(Some 480kg of mass in a ~600mm diameter configuration, at some 8-9000rpm maximum if memory serves)
It amazes me how hard it is to contain that kind of energy if it wants to get out. They're really terrifying devices.
Particularly when left in the hands of engineers who do not fully appreciate that any sufficiently rapid release of stored energy is indistinguishable from an explosion.
The balance bunker for load testing and balancing these at the shop I worked in was a pit 40 feet underground with 50 ton caps placed over top.
When the disk starts spinning all the liquid turns into essentially a solid, if there is a massive failure, simple venting could easily disperse the liquid 360 degrees or away from people, etc... (ie, a giant massive mist explosion)
I've worked with power washers that are dangerous when focused, but when the water is dispersed, it's practically harmless.
Is there some obvious thing I am missing here?
They're not good for long-term energy storage, since the fluid loses energy to friction and turbulence much faster than a solid-- think about how long a swirling vortex lasts in a cup versus how long spinning top can continue.
The thought being that since it's a safe medium to explode out, you could spin it faster than other mediums. The steel would just be a shell to hold the water, and therefore the containment vessel wouldn't have to be as robust, as the amount of steel that would fail would not be as much, and as soon as the water exploded out , the remaining steel would dramatically loose kinetic energy and possibly not even flyout.
Another comment said that 20k rpm would hold 4x the energy as 10k rpms. That implies that if you truly aren't afraid of the spinning medium hurting anyone, it would seem that water/steel hybrid would be better than solid steel, because you could spin it faster more safely.
The "secret" to dense flywheel energy storage is in the equation: Energy = 1/2 * I * w^2
Where "w" is rotational speed in radians/sec, and I is rotational-inertia / rotational-mass. So you store the most energy by rotating it as fast as possible. 20,000 RPM stores 4x the energy of 10,000 RPM.
The issue with water, or really... anything outside of steel and other such incredibly strong materials... is that water doesn't hold itself together very well. As such, the RPM limit would be smaller than a steel-flywheel.
Indeed, steel is still not used in advanced flywheels, because some forms of plastic IIRC are superior at the kinds of stresses we're talking about. Thus, these plastic / composite material flywheels, while much lighter, can spin much faster.
Leading to more efficient energy storage.
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In effect, you need every part of the flywheel designed to hold itself together to reach maximum efficiency. Water is the exact opposite. Its a lot of mass but basically no ability to handle stresses. Solid Steel would be better and would spin faster.
I want to ask then, based on other comments, that size can greatly improve the amount stored. So wouldn't a larger safer flywheel work in many places?
Also, my thought on the water as a solid, consider a steel wheel where instead of steel all the way through, put a liquid inside of a cavity. Since water doesn't compress, it would essentially be part of the same wheel, only safer in a collision? (arm-chair engineer)
I enjoy reading comments like yours so I can move on/let go of my from my silly pet ideas. :)
Well, consider that solid steel is considered too weak for a modern flywheel. So that questions why you'd be removing steel instead of adding more of it.
Modern flywheels spin really, really, really fast. Again, fast enough that modern solid steel is too weak. Newer composite materials are superior, but still need to be designed for maximum structural integrity.
Once a flywheel spins fast enough, it literally rips itself apart due to centrifugal forces. Any "cavity" weakens the structure and will break sooner.
There a few more relevant expressions for flywheels to express that you want them to be on the edge of breaking at maximum energy storage. http://large.stanford.edu/courses/2010/ph240/wheeler1/
Of course, the rotor has to be able to handle increased circumferential and radial stresses resulting from increased omega. Carbon fiber (like T1000 grade) has extremely high tensile strength (lets you spin a carbon fiber rotor very fast) but low density. T1000 rotors store more energy at lower mass than steel (or any other metal) because of the proportionality to omega^2 and their high tensile strength.
But for commercial storage, it's really about the $/kWhr/mass in terms of the overall rotor economics. By that metric, it's hard to beat a composite rotor with E-glass as the major fiber. It's cheaper than carbon fiber by easily 100X and has about 30% to 50% of CF tensile strength. It's a bit denser than CF, but its cost metric is why most modern large energy storage flywheels use glass/CF hybrid composites and magnetic bearings (for high speed and zero wear).
In the old days, they made flywheels out of solid steel, then switched to piano wire (higher tensile strength). Then fiberglass, aramids and carbon fiber happened.
Thanks for the additional math reference.
At maximum speed (maximum energy storage) you want every part of the flywheel to be about to break. By introducing a non-structural material (liquid water), you have added mass but not the strength to get to higher speeds.
Imagine filling a bucket with water and spinning it around your body. All of the force to keep the bucket from flying away has to be held by the handle of the bucket. A better bucket flywheel would be all handle... Hopefully someone else can give you a better analogy.
Ok, that makes some sense... still not feeling it, but I can see the logic. Need more math in my education perhaps.
I saw a Mazda RX3 Sport Sedan (a circuit racing class here in Australia ~25-30 years back) spectacularly blow a flywheel/clutch at the end of the main straight (at Oran Park) a long time back - it pretty much perforated all the bodywork in the plane of the flywheel - bonnet and both guards - as though it was a tearoff line on a set of stamps. We could hear bits landing 10+ seconds later hundreds of meters behind us in the parking lot. Depending on what port job he had, that could have been up around 12k rpm (if my vague 30 year old car modifying memory serves...)
I believe so, but compressor/turbine wheels don't have a lot of mass to them, so fragments don't usually escape very far if they blow up. I've seen a few compressor housings split, and the main damage is done to the engine itself really. I guess if your turbo was sitting high up and exposed, it might be a danger. Flywheels however just seem to fail more regularly from my experience.
(Also, yeah anything rotary powered deals with scary rpm's, haha!)
For anyone interested, here's some in-car footage of what we're describing here: https://www.youtube.com/watch?v=qOHUwL0zeXc and an image of what the result can look like: http://images.thetruthaboutcars.com/2011/04/LNJ11-BlowedUpPa...
The answer is to make the flywheel from a flexible kinetic lasso such that when it blows up, the forces tangle the parts in a manner that it is safely destroyed.