Electrical Equipment - Wind Energy Systems for Electric Power Generation - page 73

4.4.3.2 Superconducting Storage Devices

Superconducting magnetic energy storage (SMES) systems store magnetic field en-
ergy by a d.c. current flowing in a coil which is cryogenically cooled down below
its critical temperature by means of a refrigerator. In superconducting state the coil
resistance is zero, so that no current decay occurs and theoretically the magnetic
energy is stored indefinitely. The magnetic energy is:

E m = 1

2 LI 2

The design of coils for SMES has to take into account the inferior strain tol-
erance of conductor material, thermal contraction due to cooling and the Lorentz
forces due to the currents. Structural support is required for the coils which are
built either in solenoid or toroid form. The solenoids are less costly to manufacture,
and preferred for small SMES. The toroid, on the other hand, creates less exter-
nal magnetic force, needs less support effort and is therefore the choice for larger
coil sizes.
The system consists of the superconducting coil, a cryogenically cooled refrig-
erator, a power electronic rectifier/inverter to convert a.c. power to the coil arrange-
ment (charging) and to convert d.c. power back to the a.c supply (discharging), and a
protection device for emergency discharging. SMES systems are suitable for dyna-
mic operation. High overall efficiency values e.g. 95% are reached.
Since their invention high temperature superconductors (HTSC) are an option
beside low temperature superconductors (LTSC). Since for LTSC the typical basic
temperature is 4,2 K and the medium liquid Helium, for HTSC the respective value
is 77 K and Nitrogen as medium. Note that the critical current density of the wire
for HTSC is lower than for LTSC. Evidently the refrigeration cost depends also on
the technology used. An indicator is the losses due to heat conduction in the support
structure and terminal leads and to radiation, and its ratio to the input power of the
refrigerator.
As an example for a SMES system with LTSC wire of NbTi for 2 MJ ACCEL has
reported the following properties: Stored energy 2,1 MJ, current 1000 A, coil induc-
tance 4,1 H, flux density 4,5 T, converter intermediate d.c. voltage 800 V, nominal
discharge power 200 kW for A > 8 s, maximum power 800 kW.
A historical overview on SMES is in [Luo96]; modeling and simulation is dis-
cussed in [Ars99].

4.4.4 Mechanical Energy Storage

4.4.4.1 Water Pump Storage

Pumping water into a higher reservoir in times of low power demand, and utilize
the water flowing back to the reservoir on lower elevation through turbines when
the demand is high, is a well developed energy storage method. The machines may