International Baccalaureate

Extended Essay

Physics

The Flywheel as an Alternative Energy Storage Device for Electric Vehicles (EV): Problems Associated with the Implementation, and Possible Solutions

Candidate Name: Keith Lau

 Year 1999

IB Candidate Number: D0637036

Word Count: 3987

THE FLYWHEEL AS AN ALTERNATIVE ENERGY STORAGE DEVICE FOR ELECTRIC VEHICLES (EV): PROBLEMS ASSOCIATED WITH THE IMPLEMENTATION, AND POSSIBLE SOLUTIONS

Abstract        

Introduction        

My research question        

Maximizing the energy stored in the flywheel – in theory        

Why a flywheel is desirable to have a high angular velocity instead of a very large moment of inertia        

The maximum angular velocity of the flywheel        

The maximum energy stored        

What materials would be feasible in this application        

The leakage of energy stored        

How friction prevents flywheels from being efficient        

Magnetic bearing – and its workings        

The gyroscopic effect        

The gyroscopic effect of the flywheel        

What could be used to eliminate them        

Conclusion        

Appendix 1        

Vector nature of angular momentum (the gyroscope effect)        

Bibliography        

Books        

Articles        

Internet        

Title: The flywheel as an alternative energy storage device for electric vehicles (EV): problems associated with the implementation, and possible solutions

Abstract

A recently flourishing technology, known as flywheel energy storage (FES), could possibly overcome most of the drawbacks that are associated with chemical batteries in electric vehicles (EVs).  In this essay I will give a brief account of the development of flywheel technology, and then I will discuss the problems, which faces the developers of flywheel engineers.

I have briefly outlined the three main problems relating to flywheel development.  The first problem is maximizing the energy stored in a flywheel.  This involved equations describing the maximum velocity that a flywheel of a given material can spin at.

The centripetal force, P, which is mv2/r, would become

The tangential component of the force, F, is responsible for the tensile stress on the radial sections such as ad and bc.  The magnitude of F can be found by considering the vector diagrams of Figure 2.2.

        Figure 2.2(i) shows the direction of the forces involved, and Figure 2.2(ii) shows P resolved into two equal components F.  The angle between these two force vectors is  and so we have

Substituting in equation 2 we have

The stress produced by this force is

In terms of angular velocity (v = r) the stress is

Now we have an equation that defines the stress that the rim of the flywheel experiences at a given radius, rotational speed, and density.  The maximum tensile breaking stress that the flywheel can tolerate depends on the molecular properties of the material which it is built from.  If Sb is the tensile breaking stress of the material, then the maximum velocity that the flywheel can spin without tearing apart is given by

From this equation, we can see that if the flywheel material strength is strong, the maximum safe velocity that the flywheel can spin is increased.  Density, in this case, is the attribute that lowers the maximum velocity of the wheel.  Therefore, to extract the most speed out of a flywheel, the tensile breaking stress have to be maximized, and the radius and the density of the material minimized.  In simpler terms, we have to maximize the strength to density ratio.

The maximum energy stored

However, it might seem like equation 1 does not justify the fact that we have to maximize the strength to density ratio.  Since KE = ½I2, it seems that increasing the density would also increase the moment of inertia (since  = m / V).  But that is not the case.

The original L points in the y direction, but when it is added to ΔL, which points to the x-axis, the resultant angular momentum now points somewhere else.  This is why the wheel would swerve to the right instead of going upwards.  This property of spinning objects would affect flywheels the same way.

This phenomenon is what makes a child’s spinning top, also known as a gyroscope, precesses around a point on a table.

Bibliography

Books

1. McGillivray, Donald (1984).  Rotational dynamics.

Halley Court, Oxford: Heinemann Educational Books Ltd.

2. Giancoli, Douglas C. (1980).  Physics (4th Edition).

London: Prentice-Hall International.

3. Benham, P.P., Crawford, R.J., Armstrong, C.G. (1987).  Mechanics of engineering materials (2nd Edition)

London: Longman Group.

4. Blazynski, T.Z. (1983). Applied elasto-plasticity of solids.

London: The Macmillan Press.

5. Kleppner, Daniel., Kolenkow, Robert J. (1973) An introduction to mechanics.

Singapore: McGraw Hill Book Co.

Articles

6. Iannotta, Ben (1997).  Batteries not included.

New Scientist, Jan 11/1997, p. 30-33.

7. Rosen, Harold A., Castleman, Debroah R. (1997).  Flywheels in hybrid vehicles.

Scientific American, Oct 1997, p. 75-77.

8. McCann, Karey (1994).  Will it fly?  New flywheel technology isn’t pie-in-the-sky.

Wards’ Auto World, Vol. 30, 09-01-1994, p. 102.

9. Hively, Will (1996).  Reinventing the wheel. (Jack Bitterly is developing the flywheel automobile engine).

Discover Magazine, Vol. 17, 08-01-1996, p 58-68.

10. McCosh, Dan (1994).  We drive the world’s best electric car.

Popular Science, Jan 1994 (page number unknown).

11. Gwynne, S.C. (1996).  Business: What driving the Rosen boys?

Time magazine, 09-23-1996, p. 50.

12. Erickson, Jim (1993).  Device puts electric cars in fast lane.

St. Louis Post-Dispatch, 09-15-1993, p. 05C

Internet

13. Author Unknown (1994).  The new spin on flywheels [145 paragraphs].  (Gannett News Service, [on-line serial]).  [Online] Available: http://www.elibrary.com (this site is a searchable database, needs subscription)
14. Author Unknown (1997).  Flywheel innovation could doom battery backup [14 paragraphs].  (CMP Media Inc, [on-line serial]).  [Online] Available: http://www.elibrary.com (this site is a searchable database, needs subscription)