How Can We Improve the Efficiency and Affordability of Flywheel Energy Storage?

[HowlerMonkey]

Sure! Especially if the movie let's the wheel run slower than 390m/s, like: somewhat faster than a car...

And because I prefer this to remain a movie script, I consider burying the wheels in a pit.

What about a turbogenerator escaping its bearings, for your script? Only 150m/s, but 200t, and it runs over the ground.

Because engineers worried about this script, newer power plants with several units orient their generators so that an escaping turbogenerator doesn't smash the other unit's boiler. On older plants the orientation was random, and indeed unfavourable sometimes, including at nukes.

A series-cabled motor jumped from its bearings after running mad in my engineering school. This small one, like 1m long, destroyed the stone wall facing the motor. But hey, it has never happened with a full-sized turbogenerator - up to now.

Please take my statement at face value instead of reacting to something that is not there.

 

[Unrest]

Elastic bending at the shaft may become a limit, but the roller can be inclined (already done at turbogenerators' hydrodynamic bearings) and grinded slightly non-cylindrical.

What's wrong with hydrodynamic bearings again? Somehow I don't think bearings are an obstacle to making such a flywheel. 1% vs 3% capacity loss is insignificant at this high-level design stage.

Things like physical strength and air friction are probably real barriers - especially the latter.

And of course cost. Anybody can build a battery of flywheels that store a day's worth of energy, but not if it's uneconomical.

Another possible issue is the wide range of speeds it has to operate at. You might need a switchable gearbox, which would have to transmit those huge torques. Is that practical? I guess so but can't think of any machine which uses such a thing.

 

[Enthalpy]

Strength results in the speed I gave.

I have confidence that the flow calmer I described here keeps air loss low, but it has to be checked, and certainly developed further, as technology never works from the beginning. Without the flow calmer, air loss would waste hundreds of kilowatts, as computed with established methods.

My cost estimate is here somewhere, it summed up to 570k€ for 850kW*4h, which is economical.

Torque is small, with 850kW at 39Hz. No gear, since such speed and torque fit a reasonable motor-alternator.

I wish hydrodynamic bearings would lose only 3%! Then I may prefer them to other bearings. Computed with established methods, they are to waste 18% of the energy over 10h.

The standard roll bearings waste 3%. While this is acceptable, possible improvements should be taken. Also, if such a flywheel is used as an emergency backup, the user may want it to store energy for 3 days or 1 week, needing improved bearings.

 

[Unrest]

I have confidence that the flow calmer I described here keeps air loss low, but it has to be checked,

That would make or break it. I think you could get better feedback from people if you presented it as a design you're working on and you're trying to calculate the losses. Without making any bold claims or mentioning flywheels. Then people will have a more helping attitude and perhaps give some good ideas.

Have you considered using a flywheel privately to buy power at cheap rates and sell it when the price is higher? Eg night/day cycle. This must be unfeasible at smaller scales or people and power companies would be doing it already. But that's pretty much the only way to see it ever become a reality.

 

[Dmytry]

Magnetic bearings look feasible, it's just that I prefer the big rollers and the hydrostatic bearing. What could ruin them is aerodynamic loss at the magnetic gap: this gap must be thin, have a minimum area, and be empty. The gap alone could require vacuum operation.

A 1MW motor doesn't need 1t of magnets, sure. Far less here, at 300m/s ! An alternator is difficult when a wind turbine rotates it without a gear, but even there, no ton is needed - see Enercon:
http://www.enercon.de/de-de/ringgenerator.htm ("Choose your language" - nice Pdf)
and my electrostatic alternator and motor would use no magnet on wind turbines, hydroelectric dams, boat propeller pods. Topic "electrostatic alternator" at Saposjoint.net, Science > Technology.

Yea. Utter bullcrap on nuke advocacy sites, i just googled wind generator neodymium and sure some nucleargreen blog originated the 1ton.

Obscure to me, when combining the motor-generator with a magnetic bearing: I estimate an amorphous core material is unaffordable as it needs boron, and iron-silicon looses too much power if the induction varies at the rotation frequency - hence the design with a DC axial induction. As opposed, the motor-alternator needs some induction rotating against an iron core: I suppose this can't be the lifting flux, but a much smaller flux that affords expensive material. Then, sharing the electronics looks difficult.

Hmm. I'm thinking some windings on the rotor to create flux for lift when it is spinning. Need to try on paper several designs.

Atmospheric operation, roller bearings, steel... are all consequences of cheap and big construction. A reaction wheel for spacecraft would make the opposite choices.

Well, you have big but ultra precise construction, with very tight tolerance on it being off-axis (micrometer precision) edit: for fluid bearings at least. It won't be exact enough so it will inevitably need balancing; then the stresses in the wheel can change it's shape a little when its at high speed. Doesn't look like cheap construction to me.

Magnetic bearing and motor-generator can self-centre, if its axial flux, with tolerance of millimeters - i.e. the wheel just spins around it's centre of mass, it's ok if its a little bit off. The primary difficulty with this is in engineering, not production. It's difficult to mentally reason about stability of magnetic bearing. Math's quite complicated. For big cheap construction, prepare to do very difficult engineering.

I don't see what's the problem with vacuum operation. You are putting it in concrete pit anyway. Low grade vacuum should be enough.

The roller bearings - if it works it would be great. You'll absolutely need to allow for slight eccentricity though.

 

[Enthalpy]

[The flow calmer] would make or break [the flywheel feasibility].

Yes.

I have little doubt a laminar flow is achieved if desired, and eddy currents then dissipate far less than the Couette flow itself does. A bit unclear is what the pressure gradient along the radius does; it could augment the loss of the eddy currents, but I expect thermal conductivity to reduce the adiabatic temperature gradient, and this would block the eddy currents. Anyway, I could estimate these effects, but flow mechanics is essentially experimental.

A bit more to come about their realization.

...using a flywheel privately to buy power at cheap rates and sell it when the price is higher... must be infeasible at smaller scales or people and power companies would be doing it already. But that's pretty much the only way to see it ever become a reality.

Or it becomes feasible after some technological progress! Without good bearings and the flow calmer (or vacuum), it looks difficult. Also, big size reduces relative loss.

I see it realized as the described medium-sized units - that is, a size still transportable by road.
On a smaller scale, flywheels already store energy at uninterruptible power supplies for computers, where price is an easier constraint; better bearings and aerodynamics would be welcome.
If built as a complement to power plants, I expect the unit wheels to be even larger, and be produced in situ, by having a forge there when the site is built.

 

[Enthalpy]

...windings on the rotor to create flux for lift when it is spinning.

It's used on the Japanese Maglev train. The movement induces current - say, in the rotor - just as in an asynchronous motor. If the rotor is mainly resistive, the current creates a drag force in the rotor with respect to the field, and because the field rotate, this drag let's the rotor of an asynchronous motor follow the field with a slow slip.

If the rotor is mainly inductive, the induced current creates a repulsive force mainly. Put the gap horizontally, you get a repulsive lifting force, self-stabilizing along the vertical axis, and on all axis if the gap is a cone, a cup or similar.

The Maglev train uses superconductors, but it may not be necessary at a big size.

For the flywheel storing electricity at mains cost, I consider lift by induction is too expensive because the induction must vary at the rotation frequency or faster, and low loss would require amorphous magnetic materials which cost a lot as far as I know. Maybe I missed something, like amorphous materials without boron. This would enable other magnetic bearings designs, including active attraction designs where the induction varies within one rotation - and then a horizontal axis becomes possible.

In case you need electric power in the rotor, you can supply it without brushes by an auxiliary alternator, like the rotor gets power in turbogenerators.

From my estimates, attraction by a bold electromagnet has the least resistive losses, as compared with a coil in a field and so on.

 

[Enthalpy]

...big but ultra precise construction, with very tight tolerance on it being off-axis... Doesn't look cheap.

Magnetic bearing can self-centre with tolerance of millimeters - i.e. the wheel just spins around its centre of mass.

The roller bearings... will absolutely need to allow for slight eccentricity though.

Yes, magnetic bearings leave the wheel rotate around its centre of mass, while roll bearings force the wheel around its shaft. But manufacturing doesn't get hugely difficult:
at 39Hz, if eccentricity shall give less than 0.1G radial acceleration (acting as an undesired force on bearings), the centre of mass can be 16µm off-axis.

This is obtained by balancing, after the wheel is turned anyway so it can have ~200µm precision and hugely better concentricity. This needs a turning machine costing several M€, which makes the tore within a day but lasts 10 years. Cheap for a half-M€ storage unit.

I don't see what's the problem with vacuum operation. You are putting it in concrete pit anyway. Low grade vacuum should be enough.

I'm not basically against vacuum! But it does add its own worries. For instance, the D=6m seal ring needs a perfect shape at the contacting surfaces, which must be your metal liner outside the concrete. The liner can be turned accurately before pouring concrete, but after pouring it will be deformed.

As well, I have Japan in mind, which lacks peak production capability and would also welcome an emergency supply after an earthquake stops power plants. I'm weary - maybe without reason - against a big hermetic seal in a quake.

So provided some hardware (my flow calmer) permits operation in air, I prefer that.

 

[Enthalpy]

[Answering to Enthalpy's: "I have a reason to cite uses early: that no one gets a patent for obvious uses of my invention."]
Even if someone does patent it, is that so bad? Patents have a purpose - to promote technology development. Are you trying to stifle it?

When the real inventor gets a patent, it promotes technological development.
When someone gets a patent about someone else's invention, or about obvious developments of the invention, it promotes inventions theft and hinders progress.

So, yes, I cite the obvious developments of my ideas in order to stifle these attempts of getting undue patents, and I will go on doing so. Patents should protect non-trivial improvements, and I have nothing against if someone adds significant improvements to my ideas and patents these improvements.

For similar reasons, I create proof material for the contents of my ideas and the date I make the inventions. This includes describing the invention's capability (call it bold claims if you wish) rather than asking for help when the enabling ideas are already here.

You know, I'm an old inventor, so I know how many people try to let me develop things they imagine are more useful, and believe to go unnoticed, and as I publish an idea, tell "already exists" and "uninteresting", or claim "impossible" in order to get more details.

 

[Enthalpy]

The disks and cylindres at the flow calmer rotate fast and must resist the same centrifugal acceleration as the flywheel itself. Cold-laminated stainless sheets would be strong enough but the size makes them difficult. Alternately, fibre composites, especially (carbon fibres) graphite fibres, are even better than steel at fast rotation and can produce the big parts rather easily.

The performance gap over steel permits to use fabric or crossed unidirectional layers laid down with the impregnating matrix and overlapped. A stronger product would result from filament winding; this is usually done by a special machine, some exist for rocket parts with D=3m at least. Winding a disk can be done off-centre, which leaves a hole at the centre but loses no strength, as is done at the end of pressure vessels.

Big parts with small spacing must be stiff. Graphite is an excellent start; making a sandwich of it, for instance around a core of balsa wood (or any known material), produces very stiff parts. Or without a sandwich, the shape can stiffen a part with cylindrical symmetry: for instance a cone, or if the part has integral stiffeners parallel to the rotation - becoming stiff for the same reason as corrugated iron.

I consider holding the cylinders by the disks. A circumferential rope or thin rod made of present-day fibres holds easily the centrifugal acceleration that is difficult for steel; such a rope could pass many times through the ends of a disk and a cylinder alternately, a bit like if sewing. It can be opened within a reasonable time. When producing the disks and cylinders, handles can be made for the rope, and filament winding maintains full strength there.

I already mentioned aerodynamic skis, with some elasticity, to hold the distance between the calmer's layers. Anyway, I don't imagine a reliable prediction of the collective stability of the flow calmer, which should be experimented early in the development.

Marc Schaefer, aka Enthalpy

 

[Unrest]

When the real inventor gets a patent, it promotes technological development.
When someone gets a patent about someone else's invention, or about obvious developments of the invention, it promotes inventions theft and hinders progress.

I may have had the idea first. Years ago I thought of the different-speed disks for a very similar purpose. Also, like you I didn't do anything useful with my idea, just imagined it. I'm perfectly happy if someone else steals it. Wouldn't you be too? It'll make the world a better place if people who do productive work get paid, rather than the armchair inventors or patent-squatters. Seeing you present the same idea I had made me think it might be very common. Perhaps it could be classified as "obvious" and unpatentable.

 

[Space_Cowboy]

If I'm not mistaken, I believe I remember reading that some commercially available rotors used for this purpose, use a composite formed by coiling epoxy impregnated carbon fiber. This material is used for its extreme stress resistance. There is no way any regular steel, especially eight tons of it, will stand up to the kind of centrifugal loads you're talking about.

What's the purpose of spinning it in air? The commercially available units have an armored casing - in case the rotor shatters, and in order to be able to pull a vacuum. The goal is reduction of drag, and if you're spinning the thing in air, you've got a lot of drag to contend with. The only thing that can mitigate this, at such speeds, is a vacuum.

Also, good luck finding any bearings besides expensive and energy-sucking electromagnetic levitation bearings, that can hope to stand up to eight tons of weight.

I hate to seem like a debbie downer, but there are very good reasons why the commercially available machines of this type are designed the way they are.