Flywheel energy storage, specific energy. / General Science and Electronics / Forums

Forums

4hv.org :: Forums :: General Science and Electronics

« Previous topic | Next topic »

Flywheel energy storage, specific energy.

Move Thread

LAN_403

Patrick

Thu Nov 20 2014, 05:05PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

ive looked at flywheel energy sotorage, but struggle with the math.

for a specific case:
200 grams at the rotors' external circumference
10,000 RPM

= how many joules ?

im wondering how this compares to LiPo specific energy...

from my math of a fly wheel i get:
(10,000^2)(0.5)(0.2) = 10,000,000 J
12x40 = 480
P = j / t = W
10,000,000 J provides 480 Watts for X number of seconds ? (whats the formula for this, i feel like i goof it when i try to work it out.)

Ash Small

Thu Nov 20 2014, 06:14PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Patrick wrote ...

ive looked at flywheel energy sotorage, but struggle with the math.

for a specific case:
200 grams at the rotors' external circumference
10,000 RPM

= how many joules ?

im wondering how this compares to LiPo specific energy...

from my math of a fly wheel i get:
(10,000^2)(0.5)(0.2) = 10,000,000 J
12x40 = 480
P = j / t = W
10,000,000 J provides 480 Watts for X number of seconds ? (whats the formula for this, i feel like i goof it when i try to work it out.)

Once you take the centripetal/centrifugal forces into consideration, adding weight to the rotor tips means strengthening the rotors which means thicker section which means more drag. It also adversely affects manouverability.

Patrick

Thu Nov 20 2014, 06:22PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Yes this is what I suspect, but I keep getting a guy on a different forum Telling Me A ND Everyone Else Flywheels Are Better ThaN Batteries. BuT there's huge mass for support, and he thinks magnets will be magically light.

klugesmith

Thu Nov 20 2014, 07:14PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Patrick, by using RPM in place of velocity you failed to account for the radius of gyration
and also missed factors of 60 and 2 pi.
If you don't have a spreadsheet calculator program, get one. Cells are cheap.

As a very rough rule of thumb, the speed limit for cylindrical flywheels has a rim velocity of about the speed of sound (in air).
That means your 200 grams could be about 1 foot from the axis, and store about 10 kJ.
Your 10 MJ result would be correct if 10,000 were the velocity in m/s, which is enough to orbit the Earth but impossible in an unmagical flywheel.

This reference confirms that 13.9 watthours/kilogram is a state-of-the-art value for specific energy.

http://www.compositesworld.com/cdn/cms/ORNL Flywheel Assessment for Hybrid Vehicles 2011.pdf

(the URL with a space character failed to convert to a hot link here)

Shrad

Thu Nov 20 2014, 08:00PM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780

don't forget that such a flywheel is usually encased in a metal cage to protect lives if it shatters ;)

Ash Small

Thu Nov 20 2014, 09:00PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Keeping rotor mass to a minimum is one of the chief priorities here. The other is minimising drag.

2Spoons

Thu Nov 20 2014, 09:20PM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
Location:
Posts: 615

I worked on a flywheel storage system some years back: it was designed to store 1MJ, using a 10kg flywheel spinning at 40,000 rpm. The flywheel was about 1 foot in diameter, made from a carbon composite to stop it exploding, and run under vacuum to stop it melting. Bearings were by far the biggest problem.

Patrick

Fri Nov 21 2014, 01:08AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

klugesmith wrote ...

Patrick, by using RPM in place of velocity you failed to account for the radius of gyration
and also missed factors of 60 and 2 pi.

Oops.

klugesmith wrote ...

As a very rough rule of thumb, the speed limit for cylindrical flywheels has a rim velocity of about the speed of sound (in air).
That means your 200 grams could be about 1 foot from the axis, and store about 10 kJ.

i forgot the length, and 1 foot is a lot, plus a whole lot of kevlar. This will never beat a similar mass LiPo.

The flamer who provokes me is fine with magical conditions.

so 10kJ, would provide 480 watts for 21 seconds ?

BigBad

Fri Nov 21 2014, 03:04AM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600

There's no theoretical upper limit on the rim speed; it just gets more and more massive to go faster.

The trick is to put an exponential taper from the central axis towards the rim.

To get optimum performance you want something with a very high strength/weight ratio; Kevlar is good but there are better. CNT or graphene are potentially enormously better.

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob