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Gravitational potential energy storage - seeking comments

  1. Apr 20, 2007 #1
    I have an idea I would like to analyze but I'm practically ignorant of physics and engineering so I'm hoping you folks can give me some informative feedback.

    First, something tells me this idea is ridiculously uneconomical and/or impractical, but I'm still curious to find out and it will be a good exercise for me.

    The idea is to store energy, input and output as electricity, as the gravitational potential energy of a heavy mass. I am interested in this for off-grid autonomous facilities because it seems to me it would provide a high level of reliability and a very low level of maintenance (because of its simplicity), a high level of energy efficiency (because the mechanics would only lose a tiny amount of energy to heat and sound (?)), and it would store energy long-term with virtually no loss.

    The basic design would be: a shaft down into the earth in which the mass moves up and down. To store energy, a motor lifts the mass, and to release energy the mass is allowed to fall and drive a generator. My very limited knowledge of physics, materials, and engineering prevents me from being able to explore and analyze the idea very much, but if it could be made practical somehow, I think the benefits I mentioned above would make this type of energy storage highly attractive.

    Of course, the amount of energy stored is given by the equation

    energy = mass * gravity * height

    My target minimum for stored energy capacity is 100 kWh = 360 MJ, which gives

    mass * height = 360e6 / 9.81

    and for a reasonable (?) shaft height of 30 m, requires a mass of (approx.)

    mass = 360e6 / (9.81 * 30) = 1223242 kg

    So then, my questions are:

    Are there any types of cables and winches which can support this much weight and interface with a motor/generator? Could anything else be used to lift and drop this mass? How much would these things cost?

    What are the densest and cheapest materials which could be used for this much mass? I already looked at cement and iron but they would be way too expensive. Might rock, with a general density of 2500 kg / m^3, in the form of waste dust/pieces from industry or shale from mountain slopes, be cheap and practical to assemble into some sort of a mass block? With this density of rock, a mass with volume 489 m^3 would be required, and as a cube shape it would require a shaft width of more than 7.88 m. How much would it cost to make a 30 x 8 x 8 m shaft into the ground, for various types of ground?

    What are any other issues with the entire idea?

    Thank you very much for any feedback :)

  2. jcsd
  3. Apr 20, 2007 #2
    Hi Derick,

    It's good that you can think of an idea and invest the time in the general calculations like that, you could make a good engineer one day. :smile: Don't forget to just hang out and have fun with the guys and gals too. :wink:

    Your idea basicly would work, if for example you tie a rope to the falling mass and use it to turn an electric generator that would generate electricity. The problem, as often is the case in engineering is not with the fundumentals but with the efficiency. Just like you calculated, you'll need one hell of a deep hole and a LOT of mass to drop in there. Now if you want it to be worthwile you'll need to keep doing it for a long time to supply electricity and to do so you'll need to constantly find new mass to drop in there, plus the hole will fill up very quickly and you'll need to dig a new one. Currently Hydroelectric power stations - or just dams use this principle, the water that flows (or you could say falls, just that most of the movement is horizontal) from higher ground to lower ground (and the sea eventually) turns turbines in the dam that generate electricity using the electrical generators. The advantage of rivers of course is that we don't need to fill them in the first place and we don't have to worry about over filling the hole (or sea in this case). Actually currently we mostly use your idea in reverse - to bring things up by investing power - an elevator. So the idea is good, but we already found even better solutions to use. Don't stop inventing. :smile:
  4. May 7, 2007 #3
    I'm a bit late with this but......I think your idea is good, and so is your reasoning. This is already, and has been for some time, used in what is called pumped storage.
    Let's say that a dam has generated power all day and the water behind the dam is getting low. The inflow of water into the dam at this time of year isn't quite enough to refill the dam for the next days generation requirements. Also let's say that the electrical power being produced by steam turbine generators is declining as people go to bed, and daytime activity slows down. Steam turbine generators are most efficient when they are close to their maxium load capability.
    So here we have a situation where the electrical power demand is decreasing at night, holidays, or weekends, and the steam turbine generators (fossile fuel, nuclear) are going to have to be lowered to less efficient generating levels.
    About 40 years ago an engineer had a brilliant flash like yourself and suggested that instead of backing the steam turbine generation load down, we make a hydro electric generator that can either generate electrical power from the water flowing through the dam, or reverse itself and pump the water back into the upstream reservior of the dam to be used when need.
    To supply the needed electrical power to pump the water back into the reservior the steam turbine generators excess electrical generation capability would we used so they continue to generate at their best efficiency. Surprizingly little modification to the hydro generators had to been done to make them reverse and either generate power, or pump water back into the reservior. However there is a fair loss in efficiency in the hydro turbines because they had to be designed to do pumping in one direction, and then reverse themselves and turn the generator in the other direction to produce electrical power. And something designed to do two opposite things, never does both as well as one.
    However when all the number crunching was done, it was found that under certain conditions it paid off to do it..... if, #1 there was an abundance of off peak power generation capability that wasn't being used. #2 The peak power loads during the high load times was approaching the maximum generation capabilities of the generating system. So if the steam turbine generators could pump water back up into the reserviors of the dam, the hydro generators could operate during these high demand times, as well as normal load times, without running out of water.
    Your idea was also studied in using this same excess generation at lower loads to spin up very large flywheels, running in a vaccum to reduce their air resistance, to near, or above supersonic speeds. Then switching them over when needed to turn their stored momentum back into electrical power.
    I always liked that idea. It was clean, and more efficient than pumped storage. However, at the time, about 25 years ago, designing a flywheel large enough to be effective and hold together at such rotational speeds was
    the idea's down fall.
    Today with the advances in composites, carbon fiber, and computer design, it very probably could now be done. I have seen several buses assisted by flywheels using the buses braking energy, to help accelerate the bus again. The world definitely needs a way to store electrical power for use at another time. Batteries are not very efficient, fuel cells are good, but extremely expensive and are poisoned easily.
    A practical electrical storage system would make solar power, wind and tidal power much more attractive, and "greatly assist in reducing global warming. It is good to see "out of the box" idea's such as yours.
  5. May 7, 2007 #4


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    Welcome to PF, Derick. I agree with all of the foregoing responses. It's basically a very practical idea. A flywheel or pumped water would probably be a lot easier to implement, but your idea could be made to work. A good start might be to find an abandoned mineshaft or oilwell.
    Maybe the work done by the weight could be stretched over time by using a massive gearbox between it and the generator.
  6. May 7, 2007 #5


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    The big advantage of a pumped water system like http://www.fhc.co.uk/ffestiniog.htm is scale. Suppose you had a shaft 1 km deep and a 1000 tonne mass. That would store about 10,000 MJ of energy, or (assuming no energy loss) generate 330 MW for about 30 seconds. Compare that with the ffestiniog scheme which can generate 360 MW for "several hours". The problem of how to handle 1 km of rope, strong enough to support the 1000t mass, is another problem to think about.

    Flywheels are even smaller scale systems. The big problem there safety - how to contain the debris if something breaks.
  7. May 9, 2007 #6
    why a single huge mass
    when many smaller masses would do the same work/power
    and require small widgets to control them
  8. May 9, 2007 #7


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    I would tend to suspect that a large one would be more efficient in terms of friction and aerodynamic drag.
  9. May 20, 2009 #8
    We're doing this very thing. See our web site at http://mechanicalelectric.com [Broken].
    Last edited by a moderator: May 4, 2017
  10. May 20, 2009 #9


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    ???How does someone get a patent on something that is in every book published that relate to mechanics 101???:confused:

    Nothing but red flags in that link.:uhh:

    Numbers please,
    A. Cost of system
    B. Price of off peak power
    C. Cost to raise weights
    D. Net savings after weight has been lowered

    Did I miss something in the website?
    Last edited by a moderator: May 4, 2017
  11. May 20, 2009 #10
    All patents are specific. This one is about storing electricity per se, not the mechanics of generating electricity from mechanical force, which clearly has been done. The symmetrical in/out is what the patent covers (e.g. treating the whole thing like a black box, with plus/minus terminals on the front, acting like a battery, but being mechanical inside).

    Thanks for your support.

    Run your own numbers. Then run the numbers on solar, while you're at it. The costs represent a business model, not a technical issue.
  12. May 20, 2009 #11


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    Sorry for sounding so unsupportive, so quick.:redface:
    I do understand the black box anology, as I pretty well live in a black box enviornment.(mentally):smile:

    I'll look at your site some more, I just don't see where a gain can show, from what I looked at, and the no press, and no talk of the patent, left me feeling a little let down as to what you are promoting.
  13. May 20, 2009 #12
    It's true, there isn't a lot yet on the web site. I wasn't trying to promote it so much as to answer the thread to say, "hey, that's what we're trying to do." It's not trivial to use gravitational potential energy at a large scale, as has been pointed out. We're trying to do it, that's all :)

    Glenn Reid
  14. May 20, 2009 #13


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    Thanks, and best wishes.

    Hope you stick around, this is a very good site.

  15. May 21, 2009 #14


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    Marketspeak on your website aside (I understand that that's the nature of the VC / new company on the block game, so I won't hate the player here :smile:), is this intended to be more of a large-scale (i.e. utility backup) plan, or a house-hold / neighbourhood sort of plan? Are you able to comment on the efficiencies (line to retrieval) that you see?
  16. May 25, 2009 #15
    I had the same idea.
    Here are some additional thoughts, on the rationale, and a couple of additional comments on problems which I have encountered, and not yet completely solved:
    a. WATER: I am not writing here about using water, as some have suggested, above, as the medium to run the generator, e.g. Pelton turbine. I don't think Pelton is a good solution because of temperature problems, i.e. ice in the winter.
    I am describing the problem of RAINFALL, or SNOWMELT, which then will enter this deep hole. Keeping the water out, is a non-trivial consideration.
    b. gravity works to drop the stones, but then, to get them back up to ground level, say, 50 meters or 100 meters up, one needs some DC motors driven by sunshine, say, or a windmill, or, in the north, biomass/steam engine, or in California, thermal energy derived from volcanic sources (geothermal).
    c. the generator, in this design, is at the bottom of a hole. That's not very practical. It is much easier to service the generator, and it will need service, if it were at ground level, else one needs both stairwell, and elevator, to get to the bottom of the shaft. Accordingly, I recommend building a tower to store the stones, with the generator 50 meters below, i.e. at ground level.
    Anyway, those are my thoughts. I like your way of thinking!!!
  17. Jul 24, 2009 #16
    Sorry, I got distracted from this discussion by some "actual work" :)

    We are looking at large scale, but not utility back-up per se. Buffering of transmission, peak shaving, and buffering of intermittent renewables (wind, solar) to smooth them out. Most storage is actually used as a buffer for smoothing, in much the same way as buffers are used in network communications or graphics drawing on computers.

    The efficiencies are pretty good, actually. The losses are mostly mechanical, and in the range of 5-15% depending on exactly how it's all rigged, and at what scale, etc.

    Mechanical storage has some big pluses, such as being completely cleantech, inexpensive to manufacture, and it can hold its "charge" more or less indefinitely.
  18. Jul 24, 2009 #17


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    Clearly, the biggest benefit of mechanical storage over pumped storage would be efficiency. But I don't see how you could store a large enough amount of energy to matter in such a system. It just doesn't seem like a very practical idea. I haven't checked the math, but in post 2 it says you only get 100 kWh by raising a million kg 30m. Even if you installed such a system in a 300m skyscraper, you'd only get 1000 kwh. A skyscraper uses that much power in minutes!
  19. Jul 24, 2009 #18
    You only scale it up to a certain point, then you replicate. Just like windmills and other kinds of things that don't scale infinitely large. Pumped storage can be scaled more easily because you only need to scale the size of the bodies of water above and below. Though "easily" is all relative -- it costs about $3billion to build a pumped hydro storage plant.
  20. Jul 27, 2009 #19
    The Ludington, Michigan pumped water energy storage facility is one of the largest in the United States. See
    It has a capacity of 1,872 MW, and enough water to generate electricity for about 8 hours.
    The Nome Alaska electric storage system (40 MW for 15 minutes) is battery
    http://library.abb.com/global/scot/...8161966fc1256e3f004e0366/$File/38-43 M848.pdf
    for two reasons. Water reservoirs freeze solid in the winter, and they needed a system that could come on instantly, summer or winter, in case of an electric power system failure.
    Last edited by a moderator: Apr 24, 2017
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