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From: agn@unh.cs.cmu.edu (Andreas Nowatzyk)
Newsgroups: sci.electronics
Subject: Re: Superconducting Energy Storage
Message-ID: <1005@unh.cs.cmu.edu>
Date: Mon, 25-May-87 23:01:51 EDT
Article-I.D.: unh.1005
Posted: Mon May 25 23:01:51 1987
Date-Received: Sat, 6-Jun-87 12:49:09 EDT
References: <226@ndmath.UUCP> <7766@orchid.UUCP> <19635@sun.uucp>
Organization: Carnegie-Mellon University, CS/RI
Lines: 27
Keywords: Super Conducting, NOT perfect batteries

The low current density of the new materials (order of 1000 A/cm^2)
is already history. Samples of the new material that were created by
vacum deposition techniques showed current densities arround
10^5 A/cm^2 at 77K. This is similar/better than the values for conventional
superconductors. In addition, critical current densities of up to
5*10^6 A/cm^2 were reported for the new material when cooled to 4K.

A superconducting energy storage device is simply a coil with a
superconducting switch. The stored energy is 0.5*I^2*L, were L is the
inductivity of the coil. Using ferromagnetic materials for the coil
is useless because their permeability decreases to 1 at high magnetic
fields. The real problem is to prevent the coil from destintegration due
to the high mechanical forces on the current carrying wire.

The inductivity of a cylindrical air coil is roughly 
    L = N^2*r^2/(9r+10l) *10^-6 Henry    (l,r are in inches here)
You see that the stored energy for a fixed size coil does not depend on N
(the number of turns). Assuming a D-cell sized coil (50% conductor, rest
mechanical support and current switch): l=2", r=0.5", N=1 => L=10nH
This coil uses about 5cm^2 to carry the current => I=500KA
Total eneregy storage would be 1250 Ws. A normal NiCd rechargeable battery
of the same size has roughly 18000 Ws!

On a larger scale, things look better for SC energy storage but the large
magnetic stray field might be unacceptable.

  --  Andreas