Research shows tilt angles over 30 degrees delay solar module thermal failures

From pv magazine Global

A group of researchers from China and Italy has tested the influence of different tilt angles on the thermal failure of PV modules. They found that when using a tilt angle above 30 degrees, the time to failure increased.

Using a uniform radiant panel and an experimental setup of an inclined panel, the scientists tracked glass thermal breakage, surface temperatures, incident heat flux, and failure characteristics. They also simulated the setup in a finite element method (FEM) model, measuring average error.

“PV modules are composed of glass, a fragile material that may break or fall out quickly when subjected to fire. As a consequence of glass cracks, the combustible materials of PV panels would directly receive fire radiation and access oxygen, facilitating the failure of PV systems. What is more, once the glass in the photovoltaic module breaks and falls out, the exposed component of the photovoltaic façade may transform into a large combustible surface,” the group said. “Nevertheless, the performance of PV panels at different tilt angles has not been tested despite its widespread use and relevance.”

The group used a 600 mm x 600 mm propane-fired radiant panel as a uniform thermal source. Against it, the team placed a PV mounting setup made of a galvanized steel frame capable of withstanding temperatures of up to 1,200 C. They put 300 mm x 300 mm monocrystalline silicon PV panels inside it, including a 3.2 mm thick annealed, non-tempered glass layer. In total, 15 panels were tested, three for each inclination scenario of 0°, 15°, 30°, 45°, and 60°.

The analysis showed that, when the PV panels are exposed to thermal radiation, the glass cracks are commonly initiated at the edge of the maximum temperature difference on the fire-exposed surface. It also demonstrated that, due to the existence of converged ribbons at the bottom of PV panel, cracks are prone to initiating there rather than panel edges when the inclination was larger than 45°.

Furthermore, combustible gas bubbles were found between the glass layer and the solar cell layer, as well as between the backplate and the solar cell layer, which the scientists said could be potentially ignited with a sparker or in a real fire.

In addition, the measurements showed that the average time for PV panels’ first failure showed an increasing trend when the panels inclined from 0◦ to 60◦. More specifically, the average time for the first failure was 36 seconds, 33 seconds, 40 seconds, 62 seconds, and 71 seconds for the inclination scenarios of 0°, 15°, 30°, 45°, and 60°, respectively.

“The time to failure increased significantly when the PV panels were further tilted beyond 30 C, which is a critical inclination angle for the thermal failure of the PV panel,” the academics said. “Although failure times were different across the cases, the critical temperature differences were close, ranging from 61 to 84 C, and the majority of cracking occurred within 8–15 kW/m2.”

The group then developed a numerical FEM model of the system under the Abaqus software, limiting the simulation to 100 seconds since the first breakage of glass should occur during this period. Compared with the experimental results, the FEM model achieved an average error of 13.5% for temperature distributions and less than 15% for first failure times.

“Considering the difficulties caused by the gas bubbles of ethylene vinyl acetate (EVA) and expansion of the tedlar-polyester-tedlar (TPT), this agreement suggested that the thermal model could predict the PV panel temperature fields,” the scientists concluded. “The assumption of a three-layer rising temperature is proved suitable for the cases with large inclinations.”

The results were presented in “Performance of photovoltaic panels with different inclinations under uniform thermal loading,” published in the International Journal of Thermal Sciences. Scientists from China’s University of Science and Technology of China and Italy’s University of Trieste have conducted the study.

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