Investigating smoke dispersion in rooftop PV fires

Researchers have conducted a small-scale experiment on the behavior of smoke in PV-related fires. They have found that in homes featuring rooftops with a pitch of less than 45 degrees, residents only have about four minutes to safely evacuate.

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Scientists from Japan and Canada have assessed wind-driven smoke dispersion in rooftop PV fires in a wind tunnel setup, using 1:15 small-scale building models of a prototype home in Quebec.

“The dispersion of fire smoke from rooftops with PV raises significant safety concerns regarding residents’ exposure,” the group said. “Therefore, it is essential to investigate the phenomenon of smoke dispersion generated by rooftop PV fires, particularly through rooftop openings such as skylights.”

For their small-scale study, the researchers used helium as a surrogate for the smoke from a real fire, as the chemical element enables testing in a non-fireproof wind tunnel. For this setup, they developed novel equations for smoke-helium similarity and scaling law.

Those equations allowed the academics to equate the non-dimensional smoke temperature to the non-dimensional helium mole fraction. Non-dimensional temperature refers to a relative measurement, removing the need for a precise unit such as Celsius or Kelvin.

The small-scale model measured 53 cm in length, 75 cm in width, and 40 cm in height, representing a full-scale house of 8 meters in length, 11.2 meters in width, and 6 meters in height. Simulated smoke could enter through a skylight with dimensions of 24 cm by 8 cm in the model, representing 3.6 meters by 1.2 meters in full scale.

The smoke dispersion was tested on small-scale homes with different roof angles of 0 degrees, 15 degrees, 30 degrees, 45 degrees, and 60 degrees. At each angle, three different incoming wind velocities were tested. The researchers tested the model under different heat release rates (HRR) with winds blowing at 5.16 meters per second on a 0-degree roof.

“In comparing the results for various rooftop angles, the 15-degree rooftop angle is considered hazardous, while the 45-degree and 60-degree rooftop angles offer the highest level of safety for fire prevention,” the researchers explained. “For the 60-degree rooftop, residents have approximately 12 minutes to evacuate, while for other rooftops (less than 45 degrees), residents have only about four minutes.”

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The scientists said that while residents should move lower during building fires to reduce the risk of smoke inhalation, that may not be effective in rooftop PV fires due to “the swift and uniform smoke infiltration throughout the chamber.”

The scientists also found that lowering the wind velocity increased smoke infiltration into the building. In addition, higher HRR was found to lead to stronger smoke infiltration.

“Dry green rooftops have a maximum of 200 kW/m2 HRR, while wood rooftops have a maximum of 300 kW/m2 HRR, respectively,” they said. “Rooftop PV fires have a larger HRR (max 500 kW/m2), posing a higher risk.”

They presented their results in “Wind-driven smoke dispersion in rooftop photovoltaic fires: An experimental investigation with helium smoke,” which was recently published in the Journal of Building Engineering. The research was conducted by scientists from Japan’s Niigata Institute of Technology, as well as Canada’s University of Sherbrooke and Concordia University.

“Rooftop PV fires carry a higher risk than regular roof fires due to the relatively higher HRR and smoke dispersion,” the scientists concluded. “This indicates that when designing and selecting PV systems, the HRR value of the PV should be considered in terms of fire safety concerns.”

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