical holes 4 metres in diameter, each with with a 110 tonne floating, gasketed lead piston floating on weak brine (say 10% NaCl/ 1% ZnCl2 to avoid excessive microbial growth). They would avoid transporting the lead by casting it in situ into the mechanism. The brine would be pumped into the cylinder by their wind turbine or PV charger or something, while the offtake would drive the dynamo on demand.

Far more land-efficient, more capacity, only one moving part, and effectively constant pressure operation.

Let's go!

 

[Cutter Ketch]

Assuming that your source of energy is renewable, I admit that it has attractions, but really, the infrastructure is horrendous with lots of moving parts.

I can't say I like it, so they will have to stop.

Frankly, if they can afford all that much rail and real estate and mechanism, then I reckon they could afford, and do better, digging a couple of alternating 100 metre deep cylindrical holes 4 metres in diameter, each with with a 110 tonne floating, gasketed lead piston floating on weak brine (say 10% NaCl/ 1% ZnCl2 to avoid excessive microbial growth). They would avoid transporting the lead by casting it in situ into the mechanism. The brine would be pumped into the cylinder by their wind turbine or PV charger or something, while the offtake would drive the dynamo on demand.

Far more land-efficient, more capacity, only one moving part, and effectively constant pressure operation.

Let's go!

Sounds like you need to write a business plan!

 

[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.