Amari Mansour1, Chabchoub Mohamed Hedi2, Bacha Faouzi1. 1. Laboratory of Computer Science for Industrial Systems, INSAT, Tunis, Tunisia. 2. Computer Embedded System (CES) Laboratory, National School of Engineers of Sfax (ENIS), Sfax, Tunisia.
Abstract
Electric vehicles have recently attracted research interest. An electric vehicle is composed of two energy sources, such as fuel cells and ultracapacitors, which are employed to provide, respectively, the steady-state and transient power demanded by the vehicle. A bidirectional DC-DC converter is needed to interface the ultracapacitor to a DC bus. The pack of ultracapacitor consists of many cells in series and possibly also in parallel. In this regard, this paper introduces a comparative study between two packs of supercapacitors. The first supercapacitor pack is composed of ten cells in series but the second supercapacitor pack is composed of five cells in series and two parallel circuits. Each cell is characterized by 2.5 V and 100 F. A number of practical tests are presented.
Electric vehicles have recently attracted research interest. An electric vehicle is composed of two energy sources, such as fuel cells and ultracapacitors, which are employed to provide, respectively, the steady-state and transient power demanded by the vehicle. A bidirectional DC-DC converter is needed to interface the ultracapacitor to a DC bus. The pack of ultracapacitor consists of many cells in series and possibly also in parallel. In this regard, this paper introduces a comparative study between two packs of supercapacitors. The first supercapacitor pack is composed of ten cells in series but the second supercapacitor pack is composed of five cells in series and two parallel circuits. Each cell is characterized by 2.5 V and 100 F. A number of practical tests are presented.
Rapidly increasing population, energy consumption, and the need to reduce emissions through the conventional vehicle have motivated researchers to study the electric hybrid vehicles (EHVs) [1]. Usually, the electric hybrid vehicles architecture includes two or more energy sources with their associated energy converters as shown in Figure 1.
Figure 1
Electric vehicle system.
The main source is a fuel cell, with high energy storage capability; it is the electrochemical devices that convert chemical energy of a reaction directly into electrical energy [2]. This has slow dynamic to response under load variation and does not allow the recuperation of energy from the load [3]. The second source is the storage system; it produces the lacking power in acceleration and absorbs excess power in braking function. Batteries and ultracapacitors are employed as energy storage system in many hybrid applications. Recently, ultracapacitors have been explored better than batteries in the electrical vehicles because they present considerably higher power densities than those of batteries, and extremely higher energy densities than those of conventional electrolytic capacitors [4, 5]. The graph in Figure 2 illustrates the regions of applicability of the various energy storage systems [6].
Figure 2
Ragone diagram of energy storage sources.
Other than electric vehicles, supercapacitor can also be used as additional energy storage for hybrid wind and photovoltaic system. It charges energy when it is windy or sunny and discharges when there is no power generated from photovoltaic or wind due to the sudden passing clouds disturbance or very low wind speed [7]. Hence, it is necessary to understand the characteristics of the supercapacitor and determine these different electric models. In the literature many models have been developed such as electrochemical models and equivalent circuit ones. This paper presents a practical comparative study of equivalent circuit models of ultracapacitors used in electric vehicles. This paper is summarized as follows. Section 2 describes the ultracapacitor model. Section 3 details the topology of the boost, the operation mode, and the average model. Section 4 evaluates the simulation and the experimental results. And finally the conclusion is presented in Section 5.
2. Ultracapacitor Modeling
Ultracapacitors consist of two electrodes and an ion-permeable separator that prevents physical contact between the two electrodes [8]. They are characterized by high power density, high energy efficiency, low internal resistance, long cycle life, and fast charging/discharging time. In recent years as ultracapacitors become used more widely, several different circuit models have been proposed in the literature [9].
2.1. RC Circuit Model
The circuit schematic in Figure 3 represents the simple RC model for a ultracapacitor. It is comprised of three ideal circuit elements: a series resistor R: it is called the equivalent series resistor (ESR) and contributes to the energy loss component of the ultracapacitor during charging or discharging; a parallel resistor R: it is called the leakage resistance, and a capacitance Csc. This model is developed and validated experimentally in many works [5, 10–12].
Figure 3
Classical model.
The dynamic can be described asThe ultracapacitor is discharged with a constant current and the result is presented in Figure 4.
Figure 4
Evolution of voltage discharged with a constant current.
The equivalent series resistor is obtained through the following equation:where ΔU and Idech denote, respectively, the voltage drop which is observed at the beginning of the discharge and the discharge current.The capacitor can be expressed by The parallel resistor is calculated using where U1 and I denote, respectively, the open circuit voltage and the leakage current.
2.2. Three-Stage Ladder Model
The three-stage ladder model is shown in Figure 5. It is composed of three resistors and three capacitors. This model is developed in many references [13-15].
Figure 5
Three-stage ladder model.
To model the three-stage ladder model we choose three state variables including capacitors voltages u1, u2, and u3. The state space representation is described by the following equation:where , u = Isc, and y = Vsc.According to basic electrical circuit principles, the matrix expressions are the following:
2.3. The Dynamic Model
In this case, the equivalent circuit is composed of two parallel circuit RC. It has the structure shown in Figure 6 where u0 denotes the voltage across the bulk capacitor Csc, u1 and u2 denote the voltages of the two RC circuits, respectively, Vsc denotes the output voltage, and Rsc denotes the series resistance [16].
Figure 6
The dynamic model.
The state space representation can be derived as
3. Topology of Converter
There are many possible topologies of DC-DC converters which interface the ultracapacitor and the DC link. The DC-DC converters can be divided into two categories depending on using the galvanic insulation or not: nonisolated converter or isolated converter. The proposed converter has two basic operating modes: the buck and the boost mode. In this context, many papers have been elaborated [17-19]. Figure 7 shows the converter topology. The chosen DC-DC converter is composed of two IGBTs, two diodes, inductor, and capacitor.
Figure 7
Topology of DC-DC converter.
There are two modes. (a) Charging mode: in this case, the load delivers the energy to the capacitor through the IGBT (T2), the diode (D2), and the inductor L. (b) Discharging mode: in this case, the ultracapacitor delivers the energy to the load through the IGBT (T1), the diode D1, and the inductor L. Table 1 shows the state of any switch for two modes, where T, d1,and d2 denote, respectively, the switching period, the duty cycle in charging mode, and the duty cycle in discharging mode.
Table 1
State of the switches.
Switch
Charging mode
Discharging mode
T1
0
0d1Ts
T2
0d2Ts
0
D1
0
0d1Ts
D2
d2TsTs
0
The key factors that determine the parameters of the circuit include the limit ripple of the ultracapacitor current (δ(Isc)) and the ripple of the output voltage (δ(VDC)), the switching period (T), and the output power (Pout). In discharging mode the inductor and the output capacitor are described by the following equation:In this application, the ultracapacitor voltage is 25 V, if we choose Isc = 4 A, VDC = 125 V, T = 5e−4 s, and δ(Isc) = δ(VDC) = 5%. The boost parameters are d1 = 0.8, L = 50 mH, and C = 256 μF.To model the boost converter, we used the average method. In this case, it is assumed that all components are ideal; that is, there is no internal resistance in the circuit and the circuit components do not consume any energy. We choose two state variables including output voltage and inductor current. The system state space representation iswhere u is the vector of inputs, y is the outputs, and x is the status variables vector.During the switching period, the topology converter can be divided into two equivalent circuits.
Case 1 ().
The switch T1 is ON; the equivalent circuit can be simplified as shown Figure 8.
Figure 8
Equivalent circuit for Case 1.
We can write the state space by the following equation:The matrices A1, B1, and C1 can be expressed as follows:
Case 2 ().
In this case, the switch T1 is OFF; the ultracapacitor delivers the energy to the load through the inductor and the diode D1. The equivalent circuit can be simplified as shown in Figure 9.
Figure 9
Equivalent circuit for Case 2.
Using Kirchhoff law, the state space model is the following:The matrices A2, B2, and C2 can be expressed as follows:The average state model iswhereIf we insert (12) and (14) in (16), the matrices A, B, and C areIn MATLAB, we simulate the open loop response to verify the average model using the circuit parameter defined by (8). We initialize the system by choosing the duty cycle at 0.8 and the load resistor at 25 Ω. Figure 10 illustrates the DC link and the ultracapacitor voltage.
Figure 10
Output and the ultracapacitor voltage.
4. Experimental Validations
A test system has been developed allowing the supercapacitor to be charged and discharged and to calculate the equivalent circuit parameters. The test circuit shown in Figure 11 allows delivering a range of voltages and currents (charge and discharge mode) and hence is capable of characterizing the range of supercapacitors used in this application. The prototype consists of ten cells (maximum stack voltage of 25 V), a boost converter (IGBT module, diode, and inductor), and a load.
Figure 11
Test circuit.
The used ultracapacitor is Wima (made in Germany). These specifications are given in Table 2.
Table 2
Ultracapacitor specifications.
Parameter
Value
Voltage cell
Vsc = 2.5 V
Capacitor cell
Csc = 100 F
Resistor
Resr = 36 mΩ
Cell number
10
Two experiments are carried out using this setup. In the first experiment, the ten cells are connected in series, while the second one demonstrates the functionality of the system when the supercapacitors are connected in two parallel circuits.
4.1. Charging Mode
4.1.1. Case 1
In this case the ultracapacitors are connected in series and are charging with constant current (4.25 A). Figure 12 shows the current, voltage, and power waveforms of the ultracapacitor.
Figure 12
Voltage, current, and power waveforms of the ultracapacitors (charging mode-series).
Figure 12 shows that the charging time of the supercapacitor is equal to 66 seconds. In this case, the equivalent capacitor of the ultracapacitor module is 10.8 F. The cell capacitor and the equivalent series resistor are equal, respectively, to 108 F and 36 mΩ.The voltage cells obtained from this condition are summarized in Table 3.
Table 3
Voltage cells value.
Number of cells
Cell voltage (V)
1
2.603
2
2.609
3
2.3
4
2.571
5
2.589
6
2.538
7
2.482
8
2.484
9
2.512
10
2.451
According to this table, the voltages of the different cells are not identical; that is why it is important to use a voltage balancing circuit.
4.1.2. Case 2
In this case, the ten cells of this module are divided into two parallel circuits and charged with constant current as shown in Figure 13. The charging time is equal to 140 s.
Figure 13
Voltage, current, and power waveforms of the ultracapacitors (charging mode-parallel).
4.2. Discharging Mode
Initially, the supercapacitor pack is fully charged. It provides power to the load through the IGBT, smoothing inductance, and diode. The control circuit allows varying the duty cycle and the switching frequency.
4.2.1. Case 1: The Ultracapacitors Cells Are Connected in Series
The experimental results in discharging mode are obtained with (0.8 and 0.6) as the duty cycle and loads (25 Ω). Figure 14 presents the voltage, current, and power of ultracapacitors for two- duty cycle.
Figure 14
Voltage, current, and power waveforms of the ultracapacitors (discharging mode: series).
4.2.2. Case 2: The Ultracapacitors Cells Are Connected into Two Parallel Circuits
In this case, the ten cells of this module are divided into two parallel circuits and deliver the energy to the load through the boost and the inductor. Figure 15 shows the voltage, current, and power for two values of duty cycle (0.8 and 0.6).
Figure 15
Voltage, current, and power waveforms of the ultracapacitors (discharging mode: parallel).
In this case the current system and average model are simulated with d1 = 0.72 and then in time of 20 seconds the duty cycle changes from 0.72 to 0.82. Figure 16 shows the simulation and the experimental results.
Figure 16
Simulation and experimental results for step change of the duty cycle.
5. Conclusion
An ultracapacitor bank has been simulated and tested. The system uses a IGBT boost converter; the control of the system measures the ultracapacitor voltage, the ultracapacitor current, and the output voltage. A simple experimental methodology is proposed for a systematic validation of average models in time domain and tested the ultracapacitor performance for two connections (series and parallel). We have assumed a purely resistive load, which is not always the case in practice (acceleration and regenerative braking for an electric vehicle). It can be interesting to study the system (buck-boost + ultracapacitor) associated with an electric machine.
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