Factors Affecting Impact Resistance of Photovoltaic Glass

As an important part of photovoltaic modules, photovoltaic glass is connected with the cell sheet and backplane through ethylene vinyl acetate copolymer (EVA) to protect the cell sheet. With the development and intensified competition of the photovoltaic industry, the requirements for modules are becoming higher and higher. While focusing on the light transmittance of photovoltaic glass, more and more module manufacturers attach importance to the impact resistance test.

2. Traceability and standard status of impact performance test

According to the failure principle of impact test, according to the simulation analysis of ANSYS software, shear failure mainly occurs at the impact point at the beginning of the impact process. With the impact, the tensile wave formed by the reflection of the compression wave expands the failure area near the impact point, causing the damage to extend to the surrounding. When the force of destruction is sufficient, the whole is broken. When the sample is large, if there is no proper support around the impact point during impact, the greater the deformation of the center point at the moment of impact, the higher the probability of shear, and the easier it is to be damaged.

The gravitational potential energy is 1/2 of the kinetic energy when the body falls freely from the height of h, so the final velocity before impact can be obtained.

Using the calculation formula of momentum in physics:

P＝mv（1）

Where: P - momentum;

M - mass;

V - final speed.

According to IEC6121-2005 Ground crystalline silicon photovoltaic modules (PV): Design qualification and finalization issued by the International Electrotechnical Commission (IEC) for photovoltaic modules, Clause 10.17 "Hail test" simulates the impact under natural conditions through hail with specified weight and speed. In Table 1 test conditions: the low test conditions have a diameter of 12.5 mm, a mass of 0.94 g, and impact the module surface at a test speed of 16 m/s, Under such test conditions, the momentum is 0.015kg calculated according to formula (1) ˙ m/s； The high test condition is a ice ball with a diameter of 75mm and a mass of 203g, which impacts the component surface at a test speed of 39.5m/s. Under this test condition, the momentum is 8.019kg calculated according to Formula (1) ˙ M/s, large range.

However, due to the conditions of photovoltaic glass manufacturers, it is obviously not convenient to use "hail" for quick and convenient measurement. Using steel balls of certain quality to replace hail for impact performance test of photovoltaic glass is a more convenient, effective and feasible alternative test method. As a destructive test, impact performance cannot be fully inspected, and sampling inspection is usually adopted. The development of test methods and standards has gone through a certain process.

Note: 1. In GB15763.2-2005 Safety Glass for Building Part 2: Tempered Glass, this standard is for float glass, and the test requirements do not specify the impact surface;

2. GB/T30984.1-2015 Glass for Solar Energy Part 1: Ultra White Embossed Glass stipulates that the impact resistance test is a "type inspection item" in the national standard, and is not required to be a factory inspection item.

There is no test requirement for impact resistance in the European standard EN12150-1:2015 Building Glass - Thermally Tempered Sodium Calcium Safety Glass - Part 1: Definitions and Description. Most foreign photovoltaic enterprises have no requirements on the impact performance of glass, and domestic customers can choose either 1040g or 227g for falling ball quality. The requirements for surface stress of tempered glass at home and abroad are different. The requirements for surface stress in foreign standards (EN12150) are ≥ 69MPa, while those in national standards (GB/T15763.2) are ≥ 90MPa.

At present, there are different opinions on the impact performance test requirements, and the main controversial points are:

(1) Drop mass: 227g or 1040g;

(2) Sample size: full size or 610mm × 610mm；

(3) Judgment rules: 6 tablets are selected as one group, or only one tablet is selected as the representative of the whole batch.

According to formula (1), the momentum of two steel balls with different masses falling from a height of 1000mm is:

P1040g＝4.604kg ˙ m/s

P227g＝1.005kg ˙ m/s

If impulse ≥ 1kg ˙ M/s, it is found in IEC6121-2005 Ground crystalline silicon photovoltaic modules (PV): Design qualification and finalization that the ice hockey ball should have a diameter of 45mm and a mass of 43.9g, and impact at an impact speed of 30.7m/s. In complex natural conditions, there are many factors related to hail impact. Whether the falling ball mass is 227g or 1040g, it is used as a simulated hail test, which cannot replace the complex natural conditions and has certain limitations.

3. Factors related to impact testing

3.1 Impact of specimens with different sizes, impact frames and glass

Both the national standard and the industrial standard have provisions on the size of the sample, but there are large differences in actual operation. Some are based on the size specified in the national standard, while others are based on the actual supply size. The following tests shall be carried out: the samples shall be tempered glass under the same tempering conditions as the original sheet line, and a 1040g steel ball shall be used to impact the middle of the sample from a height of 1000mm. See Table 2 for the sample specifications, impact frame changes and test results.

When the impact sample and impact frame are both 610mm specified in the national standard × When 610mm, the sample is not easy to break; When the impact sample and the impact frame are the same size and larger, the probability of breakage is high.

3.2 Relationship between impact surface, pattern depth and impact strength

In the national standard and industrial standard of photovoltaic glass, it is required that "the impact surface is the side facing the sun in actual use". At present, 1040g steel ball is used to impact the middle of the sample from a height of 1000mm. The selected test conditions and test results are shown in Table 3.

The results show that under the same conditions, the cloth surface of photovoltaic glass impacted by falling ball is more difficult to be broken.

Further study the relationship between pattern depth and impact resistance test, select samples with different pattern depths, impact with 1040g steel balls, and continuously increase the impact height of ball falling at an interval of 5cm. The dot diagram corresponding to pattern depth and impact height of ball falling is shown in Figure 1.

Fig. 1 Scatter Diagram of Ball Drop Height and Pattern Depth

It can be seen from the above comparison that, within a reasonable range, the deeper the pattern is, the higher the impact height that can withstand the ball falling.

3.3 Relationship between tempering quality and impact strength

The measurement standard of tempered glass is 50mm × The number of particles in the area of 50mm is ≥ 40, and the corresponding surface stress is ≥ 90MPa. Randomly take 27 pieces with a specification of 3.2mm × 1954mm × For 984mm product, first measure the surface stress, and then place the sample at 610mm × The impact is carried out on a 610mm impact frame, and the stress relationship and crushing results are shown in Figure 2.

Fig. 2 Scatter diagram of surface stress and impact crushing

It is generally believed that impact strength of the glass has a positive correlation with the stress of the tempered surface, but it has not been verified in this test.

Explore the relationship between impact strength and impact times: the national standard and the industrial standard stipulate that there is one impact time, and multiple impact tests should be carried out. Selection: 3.2mm × 1954mm × 984mm is the sample, and the size of the impact frame is 610mm × 610 mm, 195 samples were tested for smooth impact with 1040 g falling ball. Considering the feasibility of the test, the author limited the impact to 5 times, and the impact crushing rate is shown in Figure 3 and Table 4.

Figure 3 Number of ball impact and breakage

The test shows that 41.03% of the tempered glass can not withstand the first three times of ball impact and will break. The probability of glass breaking will be reduced if the glass is impacted again after 3 times of falling ball impact.