5 ms·

I was thinking about this the other day, why not use a liquid as the mass? Say water? If the spinning disk is treated like the tanks in a liquid tractor trailer

I was thinking about this the other day, why not use a liquid as the mass? Say water? If the spinning disk is treated like the tanks in a liquid tractor trailer, with slots to divide the mass up evenly. 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?

cjalmeida 8y ago
Density, cost, complexity.
- RobertRoberts 8y ago
  That's too bad. But that sort of describes every technology at one point in time...
Scaevolus 8y ago
Fluid flywheels are used for hydraulic couplings in automobile transmissions. 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.
- RobertRoberts 8y ago
  Ya, I was thinking that the water wouldn't be sloshing around, only being inside a sectioned donut wheel. The water could be hermetically sealed (with some kind of preasure relief for temp changes) in chambers, no sloshing or friction issues. Externally it would look exactly like any other fly wheel. 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.
- oneiric 8y ago
  That is not a flywheel.
dragontamer 8y ago
Power-density and energy-density. 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. ----------------- 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.
- RobertRoberts 8y ago
  That is really interesting, thanks for explaining it. 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. :)
  - dragontamer 8y ago
    > 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 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.
oneiric 8y ago
You are missing something (but don't worry, it is good to brainstorm like this). 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.
- RobertRoberts 8y ago
  >A better bucket flywheel would be all handle... Ok, that makes some sense... still not feeling it, but I can see the logic. Need more math in my education perhaps.

cjalmeida 8y ago

Density, cost, complexity.

RobertRoberts 8y ago
That's too bad. But that sort of describes every technology at one point in time...

Scaevolus 8y ago

Fluid flywheels are used for hydraulic couplings in automobile transmissions. 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.

RobertRoberts 8y ago
Ya, I was thinking that the water wouldn't be sloshing around, only being inside a sectioned donut wheel. The water could be hermetically sealed (with some kind of preasure relief for temp changes) in chambers, no sloshing or friction issues. Externally it would look exactly like any other fly wheel. 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.
oneiric 8y ago
That is not a flywheel.

dragontamer 8y ago

Power-density and energy-density. 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. ----------------- 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.

RobertRoberts 8y ago
That is really interesting, thanks for explaining it. 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. :)
- dragontamer 8y ago
  > 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 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.

oneiric 8y ago

You are missing something (but don't worry, it is good to brainstorm like this). 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.

RobertRoberts 8y ago
>A better bucket flywheel would be all handle... Ok, that makes some sense... still not feeling it, but I can see the logic. Need more math in my education perhaps.