342 Ë 9 HTS Maglev bearing and flywheel energy storage system
Fig. 9.11:Flow rate and head of liquid nitrogen pump with RSB at different rotational speeds.
Tab. 9.2:Relation of rotational speed, flow rate, and head of liquid nitrogen pump with RSB.
Rotation speed (rpm) 756 1065 1298 1547 2075 2245
Flow rate (L/s) 0.0611 0.0866 0.1130 0.1319 0.1558 0.1603
Head (m) 0.0549 0.0699 0.0894 0.1094 0.1368 0.1425
2245 rpm, the current and the voltage of the DC power is 2.8 A and 18.5 V, so the power
is 51.8 W. Increase of mechanical energy of liquid nitrogen in 1 s can be calculated:
ΔE=
Q
1000 ×휌agHpump=0. 1603
1000 ×^808 ×^9.^8 ×^0.^1425 =^0.^181 (J), (9.21)
where,휌(=808 kg/m^3 ) is the density of liquid nitrogen,agthe acceleration of gravity,
and 0.1425 m is the head of the pump at 2245 rpm.
The total efficiency of the pump is
휆=^0 .181 J/1 s
51 .8 W=^0 .181 W
51 .8 W
= 0. 35 %. (9.22)
As a cryogenic pump prototype, the efficiency is low, although some mechanical
parts are not specially designed for the liquid nitrogen pump. The gap between some
parts should be redesigned to be smaller in order to reduce the leakage of liquid
nitrogen.
Second, for practical application, the displacement of the rotor due to force creep
of the RSB should be reconsidered, too. The critical current density of the HTS bulk
increases with decreased temperature (such as in liquid helium), so a lower working
temperature benefits to the performance of the RSB. More research on characteristics