Principes

In a SMES, the magnetic induction B is generated by a permanent current which circulate in the superconducting short-cut coil.  The energy volumic density associated to such a setup (B/2µ 0) can be rapidely given back.  This corresponds to very high power.  The great advantages are its excellent rendement, its very high answer time and its quasi infinite cyclability.

In so doing, the system can be divided in 3 regimes :
  1. the charge phase : the voltage is applied to the coil and the current increase up to its expected value
  2. the storage phase : the coil is short-cut and the superconducting current occurs in the coil
  3. the discharge phase : the energy is given back to the net via a current-voltage convertissor.
A priori, the injection/retirement(?) power can be based on two different principles :

fig fig

Fig. 1 : Comparison of the two basic principles for the power transfert between the superconducting coil and the extern network : (a) superconductor switch and (b) electronic switch.

*    a superconducting switch, soudé in parallel on a storage coil (Fig1a) play a short cut role or open circuit according whether the temperature is lower or higher than the critical temperature Tc.  In this later, the characteristic time to start the charge or the discharge process is at least as long as the expected time to locally heat and cool the wire (some seconds).  The state of the superconducting swith can be also remoted by a magnetic field which is able to induce a transition from the superconducting state towards the normal state.  This process is much quicker than the thermal procedure.

*   an electronic switch (MOS transistor for example) is placed in parallel with the coil (fig1b).  In this case, the injection and the retirement of the power can be done in a very quick and easy way, typically some milliseconds!  On the other hand, the persistant current as to circulate in the transistor even during the storage phase.  That means that power is dissipated in the MOS (some milliwatt).  This lost power will give the maximum storage time.


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