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Flywheels Store Electricity

  1. Jul 29, 2011 #1
    Storing energy in flywheels is old. But at a cost competitive with the power grid is uncommon. And low losses over half a day storage, using affordable technology, hasn't been done, I believe. Links to my enabling technologies (drawings need to be logged in, sorry):

    A cheap flywheel material storing much energy, and how to produce the wheel

    Here a low-loss hydrostatic axial bearing:

    There a low-loss roller bearing for horizontal axis:

    And this shall achieve low loss in normal air:

    From the cost I evaluated, it would be cheaper to build power plants to produce the mean daily electricity consumption only, and provide the peak consumption over such flywheels.

    Also nice if a country lacks electricity production capability, like Japan now. Or if a country wants to close some plants in the future, like Germany.

    The designs are to operate immediately after an earthquake with 2G upwards acceleration and 3G sidewards. Useful as an emergency supply, when power plants shut off and lines break. Few units can supply a hospital or a factory.

    Cheap storage capability for half a day means that Solar electricity (I mean: Solar thermal electricity) becomes available all the day and dependable in favourable places like California, Neguev, Atacama and many more.

    Affordable storage over a few days - as it now seems - makes wind energy dependable in places like Scotland, Brittany, Galicia, Patagonia and more.

    Marc Schaefer, aka Enthalpy
  2. jcsd
  3. Jul 29, 2011 #2
    Well then, as the other site limits images to members logged in, here are the sketches of the hydrostatic axial bearings. You may have to click on the images to enlarge them.

    Attached Files:

  4. Jul 29, 2011 #3
    And the roller bearings:

    Attached Files:

  5. Jul 29, 2011 #4
    And the airflow calmer:

    Attached Files:

  6. Aug 1, 2011 #5


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    The problem with flywheels is the speed needed to store an appreciable amount of energy; how much energy are you looking to store and using what size disc?
  7. Aug 1, 2011 #6
    Thanks for your interest!

    Good steel, for instance spring steel, is cheap and can rotate fast. I compute with 390m/s for a torus used at 1200MPa, near the mean fibre. Other alloys like X35NiCr6 can harden by quenching through the thickness of such a torus and offer margin over this stress if tempered around 300°C.

    Wheels weighing 80t for OD=5m can still be transported by road, though requiring a special transport. The stored energy is then 6.1GJ, supplying 850kW*2h during daily peak consumption hours.

    80t low-alloyed steel cost around 120k€ from a forge. Two wheels with bearings, airflow calmer, a motor-alternator, a pit, transport, assembly may sum to estimated 570k€ to provide 1.7MW*2h and this is far cheaper than providing the peak power by the power plant's oversize - nuclear plants cost presently 3G€+ for 1.4GW.

    More about steel there http://saposjoint.net/Forum/viewtopic.php?f=66&t=1974#p22398
    and about cost estimates http://saposjoint.net/Forum/viewtopic.php?f=66&t=1974#p31668

    One wheel pair has the adequate size for a small factory. A hospital or a bigger factory would need several of them.
    At a power plant, one would assemble many wheels in line to share the pit, the motor-alternator... and would probably produce the flywheel at the site, so they can be bigger.
    But even these 80t wheels make sense for 1.4GW: it takes 100 pairs of them, which fit under the car park for the power plant's workers.
  8. Aug 1, 2011 #7


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    Yikes you're talking about some seriously monster units... You're wanting to spin an 80T flywheel faster than the speed of sound! Are you going to do this in a vacuum bunker with 2m thick reinforced concrete walls?
  9. Aug 1, 2011 #8
    This will happen in normal air, because my airflow calmer reduces losses efficiently (at least according to my computations!). The sketch is in the present thread, in the third message that has drawings. Computations are on the other forum, there
    with a sort of post-scriptum there

    To catch the pieces in case of a mishap, I consider only burying the flywheels. 2m concrete wouldn't stop 20t at 390m/s. For that sake, a vertical axis could be better.

    Seriously monster unit... The rotor of a turbo-alternator for 1300MW is about D=2m L=8m of steel, hence roughly 200t, and it rotates at 150m/s.
    http://de.wikipedia.org/wiki/Turbogenerator (nicer pictures)
  10. Aug 1, 2011 #9


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    Nuclear plants are not typically oversized for peaks, they are run at 100% and the peaks are served by cheaper souces such as natural gas turbine.
  11. Aug 1, 2011 #10
    Sure! In reasonable countries that don't produce 75% of their electricity by nuclear plants...

    With the flywheel costing about 1/3 €/W if I'm not too wrong, it's still significantly cheaper than a combustion power plant.
  12. Aug 2, 2011 #11
    You're talking about energy storage, but where does the energy come from to get the wheels spinning? Are you talking about storing energy that is produced during low requirement periods and then having that available during peak requirements?
  13. Aug 2, 2011 #12
    Can you explain how your bearing concepts could lead to products will lower losses than existing hydrostatic and roller bearings? I'm deeply suspicious that adding some rollers can somehow reduce losses. How about fatigue life, misalignment, etc?

    As for the air calmer, the air will still have the same average velocity gradient either with or without the disks, so there's still going to be high viscous losses. How did you calculate it?
  14. Aug 2, 2011 #13


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    Judging from the pictures (I haven't gone to your own website) you don't have any idea what is involved in making a safe and reliable device at this scale. For example the cross section of your rotor could be politely described as fanciful, if you actually try doing some stress and rotordynamics analysis on it. There is no way you can make this successfuly out of "standard quality" forged steel. And the only sort of "airflow calmer" that will work is to put the whole thing in a low vacuum.

    The company I work for has a rig that can spin a rotor weighing about 1T with rim speeds of about 300 - 350 m/s (and a lot of smaller spinning rigs as well) Based on that experience, I think your cost estimates are out by a factor of 100, or more likely 1000.
  15. Aug 2, 2011 #14
    It's not my website. But the thread there does contain answers to some of your interrogations.

    I don't write "standard quality", quite the opposite. Why do you, if you know it's impossible?

    So did my last company, and we developed them by ourselves.

    Do I perceive here a desire to denigrate instead of discuss? I've put figures about flow losses and references to computation methods, so I don't feel a need to answer unjustified assertions. And as the inventor of several successful new technologies, I'm not naturally inclined to believe existing ones impose the solutions, feasibility or cost of new ones.

    If you're worried by the shaft's diameter on the sketches, it's certainly not to scale nor even computed. The necessary shaft is feasible, so I concentrated on wheel material and on losses.
  16. Aug 2, 2011 #15
    Yes, that's it.

    This is an easier task for energy storage, as only the difference between peak and mean power must be given back and during a limited time, so a limited energy is to be stored for few hours.

    Storing Solar-only-produced electricity for instance would mean the complete consumption during the whole night, and this translates into more energy and bigger wheels - more difficult. Or worse, in a location where Sunlight during daytime isn't certain, one would have to store for several days the full consumed power times several days - a completely different task!


    Unrest, your interrogations about the rollers and the air calmer need and deserve detailed answers, but I'll first go to bed.
  17. Aug 3, 2011 #16


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    You haven't really answered the problem of spinning this flywheel so fast. Have you ruled out storing it in a low-grade vacuum for a specific reason?
  18. Aug 3, 2011 #17


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    So, you don't like my "unjustified assertions", but you say something is "feasible" and admit you haven't computed anything about the shaft design. Yeah, right.

    So let's do an elementary calculation for you:

    You have a 2.5m radius rotor with a rim speed of 390 m/s and mass 80t concentrated around the rim.

    That gives an angular velocity of 390/2.5 = 156 rad/sec = about 1500 RPM

    The acceleration at the rim is about 2.5 x 1562 m/s2 = about 6000 G.

    Fine, those numbers are in a sensible ballpark compared with existing real machines.

    But - the diaphragm of your disk needs to withstand about 80 x 6000 = 480,000 tons force of radial load.

    And if you have a disk burst, your bearings needs to withstand a similar sized load, unless you are confident that your design will never ever fail..

    Sure, you can quibble about factors of 2 or 3 in my numbers, depending on the exact shape of your rotor, but that won't get you out of jail.

    Sorry, but it's your call to draw something that looks like "real engineering" if you want me to take any of this seriously - not a big square section ring stuck on the outside of a thin plate and the whole thing supported on wimpy little bearings.
  19. Aug 3, 2011 #18
    Certainly NOT. The tore itself resists the centrifugal force, thanks to its tangential strength, and does not rely on the diaphragm for that, of course.

    This is a computation I of course did because it tells how much energy the tore stores. You can reasonably suppose I'm able to do it.
  20. Aug 3, 2011 #19
    If a disks bursts, I don't care a bit that the bearings resist! No single flywheel has bearings designed for that. Installing the flywheel in a pit shall give protection.

    Want you to take this seriously? Well, I didn't mean you in particular, no.
  21. Aug 3, 2011 #20
    Vacuum would be possible but has serious drawbacks: it needs a huge vacuum vessel that resists big forces and is hermetic - this is costly, it adds significant failure modes, and makes operations more complicated.

    So as I found flow loss can be small in air, I clearly prefer it, and try to explain it now.
    For a wheel of OD=5m ID=3.8m L=1.11m that rotates at 28Hz = 1680/min = 177rd/s, or 440m/s at the external radius, I add per side 70 disks spaced by 5mm air. That's many disks, but composite material makes them for cheap - we're talking about a storage unit costing half a million.

    Now, speed drop across the 5mm is 6.3m/s only. Air viscosity of 15mm2/s and 18.6µPa*s gives a Reynolds number Re = 2100, meaning a laminar flow. Shear constraint is then 23mPa at R=2.5m; combine with 19.6m2 and an integral coefficient of 0.5 to get a torque of 0.57N*m only. Tripling it for the other side and for the cylindrical face, the power loss is 305W, or over 10h 0.2% of the stored energy, nice. 70*5mm are in fact an exaggeration.

    Why add the many disks, as they don't reduce the speed shear?
    - They prevent radial eddy flows. Air with a high tangential speed near the wheel feels a centrifugal force which makes it flow outwards there and inwards farther from the wheel. Losses would be 1000 times bigger.
    - A bigger speed drop over more distance would enable turbulence, with the usual losses depending on V2 instead of V, again much bigger.

    Quite a few, err, engineering details remain open... I suppose the cylinders (which must be necessary) and disks composing a layer can be tied together around the wheel using a thread of high-performance fibre. 5mm spacing over thin parts of 5m diameter may require additional measures, maybe some flexible skis between the layers, that would fly by ground effect.

    The flow calmer looks odd? Sure! Because it's new. Being useful to alternators, pumps, turbines beyond the flywheel, it can become as natural in two decades as laminations are now in transformers cores.


    Unrest, I'll answer later about the big rollers at the bearings.


    Marc Schaefer, aka Enthalpy
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