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Researchers have carried out a small-scale experiment on the behavior of smoke in fires related to photovoltaics. They have found that in homes with roofs pitched less than 45 degrees, residents only have about four minutes to evacuate safely.
Scientists from Japan and Canada have evaluated wind-driven smoke dispersion in photovoltaic rooftop fires in a wind tunnel setup, using 1:15 scaled-down building models of a prototype home in Quebec.
“Smoke dispersion from PV-powered rooftop fires raises significant safety concerns regarding resident exposure,” the group states. “Therefore, it is essential to investigate the phenomenon of smoke dispersion generated by PV-powered rooftop fires, particularly through roof openings such as skylights.”
For their small-scale study, the researchers used helium as a substitute for smoke from a real fire, since this chemical element allows tests to be carried out in a non-fireproof wind tunnel. To do this, they developed new similarity equations between smoke and helium and a scaling law.
Those equations allowed the academics to equate the dimensionless temperature of the smoke to the dimensionless mole fraction of helium. Dimensionless temperature refers to a relative measurement, eliminating the need for a precise unit such as Celsius or Kelvin.
The small-scale model measured 53 cm long, 75 cm wide and 40 cm high, representing a full-scale house 8 meters long, 11.2 meters wide and 6 meters high. The simulated smoke could enter through a skylight with dimensions of 24 cm by 8 cm in the model, which represented 3.6 meters by 1.2 meters in full scale.
Smoke dispersion was tested in 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 speeds were tested. The researchers tested the model at different heat release rates (HRR) with winds blowing at 5.16 meters per second on a 0-degree roof.
“When comparing the results of the different roof angles, 15 degrees is considered dangerous, while 45 and 60 degrees offer the highest level of safety for fire prevention,” the researchers explain. “For the 60-degree roof, residents have approximately 12 minutes to evacuate, while for the other roofs (less than 45 degrees), residents only have about four minutes.”
The scientists noted that while residents should stand lower during building fires to reduce the risk of smoke inhalation, that may not be effective in rooftop PV fires due to “the rapid and uniform infiltration of smoke throughout.” the camera".
The scientists also found that reducing wind speed increased smoke infiltration into the building. Furthermore, higher HRR was found to lead to greater smoke infiltration.
“Dry green roofs have a maximum HRR of 200 kW/m2, while wooden roofs have a maximum HRR of 300 kW/m2, respectively,” they noted. “PV roof fires have a higher HRR (max. 500 kW/m2), which poses a higher risk.” |
