PV Defects Identification through a Synergistic Set of Non-Destructive Testing (NDT) Techniques
Abstract
:1. Introduction
2. Identification of Defects, Cause and Effect, and the Need for Synergistic NDT Tools
2.1. IR Thermography for the Identification of the Nature of Degradation
2.1.1. Current Mismatch and Temperature Effect
2.1.2. Hotspots Linked to Corrosion
2.2. UVF Imaging for the Detection of EVA Degradation
2.3. Delamination of EVA/Cell Interface at the Metallisation
2.4. Corrosion in Cables and Identification by I–V Analysis
3. The Defect Diagnostics Using Cross-Correlation of the NDT Findings: Results, Analysis, and Discussion
3.1. The Proposed Diagnostics Component Based on the Deviation Analysis of the Module Electrical Parameters: Results and Analysis
3.2. The Proposed Diagnostics Component Based on the Variation Analysis of EL Images: Results and Analysis
3.2.1. Case 1: c-Si Modules Operating for 18 and 24 Years
- Breaks, cracks, and grid-line interruptions in cells were easily identified in Figure 12a–i, which show EL images of the c-Si modules no1,2,3 at various biases V. These defects contributed to I decrease, I–V distortion, Rs increase, and Rsh decrease. These may have caused small current mismatch. The quantitative effect requires I–V analysis. Cracks in the cell edges may not be identifiable in the UVF images due to photo-bleaching.
- The dark contrasts in the sequence of EL images captured from low bias V to V = Voc faded away with the increase in the bias V, and this implies that those cells or areas of cells had a lower Rsh. This is shown in Figure 12a,b,d,e and Figure 12g,h. Such dark contrasts do not appear in healthy cells.
- The EL images captured for V at around Voc and a little higher showed bright EL spots or areas along the busbar that were attributed to e−+hole recombination prevailing at these voltages. This defect increased Rs, decreased Rsh, and added a shunt diode. Such cases were numerous in the EL images, as shown in Figure 12, especially along the busbar where EVA/cell-interface delamination was the main defect finding. This did not appear in the EL images of the ODT modules in Section 3.2.2, where EVA/cell-interface delamination was not observed. However, bright spots in the EL images may also appear as a result of corrosion in the busbar, as in Figure 12b (cell position 4 from the left in the upper row), which corresponds to the cell with EVA browning and busbar corrosion in Figure 2c. In module no2, bright EL spots started appearing at lower V (Figure 12d), interpreted as damage of the p-n junction with a consequent decrease in Vm.
- EL images captured at bias V > Voc may show that dark cell contrasts reduced or disappeared, which demonstrates the presence of cell regions with δRs. If the dark contrast is sustained for any V >> Voc it implies cracks, breaks, holes, grid-line interruptions, or inactive regions in cells. Those defected cells contributed to an increase in Rs and decrease in Rsh and Isc and are numerous in Figure 12, but limited in the ODT modules (Section 3.2.2) corresponding to healthier modules. The overall Rs and Rsh of the module was determined quantitatively by the I–V analysis, whereas the increase in the Rs, δRs can be estimated as described in point 7 below.
- In Figure 12c,f,i with V > Voc some dark regions in cells were sustained. This implies regions with δRs. Figure 12g–i show a more uniform EL illumination pattern. The dark regions in Figure 12g were due to degraded Rsh and were more numerous than in Figure 12a. Hence, Rsh of no3 was lower than that of no1. At bias V > Voc in Figure 12i, the dark regions were due to δRs. In no3 there were fewer than in no1 and much fewer than in no2. That is, the Rs in no3 was lower than in no1 and no2. The above statements are in agreement with Table 2.
- The EL intensity was spatially more uniform in no3 compared to no1 and no2 at bias V > Voc in Figure 12c,f,g. In that V range, the Rs governed current I through the cells. Specifically, I vs. V was higher in no3 than no1 and no2, which implies that Rs was lower in no3 and higher in no2, which is in agreement to Table 2. The reverse is also true. In high-bias V the effectiveness in defect identification became poorer because the EL luminosity contrast reduced in almost all cells (Figure 12c,f,i).
- A combination of the deviations δVoc, δVm, and δRs with the variations of the EL images captured at bias V where the allowed current I = Isc have enriched the proposed defects diagnostics and shed light on the type of defects in the module. In general,δVoc = Vbias − Voc,nom ≥ Isc·δRsδRs = Rs,STC − Rs,nom
3.2.2. Case 2: pc-Si Modules Operating for 2 to 5 Years
3.2.3. Discussion
4. Conclusions
- Analysis of the deviations of the module electrical parameters at STC from their nominal values of δVoc, δVm, δIsc, δIm, δRs, and δRsh. This provides an insight into the impact of defects on the module electrical parameters, along with an inference on the origin of the defect to be further identified in synergy with the aforementioned NDT tools and the second component of this methodology.
- Variation analysis of EL images captured in a sequence of bias voltages, from V < Voc to V > Voc, and measuring the current allowed into the module. The analysis discloses regions in the cells with δRs, δRsh, shunt diodes, passivation issues, e−+hole recombination centres, holes, cracks, breaks, grid-line interruptions, and further defects identified in synergy with other NDT techniques. The quantification of the impact of defects is further supported by I–V analysis and the first component of this methodology.
Author Contributions
Funding
Institutional Review Board Statement
Informed Consent Statement
Data Availability Statement
Acknowledgments
Conflicts of Interest
Nomenclature
Ac | PV-cell area (m2) | Tf: Tb | PV-module temperature in the front and back side, respectively (K) |
Acor | I area in a cell with corrosion (m2) | Tpv, Tc | PV-module and -cell temperature, respectively (K). Ideally equal. |
ARC | Anti-reflective coating | Uf, Ub, Upv | The heat-loss coefficient in the front side, back side, and the whole module, respectively (W/m2K) |
EL | Electroluminescence | UVF | Ultraviolet fluorescence |
EVA | Ethylene–vinyl acetate | Vd | Voltage across the bypass diode in the module |
I, Im, Isc | The current in a PV module, the current at maximum power point, and the current at short circuit, respectively (A) | Vm, Voc | PV-module voltage at maximum power point and at open circuit, respectively (V) |
Io | Reverse saturation current (A) | Voc,c | PV-cell voltage at open circuit (V) |
Iph | Photocurrent (A) | hc, hr | Coefficients due to heat convection and IR radiation at the front side of the PV module (W/m2K), represented by hc,f + hr,f |
IT | In-plane solar irradiance (W/m2) | hc,f,hc,b | Heat-convection coefficient of the front and PV back surface to air (W/m2K) |
IR | Infrared | hr,f,hr,b | Radiative-heat coefficient of the front and back side of the PV to environment (W/m2K) |
NDT | Non-destructive testing | k | Boltzmann constant 1.3806488 × 10−23 J/K |
Pm | PV output at maximum power point (W) | m | Ideality factor of the PV-cell diode |
PL | Photoluminescence | ns | Number of cells in series in the module |
Rs | PV-module series resistance (Ohm) | q | Electron charge 1.602 × 10−19 C |
Rsh | PV-module shunt resistance (Ohm) | vw | Wind speed on the PV module (m/s) |
SRD | Solar radiation dose (MWh/m2) | δIm, δIsc, δΙο | The deviation of Im, Isc, and Io at STC from their nominal values, respectively (A) |
STC | Standard test conditions (IT = 1000 W/m2, AM1.5,Tc = 25 °C) | δVm, δVoc | The deviation of Vm and Voc at STC from their nominal values, respectively (V) |
SWIR | Short-wave infrared | δRs, δRsh | The deviation of Rs and Rsh at STC from their nominal values, respectively (Ohm) |
Ta | Ambient temperature (K) | η | PV efficiency |
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Module | Isc (A) | Voc (V) | Im (A) | Vm (V) | Pm (Wp) |
---|---|---|---|---|---|
M55 c-Si no1 and 2 | 3.35 | 21.7 | 3.05 | 17.4 | 53.0 |
SM55 c-Si no3 | 3.45 | 21.7 | 3.15 | 17.4 | 54.8 |
ODT pc-Si no1 and 2 | 8.45 | 36.9 | 7.84 | 29.4 | 230 |
ODT pc-Si no3 | 8.56 | 37.1 | 7.95 | 29.6 | 235 |
Modules c-Si | Isc (A) | Voc (V) | Im (A) | Vm (V) | Rs (Ω) | Rsh (Ω) | Pm (W) | δPm/Pm % |
---|---|---|---|---|---|---|---|---|
M55 no1 24 years | 2.550 | 20.82 | 2.496 | 16.15 | 0.85 | 106.7 | 38.71 | 28.0 |
M55 no2 24 years | 2.964 | 21.0 | 2.684 | 14.68 | 1.43 | 71.4 | 39.55 | 25.4 |
SM55 no3 18 years | 3.003 | 21.08 | 2.850 | 15.99 | 0.794 | 103.9 | 45.57 | 16.8 |
Modules pc-Si | Isc (A) | Voc (V) | Im (A) | Vm (V) | Rs (Ω) | Rsh (Ω) | Pm (W) | δPm/Pm % |
---|---|---|---|---|---|---|---|---|
ODT no1 5 years | 7.53 | 24.59 | 7.22 | 15.26 | 3.0 | 60 | 110.2 | 52 |
ODT no1 cleaned | 8.48 | 36.50 | 8.00 | 27.00 | 0.54 | 151 | 216.4 | 5.9 |
ODT no2 4 years | 8.317 | 36.67 | 8.012 | 28.04 | 0.465 | 149 | 224.6 | 2.4 |
ODT no3 2 years | 8.61 | 36.80 | 7.89 | 29.50 | 0.410 | 145 | 232.7 | 1.0 |
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Kaplanis, S.; Kaplani, E.; Borza, P.N. PV Defects Identification through a Synergistic Set of Non-Destructive Testing (NDT) Techniques. Sensors 2023, 23, 3016. https://doi.org/10.3390/s23063016
Kaplanis S, Kaplani E, Borza PN. PV Defects Identification through a Synergistic Set of Non-Destructive Testing (NDT) Techniques. Sensors. 2023; 23(6):3016. https://doi.org/10.3390/s23063016
Chicago/Turabian StyleKaplanis, Socrates, Eleni Kaplani, and Paul Nicolae Borza. 2023. "PV Defects Identification through a Synergistic Set of Non-Destructive Testing (NDT) Techniques" Sensors 23, no. 6: 3016. https://doi.org/10.3390/s23063016