Environmental Engineering Reference
In-Depth Information
Table 10.11 Practical energy storage systems
Energy storage technology
Gravimetric energy
density (kWh/kg)
Reformulated gasoline
12
Methanol
5.2
LPG - propane
8.57
CNG at 2.4 kpsi and 70 F
2.05
Li ion
0.18
NiMH
0.065
VRLA
0.053
Zinc-bromine
0.070
Lithiumaluminium iron sulphide
0.090
Sodium-sulphur
0.096
Iron-air
0.053
Nickel-iron
0.053
Flywheel at 50,000 rpm
0.015
Hydrogen (hydride)
0.40
Ultra-capacitor
0.003
and rated for 130-170 V dc on the distribution bus of the International Space
Station (ISS). The system comprises two wheels at 3 kWh each. The integrated
power and attitude control system (IPACS) also uses the PMSM drive but at
20-60 krpm and is rated 12 kW, 100 V dc at the bus and comprises four wheels, each
rated 0.5 kWh for attitude control.
The technical challenges of flywheel energy storage and attitude control include
the need for high speed, compact packaging and high efficiency. Because of the
spacecraft borne applications, the lack of thermal conductive paths for heat rejection
necessitates low losses. Adjustable speed drive (ASD) rotor control is essential for
attitude control and axial machines are preferred at high speeds, so mass is low.
Robustness requires bearingless designs and mechanical touch-down mechanisms.
The system has been spin tested to 60 krpm, dc bus regulation has been verified and
the magnetic bearings validated. Prototype flywheel structures have been fabricated
with a rating of 3.6 kW, 3.66 kWh at 53 krpm shown in Figure 10.42.
The flywheel energy storage unit is designed to spin at 53 krpm and discharge
down to 30 krpm. Natural frequencies of this system are above the maximum
speed. The titanium rotor is made stiff by its large cross section to ensure that no
natural frequencies are encountered during normal charge/discharge operations.
For the rotor with stiffness, k , and mass, m , the first natural frequency occurs at
r
k
m
w ¼
ð 10 : 65 Þ
With proper design and materials the flywheel is a feasible energy storage
device because it is non-polluting and rechargeable. Modern materials capable of
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