Modelling and validation of the electrical characteristics of super capacitor energy storages in hydrid vehicles

Abstract

[EN] Electrochemical double-layer capacitors, is also known as super capacitors or ultra capacitors, which fill in the gap between conventional capacitors and batteries. They have a high power density compared with batteries and a high energy density compared to conventional capacitors. They benefit from a high number of charging and discharging cycles, extremely low internal resistance, high efficiency, high output power, wide rage of temperature operation and improved safety. For this reason, they are very suitable for being used as a peak power unit in a Hybrid Electric Vehicle, and also combined with batteries they can improve the energy storage system for a Hybrid Electric Vehicle (HEV). In order to properly estimate and evaluate the performance of super capacitor based energy storage for an HEV, it is very important to make a simple and accurate modelling of the super capacitors, in particular the electrical characteristics. In this way, the IEC (International Electrochnical Commission) has provided several test and measurement procedures to estimate the characteristics of super capacitors. But they are not enough accurate in some case and can not model all the phenomenon occurring within the super capacitors. Moreover, in earlier works of modelling the super capacitor, very complicated mathematic equations are often used. But, in this case the simulation and validation would take a long time, and would entail several difficulties. Therefore, in this thesis, the super capacitors energy storage is simply modelled by using a variable capacitance Csc, an equivalent series resistance rsc and a parallel resistance Rp. These parameters are identified by doing several pulse current tests, and then they have been successfully validated by comparing simulation results with experimental measurements. One of the thesis conclusions is that the capacitance Csc is dependent on the voltage level of the super capacitor energy storage, and this variability has been determined. Moreover, all the defined parameter of the super capacitor model, are variable with the super capacitor ageing process. To prove this dependence, a big among of driving cycle tests have been done to the super capacitor energy storage, in order to simulate the use of the super capacitors as if they were in a passenger car. Then, more tests (using pulse current) are done in order to know the variation of the electrical parameters of super capacitors with the lifetime. Thus, comparison between new super capacitor energy storage and old one can be done. The final results achieved in this master thesis work show that super capacitors are suitable for the use as a peak power unit in hybrid electrical vehicles. The super capacitor energy storages can perfectly provide the needed peak power. With the combination of batteries and super capacitors, the electrical energy storages can be optimized to reach a very high efficiency and higher energy capacity in a smaller size.