Novel Method for Evaluating the Series Resistance of Individual Cells in encapsulated PV Modules by Means of a DLP Projection System.

J.D. Silva, Loughborough University, Loughborough, Leics LE11 3TU

A paper for publication in Solar Energy Journal

Abstract: In this work a novel method is presented for the determination of the internal series resistance of individual cells that compose an encapsulated photovoltaic module, by generating partial shading patterns using a Digital Light Processing (DLP) projector. Given the impossibility to physically access the internal cells on a PV module, there is a need on the PV industry for the development of fast and accurate systems that allow cell-level characterization. Several analytical methods have been developed to determine the internal parameters of the cells, but these methods require the projection of partial shading, which until now had been achieved by means of mechanical systems attached to the cells. The aim of this project was to develop a non-destructive measurement system for determining the operational parameters of the individual cells, by implementing high-speed image generation using a commercial DLP projector. The developed system was be able to produce automatic shading patterns that permitted the acquisition of reliable and repeatable data that could then be used to calculate the parameters using the previously developed methods. Using a projector system results in faster and less cumbersome measurements than with the previously proposed mechanical shading systems, achieving measurement correlations of up to 85%.

Keywords: PV module, Series Resistance, Partial Shading, DLP projector, LabVIEW.

1. INTRODUCTION

Commercial photovoltaic (PV) modules are manufactured as encapsulated, sealed arrays of solar cells, predominantly series interconnected. As a result, the module’s total output current is kept at relatively low values, allowing to have smaller bus bars, thus maximizing the module’s overall exposed active area, as well as keeping the total output voltages sufficiently high so that the energy produced by the module can be efficiently extracted and transported. Encapsulation of modules is a necessary feature given that they are continuously exposed to the open sky, having to overcome all weather conditions for periods that in theory should be close to 20 years or more. This also means that the internal layers cannot be intervened of modified in any way on the risk of damaging the encapsulation and potentially compromising the integrity of the solar panel.

The impossibility to alter the internal connections of the PV modules has posed a very important challenge to PV device characterisation, given that it makes it very difficult to assess the operational status of individual cells and the identification of potential faults in them. Characterization of individual cells of encapsulated modules is a significant concern in the solar industry, given that a PV module’s performance is limited by the less efficient of its cells, meaning this that if one cell underperforms, the whole module will be limited by this cell. This can cause not only reduction of the total output power but can also generate harmful overheating for the cells that significantly underperform. This phenomenon makes energy generation significantly lower than expected and in extreme cases can lead to failures of one or more of the cells, compromising the operability of the module and possibly an entire PV installation.

The main objective of this project is to develop a non-destructive automated system for assessing the performance of individual cells in a PV module, by determining the series resistance of each of the individual cells, implementing high-speed image generation using a commercial DLP projector. The developed system was be able to produce automatic shading patterns to detect the underperforming cells in an encapsulated module and evaluate their performance parameters, allowing measurements to be faster and less cumbersome than in previous work.

The method for the calculation of the series resistance of a PV module based on the IV curves generated at two different irradiances, was first published by Wolf [20] in 1963. This method became a standard for the solar industry given that even if the tests are realized on relatively low irradiances as compared to the STC, it generates Rs (series resistance) values that are extremely close to the values obtained by evaluating the PV modules under STC. This characteristic makes the method very robust and is currently one of the most commonly used series resistance calculation methods. This technique has been used in the present project as one of the main tests to be realized to the PV module, to evaluate the effectiveness and precision of the computer generated shading, by comparing the results to

those obtained by using an STC solar simulator. In [22] a modification of the Wolf method is proposed, three different irradiance IV curves are used and the results obtained for each of the curves are linearised. This modification increases the precision of the method, especially when the tests are being realized under low irradiance conditions, but with the downside of having to use different irradiances for each of the IV curves, condition that is not always achievable.

While this method has proven to be very useful, it only applies to the determination of the total series resistance of the PV module, leaving still the determination of the series resistance of the individual cells unexplored. Several researchers have used this as a base for the determination of individual resistances by implementing partial shading. In [1], Alers introduces a method for obtaining the shunt resistance (Rsh) and the short-circuit current (Isc) of individual cells of a PV module by implementing partial shading to the cells through a mesh. The technique is based on differential voltage and current measurements obtained from a set of I-V curves, acquired from PV modules with partially shaded cells establishing the relationship between the differential measurements with the series and shunt resistances of the shaded cells.

In [2] Silvestre and Chouder produce a complete method for the calculation of the series resistances of internal cells, condensing all the measurements and calculations needed for the extraction of series resistances. In this method the Rs of the cells is extracted by maximizing the dV/dI function obtained from the IV curve generated by partially shading one cell. Subsequently, the dV/dI of all the other unshaded cells is extracted from this measurements and the individual series resistances are calculated by subtracting the resistance of all the unshaded cells from the one obtained for the whole module. Tests are performed for two predefined shading values, which were achieved by implementing 3D printed meshes that could exactly simulate the desired shading levels. This process is repeated for all the cells which means that it requires a significant amount of time and effort. This was one of the main guides for the present project and provided a theoretical as well as a practical base for this work, although some changes were introduced to the method given that the results have decreased accuracy when the tests are realized under low irradiance conditions, while also other issues appear in a projector based system, which will be discussed in a later section.

A similar method for characterisation of the characteristics of polycrystalline PV cells through the I-V curves obtained under different shading conditions is also realized by De Bernardez and Buitrago in [6]. A test shading is applied to only one of the cells and the effect that this has on the PV module output is investigated. Different geometries of shading patterns are used covering many cells at a once, giving what could be the base of correlation between the changes that shading generates in the I-V curves and the deduction of the internal parameters of the cells. Shadowing for I-V measurements has also been applied to concentrating PV modules in [7], by using an electromechanical frame to shut the light entrance to the cells. The system used for the I-V curve measurements was similar to the one implemented in the current project, given that it included an automatic shading system instead of manual installation.

A technique for the measurement of shunt resistance and photocurrent of individual cells in a PV module is presented in [4] by Eisgruber and Sites, implementing laser scanning instead of partially shading the cells, which enables measuring directly the quantum efficiency, given that the number of photons and the energy incident onto the sample is known. Differential measurements for the determination of individual resistances and photocurrents are also applied by using different wavelengths for the laser beam pointed to the cells. This method offers high accuracy and a great potential for developing an automatic system, but it also requires the use of laser systems that can be costly and difficult to use, which makes the industrial large scale implementation of such technologies very hard.

The use of a projector is introduced by Alonso-Garcia et al in [3] where a DMD (digital micro-mirror device) projector is used as a solar simulator for the characterization of PV modules and determination of imperfections. The need for performing a spatial irradiance compensation is introduced, prior to the use of the system as a simulator, given that the difference in the irradiances along the projected area is mostly dependent on the relative position of the projector and the PV module. The compensation is performed by dividing the projected area into a 20 x 20 square matrix. A system composed by 20 photo detectors that measure the irradiances for each position along the x-axis was designed, and this photodetector array was moved down the y-axis so that every point was measured. A grey-scale compensated image is finally applied so that uniformity of irradiance is achieved. This calibration method is emulated in the present project with the difference that instead of using a one axis system, a two axis system is used.

A manual coordinate system has been developed so that the physical geometry of the PV modules could be fed into the control software. This is crucial so that the partial shading patterns are correctly projected over the desired cell. The projection area has been calibrated so that an irradiance uniformity of 5% is achieved over the complete projection matrix, accomplishing the characteristics of uniformity of a class B solar simulator. In order to access individual cells, different shading patterns were projected over the individual cells acquiring IV curves under different shading conditions, which the software would later analyse and generate the series resistance values for each of the cells as well as the PV module as a whole. The acquired results were then compared to those obtained by evaluating each of the cells by direct connection.

2. METHODOLOGY

The methodology for the realization of the present project followed different stages prior to the realization of the tests and measurements given that it was necessary to design and implement a computer program that could control the shading pattern generation as well as the data acquisition software so that the data could be reliably acquired and stored for the later processing and interpretation. Data analysis software was also implemented so that useful results are produced. The projection system needed to be calibrated so that the irradiance along the projection area is uniform, increasing this way the accuracy of the acquired data. The final stage was the theoretical revision and selection of the individual series measurement so that the capabilities of the system were used to the maximum extent and the future developments for the system could be explored.

2.1 Hardware Setup

The hardware setup used in the development of the project was composed by a Keithley 2040, an Acer DLP projector and a custom made c-Si PV module. The Keithley source-meter had two main purposes, it served as voltage source-current meter for the sweeping of the IV curve for each of the shading patterns and also worked as a data acquisition device by transferring all the acquired data points to the LabVIEW master program. The communication between the Keithley 2040 and the computer running the program was realized via a serial COM port at a speed of 57600 bauds.

The selected projector was an Acer P7605 DLP projector with a 370 W lamp capable of generating a brightness of 5000 ANSI lumens. The Digital Light Processing technology allows the projector to “turn off” pixels when they are being programed to project colour black, allowing to achieve a 99.5% illumination difference between the white and the black colour. This characteristic was determining for this project, given that it allows the system to project a black pixel as it was a shaded spot in the PV module. Furthermore this projector has relatively high irradiance intensity compared to other commercial products.

The PV module used in the project was custom assembled so that the terminals of each of the cells were extended to the outside of the panel, allowing to directly measure the IV curve and the series resistance of each of the cells, as every cell had its individual connections. This was made as a control function so that the results obtained from the calculation of the resistances from the partial shading tests could be compared to those obtained directly from the cells and so assess the validity and precision of the methods proposed. The PV module was composed by 6 series connected crystalline silicon cells, each with a conversion efficiency of 18%. Table 1. Shows the main characteristics of the test PV module obtained from the STC test, as well as the results obtained after testing each cell independently. A base was also built with aluminium MBS frames to support the PV module at the right vertical position during the experiments and provide stability of the sample during measurements. It also provided flexibility so that the sample can be moved forward and backwards and can be inserted in and removed from the experimental setup easily.

2.2 Control Software Development

For the purpose of controlling all the different hardware equipment and data analysis requirements, a software was designed in LabView that could control the Keithley source-meter and acquire and analyse the generated data. The software also controlled the DLP projector for the projection of the shading patterns on the sample, by superimposing them to the irradiance uniformity correction image. The control software was also able to finally extract the main operational parameters (Isc, Voc, Impp, and Vmpp) and use them afterwards to perform the calculations of the series resistances generated under the different scenarios. The information about the number, size and spacing of the cells had to be manually introduced to the software so that it could effectively project the appropriate shading patterns on the correct positions. In a future implementation, an automated system can be realised so that machine vision algorithms could be used to automatically detect the number and position of the cells, this to avoid the manual feeding of the information so that the initial configuration process can be realised in much less time.

The shading pattern generator was developed as part of the LabView control software. Its purpose is to generate a random number of black pixels in accordance with the required shading value and the portion of the PV module that is meant to be shaded. During the different tests that were executed as part of this project, the system had to be able to project any percentage of shading on any of the cells, or onto the complete module, where 0% shading means that there is no shading and 100% shading means all pixels of the specific shaded area are in the off state which means completely dark.

2.3 Projection System Calibration

The calibration of the projection system was one of the most important stages of the setup process, given that the quality of the results of the characterization system is strongly dependent on the irradiance uniformity of the projection along the area of the PV module. For this reason calibration aimed to ensure that the system should comply with the

characteristics of a class B solar simulator regarding irradiance uniformity, where a maximum irradiance spatial non-uniformity of 5% is permitted.

The spatial irradiance measurement was realized by using a system developed at CREST in Loughborough University, which allows to perform point by point precise irradiance measurements over a selected area by moving a silicon photodetector over the predefined area. The system is composed by a photodetector attached to an x-y stage system, which allows the photodetector to move along the two axis with a minimum step size of 1mm. The system also comes with its own software in the LabView environment.

For the project’s setup, the x-y stage was fixed between the projector and the sample, as is presented in figure 1b), so that the projected image perceived by the x-y stage would be an exact scale representation of the image reaching the PV module.

The XY stage was placed in such a position in front of the projector, so that all the uniformity test images were considered under the same conditions and that each of the squares that composed the measuring matrix would remain constant along the uniformity configuration. The distance between the projector and the x-y stage was 45 cm, which allowed the 20x20 cm moving parts to scan the complete projection area. Using the uniformity scan results, greyscale images were configured which had the structure of the inverse uniformity maps. By using this method the right grayscale image is defined so that when projecting this image as a background, the irradiance is uniform across every point of the projection.

Figure 1. a) Test PV module dimensions and layout b) General system layout with XY table.

2.4 Total module series resistance calculation methods.

The calculation of the series resistance of the complete PV module was realized by using the two I-V curve method under different irradiance conditions as well as the slope at the VOC point method. The use of a DLP projector makes the implementation of the two I-V curve method extremely easy, as reducing the irradiance of the projection is done by just randomly inserting a percentage of dark pixels in random positions of the projection. As it was stated in an earlier section, the apparent series resistance of a PV cell is strongly dependent on the irradiance levels to which the module is being exposed to, especially in low irradiance conditions. This means that results under the low irradiance intensity of the projector will differ from result acquired in STC conditions.

The series resistance using the slope method was calculated for three different cases; at an irradiance of 1000 w/m2 so that the series resistance at STC could be defined; at an approximate irradiance of 27.8 w/m2, which corresponds to the maximum irradiance that the projector can deliver and finally at an approximate irradiance of 16.7 w/m2 which corresponds to the irradiance delivered by the projector under a 50% shading of the full module. This was done to investigate the influence of irradiance on the series resistance measurement results.

2.5 Individual cell series resistance calculation methods when contacting single cells

The series resistances of the individual cells were evaluated under two different ways. Initially, they were evaluated by direct electrical connections to each of the cells so that it was possible to evaluate just the desired cell and discard the effect of the others. For each of the cells two different IV curves were extracted, one generated by projecting on the cell the full irradiance, and the other one was extracted by projecting on the cell a 50% shading. Value of individual cell’s series resistance were calculated from each of the slopes of the curves at V OC, as well as a value extracted using the 2 I-V curve method defined earlier. This same process was realized for each of the cells, giving a set of values that would be used as control values, so that the extraction of the parameters from the full module connection through projected shading could be assessed and evaluated.

a) b)

2.6 Individual cell series resistance evaluation methods when contacting the whole module

Two methods are used for evaluating the series resistance of individual cells in the PV module. The first methodology followed for the extraction of the individual series resistance of the cells, is based on the work developed by Kim et al in [5]. The method consists in the extraction of two IV curves of a module while partially shading the cell under consideration, each IV realized with a different shading value for the same cell. Once the IV curves are extracted, the differential resistance dV/dI for each of them is calculated, plotted against the current. The current values obtained from each of the maximum dV /dI points are evaluated on the original IV curves for each shading ratio, and a voltage value is extracted for each of the points. Then the ΔV/ΔI at this point gives the series resistance of the module excluding the series resistance of the cell under measurement. Subtracting this value from the series resistance of the whole module will result in acquiring the individual series resistance of the cell.

The second method for the determination of the individual series resistance was the two IV curve method, where a modification was introduced to the original technique. The method originally states that two different irradiance levels, one being full illumination, are to be projected uniformly over the complete module yielding the total serial resistance. In this case instead of realizing the calculations between the full module unshaded and the full module shaded, they were realized against each of the cells under 50% and 40% shading for the individual cell while the rest of the cells were fully illuminated. This was made to guarantee that the generated current for each of the curves would be completely driven by the shaded cell.

Figure 2. Graphical explanation of the differential method for individual Rs extraction.

3. RESULTS, ANALYSIS AND DISCUSSION

3.1 Shading projection validation

Before the realisation of the experiments, it was necessary to validate the darkness level of the shaded pixels, given that in theory they should be projected as completely absent of light, and the validity of the use of the projector would be jeopardized by too illuminated dark pixels. For this matter a test was defined, where two I-V curves were generated, one using a completely white background, which represents the illuminated condition, and another one projecting a completely black background, which represented the simulation of a completely shaded condition. Both I-V curves were later analysed and the short circuit current for both cases was extracted and compared, to verify the darkness level of the projected shading. This test was ran 20 times and the resulting short circuit currents were averaged. The obtained results are presented in Table 2. The resulting shading ratio of 0.3% is considered to be acceptable so the effect of the lamp on the shaded pixels is neglected in this project.

Shading Value

Average Short Circuit Current

Shaded/unshaded Ratio

0% 253 mA 0.3 %100% 0.8 mA

Table 2. Irradiance ratio between shaded and unshaded pixels.

3.2 Variation of the Series resistance with Irradiance

As it was established by Lal et al in [21], the series resistance of a PV module has a nonlinear relationship with the irradiance levels, leading to large differences on the Rs values extracted from IV curves realized at irradiances close to the STC conditions, and the ones measured at very low irradiances. As it was stated in a previous section, the irradiance levels achieved by the projector used in the project were around 5% of the standardly used 1000 W/m2, which led to very high series resistance measurements, given that at the achievable irradiances by the projector used in this project

would only allow the semiconductors to work on the non-linear sections of the irradiance Vs. Resistance curve. Future work realized with a projector system should be implemented with a system capable of generating higher irradiances so that the calculated Rs values are extracted from the more linear parts of the curve. Figure 3 shows a logarithmic fitting realized with the full module series resistance and the series resistances obtained with the projector under different shading ratios. This is consistent with the behaviour observed in previous work on the topic [21].

Figure 3. Logarithmic fitting of the relationship between the full module Rs obtained at STC and with the projector.

3.3 Projection Uniformity Correction

As it was stated in the previous section, establishing a uniform irradiance along the projected area is essential for the adequate execution of the project. Non-uniformities in the spatial distribution of incident light can yield errors in the measurements that could affect the accuracy of the measured data For this reason a technique was designed that allowed to normalize the spatial irradiance, this technique involves an x-y stage and the subsequent generation of a compensation image.

The x-y stage was configured to realize 26 steps on each of the two axis (Y axis from top to bottom and X axis from left to right) realizing 6mm steps on the Y axis and 8mm steps on the X axis. At the end of each run the control software generated an excel file with the measured irradiance on each of the 676 squares that composed the measuring matrix.

Figure 3 shows the irradiance map generated for the raw projection. The irradiance difference between the highest and the lowest point was calculated to be 47%, which is a significantly value, mainly driven by the position of the projector. The intercept between the Y and the X axis represents the top left corner of the projection, while the irradiance peak around the bottom of the projection is explained as the region that is closer to the lamp, with an incidence angle near 0°. After acquiring the irradiance map, calibration was applied to make the projection uniform. The difference between the irradiance of each of the points and the minimum irradiance point was calculated and converted into a number between 0 and 255. This represents the full span of the grey-scale, being 0 the colour black and 255 the colour white, allowing to assign a grey-scale value to each of the 675 measuring points that is in accordance to the irradiance level measured in the point.

After each of the components of the matrix was compensated to match the irradiance measured in the lowest point, an image was generated that would be used to apply the irradiance correction. the correction image was settled as the background for the projection and irradiance measurements were repeated, allowing to observe how the irradiance on each of the points had changed, and how had the maximum/minimum irradiance ratio changed. After the first compensation images were used, the ratio fell from 47% to around 13%, being this difference still too high to comply as a class B solar simulator so the process was repeated until this condition was met. After around 50 different compensation images were generated, the ratio condition was met achieving an irradiance ratio of 4.96%. Figure 4 shows the uncompensated irradiance map (a), the compensation image (b) an the corrected irradiance map with a spatial uniformity of 4.96%.

Figure 4. a) Non-compensated irradiance map, b) Grey-scale irradiance compensation image, c) resulting spatial irradiance map.

a) b) c)

3.4 Shading Pattern Generator

For the shading pattern generation a program was designed in labView that was based on the desired shading value and the portion of the PV module that was meant to be shaded the software can randomly select the pixels that are shaded and the pixels that are unshaded so that at the end the number of shaded pixels would match exactly the desired percent of shading value The image generated over the PV module was 962 pixels wide and 641 pixels high, meaning that when the image was projected over the PV module it had a pixel resolution of around 6x6 um. Figure 5 shows two different shading values, 70% and 10%, selected to be projected over the area of one cell.

The fact that the pixels were randomly selected was important given that the cells can present imperfections in their surface or there could be irregular areas on the cover glass, so if the same pattern was being projected every time the results could be affected by this imperfections. If instead the patterns are different each time, this imperfections can be neglected given that the results would be less affected by them.

Figure 5. Generated image fot two different shading values, a) 70% and b) 10% for cell 1.

3.5 Correction of the effect of the colour wheel.

DLP projectors base their technology in the projection of a beam of white light, which is divided into its red, green and blue spectrum by a colour wheel. The colour wheel is synchronized with a microchip that is composed by a matrix of aluminium micro mirrors, that each one of them has an on and off state The colour wheel rotates at a frequency of 50 Hz while the micro-mirrors mix the colours faster than the eye can see, creating the required images through the projection lens.

These special features represents a limitation for the present project, given that the colour white is represented as an addition of all the spectrum. This variation was perceived by the PV module, generating large oscillations in the measurements, which were represented as random deformations in the IV curves. Figure 6 a) shows the measured oscillations due to the waveform variation for the white colour, measured by the photodetector of the X-Y stage over one second, having sampling rate of 8000 samples per second. Each of the peaks observed on the graph is caused by the passing of a different colour and the red square shown in figure 6 a) represents one period of the colour wheel in which all of the colours are passing.

Figure 6. a) Waveform composition of the white colour generated by the projector becaue of the effect of the colour wheel. b)

structure of the six colour wheel.

In order to overcome this limitation each IV curve measurement was acquired 50 times, and then the program calculated the average value for each point, after which it generated a filtered IV curve. Although this process incremented the reliability and repeatability of the measured data, it also increased by 50 times the execution time. For future developments with projection systems colour wheel can be removed, but taking into account that removing the

a) b)

a) b)

colour wheel would also eliminate the possibility to generate the greyscale palette used in the uniformity compensation. Other options like laser-based DLP projectors or 2 chip DLP projectors could be considered for future systems.

3.6 Individual Cell Series Resistance Characterization, individually contacted

The individual cell characterization served as a control function to verify the results obtained from the full module tests. The tests were realized by using direct connections to each cell. For each of the cells one IV curve was extracted under unshaded condition, one under 40% shading and one under 50% shading. Series resistances were calculated using the three methods stated in the previous section. The results obtained for each of the cells and with each of the methods can be seen in table 2 and were used as the base to validate and assess the results obtained from the full module connection tests.

Table 2. Series Resistance of independent cells obtained by different methods.

3.7 Determination of the series resistance under full module connection.

For the extraction of the individual cell series resistance from the I-V curves of the whole module, some modifications were introduced to both of the originally selected methods, which according to the revised literature would be directed into the extraction of the series resistance of individual cells. As highlighted in figure 9 b) the resulting series resistance obtained from the modified 2-curve method, show good similarities

For the differential method originally, it was necessary to calculate the |dV/dI| for each curve and its maximum value. In the practice, this procedure was not possible because the effects of the colour wheel, which generates oscillations in the IV curve, that when derived generate swings in the dV/dI vs current graphs, that make it impossible to determine the maximum point in an objective manner. Several attempts were made to determine this point, including low pass filters to both the input current and the dV/dI function, but even though some of the plots exhibited some points that appeared to be the maximum point, this results were not repeatable and in most cases were subjective and different points could be selected as maximum for the same occasion. Given this situation, and in order to validate the technique in a different way, each of the curves were evaluated at the current at maximum power point of the fully shaded module for two reasons; first, most of the dV/dI curves showed a maximum point, that the related current was very close, or at the same point of the current at the maximum power point of the fully shaded modules at the same shading level as the individual cell; second, because even if it did not give the exact value of the series resistance of the individual cells, since it was a fixed point that would be common to the evaluation of all the cells, it had a good probability of generating a valid trend for its calculations. In Figure 7 the experimental data obtained from the application of the method proposed by Kim et al [5] is presented. As can be observed from the graph, the point of maximum dV/dI from the curves in the right is not easily definable, given that the data recorded in the I-V curves is not completely stable due to the effect of the colour wheel. In future work a different technique, such as reconstructing the I-V curve by fitting the parameters in the one diode model to provide smoother I-V curves, can be used to produce a much cleaner dV/dI curve, which could allow to determine the maximum dV/dI points. Figure 7 also presents the selected current values used to generate the series resistance of each of the cells. This values were extracted from the fully shaded module curves, and then used to calculate an evaluation of Rs from ΔV/ΔI for these selected points. The values obtained are representations of the series resistance rather than the exact values, given that the extraction of the series resistance would only have been possible by calculating ΔV/ΔI for the points projected for the max dV/dI point of each curve.

In figure 8 b), the relationship between the Isc under different shading conditions and connection methods is established. The ISC measured directly for every cell under maximum illumination has identical trend with the ISC of the module when shading the same individual cells while the rest of them are fully illuminated. This shows that the performance of each individual cell in a module can be assessed using the proposed experimental configuration. This also indicates that the partial shading on individual cells method could be used to extract the complete I-V curves of the internal cells under direct connection, using the appropriate mathematical models. Figure 9 a) presents the inverse relationship between the Short circuit current of the cells and their internal series resistance, proving that the best performing cells in terms of current generation will also be the cells that present the lowest series resistance.

Figure 7. Experimental data obtained from the execution of the method proposed by Kim et al in [5].

The experimental results presented in figures 8 and 9, show that the values extracted from the full module connection with partial shading through the projector, have a high degree of correlation with those obtained by directly measuring the values on the cells. Even though the obtained results are not equal to the results, the trends presented by the comparison of the different scenarios show that the internal parameters of the cells are being accessed and individual cells of the module can be evaluated. This results represent an important milestone for the application of projectors in the characterization of solar modules. If the flickering generated by the colour wheel could be reduced, by using advanced digital signal processing or another technology such as laser DLP projectors, the fidelity of the results would be significantly improved.

Figure 8. a) Comparison between the results obtained for the modified differential Rs and the direct cell measurement Rs. b) comparison between the Isc obtained by direct connection unshaded and the full module connection 20% individual shading.

Figure 9. a) Correlation of the results obtained for full module connection 20% individual shading and the Rs from the slope at Voc

method, direct cell connection. b) comparison between the Rs obtained by direct connection unshaded and the full module connection differential shading for individual cells.

a) b)

a) b)

4. CONCLUSIONS

This project shows that a DLP projector system can be satisfactorily used to access the series resistance of the individual cells that form a PV module, through processes that can be executed automatically and in short periods of time, avoiding the use of mechanical shading systems that can be time consuming and add uncertainties to the system. Even though the system didn’t provide exact values for the series resistance due to the variations introduced to the irradiance intensity by the colour wheel and the low irradiance levels, it was proved that the results have a high level of correlation with the actual series resistance of the cells, proving the validity of the method. In contrast, using a projector as a source of shading patterns, has proved faster and more practical than other methods, and if used with a more powerful light source it can allow to perform single cell characterization of encapsulated PV modules. A full scale system would also allow manufactures of PV modules, to detect and correct imperfection introduced to the modules by the equipment in the production lines.

5. REFERENCES

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