Environmental Engineering Reference
In-Depth Information
The capacitor mass required for a direct parallel connection is very difficult to
determine because the solution will be either energy or power limited. To examine
how this comes about, we calculate the system power available from a direct par-
allel connection of an ultra-capacitor with an NiMH battery pack. Furthermore, the
specific pulse power density for the capacitor is taken as 1,000 W/kg and for the
battery, 500 W/kg. Both values represent the high end of pulse power capability for
these technologies. Recognizing that the system voltage swing is constrained as
before, we write the equations for system power,
P
sys
:
P
sys
¼
P
batt
þ
P
cap
-
avail
¼ h
c
g
P
b
m
b
þ h
c
ð
1
s
2
Þg
P
c
m
c
ð
10
:
40
Þ
where
m
c
¼
M
stor
m
b
ð
10
:
41
Þ
Equation (10.41) represents the available mass target for ultra-capacitor. The
system power necessary to meet the peak power specification requires a starting
approximation on storage system mass. An initial approximation to storage mass is
P
pk
g
P
b
M
stor
¼
ð
10
:
42
Þ
A good starting point on storage system mass according to (10.42) would be
80 kg. Equation (10.40) is plotted in Figure 10.31 versus ultra-capacitor mass. The
interesting point is that storage system mass was increased to 100 kg to allow for
real world discharge efficiency. In this example, it is a coincidence that the capa-
citor mass is 50% of the total system mass available in order to meet the peak
power specification.
P
comb
(
i
)
P
c
(
i
)
P
b
(
i
)
6 × 10
4
10
4
4.5
×
4 × 10
4
2 × 10
4
0
0
0
20
40
60
80
100
100
1.110223 × 10
-14
m
c
(
i
)
Capacitor mass (kg)
Figure 10.31 System power of direct parallel ultra-capacitor and battery
connection
