2.1. Methods and Procedures
The overall experiment was conducted in the following order: module power output and defect verification, calculation of grade of originally applied cells, grade verification of replacement cells, power prediction, module power recovery, comparison of predicted power output and experimental results, and application of correction values. First, the defects and power output of the module to be recovered were checked via electroluminescence (EL) measurement and a sun-simulator. EL measurements are used to identify internal defects that cannot be visually identified using EL in solar cells.
Table 1 provides the nomenclature for the electrical characteristics of the module.
The current corresponding to the cell
Isc and the voltage at the same level as the module
Voc were applied for the measurement. EL images of the module were captured in several parts of a darkroom, recollected, and displayed on a screen. The EL equipment manufactured by MC Science in Korea was used for the measurements. The simulator measures the module’s
Isc,
Voc,
Pmax, etc., under the standard test condition (STC) at 25 °C, 1 Sun (1000 W/m
2), and air mass 1.5, and corrects the actual temperature to output the calculated value to the screen. The equipment used in this study was a Spire-Nissinbo Sun Simulator. The equipment was calibrated for proper use in the certification test of the photovoltaic module by receiving the AAA in three evaluation items: uniformity, stability, and spectrum. Measurements of power output from equipment are displayed in various ways, i.e., 1–4 digits after the decimal point; however, in this study, the third digit after the decimal point was rounded to two digits to maintain consistency. The CTM (cell to module) factor calculation method was applied to the power analysis of the cells used at the time of manufacturing the target samples and the review of the cells to be replaced [
40]. The grade of the applied cell was inversely calculated based on the initial power output of the module disclosed on the Internet by the manufacturer. The module power after cell replacement was predicted after checking the grade of the cell to be replaced.
The CTM coefficient k-factor calculation method was used to analyze the power of the original cell of the target samples and review the replacement cell. Manufacturing modules from cells, models, and formulas for classifying the CTM coefficient k-factor, which affects efficiency or power, and analyzing loss or acquisition mechanisms have been presented in previous research [
41,
42]. If the dimension data and rated power of a module released by the module manufacturer are the initial power outputs of the module, the module efficiency is calculated to be 13.6%. Because the module power output is calculated from the sum of the CTM coefficient k factor and the initial solar cell power in the module power output calculation model, the power output of the module can be calculated using Equations (1) and (2) [
41,
43]. The factors
i and
m in Equations (1) and (2) are variables of the routinely used pie function, and refer to the extension of the CTM factor. The CTM k-factor consisted of 15 types:
k1 (module margin),
k2 (cell spacing),
k3 (cover reflection),
k4 (cover absorption),
k5 (cover/encapsulant reflection),
k6 (encapsulant absorption),
k7 (interconnection shading),
k8 (cell/encapsulant coupling),
k9 (finger coupling),
k10 (interconnector coupling),
k11 (cover coupling),
k12 (cell interconnection),
k13 (string interconnection),
k14 (electrical mismatch), and
k15 (junction box and cabling). The meaning of
I = 3−
m in the ∏-function of Equation (1) means CTM k-factor from k
3 to k
15. Then, the sum of the cell power outputs from
j = 1−
n from the ∑-function is the number
n of cells applied to the module.
In terms of module efficiency, factors affecting the entire area of a gap module between modules are important; however, when a module is produced from a cell, a design margin (
k1) to ensure an electrical insulation distance and a loss factor (
k2) owing to the cell interval are not related to a power change. The module efficiency can be expressed by Equations (3) and (4) [
41].
Therefore, according to this model, the module efficiency is proportional to the average efficiency of the cell rather than being dominantly affected by the lowest efficiency. The average efficiency of the cell was calculated by considering the electrical mismatch loss (
k14) of the cell to predict the power output of the module to be restored. For the loss caused by the electrical mismatch of cells, studies were published prior to research on the CTM factor, and the widely known definition of RPL is expressed as the difference between the maximum power (
Pmpc) of
n individual cells connected in series to form a cell string or module. RPL can be expressed as Equation (5) from the difference between the sum of the maximum power of all cells and the maximum power of the module.
In theory, when individual cells operate completely independently, the maximum power output is denoted as
P’max, and when the average cell power output value in a group is
Pmax, the calculation of RPL
B (relative power loss of a module using Bucciarelli’s equation) is as shown in Equation (6).
The power output after cell replacement and the state inside the module were also confirmed using the EL and Sun simulators. The cell replacement process is discussed in the next section. After cell replacement, the gain factor (power increment of the replacement cell), loss factor (long-term degradation, electrical mismatch), and unidentified tolerance parts of the module track the experimental results and apply the same to the two samples, correct the power predictions, and finally compare them with the results.
2.2. Experiments
Figure 1 presents an EL image of a 6-inch 54-cell 3BB polycrystalline silicon solar module, where the hose power degrades owing to cell damage.
Figure 1a shows the first sample of 190-Wp grade, referred to as 190 A for convenience, and its appearance.
Figure 1b–d depict EL images of 190 A, the second sample of the 190-Wp class (190 B), and 190 B, respectively. As shown in images (a) and (c), a weak yellowish appearance, which was not severe, was observed. In addition, approximately six to nine dark areas were observed in the EL image (
Figure 1b) and approximately six dark areas were observed at 190 B (
Figure 1d). In the green-marked cell of (d), the dark area in the cell occurred because of poor soldering between the mutual connector and the busbar.
The modules used in this study included samples collected from commercially operated power plants; however, the current–voltage (I–V) data at the time of manufacture were unknown. Therefore, the electrical characteristics of the model disclosed by the manufacturer were assumed as the initial electrical performance.
Table 2 lists the initial electrical specifications of samples 190 A and 190 B and the electrical data of the failed samples after a certain period of operation. As confirmed in the EL image, the FF was severely degraded by the damaged cells in the middle of the string. For 190 A and 190 B, the power decreased by −21.69% and −26.47%, respectively.
Figure 2 displays the I–V and voltage–power (V–P) curves of modules 190 A and 190 B. The I–V curves appear step-shaped, while the V–P curves have two or more multi-peaks, which is a typical form caused by the decrease in Isc due to the cracking of a specific cell in a cell string [
44].
Figure 3 shows the process of removing the broken cells of 190 A and 190 B cells and replacing them with new cells. (a) First, the module is placed on a hot plate to heat the sun-side and soften the EVA, then, the back sheet is removed from the edge. (b) When the back sheet is completely peeled off, (c) the tape attached to fix the cell-string gap was removed. If it is a material such as EVA, it does not require removal; however, for a tape using polyethylene terephthalate (PET) as a basic material, a gap is formed between the tape and cell owing to the loss of adhesion.
When cleaning the back sheet removal surface or the cell removal area using ethanol or isopropyl alcohol (IPA), the permeated organic solvent may cause solvothermal swelling in the lamination process, or gas may accumulate to cause swelling [
32].