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PSL Projects : Control Strategies Development
of an Actively-Controlled Hybrid Fuel Cell/Battery Power Source
New high-energy density, high-power density power sources are
more and more attractive to applications of modern handheld electronic devices
and larger machines such as electric vehicles or aircraft on both commercial and
military markets. In most case, current technology batteries by themselves are
insufficient to provide the long-term power (energy between refuelings) that
these systems require. Fuel cells of reasonable size may provide the necessary
energy, but are then unable to provide the high peak power occasionally demanded
by these systems. Hybrid systems composed of fuel cells and batteries combine
the high energy density of fuel cells with the high power density of batteries.
Provided that the temperature was not too cold, the battery could enable instant
cold-start operation since it can provide a majority of the load power
requirement while the fuel cell is warming up. A hybrid system could allow both
components to be of smaller dimensions and to operate with higher efficiency
since neither would have to provide full load and capacity. The battery could
also condition the power output from the fuel cell to provide a voltage range
that is acceptable to the equipment since most devices are already designed to
accommodate the source characteristics of a battery (i.e., voltage fluctuation
from full charge to depleted).
As an alternative to the passive hybrid, a DC/DC power
converter could be placed between the fuel cell and the battery, which would
greatly augment the peak output power while reducing the system weight and
volume. In the active fuel cell/battery/converter hybrids, the positions of the
fuel cell and the battery determine the terminal voltage and power
characteristics of hybrid power sources. Two possible configurations of active
hybrid fuel cell/battery power sources are briefly discussed here.
Two configurations of the active hybrid fuel cell/battery power source
In both configurations, the power shared by both components could then be actively controlled. However, control of the power converter in such systems becomes very complicated, which actually is a multi-objective control issue. Rather than being controlled to serve as a sole voltage regulator or current regulator, the power converter is required to regulate and balance the power flow between the fuel cell and the battery to satisfy the load power requirements while ensuring the operation within any limitations of the electrochemical components such as battery over-charge/over-discharge, fuel cell current limit, etc. The only control input is the duty cycle of the power converter. By changing the duty cycle, the output current of the fuel cell and the current (or voltage) of the battery can be regulated, but not independently. The control strategy that we describe here has three regulation modes in both configurations: constant fuel cell current (CFCC) mode, constant battery current (CBC) mode, and constant battery voltage (CBV) mode.
States:
CFCC: Constant Fuel Cell Current mode
CBC: Constant Battery Current mode
CBV: Constant Battery Voltage mode
DISC: Disconnection of the load
Conditions of Events:
1: Power on
2: Ib > Iref
3: Ifc > Irated
4: Vb > Vref
5: Vb < Vref, Ib > Iref (This rarely happens)
6: Ifc > Irated
7: Vb > Vref
8, 9, 10: Ib < Idisc (for instance, Idisc = -4 x Iref. This happens under very heavy load)
State machine representation of the control strategy for active hybrid fuel cell/battery power sources
The VTB simulation tool can be used to study the performance of the active hybrid power sources.
VTB schematic view of the active hybrid fuel cell/battery power source
Currents from the fuel cell stack, from the battery, and to the load
Voltages of the fuel cell and the battery (the voltage of the load equals to the battery voltage)
Change of the regulation mode (1: CFCC, 2: CBC, 3: CBV)
Experimental tests are conducted to validate the control strategies of the
hybrid power sources.
Experiment platform of the hybrid power source
Zoomed version of currents of the fuel cell and the battery
Zoomed version of voltages of the fuel cell and the battery (Top: fuel cell voltage, Bottom: battery voltage)
The following table summarizes a comparison of three different power sources.
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Fuel Cell Alone |
Passive FC/Battery Hybrid |
Active FC/Battery Hybrid |
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System Configuration |
Fuel Cell: H Power D35 |
Fuel cell: H Power D35 Battery: 5 ×2 (US18650) |
Fuel cell: H Power D35 Battery: 4 ×2 (US18650) DC/DC power converter |
|
System Weight (g) |
2900 |
3350 |
3266 |
|
System Volume (cm3) |
5092 |
5092 |
5092 |
|
Power capacity (W) |
35 |
70 |
140 |
|
Specific power (W/kg) |
12.1 |
20.9 |
42.9 |
|
Power density (W/cm3) |
6.9e-3 |
1.37e-2 |
2.75e-2 |
