106 CHAPTER 5 CIRCUIT MODELING METHODOLOGY FOR THIN-FILM PHOTOVOLTAIC MODULES 5.1 INTRODUCTION In this Chapter, the constructional details of various thin-film modules required for modeling are given. The present state of modeling the thin-film modules is reviewed and the difficulties in modeling of thin-film PV modules are narrated. The modeling of the amorphous silicon PV module is taken up and the Simulink circuit model is developed. The high temperature operation capabilities of the amorphous silicon module are examined. 5.2 IMPROVEMENTS NEEDED IN THE PV MODULE CONSTRUCTION The improvements needed in the construction of the thin-film PV module are 1. Thin-film for improving the efficiency. 2. p-i-n diode for improving the voltage output. 5.2.1 Thin-film for Improving the Efficiency of the PV Module Zeman (2003) mentioned the need for thin-film PV module for improving its efficiency and the same is explained below.
109 non-silicon based thin-film technologies are cadmium telluride (CdTe) and copper indium selenide (CIS). The new concept devices under research stage are the solar concentrator systems, organic-based PV cells and quantum cells. Deb (2000) specified two requirements to increase the efficiency of the PV cells. The first is to select the appropriate semiconductor materials with specified energy gaps to match the solar spectrum and optimizing their optical, electrical, and structural properties. The second is to achieve more effective charge collection as well as better utilization of the solar spectrum with single and multi-junction approaches. Figure 5.2 Classification of thin-film PV technologies
110 A solar cell s efficiency depends on the band gap (E g ) of the material from which it is constructed. The band gap is determined by the intrinsic material and the doping level. For a solar cell to produce the maximum electricity for a given illumination level, the cell material used should be chosen based on the spectrum of light. 5.3.1 Maximizing the Efficiency of Thin-film PV Module Figure 5.3 shows the reflection and absorption activities of the photons in a solar cell (Bossert et al 2000). To maximize the power rating of a solar cell, it is necessary to increase the absorption and decrease the reflection in materials through improved structure development. Figure 5.3 Behavior of light shining on a solar cell
111 5.3.2 Advantages and Disadvantages of Thin-film PV Modules Thin-film PV modules are efficient and high performing with better energy output in terms of kwh. They use lesser materials of lower cost, have reduced processing steps, and are versatile and flexibile, and give good performance at high temperature. Their disadvantages are the degradation of performance over time, higher total costs and scarcity of raw materials (Bossert et al 2000). 5.4 THIN-FILM PV MODULES The major thin-film technologies in large-scale production are the amorphous silicon (a-si) modules, the multi-crystalline CdTe modules, the CIS modules and the copper indium gallium diselenide (CIGS) modules. Their constructional details are needed for modeling purposes and hence presented in the following sections. 5.5 AMORPHOUS SILICON (a-si) MODULES Amorphous silicon (a-si) is non-crystalline form of silicon. It is hydrogenated amorphous silicon carbide with the chemical symbol asi 1-x C x :H.The gap between the "optical gap" and the band gap, can be widened by increasing the concentration of carbon in the alloy. Thus, the light efficiency of the solar cells made with hydrogenated amorphous silicon carbide layers is increased. To improve the efficiency and solve the degradation problems, multiple junction a-si devices have been built and their test results are shown in Figure 5.4 (Carlson et al 1976). This improvement is due to the absorption of different wavelengths from the solar irradiation (from short to long
112 wavelength) and reduction in the recombination losses. The amorphous silicon solar cells are based on the double junction p-i-n structure, consisting of a thin p-type doped layer, a central intrinsic i-type layer as the photon absorbing layer, and a thin n-type doped layer, as shown in Figure 5.5. Figure 5.4 Variation of the output with the insolation for multiple junction a-si Figure 5.5 Improved structure of an amorphous silicon thin-film cell
113 The intrinsic "i" region has more charge carriers than the "p" and "n" regions, which helps to increase the diode current. The depletion region is within the intrinsic region of the p-i-n diode and is larger than the p-n diode. This results in the increase of the electron-hole pairs generated by an incident photon. The electrical transport in the i-type layer is assisted by an electric field. These facts aid the temperature performance of the a-si cell. The STC rated efficiencies of the a-si module are around 6-7%. The transparent conducting oxide (TCO) helps to decrease the resistance at the top of the cell. The glass superstrate is used for depositing a different material layer. 5.6 ADVANTAGES OF a-si MODULES The a-si PV modules have several attractive benefits in terms of energy production, cost, suitability, and environmental attributes (Jansen, 2011). The spectral matching of the a-si cell response to the solar spectrum gives it an advantage of increased power production at the time when the Sun is at its brightest (at the midday or in the summer), which are also the times when the electricity is highly valued. At these times, the Sunlight is rich in blue light. The a-si PV module has a higher spectral response to the blue light than to the red light due to its higher band gap energy level. Up to 6 % increase in the performance of the a-si modules from winter to summer, due to the spectral effects, was found. The savings in the module cost are offset due to the higher balance of system costs and increased operational and maintenance costs. As a result, the crystalline PV system has a total installed cost of about 10 % lower than the cost of a-si based system. Hence, the price of amorphous silicon solar modules is marginally lower than that of crystalline silicon modules for a given stabilized power output.
114 5.7 CdTe PV MODULES In 1950, it was found that the CdTe has a band gap about 1.5 ev, which helps in increasing the optical conversion to electricity with the absorption of different wavelengths from the solar irradiation. A hetero junction cell is made with a p-type CdTe and an n-type cadium sulfide (CdS). The CdTe cells have larger area, as the module has an additional layer called the transparent conducting oxide (TCO) instead of a metal grid. Figure 5.6 shows the layered structure of a CdTe cell. Figure 5.6 Layered structure of a CdTe cell The higher efficiency of the CdTe cells, by above 15 %,is achieved by adding a resistive tin oxide buffer layer with a TCO stack. Figure 5.7 shows the typical 75Wp CdTe PV module (NREL 2011).
115 Research is in progress to increase the efficiency of the CdTe cells with improved doping of the CdTe and the cadmium chloride crystallization process. Cost reduction has to be made through improvements in broader substrates to reduce the capital costs. Thinner layers are in trial to save the material, electricity and throughput time. Figure 5.7 Typical 75 Wp CdTe PV module 5.8 OTHER ISSUES WITH CdTe MODULES Other issues with CdTe modules are the toxicity of cadmium, price vulnerability and market viability. Due to these problems, the development of this type of module is facing a major hurdle.
116 5.9 CIGS PV MODULE 5.9.1 Properties of CIGS Copper indium gallium selenide (CuIn1-xGaxSe2 or CIGS) is a semiconductor with direct band gap, which is useful for the manufacturing of PV cells. A thinner film is required due to the strong absorption of Sunlight in the semiconductor. The CIGS absorber along with electrodes is placed on a glass backing. CIGS has layers of copper, indium, gallium, and selenium. The band gap varies continuously from about 1.0 ev in copper indium selenide to about 1.7 ev in copper gallium selenide. 5.9.2 CIGS Photovoltaic Cell Figure 5.8 shows the structure of a CIGS PV cell. The substrates used are glass and metal foils. A molybdenum (Mo) layer is deposited by sputtering it in the rear contact. With the molybdenum deposition, a p-type CIGS absorber and an-type intrinsic ZnO layer is added. The intrinsic ZnO layer improves the cell performance by preventing the contact between the conductive ZnO layer and the CIGS layer. The Al doped ZnO serves as a transparent conducting oxide to collect and move more electrons out of the cell with the absorption of less light. CIGS films are manufactured by vacuum-based process by co-evaporating copper, gallium, and indium onto a substrate at room temperature, then annealing the selenide vapour to form the CIGS structure.
117 Figure 5.8 Structure of the thin-film CIGS cell Figure 5.9 shows typical 70 Wp CIGS PV module (NREL 2011). Production of these modules involves the deposition layer being cut into a series of parallel-connected strips. A further transparent protective cover is applied to the module. This sandwich construction is then sealed against the ingress of moisture. Rigid physical support is necessary to prevent fracture of this fragile structure.
118 Figure 5.9 Typical 70 Wp CIGS PV module 5.9.3 CIS PV module The CIS PV module is the subset of CIGS PV module without the copper gallium-selenide and has poor voltage capability. 5.10 COMPARISON OF CIGS WITH SILICON AND OTHER THIN-FILMS The silicon cells are based on a homo junction and the CIGS cells is a hetero junction system. CIGS is cheaper due to the lower material and less fabrication costs. As a direct band gap material, CIGS has very strong light absorption. The use of gallium increases the optical band gap of the CIGS layer as compared to the pure CIS, thus increasing the open circuit voltage. Further, gallium is added to replace as much indium as possible due to the gallium s relative availability compared to the indium. The commercial production of the CIGS with 13 % efficiency has begun.
119 5.11 MODELING OF THE THIN-FILM PV MODULES On going through the constructional details of various thin-film modules, the structure of thin-film cells are more complex junction systems. Gupta et al (2012) proposed an improved two-diode model common to the mono-crystalline, the multi-crystalline and the thin-film PV modules. In the two-diode model, it was assumed that the photon current is equal to the short circuit current given in nameplate details of the module. The obtained curves in their work were compared with datasheet values provided by manufacturers of the mono-crystalline, the multi-crystalline and the thin-film PV modules. The authors developed the equations for all the parameters of the two-diode model including the series and the parallel resistances. Finally, the equation of the I-V characteristics was solved using the Newton s method for rapid convergence. The parameters I ph, I o1, I o2 R s, R sh and A vary with temperature and irradiance and depend on the manufacturing tolerance. They were used in equations and numerical methods. Finally curve fitting was used to get the curve. Kandil et al (2011) worked on the effects of the temperature and the radiation intensity on the parameters of CIS PV module using an outdoor experimental setup. The values of the module parameters namely series resistance R s and shunt resistance R sh, were determined from the I-V measurements at different irradiation intensity and temperature ranges. The singe-diode five-parameter PV model was adapted in their study to determine the module parameters that are used to simulate the characteristic of the thin-film solar cells at different weather conditions. Their model added the R sh to the four-parameter model making it applicable for crystalline as well as thin-film PV solar cells. They also found that the amorphous and the CIS PV cells exhibit a pronounced slope at the short circuit point, and therefore a finite value of shunt resistance was used. The value of R sh was calculated
120 by an iterative procedure and different values of resistances were tested to get the most appropriate value. In their experimental setup, the variable resistance was connected across the CIS PV module and the nonlinear curve-fitting program IVFIT was used to fit the experimental I-V data. Figure 5.10 shows the measured and predicted I-V characteristics of the CIS module (T amb = 20 C) obtained in their work. Figure 5.11 shows the effect of the temperature on the I-V curve of the CIS module. Different curves were obtained around noon on different days at different module temperatures and fitted in the curve showing the variation of I-V characteristics with temperature for CIS module (G=800 W/m 2 ). Figure 5.11 clearly indicates that the voltage V is more temperature dependent than the short circuit current I. Figure 5.10 Comparison of the measured and the predicted I-V characteristics for the CIS module (T a = 20 C)
121 Figure 5.11 Variation of the I-V characteristics with temperature for the CIS module (G = 800 W/m 2 ) For the CIS module, the authors used the orthogonal distance regression (ODR), a mathematical method, for fitting the measurements. This technique also computes the I-V curve parameters of I PV, I o, R s, R sh and ideality factor n, which are the measure of the cellular imperfection. Figure 5.12 shows the variations of the parameters of the CIS module with radiation intensity at module temperature of 20 C.
122 Figure 5.12 CIS module parameters variations with radiation intensity at the module temperature of 20 C At 20 ºC, the ideality factor n varies from 2.3 at 250 W/m 2 to 1.55 at 850 W/m 2. Similar variations in I 0 are from 4.5*10-4 A to 0.3*10-4 A, R s from 1.3 to 0.9, while R sh increases from 50 to 200 and then decreases to below 10. Thus, they concluded that the wide variations found in the parameters of the thin-film modules have to be taken care of in the modeling. Figure 5.13 shows the variations of the CIS module parameter with radiation intensity at the module temperature of 40 C.
123 Figure 5.13 CIS module parameters variations with radiation intensity at the module temperature of 40 C From the above work, it can be seen that both the series and the shunt resistances decrease with the increase in insolation and temperature. This results in a large voltage drop at the high temperature operation of the CIS PV module. Zdanowicz et al (2003) presented a large number of I-V curves data acquired in natural outdoor conditions during long-term monitoring at the Solar Lab, Department of Micro system, Electronics and Photonics, University of Technology, Wroclaw, Poland. Zdanowicz (1994) proposed special purpose software allowing numerical fit of the measured I-V curves to either of the one-diode or the twodiode model. The software automatically imports I-V data stored in the Solar
124 Lab s database enabling fitting of a thousand curves within a reasonably short time. Prorok et al (2006) and Werner et al (2006) proposed the two-diode model for the CIGS as well as the CdTe thin-film modules. From the actual voltage and the current measurements, the module I-V and P-V curves are drawn through the curve fitting process using special software and the module parameters are extracted. Detailed graphs of the module parameter variations with respect to a wide range of temperatures were shown for CIS, CdTe and CIGS thin-film modules. In the above works, the nameplate details of these modules were not provided. Their work can be taken as an example of scarcity of reliable data in the area of thin-film modules. Being a very high cost research work, manufactures are constrained and do not provide the data even to standard research journals. However, this I-V curve research is essential for boosting the consumer confidence and to aid in marketing of thin-film PV modules. Garcia et al (2011) described the application of a nonlinear regression technique to model the CdTe thin-film solar modules. In this work, both single-diode and the two-diode equivalent circuits were taken for analysis. However, they had taken the nonlinear equation to fit the I-V curves provided by the manufacturer. The symmetrized Gompertz curve equation was taken and the three parameters of the equation were determined with open circuit voltage and the short circuit current of the CdTe thin-film PV module. Set of curves for both the single-diode and the two-diode models were drawn.
125 5.12 MODELING OF THE AMORPHOUS SILICON PV MODULE Ghoneim et al (2011) investigated the effects of the temperature and the radiation intensity on the performance parameters of the amorphous hydrogenated silicon (a-si:h) PV module with an outdoor experimental setup. The values of the module parameters, namely, the series resistance R s, shunt resistance R sh, the diode ideality factor n and the reverse saturation current I o were determined from the experimental I-V measurements at different irradiation intensity and temperature range. The single-diode fiveparameter photovoltaic model was adapted in their study to determine the module parameters to simulate the characteristic of thin-film a-si cells at different weather conditions. Their model added R sh to the four-parameter model making it applicable to both the crystalline and the thin-film PV solar cells. They also found that the amorphous and CIS photovoltaic cells exhibit a pronounced slope at the short circuit point, and therefore a finite shunt resistance was taken in their work. The value of R sh was calculated by an iterative procedure and different values of resistances were tested to get the most appropriate value. TRNSYS software was used to collect the hourly data and the nonlinear curve-fitting program IVFIT was used to fit the experimental I-V data. For the a-si:h module, the authors used the orthogonal distance regression (ODR) for fitting the measurements and at the same time computing the I-V curve parameters I PV, I o, R s, R sh and ideality factor n, which are the measure of the cellular imperfection. Figure 5.14 shows the variations in the a-si:h module parameters with radiation intensity at the module temperature of 28 C.
126 Figure 5.14 Module parameters variations of the a-si:h with radiation intensity at the module temperature of 28 ºC At 28 ºC, the ideality factor n decreases from 3.9 at 250 W/m 2 to 3 at 850 W/m 2 and again increases to 3.5 at 1000 W/m 2. Similar variations in I 0 results in decrease from 1.5*10-4 A to 0.7*10-4 A at 850 W/m 2 and again increase to 1.2*10-4 A at 1000 W/m 2. R s increases from 6 at 250 W/m 2 to 12 at 850 W/m 2 and again decreases to 9 at 1000 W/m 2. R sh decreases from 300 to 100. Figure 5.15 shows the variations of the amorphous hydrogenated silicon module parameter with radiation intensity at the module temperature of 48 C.
127 Figure 5.15 Module parameters variations of the a-si:h with radiation intensity at the module temperature of 48 ºC At 48 ºC, the ideality factor n decreases from 4 at 250 W/m 2 to 2.9 at 850 W/m 2. Similar variations in I 0 are from 5.5*10-4 A at 250 W/m 2 to 1.0*10-4 A at 850 W/m 2. R s increases from 11 at 250 W/m 2 to 11.2 at 800 W/m 2 and again decreases to 10.9 at 850W/m 2. R sh decreases from 250 to 100. These types of wide variations found in the module parameters with radiation intensity of thin-film modules have to be taken care of in the modeling of thin-film modules. 5.13 I PV MODEL OF THE AMORPHOUS SILICON PV MODULE 5.13.1 Reference model While just the specifications given in nameplate details are enough for modeling of the standard polycrystalline PV modules, thin-film PV
128 modules require detailed literature on the module parameters and their variations. Due to the above requirements, the model used by Ghoneim et al (2011) in their work, is used as the reference model for the circuit modeling. Table 5.1 gives the electrical characteristics data of a 10 Wp amorphous silicon PV module. Figures 5.15 and 5.16 show the wide variations of the module parameters. However, these parameter values are nearly constant for insolation above 900 W/m 2. Hence, the parameter values for insolation of 1000 W/m 2 is used for simulation and given in Table 5.1. Further, the objective is to find the module performance at the high temperature operation with insolation at 1000 W/m 2. The model is valid only for verification at 1000 W/m 2 and at the high temperature operation. For module performance at lower insolation, the parameter variations have to be accommodated in the modeling. Berkel et al (1993) gave the details of the p-i-n a-si diode properties. At low bias, a well-defined exponential region exists, described by a non-integer quality factor A between 1.2 and 1.7. With increasing temperature, the quality factor decreases. Hence, the diode quality factor A is taken as 1.3. 5.13.2 Equivalent Circuit and Equations of the Amorphous Silicon PV Module Werner et al (2006) and Gupta et al (2012) used the single-diode model for the simulation of thin-film modules. Hence the single-diode model, shown in Figure 2.5, is taken for modeling the amorphous silicon PV module. Equations (2.1) to (2.4) were used for modeling. The main difference here is that the series and shunt resistances are not neglected for the amorphous silicon PV module.
129 Table 5.1 Electrical characteristics of the 10 Wp amorphous silicon PV module Description Rating Rated Power 10 Wp Short circuit current ( I SCr ) 0.9 A Open circuit voltage ( V OC ) 21.5 V Voltage at Maximum power (V mp ) 17.00 V Current at Maximum power ( I mp ) 0.7 A short circuit current temperature co-efficient 0.0003 E go 1.2 Diode ideality factor A 1.3 Series resistance of the module 3 Shunt resistance of the module 100 Total number of cells in series (N s ) 30 Total number of cells in parallel (N p ) 1 Note: The electrical specifications are under test conditions of irradiance of 1 kw/m 2, spectrum of AM 1.5 and cell temperature of 25 ºC. 5.13.3 Module Output Current I PV The series and parallel resistances are taken into account for obtaining the current output I PV of the single-diode model of the amorphous silicon PV module. In this case, the model given in Figure 2.12 is modified and is given in Figure 5.16.
130 Figure 5.16 Model for the module output current I pv of the a-si module 5.13.4 I PV Model Block of an a-si PV Module With an improved I PV model, the I PV model block of the a-si PV module is formed through the subsystem combination of MATLAB/ Simulink models of all the four equations. 5.14 OUTPUT CHARACTERISTICS OF THE a-si PV MODULE The simulation setup given in Figure 2.16 is used for obtaining the output characteristics. Figure 5.17 shows the I-V output characteristics of the a-si PV module at the constant irradiation of 1000 W/m 2 and at the constant temperature of 25 ºC.
131 Figure 5.17 I-V characteristics of the a-si PV module at the irradiation of 1000 W/m 2 and 25 ºC Figure 5.18 gives the P-V output characteristics of the a-si PV module at the constant irradiation of 1000 W/m 2 and at the constant temperature of 25 ºC. Figure 5.19 shows the output I-V characteristics of the a-si PV module for varying temperature and constant irradiation of 1000 W/m 2. Figure 5.20 shows the output P-V characteristics of the a-si PV module for varying temperature and constant irradiation of 1000 W/m 2.
132 Figure 5.18 P-V characteristics of the a-si PV module at the irradiation of 1000 W/m 2 and the temperature of 25 ºC Figure 5.19 I-V characteristics of the a-si PV module for varying temperature and constant irradiation of 1000 W/m 2
133 Figure 5.20 P-V characteristics of the a-si PV module for varying temperature and constant irradiation of 1000 W/m 2 5.15 CIRCUIT MODEL OF THE a-si PV MODULE The circuit model block of the a-si PV module is formed in the same way as discussed in section 2.15. Figure 5.21 shows the detailed circuit model of the a-si PV module. I PV model block of the a-si PV module is placed in the circuit model to supply the module current for the input insolation and ambient temperature and the diode voltage is set to the a-si module open circuit voltage of 21.5 V. Figure 5.21 Detailed circuit model of the a-si PV module
134 5.1 6 VALIDATION OF THE a-si CIRCUIT MODEL The exact experimental verification of the developed circuit model is not always practical due to the following reasons. i. Thin-film PV modules are very costly. ii. The performance has to be found for a wide variation in irradiation and temperature. However, these performance variations are not always present when the experiment is carried out. Hence, a simplified validation procedure for the model is the need of the hour. In section 2.17, the validation of the Simulink circuit model of the mono crystalline PV module at three remarkable points of the I-V curve closely follows the experimental results. Thus the validation through a simple simulation circuit at the three remarkable points with nameplate details is equivalent to the experimental one. The same is carried out in the following sections of the developed circuit model of the a-si PV module. 5.17 VALIDATION OF THE a-si CIRCUIT MODEL THROUGH SIMULATION The a-si circuit model is validated at the three remarkable points, namely at V=0, at V=V oc and at V=V mp. 5.17.1 Remarkable Point at Short Circuit, V=0 Figure 5.22 shows the a-si circuit model with simulation results that is used for the short circuit point where V=0. As per the nameplate at V=0, the rated current at I=I sc =0.9 A.
135 Simulated value=0.8738 A Error in current=0.9-0.8738=0.0262 A % Error in current =(0.0262/0.9)*100=1.96%=2 % Figure 5.22 Simulation circuit with the a-si circuit model at V=0 5.17.2 Remarkable Point at Open Circuit, I=0 Figure 5.23 shows the a-si circuit model that is used in the open circuit simulation circuit, where I=0. As per the nameplate at I=0, the rated voltage V=V oc =21.5 V. Simulated value=21.23 V Error=21.5-21.23=0.27 V Error %=(0.27/21.5)*100=1.256 %
136 Figure 5.23 Simulation circuit with the a-si circuit model at I=0 5.17.3 Remarkable Point at Maximum Power Point Figure 5.24 shows the a-si circuit model that is used in the variable resistance simulation circuit to obtain the maximum power point operation. The resistance of the circuit is varied from 0 to 100 and the results obtained are tabulated in Table 5.2. Figure 5.24 Simulation circuit with the a-si circuit model at MPP
137 Table 5.2 Results of the simulation of the a-si circuit model for variable resistance at insolation of 1000 W/m 2 and the temperature of 25 ºC S.No. Resistive load PV Current PV Voltage PV Power ) (A) (V) (W) 1 0 0.8738 0 0 2 1 0.8654 08654 12.5 3 5 0.8333 4.167 3.472 4 10 0.7964 7.964 6.343 5 15 0.7624 11.44 8.718 6 20 0.7257 14.51 10.53 7 22 0.7079 15.51 10.93 8 23 0.6924 15.93 11.03 9 23.5 0.6857 16.11 11.05 10 23.9 0.6802 16.26 11.06 11 24.3 0.6745 16.39 11.05 12 50 0.3855 19.28 7.431 13 100 0.2028 20.28 4.112 14 Open circuit 0 21.23 0 Table 5.2 shows that the maximum power point operation is at V mp =16.26 V, I mp =0.6802 A, and P mp =11.06 W. This corresponds to the circuit resistance of 24.9 at the maximum power point operation. The nameplate details of the a-si PV module gives V mp =17.0 V, I mp =0.7 A, and P mp =11.9 W. This corresponds to the circuit resistance of 24.2857 at the maximum power point operation.
138 Rated power at P mp = 11.9 W Simulated value = 11.06 W Error in power = 11.9-11.06 = 0.84 W % Error in power = (0.84/11.9)*100 = 3.694 = 7.06 % Rated voltage at V mp = 17.0 V Simulated value = 16.26 V Error in voltage = 17.0-16.26 = 0.74 V % Error in voltage at the maximum power point = (0.74/17)*100 = 4.353% = 4.4 % Rated current at I mp = 0.7 A Simulated value = 0.6802 A Error in current = 0.7-0.6802=0.0198 A % Error in current = (0.0198/0.7)*100 = 2.828% = 2.9 % Further, Table 5.2 exhibits that the simulated values of the a-si circuit model follows the I-V curve as given in Figure 5.19. Table 5.2 values represent the circuit operating points and the simulation values are lower than the nameplate values. With the above simulation results, it is found that the developed a-si circuit model has reasonable accuracy. 5.18 HIGH TEMPERATURE PERFORMANCE OF THE a-si PV MODULE The high temperature performance of the a-si PV module can be found out through the simulation of its circuit model. Hence, the circuit model
139 of the a-si PV model is tested with the variable resistance simulation circuit, as shown in Figure 5.25. The resistance of the circuit is varied at 45 ºC around the maximum power point and results are tabulated in Table 5.3. Figure 5.25 Simulation circuit with the a-si Circuit model at 45ºC Table 5.3 Results of the simulation of the a-si circuit model for variable resistance at insolation of 1000 W/m 2 and the temperature of 45 ºC S.No. Resistive load PV Current PV Voltage PV Power ) (A) (V) (W) 1 21 0.7022 14.75 10.35 2 22 0.6868 15.11 10.38 3 23 0.6707 15.43 10.35 4 23.9 0.6558 15.67 10.28 At the ambient temperature of 45 ºC, the maximum power point operation shifts to V mp =15.11 V, I mp =0.6868 A, and P mp =10.38 W. At higher temperature, the reduction in power is mainly due to the reduced voltage.
140 The resistance of the circuit is varied at 65 ºC around the maximum power point, as shown in Figure 5.26, and the results are tabulated in Table 5.4. Figure 5.26 Simulation circuit with the a-si circuit model at 65 ºC Table 5.4 Results of the simulation of the a-si circuit model for variable resistance at insolation of 1000 W/m 2 and the temperature of 65 ºC S. Resistive load PV Current PV Voltage PV Power No. ) (A) (V) (W) 1 19 0.7122 13.53 9.637 2 19.5 0.7038 13.72 9.658 3 20 0.6951 13.9 9.663 4 20.5 0.6882 14.07 9.654 5 22 0.6592 14.5 9.56
141 At the ambient temperature of 65 ºC, the maximum power point operation shifts to V mp =13.9 V, I mp =0.6951 A, and P mp =9.663 W. At high temperature, the power reduction is mainly due to the reduced voltage. The voltage, current and power variation at the maximum power points at 25 ºC, 45 ºC and 65 ºC are tabulated and compared in Table 5.5. Table 5.5 Comparison of the MPP variation at the high temperature operation at 1000 W/m 2 S.No. Temperature (ºC) PV Current (A) PV Voltage (V) PV Power (W) % Voltage variation 1 25 0.6802 16.26 11.06 Nil 2 45 0.6868 15.11 10.38 7 3 65 0.6951 13.9 9.663 14.5 The base voltage of the module is taken as 16.26 V and the percentage variation in voltage is calculated. The large voltage variation at the high temperature indicates the need for a PI controller with a boost converter for continuous operation of the PV power plant. 5.19 BLUE LIGHT EFFECT ON THE a-si PV MODULE At the brightest time in the midday, the Sunlight is rich in blue light. Accordingly, the a-si has a higher spectral response to the blue light than to the red light due to its higher energy gap. Up to 6 % increase in the performance of the a-si modules from winter to summer due to the spectral effects was obtained. At the same time, the c-si module performance decreased by 3 % from winter to summer, solely due to the changing spectral content of the incident Sunlight (Guechi et al 2007).
142 To estimate the blue light performance of the a-si module, the circuit model is simulated with insolation of 1200 W/m 2 at temperature of 65 ºC, as shown in Figure 5.27. Figure 5.27 Simulation circuit with the a-si circuit model at 1200 W/m 2 and temperature 65 ºC Table 5.6 Blue light performance with the a-si circuit model at the insolation of 1200 W/m 2 and the temperature of 65 ºC S.No. Resistive load ) PV Current (A) PV Voltage (V) PV Power (W) 1 15 0.8829 13.24 11.69 2 15.5 0.8703 13.49 11.74 3 16 0.8572 13.72 11.76 4 16.5 0.8438 13.92 11.75 In Table 5.7, the values in the first row are taken as the base values. % of improvement of power for 1100 W/m 2 = ((10.72-9.663)/9.663)*100 =11 % % of improvement of power for 1200W/m 2 = ((11.76-9.663)/9.663)*100 =21 %
143 Table 5.7 Calculation of the improvement due to the blue light performance at the temperature of 65 ºC S.No. Irradiation PV Current PV Voltage PV Power (W/m 2 ) (A) (V) (W) 1 1000 0.6951 13.9 9.663 2 1100 0.7864 13.63 10.72 3 1200 0.8572 13.72 11.76 Higher spectral response to the blue light of a-si module is already available in literature. The above simulation response of Simulink circuit model validates the same. The voltage reduction at the high temperature indicates the need for the voltage boosting circuits. 5.20 SIMULATION ARRANGEMENT TO FIND THE HIGH TEMPERATURE OUTPUT OF THE a-si PV MODULE The a-si circuit model is used to find the output of the a-si PV module at the blue light and at the high temperature. Simulation is done for 1100 W/m 2 and 1200 W/m 2 inputs. The detailed circuit arrangements are shown in Figures 5.28 and 5.29 with the developed a-si circuit model of the PV module and the MPPT control unit. V in and I in are taken as the inputs to the MPPT unit and the duty cycle is obtained as the output. With the MPPT control circuit shown in Figures 5.28 and 5.30, simulation is carried out with irradiation of 1100 W/m 2 and 1200 W/m 2 respectively, at the high temperature of 65 ºC. The input-output voltages are shown in Figures 5.29 and 5.31.
144 Figure 5.28 Simulation arrangement to find the high temperature performance of the a-si PV module at 1100 W/m 2 Figure 5.29 Variation of the voltage, the current, and the power for insolation of 1100W/m 2 and the temperature of 65 ºC
145 Figure 5.30 Simulation arrangement to find the blue light performance of the a-si PV module at 1200 W/m 2 With the MPPT control circuit shown in Figures 5.28 and 5.30, simulation is carried out with irradiation of 1100 W/m 2 and 1200 W/m 2 respectively, at the high temperature of 65 ºC. The input-output waveforms are shown in Figures 5.29 and 5.31.
146 Figure 5.31 Variation of the voltage, the current, and the power for insolation of 1200W/m 2 and the temperature of 65 ºC The quality of the output of a-si module as given in Figure 5.31 is better than the c-si module, as discussed in Chapter 4. With this better quality of the output, the tripping of inverters at high temperature operation can be avoided and the overall output of a-si PV module is improved. 5.21 CONCLUSION In this Chapter, the constructional details of different types of thin-film modules required for circuit modeling is provided. Thin-film module parameters vary with irradiation and temperature and the difficulties in modeling of thin-film PV models due to these variations is presented with the literature survey.
147 The amorphous silicon PV module is taken to explain the circuit modeling procedure. The developed a-si circuit model is verified for its accuracy with simulation circuits. The blue light performance of the a-si PV module at high temperature is explained with the simulation of circuit model. The quality of DC output obtained at high temperature operation of the a-si PV module is presented in this Chapter. The high temperature operation is necessary in India especially in Tamil Nadu. Hence, the DC power processing circuits are required. It can be concluded that for the smooth and efficient operation of the PV modules in India and other high temperature countries, dedicated DC-DC converter power processing are required.