Solar cells increase the risk of fire spreading on flat roofs

Published: 13.10.22

Solar panels mounted on flat roofs can spread fire in the gap between the roof and the panel. Jens Steemann Kristensen, who is a Research Consultant at DBI – the Danish Institute of Fire and Security Technology, has written a PhD thesis on the issue.

European dependence on Russian gas has made the EU see the light. As part of the REPowerEU strategy, 320 GW (gigawatts) of new solar power is to be established by 2025, more than twice the current level, and almost 600 GW by 2030.

A large proportion of the GW will be installed on the roofs of commercial and public buildings. In fact, it is an EU requirement that all public buildings with a suitable roof over 250 m2 must have solar panels by 2027. The only problem is that solar panels mounted on the flat roofs that commercial and public buildings typically have creates a risk of fire spreading that does not exist without the solar panels.

- Based on current test methods, a roof structure without solar panels is designed so that it cannot cause the spread of fire. But when solar panels are installed on the roof, a cavity is created between the existing roof structure and the back of the solar panels, which allows fire to spread in the event of ignition. This means that fire can spread via the roof and disregard fundamental requirements for fire safety in construction, explains Jens Steemann Kristensen, who is a Research Consultant at DBI and has written a PhD thesis on the issue at the University of Edinburgh.

Fire spread across fire sections

Initially, it is not the solar panels that ignite and cause the spread of the fire, because they will usually have the thermally stable fluoropolymer on the reverse side. On the other hand, the reverse side reflects the flames, increasing heat radiation to the roof structure, which can release a critical concentration of flammable pyrolysis gases, which can cause the spread of fire. This is something Jens Steemann Kristensen has documented in a series of fire tests, and there are also numerous examples of this from other countries. This spring, for example, a severe fire broke out on the roof of the We The Curious Science Centre in Bristol, England, where the fire spread under the solar panels on the roof.

- And in 2017, a forklift truck caught fire in a large warehouse in Norway. The fire broke through the roof, where it spread from one fire section to another under the solar panels, says Jens Steemann Kristensen, who describes the ignition source as atypical, because it is usually components relating to the photovoltaic system that create an arc.

- Together with a Malaysian team, I’ve calculated that 29 fires – both small and large – occur per gigawatt of capacity. 67% of the fires relate to components in the solar panels, and 33% relate to external ignition sources, explains Jens Steemann Kristensen, who mentions that the calculation is only based on a very limited amount of data, which is why the calculations are considered the best possible attempt to quantify the issue.

No test methods

In light of the EU’s strategy for more solar energy, Member States are therefore looking at a future with potentially more roof fires due to photovoltaic systems. So, what’s the solution? Jens Steemann Kristensen lists a number of possible measures: a greater distance between roof and solar panels, better fire protection of the roof structure, fire belts between the solar cell arrays, increased training requirements for installers of photovoltaic systems, etc. But regarding the situation in Denmark, he appeals first and foremost to the Danish Housing and Planning Authority to get on board.

- Current test methods do not take into account how the installation of solar cells fitted on buildings – Building Applied Photovoltaics (BAPV) – affects the fire dynamics of large roof structures, as the solar panels are not considered part of the building, but rather an add-on technology, says Jens Steemann Kristensen, continuing:

- There are probably some fire safety officers who are familiar with the problem, but they can only know so much. If there are no requirements from the authorities and the construction is in accordance with current legislation, the question is how much interest the developer has in doing something about it.

As things stand, the installation of photovoltaic systems ends up on flat roofs in no-man’s land.

- There are two worlds colliding. The construction industry and the electronics industry must work together. Who is responsible for creating the standards? Is it the construction people, who’d say that it’s nothing to do with their buildings, or is it the electronics people, who’d say that there’s nothing wrong with their products, says Jens Steemann Kristensen, emphasising why the problem requires central regulation:

- The electronics industry has created a standard according to which solar panels are tested. The test method examines the spread of fire on roof structures combined with solar panels, which is exactly what is needed. But this happens with a non-flammable roofing material, which is therefore not representative of a flat roof.

Private residences do not pose a similar problem

Rising electricity prices have caused interest in private photovoltaic systems to explode, but according to Jens Steemann Kristensen, this should not engender the same concern as large systems on flat roofs on commercial and public buildings. On the one hand, many private photovoltaic systems are installed on roofs clad with tiles or cement roof tiles, and on the other hand, there is less risk of ignition.

- The probability of ignition is related to the number of components in the photovoltaic installation. In a large photovoltaic installation, you have significantly more components, says Jens Steemann Kristensen, adding:

- Frequency is a function of capacity and not a function of system. So that means that the probability is significantly less with fewer components. Having said that, you should be aware of the requirements that apply to the installation of the individual components. For example, the inverter should not be installed in direct sunlight, in damp conditions or on a flammable material.

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Jens Steemann Kristensen
Research Consultant

+45 50 80 74 41