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I've done some web searches, but I don't see anything very current on how close we are to having a home energy storage flywheel system that's comparable in price and performance to a battery system.

Also, as a bonus, what is the current state of a domestic-scale flywheel system in terms of maximum energy storage, power output, and usable energy (maximum energy minus minimum energy -- assuming there's a minimum speed they must maintain, unless there's not)?

  • 1
    As others have said - there is a lot of work on flywheel storage, but I don't think anybody is planning to deploy them in domestic settings. – Flyto Jun 19 '17 at 13:57
  • en.m.wikipedia.org/wiki/Flywheel_energy_storage High power, short term, low capacity seem to be the main characteristics. For domestic applicartions you typically care more about capacity and the time you can keep the energy than about power. – Martin Maat Jul 2 '17 at 8:25
  • Velkess is a name that was going around a few years ago. The company was looking to raise about $50,000 to develop a residential flywheel energy storage system. Eventually, they send the money back because the project didn't end up being viable. – LShaver Feb 27 at 21:04
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Lets check the pros and cons on flywheel energy storage and whether those apply to domestic use (source):

Compared with other ways to store electricity, FES systems have long lifetimes (lasting decades with little or no maintenance;[2] full-cycle lifetimes quoted for flywheels range from in excess of 10^5, up to 10^7, cycles of use),[5] high specific energy (100–130 W·h/kg, or 360–500 kJ/kg),[5][6] and large maximum power output. The energy efficiency (ratio of energy out per energy in) of flywheels, also known as round-trip efficiency, can be as high as 90%. Typical capacities range from 3 kWh to 133 kWh.[2] Rapid charging of a system occurs in less than 15 minutes.[7] The high specific energies often cited with flywheels can be a little misleading as commercial systems built have much lower specific energy, for example 11 W·h/kg, or 40 kJ/kg.[8]

  • long lifetime -> that would be a nice thing but not of the utmost priority. With 100,000 cycles (minimum taken from Wikipedia) it significantly outperforms rechargable battiers such as Tesla's Powerwall 1 with 5,000 cycles within warranty. Other sources report a design life time of 30 years (though for utility scale storage systems not for residential use)
  • capacity starting from 3 kWh -> A few to a few tens of kWh would put it well in a useful range for domestic use.
  • Specific energy of 11 Wh/kg (typical commercial system, value taken from Wikipedia) -> With an usable capacity of 13,5 kWh at 120 kg system weight Powerwall 2 outperforms commercial flywheels tenfold.
  • price -> While I did not find any good numbers for small home-use flywheels but I somewhat doubt they beat the 5.5k US-$ for a Powerwall 2.
  • round-trip efficiency (charge/discharge) of up to 90% -> in the range of what Powerwall claims (92.5%, source)
  • large maximum power output -> Again, no numbers for smale-scale systems but a high power throughput is not a primary concern for domestic use (7 kW peak; 5 kW continuous of the Powerwall seem quite reasonable for that use case).
  • safety -> It's hard to compare the risk from batteries (fire) to catastrophic failures of flywheels (kill'em all projectiles from a disc turning at 50,000 rpm).

In conclusion: with all the effort put into development of rechargeable batteries and upscaling of a whole industry to massproduce and market them I would doubt that flywheels are ever going to fly (pun intended) any time soon in a residential setting. I would rather expect to see them as grid energy storage in utility-scale levels that would benefit from the advantage of a high power output and where a higher capacity warrants the complexity of such systems (moving parts, vacuum, mag-lev).

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    Re specific energy: For a stationary system, energy stored per unit volume probably is more relevant that energy stored per unit mass. – Solomon Slow Jun 25 '17 at 18:23
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Domestic flywheels are unlikely to happen for 3 reasons:

  1. They must be heavy to store significant energy. If you need a crane to install one at your house it’s never going to be super cheap, even with high volume manufacture.

  2. The risk of the spinning mass fracturing requires special safety precautions - commercial operators put them in the ground but that would be super expensive on a domestic scale. Note that this isn’t the same as failed bearings where it might grind to a noisy halt. We’re talking heavy projectile at great speed.

  3. They don’t store energy for long 20-50% loss in two hours according to Wikipedia. In a typical solar powered home set up, to boil the kettle just before dawn you’d need to have stored ~13times that energy at sunset. You’d be better off having paid the extra to get it from the grid.

Now if you’re off the grid, that’s a different story. Storage Batteries have trouble with high power - short duration loads such as when your fridge compressor starts up. A very small flywheel could help here in conjunction with normal batteries. It being very small eliminates the problems of weight, safety and energy loss over time. But the right battery already does the job cheaper - a low capacity, high power battery like the one in your car (wired correctly; don’t try it!).

So for it to be cheaper it’d take a revolution in low cost, high speed bearings in tandem with a stalling in high drain battery development; and it’d still only be a very niche market.

  • Welcome to Sustainable Living! Nice answer. Could you please link to the Wikipedia page where it says that flywheels have 20-50% energy loss in 2 hours? – THelper Jun 6 at 13:06
2

My knee jerk reaction:

From what I know of mechanical systems, anything mechanical introduces losses due to friction in the form of heat, sound, and vibration. How would you even begin to use the mechanical energy stored in a flywheel? Simply storing the energy induces massive losses. Transferring the energy to its appliances introduces transfer problems such as losses, torquing rods, etc.

My reading after I wrote my knee-jerk reaction: Article showing state-of-art flywheel progress, with efficiency of 85%. Flywheels operate in vacuum. https://www.scientificamerican.com/article/new-flywheel-design/

Big $$ spent by Boeing on specialty-use flywheel: https://www.uaf.edu/files/acep/BoeingFlywheelOverview_06_20_2012.pdf

Afterthought: This seems to be something for specialty use. The promising flywheels seem to need uber-clean, highly maintained conditions. This is unlikely to be something for use in the home anytime soon. I expect that it is already in prototype experimentation in some factory.

1

TL;DR - Because of how flywheel energy storage scales it is unlikely that significant efforts will be made to develop the technology for home use.

This is similar to the case for windmills, where the power output increases as the square of the diameter, and the cube of the wind speed (which itself doubles roughly every 20m of elevation).

From the Engineering Toolbox, the equations governing flywheel kinetic energy are:

E_f = 1/2 × I × ω²
I   = k × m × r²

where:

E_f = flywheel kinetic energy
I   = moment of intertia
ω   = angular velocity (measured in radians/second, proportional to RPM)
k   = inertial constant (a value from 0 to 1 which depends on the flywheel shape)
m   = mass of the flywheel
r   = radius of the flywheel

If we were to assume all flywheels have the same shape, roll all the constants together in to some value K, and combine the two equations, we'd get something like this:

(flywheel kinetic energy) = (K) × (RPM)² × (mass) × (radius)²

Thus to maximize the energy storage of a flywheel we would focus on making it larger (increasing the radius) and faster, as the total energy will increase proportionally to the square of these factors. Note from @Ghanima's answer we know that efficiencies are already greater than 90%, so there isn't much potential there.

  • double the radius, quadruple the energy
  • double the speed, quadruple the energy

There's a fun calculator you can use to see this in practice.

For domestic applications, making things larger is not an option -- it has to fit in a home. Thus to advance the tech for home use, designers would have to increase the speed.

Increases in speed are limited by technology costs and electricity supply -- faster motors cost more, and require a higher current input than may be available in a typical home. Given that energy potential increases for this application are already limited by size constraints, it simply doesn't make sense to put more money into the motor and the electricity supply.

  • It's not the RADIUS but the angular momentum that matters. That means you can increase the RADIUS, or increase the MASS. Most commercial flywheel systems use a drum-shape to allow the flywheel to be extruded vertically instead of increasing the radius. A flywheel storage system is also almost identical in many respects to a power generator - it's often built around a magnetic coil so the motor and the output generator are the same piece of hardware, with simple switching reversing the circuit to switch between spin-up and discharge. – Steve K Mar 28 at 1:12
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The loss through storage due to friction by using a flywheel seems like it wouldn't matter much. In Florida there is plenty of sun. Who cares if some energy is lost. Anything collected is free. We as homeowners don't need a high tech composite spinning at high speeds. We should put a heavy steel one in the ground only turning at a safe slow speed. It would be a one time cost compared to batteries, that have to be replaced often. I'm sure the battery makers don't want to see flywheels on the market. It would be bad for their business. Flywheel is a new technology. The battery makers will make it seem like it's unsafe, but it's the perfect solution to energy storage. Municipalities won't go for it either, without a lot of money for licensing and even insurance. They're afraid of their own shadow. God forbid there's a spinning heavy disk in the ground outside my window. You'll never see them domestically. No company could ever guarantee that they'd be safe. The energy companies also don't want individuals to be self-sufficient. They want us to buy energy from them. They have a good thing going for themselves and they don't want anyone rocking the boat. Imagine thousands of homeowners not needing to pay an electric bill anymore. I visualize a one ton flywheel about 15 feet in diameter turning at 100 - 200 rpm. It would probably have to be in a cement enclosure, and in Florida a sump pump to keep it dry.

  • 1
    A 1,000kg, 5m, 200RPM flywheel would store 685,567J of energy if it was shaped like a disc. That's 0.19kWh of energy — enough to boil the water for about seven (7) cups of tea or run a typical airconditioner for about 10 minutes. I think you might be over-estimating how much energy these things can store. – Tim Aug 16 '18 at 3:49
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    This does not answer the How close are we? question and has more opinions than facts. – Jan Doggen Aug 16 '18 at 9:58

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