BUILT ENVIRONMENT JOURNAL

Solar roof fire risks

As installation of photovoltaic modules on roofs has increased, so has the potential for fire. How can products be tested to minimise this risk?

Author: Craig Ross

23 November 2019

In the course of use, new construction technology, products and applications can develop defects that were not apparent when they were commissioned. These can include, for example, problems such as bimetallic corrosion that result from component combinations, or health and safety issues such as those discovered with asbestos. As awareness grows of such risks, further testing can be carried out to inform changes in regulations and the applications themselves to avoid issues recurring.

One such problem that has recently emerged is the fire risk caused by solar photovoltaic (PV) modules in roof applications. Although PV is by no means a new technology and modules have been installed on roofs for years, recent reductions in the capital cost of units, increases in conventional energy bills and concern about carbon emissions have led to a significant increase in use globally, especially in the commercial sector.

However, there has also been a significant increase in the number of fires involving roof-mounted PV systems prompting several investigations. These include a BRE report that was published in May 2018, and more recently a series of tests carried out at the Technical University of Denmark.

BRE investigated 25 recent and 33 historic fires in the UK involving PV and summarised the causes; fire investigations are often inconclusive, but it found that 32 of these incidents probably originated in PV systems, while a further 14 possibly had.

It also found an even split between fires in domestic buildings and those in other premises. Fortunately the number of injuries or psychological trauma in fires caused by PV systems has been low, with eight recorded in the entire BRE data set. To help ensure that information on such types of fire is more consistent, BRE has created a database to record future incidents.

The researchers in the Danish study meanwhile identified that PV installations can, thanks to the substantial DC power they generate, increase the probability of ignition when electrical malfunctions result from installation errors, natural wear and tear or using the product beyond its specified service life.

The report also cites work by the US National Fire Protection Association, which shows that PV installations can be a hindrance to firefighting in some instances. Acting effectively as a second layer of roof covering, modules can inhibit fire suppression, as well as complicating health and safety risks because systems can remain live during firefighting.

"There has been a significant increase in the number of fires involving roof-mounted PV systems, prompting several investigations"

The main issue NFPA found, however, was that PV units can actually promote the spread of fire through the cavity that is created between them and roof finishes. The researchers concluded that there is a critical minimum distance for this gap below which fires can develop through rapid flame spread, significantly increasing risk. The test results indicate a critical gap distance of at least 170-200mm, but NFPA recommends further studies are carried out for specific applications.

Ultimately, this research shows how current test procedures for PV units, which focus mainly on the combustibility of materials, are limited. A more realistic approach is required to help us understand how new applications affect the propagation of fire, which in turn will inform how we manage it.

As new products can change the fire dynamics of construction, they should be tested in situations that reflect their intended application as far as is practicable. This should take account of both the materials and fixings, as well as any unique design features, in order to understand how the components of PV units are likely to react in actual installations.

Craig Ross MRICS is RICS associate director of the built environment cross@rics.org

Related competencies include: Fire safety, Sustainability

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