As a product working for 25 years or even 30 years, generating electricity continuously for PV module is essential to maximize the value of customers. Therefore, the module should be designed in consideration of the ability to resist extreme climate throughout the full life cycle. To verify the ability of the module to resist external mechanical stress, LONGi and TÜV NORD jointly carried out a wind tunnel test to verify the ability of the module under a dynamic load, and LONGi also carried out a static load test on the large-size module (LONGi Hi-MO 5, 2.26×1.13m) and the oversized module (2.38×1.30m) to simulate the bearing capability of the module under outdoor snow load. The test results showed that the stiffness of both monofacial and bifacial Hi-MO 5 is better than that of the oversized modules and can pass the thresher test at the wind speed of 60 m/s. The static load test showed that the deformations of both monofacial and bifacial Hi-MO 5 modules are less than 45 mm, and their anti-crack abilities are excellent.
2. Dynamic Load Test - Wind Tunnel Test
- Basic test of wind tunnel
Fig. 1. a. Time Domain graph for Vibration Acceleration of Bifacial Module;
b. Frequency Domain for Vibration Strength of Bifacial Module
The vibration acceleration and vibration strength of the large-size module and oversized module were tested in the wind tunnel laboratory respectively. It can be seen that the acceleration of the black curve (oversized module) is obviously greater than that of the red curve (large-size module) during the change in wind speed, which further indicates that the vibration amplitude of the oversized module is bigger than that of the large-size module. Therefore, the vibration amplitude of the oversized module is greater than that of the large-size module in the case of the consistent wind speed.
On the other hand, the stiffness of the module is positively correlated with its natural frequency, namely, the lower the natural frequency of the module, the lower its stiffness. The lower stiffness means that the lower energy required for wind-induced vibration of the module under outdoor wind load, which means the greater risk of its failure. As shown in the above figures, the natural frequency of the large-size module is higher than that of the same type of oversized module, so its stiffness is also higher than that of the same type of oversized module. According to the calculation of the stiffness matrix, the structural stiffness of the large-size bifacial module is 1.3 times that of the oversized bifacial module. The reliability of the module is based on its stiffness. The reduction of stiffness would cause a decrease in the reliability, leading to a higher risk for the power plant, which may be solved by increasing the design margin, but at the same time, the cost of the power plant will be increased.
- Thresher test of wind tunnel
In order to test the characteristics of the module in extreme cases, a thresher test was further carried out, i.e., the wind speed is increased until the module is damaged. The pre-compliance testing was carried out on the wind load capacity of the module, which is different from the static and dynamic load tests in the laboratory in that: the reciprocating vibration frequency of the module caused by the wind tunnel test is greater than the laboratory conditions, and the deformation inertia force of the module is found in the tests. (Note: The wind load frequency is lower than the natural frequency of the module, and the resonance does not occur in the module during the test.)
Fig. 2. At 45 m/s Wind Speed Test Oversized Bifacial Module Failed, and the Large-size Bifacial Module Passed 60 m/s Wind Speed Test
This test showed that when the wind speed is 45 m/s, the oversized module has failed, and the bolted screw hole is deformed and broken. Notes: The limit value of the test does not mean that the failure actually occurs only when the limit value is reached, and the actual parameters such as local gustiness factor and pulse wind conversion are also need to be considered; therefore, the failure may also occur below hurricane (32-37 m/s). However, the large-size module can still pass the test when the wind speed is continuously increased to 60 m/s. The above data shows that the performance of the large-size module at the extreme wind speed is at least more than 30% of that of the oversized module.
This wind tunnel test has been completed together with TÜV NORD laboratory. The test report on test results has been issued.
3. Static load test
- Load capacity of single-glazed module
The primary factor that determines the static load capacity of the PV module is glass, followed by the frame. From the perspective of module cost and weight control, the glass thickness of the monofacial module is still kept at 3.2 mm, but its deformation is also increased as its area increases significantly, thus increasing the number of cracks of cells and affecting the power output of the module. In this case, two reinforcing ribs are particularly added on the back of the oversized monofacial module, so as to logically control the deformation of the module under the static load. However, the reinforcing ribs are only helpful for the front load, and the deformation still increases when the module is subjected to a negative load. To study the above situation, the static load test was respectively carried out on the large-size monofacial module without reinforcing ribs on its back and the oversized monofacial module with two reinforcing ribs on its back at -2400Pa. The measured deformation of the large-size monofacial module is 43.5 mm, while that of the oversized monofacial module is 67 mm. After the test, the EL image further shows that the number of cracks of the oversized module is 6 times that of the large-size monofacial module. The above tests verified that the schematic design of the reinforcing ribs is not helpful for negative wind pressure, and the reinforcing ribs can only help the oversized module meet the IEC test standard for positive pressure (5400 pa).
Fig. 3 EL Comparison of large-size and oversized monofacial modules at -2400Pa
- Load capacity of bifacial module
Similar to the static load test of the monofacial module, the static load test (-2400Pa) was also carried out on the large-size and oversized bifacial modules. The test results showed that the deformation of the large-size bifacial module is 38.5 mm, while that of the oversized bifacial module is 63 mm. The figure below shows that the deformation of the oversized module is much larger than that of the large-size module. Therefore, Hi-MO 5 has obviously more excellent performance against deformation and crack.
Fig. 4. Comparison of Deformations of Large-size and oversized bifacial Modules at -2400Pa
According to the above dynamic and static load tests, Hi-MO 5 has an excellent performance in both the stiffness and resistance to extreme wind speed damage, deformation and crack. The module size of 2.26m*1.13m is proved to be mature and reliable through the test, which effectively ensures the reliability of the product and can effectively cope with "once-in-a-century" hurricane or blizzard throughout the life cycle, thereby bringing long-term benefits to customers and ensuring the safety of the power plants.