Researchers from the University of Applied Sciences and Arts of Southern Switzerland have conducted a series of tests to assess the impact of ice balls on solar modules with the aim of improving the standards for measuring their hailstone safety resistance.
In the paper “An experimental investigation of ice ball impact behavior to improve PV panel hailstone safety,” published in the International Journal of Impact Engineering, they explained that their analysis considered that natural hailstones can have diameters ranging from 5 mm to 100 mm, with final velocities reaching between 10 m/s and 50 m/s.
They also took into account various ice balls' velocities and temperatures by using a modified version of the Split Hopkinson Pressure Bar (SHPB), which is commonly utilized for testing the dynamic stress-strain response of materials. Spherical ice specimens with diameters ranging from 25 mm to 90 mm were prepared. A gas gun was used to achieve theoretical speeds of 25, 50, 75, and 100 m/s. Four different ice temperatures were tested -4 C, -10 C, -20 C, and -28 C.
“A total of three setups were used to measure the load-time history signals of ice ball impact,” the scientists stressed. “The first consists of an aluminum bar, 30 mm in diameter and 1.5 m long, while the second and the third consist of a 3 m long aluminum bar, 30 mm and 60 mm in diameter, respectively. The longer setups were used to increase the pulse recording time, thus avoiding the overlapping of reflected waves and testing bigger ice balls.”
Image: University of Applied Sciences and Arts of Southern Switzerland, International Journal of Impact Engineering, CC BY 4.0
The research group carried out over 100 tests applying various diameters, velocities, and temperatures, with impact tests at 45° being conducted to examine the impact angle influence. The analysis of the tests' results showed that the size, impact velocity, temperature and collision angle of the ice balls have a “significant” influence on impulse measurement, with increasing size and impact velocity being directly related to increasing peak force and its associated time.
“When the temperature is lower, the peak force is higher and the peak time is shorter. When collision angles were varied, the maximum load (peak) was affected by impact angle, but not the time at which it occurred,” the academics explained. “A refined analysis, incorporating both dynamic effects, such as tensile strength under dynamic conditions, and impact angle effects, reveals a consistent trend, highlighting the fact that results from tests across multiple impact angles can now be accurately represented within the linear (log–log) graphs, highlighting the reliability of the trend.
The researchers concluded that the dynamic and static characterization of the proposed tests are very difficult to achieve and stressed that each variation in the test setup can lead to different results. “For the purpose of replicating the effect on the panels or gaining a more complete understanding of the problem, it is necessary to examine the impact on less rigid surfaces,” they added.
Looking forward, the research group is planning to compare the load-time history signals generated by ice ball impacts on a rigid aluminum bar with those observed on PV modules equipped with strain sensors on their rear side.
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