Modeling Photovoltaic System Using MATLAB/Simulink

MODELING PHOTOVOLTAIC SYSTEM USING MATLAB/SIMULINK Soumia KARIM, Nadia MACHKOUR, Mourad ZEGRARI, Abdelhafid AITELMAHJOUB, Zakaria SABIRI Soumia93.karim@gmail.com, nadia.machkour@gmail.com, mouradzegrari@yahoo.fr, aitelmahjoub@gmail.com, sabiri.zakaria@gmail.com Structural Engineering, Intelligent Systems & Electrical Energy Laboratory, ENSAM Casablanca, Hassan II University of Casablanca, Avenue Nile, 20000, Casablanca, Morocco. Abstract The solar cell is the device which converts light into electricity. Many models of solar cells had been proposed since the beginning of the solar energy exploitation. This paper presents a mathematical modeling of the three single diode models, more than that this work shows the effect of irradiance and temperature on the I-V and P-V characteristics of the PV module. In order to increase the power extracted from the solar system, it is necessary to operate the PV system at the maximum power point (MPP). This paper focuses on the Perturb and Observe method to track the MPP. Then we have simulated the overall system with the P&O algorithm using MATLAB/Simulink software due to its frequent use and its effectiveness. Keywords– PV module; I-V and P-V characteristics; mathematical modeling; DC/DC converter; MPPT; Perturb and Observe method. 1. Introduction The energy of solar radiation is directly utilized to convert the solar radiation into electricity. This direct conversion is described as a photovoltaic (PV) energy conversion because it is based on the photovoltaic effect. It takes place in a semiconductor device that is called a solar cell. The modeling of this device necessarily involves a judicious choice of equivalent electrical circuits. To develop a precise equivalent circuit for PV cell, it is necessary to understand the physical configuration of the electrical elements of the PV cell, and their electrical characteristics. According to this philosophy several mathematical models are developed to present the non linear behavior of the PV cell [1]. Such as: The single diode model, and the two diode model. In this paper we have detailed the first model. For large scale generation of solar electricity the solar panels are connected together into a solar array. The solar panels are part of complete PV system. To receive the maximum solar irradiation many Maximum Power Point Tracking methods are developed in literature [2]. The most utilized one is P&O. which is presented in this work. 2. PV module 2.1. Presentation of the PV module The most common material for the production of solar cells is Silicon (Si). The Silicon is obtained from sand or quartz, which is the most common elements in the earth’s crust, so there is no limit of the availability of raw materials. Basically three types of technology are used in the production of photovoltaic cells: Monocrystalline, polycrystalline, and amorphous silicon. The status of a photovoltaic cell technology depends on the efficiency. Some of previous researches have done analysis of performance parameters of the different types of PV modules [3]. Solar cells can be classified as a semiconductor device. When semiconductor materials are exposed to light, solar irradiation penetrates to the semiconductor crystal which causes significant number of free electrons in the solar cell. This is the basic reason of production a DC current due to Photovoltaic effect Fig.1. The power produced by a solar cell is so low (1-1.5w) so to get a practical voltages and currents, solar cells must be connected in series or parallel to create PV module, while ensuring their electrical isolation and protection against external factors. Such as: Humidity; Rain; Dust; Corrosion; and Mechanical shock. Fig.2 Equivalent circuit of ideal solar cell This model does not take into account the internal losses of the current. The output current (Ipv) of the ideal solar cell model with single diode connected in module is obtained by Kirchhoff law: Fig.1 Construction and principle of operation of PV cell [4] 2.2. Parameters and mathematical modeling of the PV module The main parameters that are used to define the conversion efficiency η of solar cells (Eq.2) are the MPP (Maximum Power Point) Pmax, the short circuit current Iscr, the open circuit voltage Voc, the fill factor FF (Eq.1). These parameters are obtained from the characteristic I-V of the solar cell. Additionally, the performance of a solar cell strongly depends on the temperature and the total irradiance S. (1) (2) Equivalent circuit models define the characteristic I-V of the PV generator. Three equivalent circuit models can be used to model a photovoltaic cell with single diode: The ideal equivalent circuit, solar cells with series resistance Rs, and solar cells with series Rs and shunt Rsh resistance. 2.2.1. Ideal equivalent circuit The ideal equivalent circuit of a solar cell with single diode model is a current source in parallel with a diode. This model is shown in Fig.2. (3) Where Np is the number of cells connected in parallel, Iph is the photo generated current (4), Id is the diode current (5). (4) (5) I0 is the diode saturation current (6), Ki is the short circuit temperature coefficient, q is electron charge (1.602 x 10-19 C), K is Boltzmann constant (1.3805 x 10-23 J/K), A is the ideality factor which depends on PV cell technology, Ns is the number of cells connected in series, Tak is the actual cell temperature (K). (6) Irs is the diode reverse saturation current (7), Trk is reference temperature in Kelvin (298K), Eg is the band gap of Si. (7) 2.2.2. Solar single cell model with series resistance (10) This model is more practical because it involves the series resistance Rs as shown in Fig.3. So the output current became: (11) Fig.3 Equivalent circuit of single model solar cell with series resistance When Rs is taken into consideration, the output current should take the next form: (8) 2.2.3. Solar single cell model with series Rs and shunt Rsh resistance 3. Maximum Power Point Tracking The output of the solar cell is not maximum at all times because the I-V and P-V characteristics are non linear, and greatly depend on irradiation (S), and temperature (T) [5].These environmental conditions varies randomly. So the MPP position is continuously changed. Thus, boost the need to device methods that would compel the operating point to coincide with the maximum power is available from the source Fig.5. This model includes the internal losses of the current. These losses are modeled by two resistances: Shunt resistance Rsh, and series resistance Rs. The electric scheme equivalent to this model is shown in Fig.4. Fig.5 Typical I-V and P-V curves for PV module [6] Fig.4 Equivalent circuit of single model solar cell with series and shunt resistance In this case the characteristic equation can be directly deduced by using the Kirchhoff law: (9) And the shunt current is given by: Many Maximum Power Point Tracking methods are developed in literature. The more used ones are Perturbation and observation (P&O), Fuzzy-Logic (FL), Hill-climbing (HC), Incremental conductance (IncCond), and Neutral Network (NN). Each method has its own advantages and disadvantages concerning the tracking precision, the tracking speed, the component cost, and the implementation complexity. In this paper the MPPT algorithm used is the P&O (Perturb and Observe). Fig.6 shows the whole power system. This model has been used in many applications for designing better efficiency PV system in the literature [7]. A perturbation is provided to the PV module or array voltage. This would translate to an increase or decrease in the power. If an increase in voltage leads to an increase in power, this means that the operating power point is in the left of the maximum power point. Hence the voltage perturbation is required to watch the right to reach the maximum power point. Inversely if an increase in voltage leads to decrease in power, this means that the operating power point is in the right of the maximum power point. Thus the voltage perturbation is required to watch the left to reach the maximum power point. In this way the algorithm converges to obtain the maximum power point over several perturbations. Fig.6 Model of the overall PV system P&O algorithm The direct maximum power point tracking technique is the Perturb and observes (P&O). It is the most commonly used technique to track the maximum power point due to its simple structure. Its algorithm is shown in Fig.7. The algorithm used simple feedback technique. In this approach, the power (P) is computed using the measured values of the voltage (V) and current (I) of the PV array. 4. Simulation and Results Reference model The type of the PV panel cells used in this study is Polycrystalline silicon. The number of modules in series Ns is 36; the number of modules in parallel Np is 1. The main electrical characteristic of this PV panel are shown in table1. Parameter Value Maximum power Pmax 244.72 W Maximum voltage Vm 32.2 V Current at max power Im 7.6 A Open circuit voltage Voc 40.8 V Short circuit current Iscr 8.22 A Table1. Electrical characteristics of the reference PV model The PV system model which developed in this paper is designed by using the mathematical equations of the equivalent circuit of solar cell model shown in Fig.4 with DC-DC boost converter and P&O MPPT algorithm. Fig.7 P&O algorithm Fig.8 I-V characteristic of the PV model at 1000 W/m2 Fig.11 Variation of duty cycle Vs Time Fig.9 P-V characteristic of PV model at 1000 W/m2 Fig 12: output power vs. time The irradiation is maintained constant at 1000 W/m2 while the temperature is varied between 25 and 45 °C. Fig.8 and Fig.9 show the simulation results of I-V and P-V characteristics respectively under the same conditions. The voltage and the power decreases with the increase of the temperature. Fig.10 I-V characteristic of the PV model at T= 25°C Fig.10 and Fig.11 show the MATLAB results of the PV characteristic at T= 25°C and by varying the irradiation (100W/m2, 500W/m2, 1000W/m2). The influence of irradiation on maximum power point is clear, the power of the module increases with increasing the solar irradiation level from 100 W/m2 to 1000 W/m2. The output current and voltage from a PV array is given as input to MPPT (P&O), and the duty cycle is given as a control signal to the converter Fig.12 shows the output respond of the PV power. Conclusion Fig.11 P-V characteristic of the PV model at T= 25°C In this paper, we have presented a mathematical modeling of the three types of the single diode photovoltaic cell, and described their equivalent circuits. The results show that the power generated by the PV system is dependent on solar irradiance and the temperature parameters. This leads to add other power electronics between PV modules and load/grid. Such as DC-DC converter based on maximum power point tracking. The chosen model is implemented under MATLAB/Simulink environment, and the MPPT method used in this paper is Perturb and Observe. References [1]O. Gergaud, B. Multon, H. Ben Ahmed “Analysis and Experimental Validation of Various Photovoltaic System Models” 7th International ELECTRIMACS congress, Montereal, August 2002. 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