Electrothermal Modeling of a Module of Ultracapacitors

Ultracapacitors are energy storage devices that can be used in a stand-alone fashion or connected in parallel with batteries to form a hybrid power system. The state of charge of the ultracapacitors depends on the temporal evolution of the current profile, the initial state of charge, and their temperature. The ultracapacitor internal resistance is temperature dependent since the electrolyte resistance depends on temperature. Therefore, in order to determine the state of charge of an ultracapacitor, one needs to obtain the temperature within the ultracapacitor. The purpose of this study is to generate an electrothermal model of an available module of ultracapacitors and implement it in a multiphysics coupled circuit form. The model parameters are calculated by using simple approximations of thermal capacitances and thermal conduction resistances, and by using finite element simulations to determine the natural convection thermal resistances. A comparison of the results with finite element simulations in COMSOL is performed to determine the accuracy of the model.