Fire-induced re-radiation underneath photovoltaic arrays on flat roofs

The impact of the reflection of fire-induced heat from a gas burner was studied experimentally to gain knowledge on the interaction between photovoltaic (PV) panels and a fire. The heat flux was measured in a total of eight points at the same level as the top of the gas burner. The gas burner was placed underneath the centre of a PV panel and the eight points were measured in symmetrical pairs of two at four different distances from the burner. The heat release rate from the gas burner was increased stepwise every four minutes. The measurements were made underneath a PV panel installed in a geometry similar to a commercial East-West orientated mounting system and was compared to a baseline test without the re-reflection from the PV panel. A significant increase of the received heat flux was noticed and the trend indicated an ascending percentage-wise difference as a function of an increased heat release rate. Contrary to the basic view factor theory, the received heat flux was higher underneath the most elevated part of the PV panel, and this occurred due to two important flame related reasons: 1) the deflection of the flame towards the most elevated part of the panel, resulting in an increased amount of radiation from the flame towards the surface; 2) A non-homogeneous distribution of the temperature on the PV panel surface, due to the deflected flame, and thereby a non-homogeneous emission from the heated PV panel. Finally, it was seen that two similar tests conducted with respectively a brand new PV panel and a PV panel tested for the fourth time, showed very comparable results, except during the period when the thin combustible film underneath the new PV panel was burning. This resulted in a higher heat flux during that period and implies that the results presented herein are conservative in that they are lower than what can be expected in case of a real fire hazard, where the PV panel is by definition involved in the fire for the first time. It can be concluded that PV panels can have a significant contribution in roof fires, as they stimulate fire spread over the roof on which they have been mounted. These findings emphasise that the risk related to the installation of PV panels is not only associated with the increased fire load and possibility of ignition, but largely also with the changed fire dynamic surroundings of the roof construction.

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