Environmental Engineering Reference
In-Depth Information
Physical level energy storage combinations can be implemented as either direct par-
allel, that is the tandem connection, or in active parallel as discussed in the previous
section. Unfortunately, the simplicity of tandem connections is outweighed by a con-
strained SOC window on the ultra-capacitor, whereas the power electronic interface in
the active combination enables the widest possible scope in applications that benefit
from decoupled power and energy. Use is made of a state-space average model of the
bidirectional buck-boost power converter that buffers the ultra-capacitor. The aver-
aged model facilitates the evaluation of actively combining the ultra-capacitor with a
lithium ion battery versus different EMS. The results of various EMS that provide
decoupled power and energy while simultaneously meeting the requirements for a
seamless transition between boosting and regenerative energy capture, all the while
dynamically managing the ultra-capacitor voltage swing and battery clamped fixed
voltage bus, are demonstrated. For the BEV case study evaluated here, the vehicle
characteristics and added battery mass are summarized in Table 10.7. For more details
refer to the steps discussed in Section 4.2, Chapter 4.
Table 10.7 Vehicle parameters used in the performance simulations
Parameter
Description
Rating
Parameter
Description
Rating
Battery pack
44S
1P
200 Ah
144 V,
28 kWh
Aerodynamic
C
d
(#)
0.25
Battery mass
kg
303.6
Rolling
resistance
R
0
(kg/kg)
0.08
Vehicle mass
kg
920 (1,127)
Tyre rolling
radius
P205/50R16V
r
w
(m)
0.4
ICE power
kW
125
Frontal area
A
f
(m)
1.96
Step 1
: Model the ESS components and the vehicle. Simulate over the desired
drive schedule to obtain propulsion power as a function of vehicle velocity, P(V).
Step 2
: Import file P(V) into the ESS simulator (ANSYS/Ansoft Simplorer or
MathLab Simulink) and develop the energy management system (EMS) strategy.
Step 3
: Evaluate the dynamic performance of the ESS for the specified vehicle,
EMS and drive schedule. Quantify the key performance attributes of the ESS and
build a comparison summary table.
Table 10.8 summarizes the main energy storage components used in this example.
10.2.5 Ultra-capacitor cell balancing
In order to store the maximum amount of energy in an ultra-capacitor, its voltage must
be near or at maximum tolerable levels. Ultra-capacitors having organic electrolytes
are typically confined to voltage stress levels less than 2.7 V across any given cell in a
series string. When the voltage exceeds 3 V the cell will gas and when the voltage is
raised above 4 V the cell will burst in a short time after application. Common cell
balancing techniques range from passive component, dissipative, equalizers to active
component, non-dissipative equalizers. Several examples of each type will be given to
acquaint the reader with available techniques and their relative merits.
