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An international research team has observed how fire spreads in photovoltaic systems installed on flat roofs and has discovered that a certain amount of energy is needed for a fire to start and spread in the cavity beneath the solar panels.
A Slovenian-Norwegian research group has investigated how fire could spread in photovoltaic systems deployed on flat roofs and found that the separation distance between the panels and the roofs, as well as the size of the ignition source, are key factors.
“Our work is based on previous knowledge from experiments conducted by Underwriters Laboratories (UL) more than ten years ago,” said lead author Reidar Stølen, who told pv magazine . “The main difference to their work is that we placed the ignition source underneath the PV modules to represent a fire caused by an electrical fault in the PV installation.” “The UL experiments looked at larger external fires that spread to PV modules on pitched roofs.”
The research group showed that for a fire to start and spread in the cavity under a photovoltaic installation, a certain amount of energy is needed. “This means that if the energy released by an electrical fault and the ignition of nearby materials is sufficient, the fire will not spread,” Stølen continued. “The amount of energy needed depends on the geometry, such as the distance between cavities, but also on the cavity materials.”
The researchers varied the distance of the cavity, but used the same slope and materials in all tests at different scales. In the experimental scenario, they used steel photovoltaic modules, with no wiring or other materials in the cavity, and a bituminous roofing membrane on a chipboard substrate at a 30-degree slope. “Other constructions will likely require a different minimum initiating energy to start a spreading fire,” Stølen said.
With the available fuel load, the duration of the fire was limited to about 20 minutes on each part of the roof. According to the research team, the roof construction with 22 mm chipboard had sufficient fire resistance to prevent the fire from spreading to the attic. “This shows that a very robust construction is not necessary to keep the fire out of the building,” says Stølen. “However, with more fuel in the construction, the fire duration and fire resistance would have to be higher.”
The scientists also observed another mechanism of fire spread through melted and burning bitumen. “Compared to previous research on flat roofs, we observed the same kind of effects by cavity distance,” added Stølen. “The main difference would be that with the pitched roof we experienced a much faster flame spread, as the plume was driven by buoyancy up the slope of the roof. In the flat roof experiments, the fire spread in all directions and was more influenced by the wind.”
Their findings are available in the study “ Experimental study of fire propagation on sloped roof with building applied photovoltaics”, published in the Journal of Physics . The research group included academics from the Norwegian company RISE Fire Research AS and the Slovenian National Institute for Building and Civil Engineering (ZAG).
The latter recently conducted a series of tests to assess the fire safety parameters of vertical rooftop PV systems and found that these installations offer improved parameters compared to conventional rooftop arrays. The former conducted a series of experiments indicating that the distance between solar modules and roof surfaces could be a crucial factor in PV system fires. |
