The characterization of the electric double layer capacitor (EDLC) and its application in micro grids using the hybrid energy storage system (HESS)

Abstract

Electrical energy storage is very important. Energy storage devices such as batteries and capacitors can be used to store energy. In this dissertation we investigate the use of a supercapacitor or electric double-layer capacitor (EDLC) to store energy. The EDLC, also known as a supercapacitor, is a new type of electrical energy storage device used across many different fields in engineering. Firstly, two EDLC’s of values 300 F and 400 F were characterized by connecting them in a laboratory experiment to produce their charge/discharge profiles under constant current. The EDLC’s datasheet parameters including the capacitance and equivalent series resistance were then derived from the acquired charge/discharge profile. In addition to this, the experimental charge/discharge profiles were used to determine the parameters of the chosen EDLC's using the equivalent circuit model called the "two branch" or Faranda model. The derived "two branch" equivalent circuit parameters were then used to design a Python/MATLAB/Simulink (PMS)-hybrid model of the EDLC's for simulation purposes. The PMS model used the derived parameters as inputs and produced simulated charge/discharge profiles for both the 300 F and 400 F EDLC’s. By superimposition, the simulated and experimental charge/discharge profile were compared to evaluate the accuracy and validity of the proposed PMS model. This was done to enable future simulation in conjunction with a battery in a hybrid energy storage system (HESS) to show the potential of the EDLC in improving battery lifespan. Following the above, a 65 F, 16.2 V EDLC supercapacitor was sourced due to its midrange capacitance and voltage close to a 12 V DC supply. This was done to investigate whether the EDLC can help in reducing stress and prolong the battery lifespan in a hybrid energy storage system (HESS). A laboratory experiment was setup for the EDLC to produce its charge/discharge profile at a constant current of 5 and 10 A. As was done with the previous 300 F and 400 F, the 65F, 16.2 V Faranda or “two branch model” mathematical parameters were extracted from the experimental charge/discharge profile in preparation for its modelling. In keeping with the laboratory experiment, the EDLC was connected with a 12 V lead-acid battery and a load to form a HESS, which was tested under difference load conditions. Experimental battery data such as voltage, current, state of charge (SOC) under HESS and battery energy storage system (BESS) without the EDLC were compared. To simulate a HESS, the extracted parameters from the 65 F, 16.2 V EDLC were used as inputs to design the Python/MATLAB/Simulink (PMS) model of the EDLC. The PMS model of the EDLC was then used as a subcomponent in a HESS system modelled in MATLAB/Simulink. Using constant load conditions, the battery’s voltage, current, power and state of charge (SOC) were analyzed for a BESS and HESS systems similar to what was done in the experimental setup. Experiment and simulation were then compared. In conclusion, due to the high variation of battery parameters in the BESS setup for both experiment and simulation, and stability of the same battery values in the HESS setup, the EDLC in the HESS setup indeed reduces stress on the battery and prolongs battery lifespan. The unique contribution of this work lies in modelling the EDLC using python programming language before integrating the python algorithm to MATLAB and Simulink to form the Python/MATLAB/Simulink (PMS) – hybrid model of the EDLC.