Another 'Gravity Battery' Question

[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.

 

[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.