Environmental Engineering Reference
In-Depth Information
diffi cult to calculate and has been extensively researched. However, nearly all of
this research was for AC current applications such as HTS power cables and trans-
formers, where the HTS wire current and magnetic fl ux density is purely AC, and
small relative to the DC critical current value, such as in [36]. In this situation there
will also be hysteresis losses in the magnetic substrate of the 2G HTS wire. The
2 MW generator in [37] has a permanent magnet fi eld and pure AC current in the
HTS stator winding. In the Converteam HTS generator the HTS wire is operating
with a DC current and in a very high DC magnetic fl ux density, not far from the
wire critical current, with a very small (compared to the pure AC studies, and even
slammer compared to the DC component) AC component superimposed. The 2G
wire magnetic substrate would be fully saturated in this case, and would not expe-
rience hysteresis loss, but these will still be losses due to the changes in trapped fl ux
within the superconductor (also known as hysteresis) and due to eddy currents in the
wire substrate. Converteam Ltd. have commissioned the University of Cambridge,
UK to carry out a theoretical analysis of AC loss for wire under these conditions
backed up by tests using a variable temperature insert in a 5 T LTS magnet.
6.4.4 Cooling of HTS coils
It is essential that during operation the HTS coils are maintained at a temperature
such that there is suffi cient margin between the operating fi eld current and the
critical current of the wire. In order to minimise the power input to the cryocooler
and make best use of its cooling capacity a temperature difference as small as pos-
sible between the cryocooler and the HTS coils is desirable.
Past HTS motor projects have used either closed circuit helium gas circulation
[38], or phase change neon cooling systems. The neon-based systems, such as
described in [39] condense the neon gas at the cryocooler at its boiling temperature
of 27.2 K. Liquid neon is then supplied to the rotor and allowed to evaporate,
removing heat from the rotor in the process, and returning to the cryocooler as a
gas. This type of system has the advantage that it is a very effective cooling pro-
cess and can operate as a thermosiphon, with no mechanical assistance to the cir-
culation. One disadvantage to such a system is that the cryocooler cold head
temperature varies with heat load, and it is necessary to introduce a heater to the
system to prevent the temperature from dropping to 24.6 K and freezing the neon,
wasting cryocooler power. A second disadvantage is that the coolant temperature
is fi xed at 27 K, and it is expected that with 2G HTS wire, that the operating tem-
perature could be considerably higher, probably in the range 40-60 K. A third
disadvantage is that cooling will be non-uniform when the rotor is stationary. This
could cause undesirable stresses in coils and their support structure.
A helium gas circulation system was chosen for the HTS wind generator. While
this had the disadvantage of requiring assisted circulation, it offered complete fl ex-
ibility in the choice of operating temperature. Heat was transferred between the
HTS coils and the cold helium in the rotor cooling circuit by conduction. In order
to calculate the heat fl ow and to determine the coil operating temperature, it was
necessary to use detailed computational fl uid dynamics (CFD) and thermal FE
models that also had to take into account the larger (order of magnitude) variation
Search WWH ::
Custom Search