Investigation of EVA Accelerated Degradation Test for Silicon Photovoltaic Modules

1. Introduction

The amount of waste panels is gradually increasing, and the generation of waste panels in the future is expected to increase substantially due to the end of active panels and the evolution of new PV panels.

In Fig. 1, the growth starts with 5% in 2020 (i.e. annual waste of 220,000 ton compared to 5 million ton in new installations). However, it increases over time to 4~14% in 2030 and 80~89% in 2050. In 2050, 5.5~6 million tons of PV panel waste is predicted in comparison to 7 million tons in new PV panel installations.^[1]

Fig. 1.

Estimated cumulative global waste volumes (million t) of end-of-life PV panels[1]

Supplied solar modules are certified by the International Electrotechnical Commission (IEC) standards. However, even in modules that have been certified through the IEC test, many problems are constantly found and has to be discarded before the end of the warranty period.

The main failure modes found in silicon solar modules are corrosion, delamination, discoloration, cracks, and breakages. According to data reported by NREL in 2017, the most common degradation mode in modules used up to now was encapsulant discoloration (45%) followed by delamination (14%), hotspots (11%), fractured cell (9%), diode, and junction box (8%), and glass breakage (4%). It can be seen that encapsulant discoloration and delamination are major problems accounting for 60% of the total failure modes. Thus, it is not practical to consider that the increase in waste panels can be prevented by planning an experiment to analyze only encapsulant degradation, which is one of the most common problems in recent times.^[2]

The encapsulant serves as structural support, electrical isolation, physical isolation, and physical protection from external harmful environmental factors for the solar module.^[3] In addition, to allow sunlight penetration into the solar cell, the encapsulant is made of a transparent polymer. Materials used as encapsulants include EVA, POE, TPU, PVB, PDMS, and silicone, among which EVA is the dominant encapsulant in PV applications due to its low cost and high thermal stability.^[4,5] Figure 3 shows the percentage of world market share in 2020 in each of the different material categories.^[6]

Fig. 2.

Pie chart of the most significant degradation modes for all years[2]

Fig. 3.

The percent of world market share in each of the different material categories

In order to design an accelerated test to evaluate the solar modules, it is necessary to analyze the degradation of the encapsulant, which is a major part of degradation, especially EVA, which is used most. Therefore, in this paper, we intend to summarize the research trends by reviewing the types, causes, and test methods of EVA degradation and to suggest future research plans.

Fig. 4.

Delamination in solar module

2. EVA Degradation

2.2 Discoloration (Browning)

The main cause of power loss in crystalline silicon PV modules is the reduction of short circuit current. One of the mechanisms that can reduce short-circuit current in this way is the discoloration of the encapsulant.^[11] Specialized Technology Resources, Inc. (STR, formerly Springborn Laboratories) reported that the observed EVA discoloration was the most severe in modules deployed in areas with high operating temperatures and high irradiation.^[12] According to the study of G. Oreski et al., the main cause of discoloration is UV radiation and moisture exposure combined with temperatures above 50℃ which cause changes in the chemical structure of the polymer.^[13] UV light below 360 nm generates free radicals in the presence of oxygen which cause photothermal degradation of polymers.^[14] The photothermal degradation of EVA eliminate acetic acid which are the responsible chromophores for discoloration.^[15] The degradation reactions of EVA are shown in Fig. 5.^[14] In general, the discoloration of EVA proceeds from light yellow to dark brown and follows a sigmoidal pattern.^[16] If discoloration proceeds, it prevents the absorption of solar power, causing power loss, and the heat dissipation is not smooth which makes the operating temperature rise, accelerating the degradation. Since discoloration accelerates in a sigmoidal pattern until the critical point is reached, it is expected that long-term life expectancy can be predicted by analyzing the initial degradation rate and finding the threshold point.

Fig. 5.

EVA photothermal degradation mechanism

3. IEC Test

IEC 61215 specifies the requirements for design qualification and type approval of crystalline silicon modules for operation in general outdoor climates. IEC 61215 divides tests as shown in the figure and proceeds sequentially. Among them, the tests that can affect the encapsulant are damp-heat test, humidity freeze test, thermal cycling test, and UV test, and each test condition is in Table 1.

Table 1.

Conditions of IEC 61215 test

Fig. 6.

Schematic of IEC 61215:2016 test

This is insufficient time for evaluation of long-term use of solar modules. It is necessary to extend the test time to proceed with certification or to redesign the test in consideration of conditions that can accelerate degradation.

4. EVA Accelerated Test Method and Analysis

4.1 Damp Heat Test and Humidity Freeze Test

Moisture is well known to cause problems with polymers and adhesions. In particular, EVA has a high moisture absorption rate, so moisture stability is poor.^[17]

Commercial product EVA has been found delamination considerably from glass substrates after prolonged immersion in an 85℃ water bath or exposure to damp heat.^[18] F. J. Pern et al. reported the mechanisms for enhanced EVA adhesion during damp heat exposure are attributed to follows: (1) An increase in the siloxane bonding density via the smaller silane molecules of cross-linking vinyl or dipodal silanes, (2) a presence of hydrophobic fluorosilane, (3) a dominant use of vinyl benzyl silane (with vinyl and hydrophobic benzyl), and/or (4) a presence of low-concentration aminosilane serving as a catalyst.^[18] The mechanism of damp heat degradation in the paper reported by Xu-Ming Shi et al. shows that acetic acid is generated as a by-product.^[19] Acetic acid can accelerate degradation because it promotes corrosion. According to a study by C. Peike et al., as a result of the damp heat test for 2,000 hours at 80℃ and 90℃, it was confirmed that the series resistance increased and the slope of V_OC decreased due to grid corrosion by delamination occurring only at 90℃.^[20] Although the test was conducted twice as long as the IEC test, degradation did not occur at 80℃, and only at 90℃. It can be seen that the condition of IEC test, 1,000 hours at 85℃, may not be sufficient for the degradation.

Humidity Freeze Test (HF) endures a DH condition (85℃, 85% RH) for 20 hours, then cool down the temperature cycle lasting 4 hours to -40℃ and rises back to + 85℃. A sequence of 10 cycles (10 days) is required to meet IEC 61215 requirements.^[21] However, according to J.I. Hanoka et al.’s paper, there is no noticeable power drops even after 75 HF cycles.^[22] This test result also shows that the IEC test is insufficient for analysis of degradation for long-term use.

Table 2.

Comparison of test conditions and acceleration factor

In order to test delamination, it is necessary to provide moisture, maintain a high temperature for a long time, or repeat the cycle many times with various temperature conditions.

5. Conclusion

In order to solve the problem of increasing waste panels, we have focussed at the encapsulation material, especially EVA, which is one of the parts to be studied intensively in the module technology. According to previous studies, the main causes of EVA degradation are known as moisture, heat, and UV. Specifically, it was confirmed that moisture causes delamination, and UV cause discoloration. However, through several studies, it can be seen that the conditions and test time of the IEC test are insufficient to confirm and analyze the degradation. To compensate for this, it is necessary to add conditions that can accelerate the test or increase the test time. The discoloration due to UV, which is the cause of the decrease in output power, can be accelerated by temperature and irradiance. By controlling each variable and increasing the acceleration factor, long-term life expectancy can be predicted in a short time. However, to design an accelerated test for the application to an eco-module, a more accurate prediction of degradation and lifetime will be possible by designing the test and analyzing it under conditions similar to the outdoor environment of each region. Therefore, future research should be designed to enable long-term analysis in short time by combining each acceleration factor affecting the acceleration test and considering the similarity with the results in the actual outdoor environment.

Acknowledgments

This research was supported by grants from the New & Renewable Energy Technology Development Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) funded by the Korean Ministry of Trade, Industry and Energy (MOTIE) (Project No. 20203030010060 and 20194010000090).

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