Undergrad Another 'Gravity Battery' Question

[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!