Matlab/Simulink and Experimental Studies of Shading Effect on a Photovoltaic Array

Matlab/Simulink and Experimental Studies of Shading Effect on a Photovoltaic Array

Mourad Talbi1

1Laboratoire de Nanomatériaux et des Systèmes pour les Energies Renouvelables

Center of Researches and technologies of energy of Borj Cedria

Tunis, Tunisia

Nawel Mensia2, 1Fehri Krout, 2Radhouane Chtourou

2Laboratoire Photovoltaïque

Center of Researches and technologies of energy of Borj Cedria

Tunis, Tunisia

AbstractPhotovoltaic systems are vulnerable to the change of climatic conditions (temperature and irradiance). They cause a declination in power generation efficiency and reliability. The variation is commonly caused by uniform and non-uniform partial shading. The aim of this paper is to present the non- uniform partial shading impact on the performances of a photovoltaic generator. Hence, a theoretical modeling of a generator composed of two series photovoltaic panels, is presented. The first part provides literature review with mathematical model. The second section offers details of the simulation analysis under diverse partial shading conditions.

Keywords Photovoltaic systems, Shading, PV panels, (P-V) characteristic, (I-V) characteristic.

1. INTRODUCTION

   The quick renewable development, green and clean energy technologies play a very important role in clean application particularly in electric power generation. Energy produced from renewable resources including wind, biofuels, sun and others is named renewable energy [1]. The most popular form of renewable energy is solar energy. It provides electric energy directly by employing PV modules then MPP Tracker (MPPT) is used in order to maximize the efficiency of the PV system. The PV system has a single MPP at the peak values of current and voltage [2]. The power yield of PV modules is a function of different weather conditions including temperature [3], solar irradiance [4] and partial shading [2]. In this paper we will be interested in a model of a PV Module containing 36 solar cells. This model was proposed by Sanjay Lodwal and presented in matworks [5] and we have made one modification on it by introducing the technique of the series resistance proposed in [6]. After this modification we connect in series two modified models and we study the effect of partial shading on one of them. Section 2 is dedicated to present the equivalent circuit to a PV cell. The effect of Partial Shading on a PV cell is explained in section 3. Matlab/Simulink numerical simulation of one PV module exposed to different shading effects is presented in section 4. Finally, the conclusion is given in Section 5.

2. PHOTOVOLTAIC EQUIVALENT MODEL The PV array consists of solar cell stacks. Solar cell

   transforms light into electricity. In figure 1, is displayed a solar cell equivalent circuit. It is simply consists of a photo current (IPH), a diode, a shunt or parallel resistor (RSH) and

   an internal resistor (RS). The current at the terminal of the solar cell is expressed as follows [2]:

   (1)

   With is the saturation current, is the constant of Boltzmann, is an ideal factor and is the Kelvin temperature.

   Figure 1. Equivalent circuit of a PV cell.

   In a simple representation of a PV cell, series resistance, and shunt resistance [7]. This ideal case is practically impossible; though, many research works aim to reduce the effect of both the shunt and series resistances. Common cell yields below at , which is significantly low. For larger output power values, a PV array is employed. It is it is constituted of a number of modules which are connected in series and parallel arrangements. Each of these modules is constituted of of PV cells connected in parallel and series. PV modules are simulated employing either physically using SimPowerSystems toolbox or mathematically using math function. Generally, the mathematical approach is easier to use than the physical model. On the contrary to physical model, in the mathematical one, to have a series-parallel combination for a PV cells, there is no need for block diagram repetition [7, 8]. Consequently in [7], was built the system that depends on mathematical modeling.

3. EFFECTS OF PARTIAL SHADING ON PV The functioning of a photovoltaic array is impacted by

   solar irradiance, temperature and shading and array configuration. Often, the PV arrays get shadowed, partially or

   wholly, by towers, trees, utility and telephone poles, adjacent buildings and the moving clouds. The situation is of special interest in case of big PV installations such as those used in distributed power generation systems. Under partly shaded conditions, the photovoltaic characteristics get more complex with more than one peak. Yet it is very crucial to understand and predict them in order to draw out the maximum possible power. Here, we present a MATLAB-based modeling and simulation scheme desirable for studying the and characteristics of a photovoltaic array.

   In this section is described the procedure used to simulate the and characteristics of a partially shaded PV array. It is of importance to understand how the shading pattern and the PV array structure are defined in MATLAB using the proposed scheme. The PV array is configured as a combination of six series of PV modules connected in three parallel connections. Each set of PV modules operate under different solar radiations and different cell temperatures. The first set is under solar radiation of and cell temperature of , the second set is under solar radiation

   of and cell temperature of and the third set is under solar radiation of and cell

   temperature of . Based on these conditions the

   simulations illustrating the PV characteristics is shown in Figure 2 with three different multiple peaks. The maximum peak is called as global peak and the remaining two peaks are named as the local peaks.

   The PV output power is affected by different factors such as partial shading [9], solar insolation, temperature and the configuration of the PV arrays. In [7] Khaled Matter et al., discussed the effect of several shading condition on the PV arrays that may occur due to presence of trees, buildings and clouds [10, 11]. The power-Voltage (P-V) characteristic curves of the PV system with full insolation exhibits nonlinearity with one MPP [6]. This complexity increases with changing insolation conditions [2, 6]. Under conditions of partial shading, some of the PV cells which collect even irradiance, will work with maximum efficiency. In the series structure, a uniform current is passing in each cell. Consequently, the cells experience shading run in reverse biasing to yield equal current leading to the decrease in the MPP value. To solve this problem, a bypass diode is connected to selected cells in the series configuration [9]. The addition of bypass diodes modifies the array characteristics. Under partial shading conditions and in the presence of the bypass diodes, many local MPP emerge. The bypass diodes produce a short circuit around the shaded cells permitting the current produced from unshaded cells to flow; consequently, the array current and heating losses are reduced [9-12].

4. MATLAB/SIMULINK NUMERICAL SIMULATION OF ONE PV ARRAY EXPOSED TO

   DIFFERENT SHADING EFFECTS

   In this paper, we used a Matlab simulink for investigating the characteristics curves of a PV array subject to diverse shading circumstances. This PV array is as previously mentioned constitutes of two PV modules connected in series. Each of them is a model of PV module contains 36 solar cells. This PV array is presented in Figure 2.

   Figure 2. The Simulaton block diagram of the PV array experiencing different shading conditions.

   Where the Sub PV Module 1 or 2 is illustrated in Figure 3 with designates the Irradiation. For the partial shading of the Sub PV Module 2, we have chosen equal to

   500 while it is equals to for the Sub PV Module 2.

   The model of those solar cells can be determined by choosing all the parameters existent in the block parameters of each solar cell. These block parameters are in number of 5 and are listed in Table 1.

   Figure 3. The Sub PV Module 1 or 2 contains 36 solar cells connected in series.

   The model of those solar cells can be determined by choosing all the parameters existent in the block parameters of each solar cell. These block parameters are in number of 5 and are listed in Table 1.

   Parameter	Value
   Short-Circuit Current: Isc	8.9 A
   Open-circuit voltage: Voc	22.75V
   Quality factor: n	1.2
   Series resistance: Rs	0.0125
   Irradiance:	Its value is variable

   TABLE 1. THE BLOCK PARAMETERS OF EACH SOLAR CELL

   We have used in this work the computation procedure of the series resistance described in [12] in order to compute for each solar cell. This procedure is summarized as follow [12]:

   (2)

   Where is the number of cells connected in series and in this work, it is equals to 36. The quantity is expressed as follow:

   (3)

   Where with is the voltage

   in open circuit and in this work is equals to . the

   current is expressed as follow:

   and is the temperature in Kelvin and is expressed as follow:

   The constant is the Boltzmann constant and is equals to and is the electron charge and is equals

   to . The factor is the Quality factor and is equals to 1.2 (Table 1).

   Figures 4 to 13 illustrated the and characteristics in the following cases:

   ,

   • (parti

   al shading),

   (parti

   al shading),

   (parti

   al shading),

   (parti

   al shading).

   Where is the irradiance of the Sub PV Module 1 and is the irradiance of the Sub PV Module 2.

   9 300

   8

   250

   7

   6 200

   5

   150

   4

   3 100

   2

   1

   0

   0 5 10 15 20 25 30 35 40 45 50

   Figure 4. characteristic in the case:

   (without shading).

   50

   0

   0 5 10 15 20 25 30 35 40 45 50

   Figure 5. P-V characteristic in the case:

   (without shading).

   For maximum irradiances Ir1 and Ir2 equal to 1000W/m² (Figures 4 and 5) and a constant ambient temperature (25°C), the maximum PV power reaches approximately . The corresponding voltage and

   current are, respectively, and .

   9

   8

   7

   6

   5

   4

   3

   2

   1

   250

   200

   150

   100

   50

   0

   0 5 10 15 20 25 30 35 40 45 50

   Figure 7. characteristic in the case of partial shading:

   0

   0 5 10 15 20 25 30 35 40 45 50

   Figure 6. characteristic in the case of partial shading:

   9 120

   8

   100

   7

   6 80

   5

   60

   4

   3 40

   2

   1

   0

   0 5 10 15 20 25 30 35 40 45

   Figure 8. Characteristic in the case of partial shading:

   (partial

   20

   0

   0 5 10 15 20 25 30 35 40 45

   Figure 11. characteristic in the case of partial shading:

   160

   140

   120

   100

   80

   60

   40

   20

   0

   shading).

   0 5 10 15 20 25 30 35 40 45

   Figure 9. Characteristic in the case of partial shading:

   9

   8

   7

   6

   5

   4

   3

   2

   1

   0

   120

   0 5 10 15 20 25 30 35 40 45

   Figure 12. characteristic in the case of partial shading:

   9

   8

   7

   6

   5

   4

   3

   2

   1

   0

   0 5 10 15 20 25 30 35 40 45

   Figure 10. characteristic in the case of partial shading:

   100

   80

   60

   40

   20

   0

   0 5 10 15 20 25 30 35 40 45

   Figure 13. characteristic in the case of partial shading:

   The obtained results show that the maximum PV power is affected by the partial shading. Two local peaks appear on the P-V and I-V characteristics. These peaks vary with the level of the partial shading. During partial shading, each module is exposed to different irradiances. Thus, each panel has its own maximum (peak) power. Figures 6 to 13 presents the output (I-V and P-V) curves of the PV generator composed of two panels. It shows, for various Ir1, the array characteristics exhibiting two power peaks, for each case of level partial shading.

5. CONCLUSION

This paper presents a study of a partial shading impact on the PV panel characteristics. We demonstrate that partial shading results a substantial degradation in the output power causing global and local maximum peaks in the P-V characteristic curves. For this reason, appropriately rated bypass diodes are commonly employed to preserve solar array power. Moreover, the use of a maximum power point tracking algorithm to extract the global PV power is not an efficient solution. We suggest a unified controller which makes each panel to operate at its maximum power.

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