PVTIME – As the size of PV modules increases, the ability of a product to withstand extreme climatic conditions throughout its life cycle has been the subject of intense scrutiny within the industry. LONGi’s Hi-MO 5 (2.6m2) bifacial module has recently passed the China General Certification Center (CGC)’s inhomogeneous snow load test, which simulates a beamless installation, where the back of the module does not touch the beam during deformation. The results indicated that, for five Hi-MO 5 modules, critical snow load (at point of failure) reached or exceeded 6400Pa, with the highest load reaching 7400Pa. Considering the safety factor, the inhomogeneous snow load of the Hi-MO 5 was 4341Pa, although, for 3.1m2 modules tested at the same time, the average critical snow load was 4600Pa, 3066Pa when taking into account the safety factor. Following wind tunnel, hail and ultimate dynamic load testing, the super-high reliability of LONGi modules in terms of size and structural design has again been verified.
Figure 1. Inhomogeneous snow load testing conducted at CGC lab
1. Significance of inhomogeneous snow load testing
Figure 2. Typical real snow load cases
Compared with real snow load characteristics, mechanical load testing is unable to apply extreme stress to the framing section at the lower part of a module at an inclined exposure. Snow loads tend to creep downhill and invade the potential space between frame edge and top surface, the ice formed by compression of the lower snow areas pushing against the exposed tip of the frame, making the load on the modules inhomogeneous. As length and width of a module increases, the local pressure and bending moment caused by an inhomogeneous snow load increase significantly, making it important to evaluate the performance of larger-sized modules under such conditions.
2. Methodology of inhomogeneous snow load testing
According to the IEC 62938:2020 inhomogeneous snow load testing standard, a step-distributed non-uniform load is applied to 2/3 of the length of a module at a specified inclination angle (usually 37°), with the load gradually increased to measure the critical point at which the module is damaged, the sample result of five modules subsequently used to calculate the average. Finally, the inhomogeneous snow load is obtained by applying a safety factor of 1.5.
Figure 3. Inhomogeneous snow load testing apparatus at CGC and distribution of load on the test specimen at an inclination according to IEC 62938:2020
To illustrate this experiment more clearly, the detailed information is as follows:
- Initially applying Sk = 2400Pa load (the minimum design qualification for PV modules according to the IEC 61215-2 static mechanical loading test (MQT 16), without inclination angle) to 2/3 of the length of a module at a 37° angle of inclination. Since there is a degree of inclination, the actual module load should consider this factor. A shape coefficient of µi = 0.61 (at 37°) is therefore applied.
- SA = Sk* µi = 1464Pa can then be calculated, distributed over a length of 2/3 of the module (l). This is also called the ‘snow load on the roof.’
- Finally, the lower edge of the module represents the eaves of a roof and this also needs to be considered, with linear load SE distributed across the bottom area of the inclined module over half its vertical length (l), as seen in figure 3. SE is then calculated and increased according to the following formula:
Where:
SE is the snow load of the overhang depending on eaves, in kN/m;
SA is the snow load on the roof, in kN/m2;
γ is the specific snow weight, in kN/m3.
3. Hi-MO 5 inhomogeneous snow load test results
Larger-sized bifacial modules are currently applied mainly in portrait installation. In order to avoid the influence of the beam on backside power generation, the modules are usually raised by clamps to increase the distance between the back glass and the beam, with the installation method for this test similar to the above scenario. The backside of the module does not touch the beam during the test, which is more critical than for a conventional test.
Figure 4. Deformation of lower edge relative to center point for different size modules conducted at CGC
In addition to obtaining the critical load of a module, this test also records deformation of the lower edge relative to the module’s center point under different loads. Hi-MO 5 not only has a significantly higher snow load tolerance than a 3.1m2 module (2384×1303mm), but also presents significantly lower deformation under a specific pressure. The critical load of a 3.1m2 module is low, with deformation exceeding 50mm under a load of 3600Pa. In practical application, long-term excessive deformation may potentially lead to problems such as cracks in the cells, deformation of the frame and glass breakage.
Table 1. Deformation recorded for different size modules conducted at CGC
In summary, module size is strongly linked to mechanical properties. Whether extreme dynamic load, hail, wind tunnel or inhomogeneous snow load testing, all tests indicated that size should have reasonable limits. With its optimal sizing insight and reliable design, Hi-MO 5 performs very well in extreme climatic conditions and LONGi will adhere to its product design concept based on LCOE, paying attention to the application performance of a product throughout its life cycle, and continuing to develop technologies and products with customer value.
