International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 3, March 2013) Modelling and Simulation of photovoltaic module considering single-diode equivalent circuit model in MATLAB Dominique Bonkoungou1, Zacharie Koalaga2, Donatien Njomo3 1,2 Laboratory of Materials and Environment (LAME)/ Faculty of Sciences/ University of Ouagadougou/P.O. Box 7021 Ouagadougou/ Burkina-Faso 1,3 Environmental Energy Technologies Laboratory (EETL)/ Faculty of Sciences/ University of Yaounde 1/ P.O. Box 812 Yaounde/ Cameroon Among these methods, in view of its quadratic convergence and enhanced accuracy, Newton Raphson’s method remains attractive with the number of variables being limited to five parameters and their partial derivatives easily obtainable. In this work, we elaborate a MATLAB script file program, which uses Newton Raphson’s method to compute the five parameters of the single diode model of illuminated solar cells. The results obtained by simulation show the consistency between the data and obtained the parameters given by the manufacturer, namely: short circuit current (I SC), open circuit voltage (VOC) and maximum power point (Pmpp). For instance, these parameters can be used for quality control during production or to provide insights into the operation of the devices, thereby leading to improvements in devices. Abstract— This paper presents a photovoltaic (PV) cell to module simulation model using the single-diode five parameter models. The model was implemented in MATLAB software and the results have been compared with the data sheet values and characteristics of the PV module in Standard Test Conditions (STC). Parameters values were extracted using Newton Raphson’s method from experimental Current (I)-Voltage (V) characteristics of Solarex MSX60 module. The results obtained are in good agreement with the experimental data provided by manufacturer. The approach can thus, be very useful for researchers or engineers to quickly and easily determine the performance of any photovoltaic module. Keywords— Matlab Photovoltaic, Simulation, software, Newton Raphson, I. INTRODUCTION An accurate knowledge of solar cell parameters from experimental data is of vital importance for the design of solar cells and for the estimates of their performance. Thus, different solar cell models have been developing to describe their electrical behavior, but the electrical equivalent circuit is a convenient and common way in most simulation studies. The five parameters of interest in the equivalent circuit are the photo-current (IPV), series resistance (RS), diode saturation current (I0), parallel resistance (RSH) and the ideality factor (A). The currentvoltage relationship of a solar cell is described by a mathematical equation that is both implicit and nonlinear, therefore; the evaluation of these parameters has been the subject of investigation of several authors. While some authors use numerical analysis methods to solve the implicit nonlinear equation of I-V relation [1-3], others use analytical methods with a series of simplifications and approximations [4-6]. II. MODELLING OF PHOTOVOLTAIC MODULE Solar cell is basically a p-n junction fabricate in a thin wafer or layer of semiconductors. The electromagnetic radiation of solar energy can be directly converted to electricity through photovoltaic effect [7]. When exposed to sunlight, photons with energy greater than the band-gap energy of the semiconductor are absorbed and create some electron-hole pair proportional to the incident irradiation. Under the influence of the internal electric fields of the p-n junction, these carriers are swept apart and create a photocurrent which is directly proportional to solar irradiation [8]. Naturally, PV system exhibits a nonlinear current-voltage (I-V) and powervoltage (P-V) characteristics which vary with the radiant intensity and cell temperature. The dependence of power generated by a PV array with changing atmospheric conditions can readily be seen in the I-V and the P-V characteristics of PV arrays as shown in figure 1. 493 International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 3, March 2013) II-1. Ideal Photovoltaic model To develop an accurate equivalent circuit for a PV cell, it is necessary to understand the physical configuration of the elements of the cell as well as the electrical characteristics of each element. The ideal equivalent circuit of a PV cell is a current source in parallel with a singlediode. The configuration of the simulated ideal solar cell with single-diode is shown in figure 2 [9]. 20 18 1000 W/m2 16 T= 28°C 14 Array Current (A) 800 W/m2 T=56°C 12 2 600 W/m 10 I 8 400 W/m2 6 ID 4 200 W/m2 2 0 0 5 10 15 20 25 30 35 40 45 V D IPV G 50 Output-Voltage (V) a) I-V characteristics at various irradiance and temperatures (T = 28°C & 56°C) Fig.2: Ideal PV cell with single-diode The equation for the output current is given by: 350 I I PV I D 300 1000 W/m2 V I D I 0 exp AV T Where 250 Array Power (Watt) 800 W/m2 T=28°C 600 W/m2 200 Then equation (1) becomes: T=56°C 400 W/m2 150 V I I PV I 0 exp AV T 200 W/m2 100 1 (2) I PV is the current generated by the incidence of light; 50 0 1 (1) 0 5 10 15 20 25 30 35 I0 40 is Output-Voltage (V) VT b) P-V characteristics at various irradiance and temperatures (T = 28°C & 56°C) the diode reverse bias saturation current; Ns * k * T is the thermal voltage of a PV module q having Ns cells connected in series; q is the electron charge; k is the Boltzmann constant; T is the temperature of the p-n junction and A the diode ideality factor. A PV cell can at least be characterized by the short circuit current (ISC); the open circuit voltage (VOC) and the ideality factor A. The output of current source is directly proportional to the light falling on the cell. For the same irradiance and p-n junction temperature conditions, the short circuit current (ISC) is the greatest value of the current generated by the cell [10]. The short current is given by: Fig.1: Simulated characteristics of photovoltaic array. The nonlinear nature of PV systems is apparent from fig.1. We note that the array current and power depend on the array terminal operating voltage. Fig.1 illustrates the dependence of I-V characteristics on temperature and irradiance for a sample module. The trends evident in these plots are similar for other PV module, namely, the strong effect of irradiance on short circuit current and of temperature on open circuit voltage, and the weaker effect of irradiance on open circuit voltage and of temperature on short circuit current. For V=0, ISC = I = IPV (3) 494 International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 3, March 2013) Likewise, for the same irradiance and p-n junction temperature conditions, the open circuit voltage (VOC) is the greatest value of the voltage at the cell terminals and it can be written as: I V VOC A * VT ln 1 SC I0 I V VOC A * VT ln 1 SC I0 V I * RS P V I SC I 0 exp A *VT For I = 0 1 (4) (5) RS V D G Fig.3: PV circuit model with single-diode and series resistance For the same irradiation and temperature conditions, the inclusion of a series resistance in the model implies [11] the use of a recurrent equation to determine the output current in function of the terminal voltage. The I-V characteristics of the solar cell are given by [12]: I I PV V I * RS I 0 exp A *V T 1 I ID I SC I I PV for V = 0 1 ISH RS (6) IPV D RSH V G A simple iterative technique initially tried only converged for positive current [13]. The Newton Raphson’s method converges more rapidly and for both positive and negative currents [14]. In this case, the short circuit current ISC is given by: I * RS I 0 exp PV A *VT (9) II.3.1. Photovoltaic model with single-diode, series and parallel resistances Photovoltaic cell models have long been a source for the description of photovoltaic cell behavior. The most common model used to predict energy production in photovoltaic cell modeling is the single diode lumped circuit model [15]. In the single diode model, there is a current source parallel to a diode. The current source represents light-generated current IPV that varies linearly with solar irradiation. This is the simplest and most widely used model as it offers a good compromise between simplicity and accuracy [16, 17]. Figure 4 shows the single diode equivalent circuit model of PV cell which is commonly used in many studies and provides sufficient accuracy for most applications. I IPV 1 II.3. Photovoltaic model with series and parallel resistances The PV model devices are basically represented in two different models with series and parallel resistances: Single-diode model with series and parallel resistances and double diode model with series and parallel resistances II.2. Photovoltaic model with single-diode and series resistance More accuracy and complexity can be introduced to the previous model by adding a series resistance. The circuit diagram of this model is shown in figure 3. ID (8) And the output power is given by: And at the same conditions, the output power is given by: V P V I SC I 0 exp A *V T for I = 0 Fig.4: PV circuit model with a single-diode, series and parallel resistances As mentioned previously, equation (6) doesn’t adequately represent the behavior of the cell when subjected to environmental variations, especially at low voltage [14]. A more practical model can be seen in figure 4, where RS represents the equivalent series resistance and RSH the parallel resistance. According to [18] and based on the equivalent circuit of a photovoltaic panel, its characteristic equation is deduced. (7) Normally the series resistance is small and negligible in computing (eq.7). Hence, it uses equation (2) as a good approximation of equation (7).The open circuit voltage VOC can be written as: 495 International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 3, March 2013) V I * RS I I PV I 0 exp A *VT V I * RS 1 RSH I 01 is the reverse saturation current due to diffusion; I 02 is the reverse saturation current due to recombination in the space charge layer. A1 1 is the diode D1 ideality factor and A2 1.2 is the diode D2 Where (10) This model yields more accurate result than the PV model with series resistances RS but at the expense of longer computational time. ideality factor. All others parameters are explained previously. The light-generated current of the module depends linearly on solar irradiation and is also influenced by temperature [15, 21] according to equation (15): II.3.2. Photovoltaic model with two diode, series and parallel resistances In this model, an extra diode is attached in parallel to the circuit of single-diode model (fig.4).This diode is included to provide a more accurate I-V characteristic curve that considers for the difference in flow of circuit at low current values due to charge combination in the semiconductor’s depletion [19, 20]. I PV I PV ,n K i * T * IPV ID1 ID2 ISH (15) I PV ,n is the light-generated current of the Where I G Gn module at Standard Test Conditions (STC). The diode saturation current I 0 dependence on RS RSH temperature can be expressed by [16]: V 3 q * Eg T I 0 I 0,n * n * exp T A* K G Fig.5: PV circuit model with two-diode, series and parallel resistances Where The accuracy of these models is more than the singlediode model but there are some difficulties to solve the equation. For simplicity, the single diode model of fig.4 is preferred and is used in this work. The basic equation (6) of two-diode model of the PV cell is given by the following equation: I I PV I D1 I D 2 where I D1 1 1 1.12 eV for the polycrystalline Silicon at 25°C) I 0,n the nominal saturation current expressed by equation (17) at Standard Test Condition (STC) [16]. I 0,n (12) I SC,n VOC ,n exp A *V T ,n (17) 1 Now, the single diode model of PV device can be improved by modifying the above equations (13) I 0,n After the combination of equation (12) and (13) and the inclusion of additional parameters RS and RSH, equation (11) becomes: V RS * I V RS * I V RS * I 1 I 02 * exp 1 I I PV I 01 * exp A1 * VT A2 * VT RSH (16) E g is the band-gap energy of the semiconductor [12, 16, 22] and (11) V I 01 * exp A1 *VT V I D 2 I 02 * exp A *V 2 T ( Eg 1 1 Tn T VOC ,n K v * T exp A * VT ,n Where (14) 1 I SC,n is the short circuit current ; VOC ,n the open circuit voltage ; irradiance and Conditions. 496 (18) I SC, n K i * T VT ,n the thermal voltage ; Gn the Tn the temperature , all at Standard Test International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 3, March 2013) Therefore, the series resistance RS, which represents structural resistances of photovoltaic panel [16, 20], has a strong effect in the voltage-source region. In turn, the shunt resistance RSH that accounts for current leakage in [10] the p-n junction, is of great importance in the current-source region and the maximum power point appears to be compromise of the hybrid behavior of the cell between both voltage and current-source region. K i and K v are short circuit temperature coefficient and open circuit temperature coefficient respectively. Since the saturation current has strong temperature dependence; equation (18) results in a linear variation of I 0,n with respect to temperature T. The model’s validity with the new equation has been tested experimentally [16]. III. DETERMINATION OF MODEL PARAMETERS IV. METHOD TO DETERMINE THE UNKNOWN PARAMETERS All model parameters can be determined by examining the manufacturer’s specification of photovoltaic products. The performance characteristics of a PV module depend on its basic materials, manufacturing technology and operating conditions. The most important points widely used for describing the cell electrical performance are: the short circuit point where the current is at maximum (short circuit current ISC) and the voltage over the module is zero; the open circuit point where the current is zero and the voltage is at maximum (open circuit voltage VOC); the Maximum power point where the product of current and voltage has its maximum. The power delivered by a PV cell attains a maximum value at the points (Imp, Vmp). The aforementioned equations (10) and (14) are implicit and nonlinear; therefore, it is difficult to arrive at an analytical solution for a set of model parameters at a specific temperature and irradiance. Models that use constant parameters have been proposed [22]. The five parameters (IPV, I0, A, RS and RSH ) model as seen in equation (10) assumes that the dark current of a PV system can be described by a single exponential dependence modified by a diode quality factor A. The values of the five parameters in the equation (10) must be determined to reproduce the I-V curve of a PV system. This requires five equations containing five unknowns that should be solved simultaneously to obtain the values of the parameters [19, 20]. G .Walker [7] has further simplified this model by removing the shunt resistance RSH to obtain a model as the four parameters model. This model reliably predicts the performance of single crystal and polycrystalline PV systems. The four parameters model assumes that the slope of the I-V curve is flat at the short circuit condition 2 Output Current(A) X: 0 Y: 1.95 X: 24.24 Y: 1.83 1.5 X: 0 Y: 0.75 1 X: 22.38 Y: 0.7038 X: 0 Y: 0.375 0.5 dI 0 dV V 0 X: 20.97 Y: 0.3519 X: 26.63 Y: 0 0 0 5 10 15 20 X: 24.8 25 Output Voltage(V) Y: 0 (19) However, this assumption is not valid for amorphous PV systems. The short circuit I-V is finite and negative, so the four parameters model can’t reproduce exactly the I-V characteristics of amorphous silicon. As mentioned previously, there are key points on the I-V curve of a photovoltaic cell. For the five parameters model, the first equation is derived from open circuit condition where I =0 and V = VOC. Equation (10) becomes 30 X: 28.67 Y: 0 Fig.6: A typical I-V plot for a three solar cells. The open-circuit voltage and short-circuit current are labeled by VOC and ISC respectively. The point of maximum power is denoted by (Vmp, Imp). Typically, three points (ISC, 0), (VOC, 0) and (Vmp, Imp) are provided by the manufacturer’s datasheet at Standard Test Conditions. An accurate estimation of these points for other conditions is the main goal of every modeling technique. From the aforementioned models, it is obvious that the PV cell acts as a current-source near the short circuit point and as a voltage-source in the vicinity of the open-circuit point. V 0 I PV I 0 ¨*exp OC A *VT VOC 1 RSH (20) The second equation occurs at short circuit condition where I = ISC and V =0. Then equation (10) becomes 497 International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 3, March 2013) I *R I SC I PV I 0 * exp SC S A * VT I SC * RS 1 RSH After substituting in equation (23) the following equation is obtained: (21) The measured current voltage pair at the maximum power point can be substituted into equation (10) to obtain the third equation where I = Impp and V= Vmpp Vmpp I mpp * RS Vmpp I mpp * RS 1 I mpp I PV I 0 * exp A *VT RSH The five parameters (IPV, I0, A, RS and RSH) can be obtained simultaneously solving these equations in MATLAB using iterative method like Newton Raphson’s method to solve system of nonlinear equations. For notational convenience, the following can be defined: These three equations are obtained using the key points. In order to get another two equations, we can differentiate equation (10) with respect to V; thus we get: RS 0 I 0 1 I 0 * VT dI * 1 dV V * RS exp OC I 0 A *VT 1 dI * 1 R p dV RSH 0 * RS I 0 V 1 dI * RS exp OC * 1 * RS A * V R dV V 0 V 0 T p A We get I mpp dI dV Vmpp I ISC Vmpp I mpp * RSO VOC Vmpp V VT * ln I SC I mpp ln I SC OC R RSH SHO I mpp V I OC SC RSHO The rest of the initial of the parameters can be found from the following equations [11, 24] (25) RSH RSH 0 (33) VOC I0 I SC R SH An addition equation can be derived using the fact that on the P-V characteristic of a PV system at the maximum power point, the derivative of power with voltage is zero V V (31) (32) (26) mp p d I * V dI dP I *V 0 dV dV dV I I mp p dV dI (24) The power transferred from the P-V device at any point is given by: P I *V (30) V VOC Based on the work [21, 22], R S0 and RSHO can be obtained experimentally from the I-V curve. Thus the initial can be calculated by calculating the diode ideality factor [23]: Again at the short circuit point on the I-V curve I = ISC and V = 0, dI dI after substituting in equation dV dV V 0 (23), we obtain: 1 dI dI I 0 * * 1 dV V 0 VT dV dV dI (23) Again at the open circuit point on the I-V curve, V = VOC and I = 0, therefore dI dI after substituting dV dV I 0 in equation (23) we obtain the following results: dI dV Vmpp 1 I mpp Vmpp I mpp * RS 1 I mpp 1 I 0 * 1 * RS * exp * RS A *VT R SH Vmpp VT Vmpp (29) (22) 1 dI dI V I * RS 1 dI I 0 * 1 * RS exp 1 * RS dV V dV A * V R dV T SH T I mpp RS RS 0 VOC A *V exp T VOC A * VT exp A *V I0 T RS I PV I SC * 1 R SH (27) (35) I SC * RS A *V I 0 exp T To compute the five parameters I PV , (28) (34) 1 (36) I 0 , RS , A and RSH which are necessary to apply equation (10), the above equations (32)-(36) have been used. 498 International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 3, March 2013) Finally, the equation of I-V characteristics is solved using the Newton Raphson’s method. VI. RESULTS AND DISCUSSION In order to validate the modeling and simulation method presented above for PV module, the calculated values and experimental values are compared for a commercial polycrystalline silicon cells from Solarex MSX60 module, composed of one parallel string of 36 solar cells. The electrical characteristics of Solarex MX60 module for G =1000W/m2 and T=25°C, at Standard Test Condition can be seen on table 1. V. NUMERICAL METHOD FOR PV CELL MODELING In many Physics, Chemistry and Engineering problems, expression where linear and exponential responses are combined appear. For instance, the implicit transcendental equation (10) of the PV cell five parameters model I-V characteristic can’t be solved explicitly for its output current or voltage using the common elementary functions. This has prompted many attempts using iterative or analytical approximations of such systems [25]. The Newton Raphson’s method, which is widely used for obtaining roots of implicit transcendental equations, is popular in iterative computational applications because of its simplicity and fast convergence [23, 26]. Newton Raphson iteration is a numerical technique used for finding approximations to real roots of the equation f ( I ) 0 given in the form of an iterative equation: I n 1 f (I ) In ' n f (I n ) Where n denotes Table.1 Solarex MSX-60 specifications (1kW/m², 25°C) n th Short-circuit current (Isc) 3.8 A Open-circuit voltage (Voc) 21.1 V Temperature coefficient of (0.065±0.01)% /°C short-circuit current (KI) Approximate effect temperature on power Nominal operating temperature (NOCT) V I * RS (38) I0 0 RSH V I * RS I 0 * RS R S * exp A *VT A * V R T SH 60 W Temperature coefficient of -(80±10)mV/°C open-circuit voltage ( KV) iteration reaches an acceptably small value. Using equation (37), the output current of a PV cell can be calculated by modifying the I-V relation of five parameter model in equation (10) as follows: f ' (I ) 1 Typical peak power (Pmpp) Current at peak power (Imp) 3.5 A and f ' ( I ) d f ( I ). This iterative process can be dI concluded when the difference between I n 1 and I n V I * RS f ( I ) I I PV I 0 * exp A *VT Specifications Voltage at peak power (Vmp) 17.1 V (37) the Characteristics of -(0.5±0.015) %/ °C cell 47±2°C Table.2: The calculated data of the parameters for Solarex MSX-60 at 25°C, AM1.5, and 1kW/m². (39) Parameters Calculated Values I0 1.859 x 10-7 A IPV 3.8119 A RS 0.180 Ω RSH 360.002 Ω A 1.360 By using the above equations, the following output current is computed iteratively V I n * RS V I n * RS I n I PV I 0 * exp I0 ( 40) A *VT RSH I n1 I n V I n * R S RS I *R 1 0 S * exp A *VT A *VT RSH 499 International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 3, March 2013) 4 3 Experimental Values Calculated Values 2 1 0 (Vmp, Imp) X: 0 Y: 3.8 0 5 10 15 20 Output Voltage(V) 3 Output Current(A) Output Current(A) 4 25 2 1 30 0 10 15 20 X: 21.1 25 30 Output Voltage(V) Y: 0 Fig.8: I-V Curve simulated at Standard Test Conditions (T=25°C, G=1000 W/m2) 2 Fig .7: I-V curve for MSX60 at 25°C, AM1.5, and 1000 W/m comparison between experimental values and calculated values. Fig.7 reveals consistency between experimental results and predicted results. It is obvious that the calculated values are in good agreement with the experimental values provided by panel manufacturers. Fig.8 shows the I-V characteristics at Standard Test Conditions where the three most important points, maximum power point (V mp, Imp), short circuit current (0,Isc) and the open circuit point (Voc,0) data are shown after the computation of all the five parameters model with numerical method. The model curves exactly match with the experimental values at the three points provided by data sheet in table 1. Fig.9 illustrates the dependence of I-V characteristics on temperature and irradiance for a solarex MSX-60 module. The fig.10 shows the P-V characteristics of the PV module with varying irradiance at constant temperature. From the graph when the irradiance increases, the output current and voltage also increases. This result shows the net increase in power output with irradiance at constant temperatures. Furthermore, it is known that for a certain PV array, the voltage –power characteristics are fixed for each insolation without intersection, as shown in fig.10. Therefore, for any given PV voltage and power, the corresponding insolation can be estimated. Also, in fig.11 and fig.12, the P-V and IV characteristics under constant irradiance (G=1000 W/m2) with varying temperature are presented, respectively. From these figures, when the operating temperature increases, the output current increases dramatically while the output voltage decreases marginally, which results in a net reduction in power with a rise in temperature. 0 5 4 1000 W/m2 800 W/m2 3.5 600 W/m2 400 W/m2 Output Current(A) 3 200 W/m2 2.5 2 1.5 1 0.5 0 0 5 10 15 20 25 Output Voltage(V) Fig.9: Curve I-V at T=25 °C for various irradiance levels 60 1000 W/m2 800 W/m2 50 Output Power(W) 600 W/m2 400 W/m2 40 200 W/m2 30 20 10 0 0 5 10 15 Output Voltage(V) 20 Fig.10: Curve V-P at T=25 °C for different irradiances 500 25 International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 3, March 2013) REFERENCES Output Power (W) 70 [1] the temperature is 60 from 0°C to 75°C with an interval of 50 25°C [2] 40 [3] 30 T=75°C 20 T=0°C [4] 10 0 0 2 4 6 8 [5] 10 12 14 16 18 20 22 24 26 Output Voltage (V) Fig.11: Curve P-V at G=1000 W/m2 for various Temperatures [6] 4 [7] Output Current (A) 3.5 3 [8] 2.5 the temperature is from 0°C to 75°C 1.5 with an interval of 25°C [9] 2 [10] T=0°C 1 T=75°C 0.5 0 [11] 0 2 4 6 8 10 12 14 16 18 20 22 24 26 Output Voltage (V) Fig.12: Curve I-V at G=1000W/m2 for various Temperatures [12] VII. 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