Abstract
Fuel Cell Electric Vehicles (FCEV) have higher efficiency and lower emissions compared with the internal combustion engine vehicles. But, the fuel cell has a slow dynamic response; therefore, an auxiliary power source is needed during start up and transient conditions. Batteries and ultracapacitors can be used as auxiliary power sources. Batteries have high energy density and ultracapacitors have high power density. By using both the battery and ultracapacitors as the auxiliary power sources of the FCEV, the performance and efficiency of the overall system can be improved. In this system, there is a boost converter, which steps up the fuel cell voltage, and two bi-directional DC-DC converters, that each of them couples the battery and ultracapacitors to the bus. Each bi-directional converter has two modes of operation. During the buck mode the battery/ ultracapacitors will charge through the fuel cell or regenerative braking. During the boost mode they will supply the load. These converters should be properly controlled. Besides, a control strategy is needed to select the most suitable power source for supplying the load at each operating point, due to the efficiency of the system and demanding power. In this paper the fuzzy control method has been employed for controlling the whole system. Three kinds of fuzzy controllers are needed for this system. One of them is the main controller, which selects the switches that should be activated at each moment. The second one is the boost controller, which has 3 inputs and is needed for three switches.
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