Environmental Engineering Reference
In-Depth Information
SOC controller actively manages the ultra-capacitor energy and recuperates the
maximum possible regenerative energy.
Table 4.7 Performance comparison of lithium ion battery alone versus active
combination
Component
I
mx
I
mn
I
pp
I
avg
I
rms
U
b
_pp
U
b
_rms
U
c
_pp
U
c
_rms
Battery only
208.2
136 344.3 11.77 41.8 17.4 2.5
Battery plus ultra-capacitor combined via dc/dc converter
Active Combination
Battery Fraction
133.7
24
157.9 10.9
21.7
8.75
1.5
Converter Output
96.7
129.4 226
0.47 25.7
Ultra-capacitor
Fraction
156.8
204.2 361
0.57 38.3
90.6
98.5
For the active combination of 78 Wh of ultra-capacitor, 58 Wh useable and
28,000 Wh of lithium ion energy, a dramatic reduction in battery currents and
terminal voltage ripple is realized.
Table 4.7 shows that virtually all the available regeneration energy is routed to
the ultra-capacitor except for a small threshold of
12 A for minimizing converter
operation at very low power and its consequent inefficiency. This particular
implementation also has the ultra-capacitor pack voltage only slightly lower than
the lithium ion pack voltage so the converter operates over very shallow boost and
buck gains. There is great flexibility here to operate with even lower or higher
voltage on the ultra-capacitor relative to the lithium ion pack. The choice depends
on available cells/modules and packaging constraints.
Example 3:
Using (4.6) and voltage levels taken from Figure 4.14(c) at the fol-
lowing time points, calculate the duty cycle boosting,
d
1
, and buck mode,
d
2
, that
satisfy the battery voltage of 150 V.
Given:
U
c
=76Vat
t
= 480 s and
U
c
= 110 V at 510 s.
d
1
¼
ð
U
d
U
d
¼
ð
U
c
Þ
150
Þ
150
¼
0
:
493
76
d
2
¼ð
1
d
1
Þ¼
0
:
507
4.2.4 Torque and power
Motor-generator capability curves for torque and power define the peak operating
capability of the hybrid electric system. It is necessary to be clear in understanding
that the capability curve defines the operating bounds of the hybrid ac drive system.
Figure 4.15 shows the defining characteristics of the torque-speed envelope
regardless of M/G technology.
