Environmental Engineering Reference
In-Depth Information
combined capacitor plus battery energy will match the system energy requirements
when the ultra-capacitor mass is near 100 kg. The system is then clearly a small
battery, large capacitor solution.
400
357.5
E
cap
(
i
)
E
bat
(
i
)
10
E
req
200
0
0
0
1.221245 × 10
-14
50
100
150
110
m
c
(
i
)
Ultra-capacitor mass (kg)
Figure 10.32 Direct parallel connection ultra-capacitor mass to meet
energy constraint
The ultra-capacitor mass for the direct parallel connection is calculated by
substituting the value for battery mass from (10.46) into (10.40). The resulting
expression is then set equal to the specified peak power. The result after sub-
stituting for battery and ultra-capacitor specific power is
P
pk
h
c
g
P
b
m
b
h
c
ð
1
s
2
m
c
¼
ð
kg
Þ
ð
10
:
47
Þ
Þg
P
c
The total system mass is given by (10.41) after rearranging for
M
stor
. In this
configuration the ultra-capacitor mass computes to 109.7 kg and the total system
mass becomes 111.2 kg.
A more realistic configuration is the independent power converter interface to
the ultra-capacitor shown in Figure 10.27(b). In this connection the battery system
bus voltage constraint remains, but the ultra-capacitor voltage is permitted to droop
substantially lower. This means more energy is available from the ultra-capacitor,
so its mass can be reduced well below that obtained for the direct parallel con-
nection. In fact, if the voltage ratio used in (10.40) can be set to 0.33 from its value
of 0.78, it can yield a substantial increase in capacitor available energy. This con-
figuration is now explored in more detail.
There is a rather interesting relationship for
P
/
E
of an independent processor
ultra-capacitor connection. To describe this, the mass of the ultra-capacitor is taken
as its available power (i.e. the ratio of system peak power demand minus battery
