Sizing Considerations for Ultracapacitors in Hybrid Energy Storage Systems

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Ultracapacitors are energy storage devices that can be connected in parallel with batteries to create a hybrid power system. The hybrid configuration provides a higher power density compared to a single battery and leads to an extended life of the batteries. An important issue is the voltage regulation of a battery supplied dc bus subjected to a long high power transient. In the aerospace field the bus voltage must be maintained within certain margins. Batteries subjected to high power transients generate terminal voltages with voltage dips that can exceed the voltage regulation band. The purpose of this study is to see to what extent an ultracapacitor placed in parallel with a battery can assist in maintaining a bus voltage within the regulatory limits, and develop a closed form mathematical expression assuming the injection of constant power pulses. This expression can be used to size the ultracapacitor, to meet a specific voltage regulation requirement, for a given battery, or to give constraints on voltage requirements that can be achieved.

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