PV modules on buildings – Outlines of PV roof samples fire rating assessment

Highlights

• Fire Safety of Photovoltaic System.

• Review of PV module international test protocols.

• A novel fire behavior test protocol for PV modules.

• The assessment proposed test focuses on the fire behavior of the PV roof sample.

Abstract

The photovoltaic (PV) systems fire risk has grown up reaching a size that is no more negligible. PV fire events have happened mostly on systems installed on residential and industrial buildings and they have set in motion a process mainly aimed at developing product and installation regulations focused on the minimization of that risk. Such a process is growing up with the participation of different actors (authorities having jurisdiction, standardization bodies, PV modules manufacturers, installers, etc.). In this regard, in Italy a first result has been achieved by some new regulations issued by the Italian National Fire Rescue and Service – CNVVF – of the Ministry of Interior focusing on PV modules and roofs coupling, which contain procedures to categorize when a PV installation could be considered safe. To this end, such measures contain reminders to some national reaction to fire technical standards which, however, were developed in the past for products other than PV modules (e.g. wall panels, etc.). The research activity summarized in this paper was carried out in the last years and it was focused essentially on: analysis of Italian and European legal regulations and technical standards about the PV fire risk and identification of their shortcomings; development of new test protocols focused on PV roofs fire rating (based on some of the existing construction products harmonized standards). In particular, the test protocols are using PV roof samples with both PV modules and roof specimens.

Introduction

The rise in photovoltaic installations figures has increased over the last years in many countries (2015, global installed capacity 227,1 GWp, [1]), including Italy (e.g. end of 2015, Italy achieved a total installed capacity of 18.892 GWp, +0.298 GWp over the previous year, 688,398 systems in operation, [2]) and it has been accompanied by cases of accidental combustion whose number has been growing (2012, Italy: 479 interventions of Fire Departments [3] - Fig. 1; 2012, Germany: 390 fires of this type [4]).

Because of that, standard development processes started at International level (IEC TC 82 “Solar photovoltaic energy systems”), European level (CENELEC TC 82 “Solar photovoltaic energy systems” - WG01 “Wafers, cells and modules”) and National level (Italy: CEI TC 82 “Systems of photovoltaic conversion of solar energy” - WG11 “The fire risk in photovoltaic plants”, USA: UL) [5].

Furthermore, the result of a series of tests to investigate whether and how the presence of PV mounted array could affect the fire class of common roof material has been reported in Ref. [6]. The tests were conducted to examine combined effects of modules and roof coverings as a system when exposed to fire using the test rig configurations of UL 790 standard to evaluate different combinations of modules, stand-off heights, and roofing materials. The experimental study of the “Flammability Testing of Standard Roofing Products [6]” points out the increased temperature and heat flux due to “channeling effect” through which the PV module holds hot gases and flame closer to the roof surface; therefore the presence of either “Class C” or “Class A” modules mounted above “Class A” roof materials resulted in the roofing assemblies failing to meet the “Class A” spread of flame test requirements. In order to better understand the “channeling effect” of rack-mounted PV on the fire rating of roof assembly some experiments were conducted and reported in Ref. [7].

The tests in Ref. [7] have been focused on the determination of the minimum gap between the PV and the rooftop to maintain the roof covering original fire rating and to collect data on the effect of sheet metal interposed to block the passage of flames between the PV module and the roof assembly. Other experimental tests have been conducted to evaluate the PV module specific positions (low slope and steep slope) on roof decks during spread flame tests [8].

Meanwhile, several research programs have been formulated in order to support the standards development and the processes of manufacturing and installation. One of those is the program carried out by RSE S.p.A. as part of the research for the Italian Electrical System. Initially, in such research program, a brief analysis of the PV systems fire cases (about PV on buildings, but without BIPV systems) and a study of existing legislation and technical standards were performed.

The collaboration with standardization bodies such as CEI (Italian Electrotechnical Committee) and CENELEC (European Committee for Electrotechnical Standardization) was part of the activities during the program. In particular, about that, the CENELEC TR 50670: 2016 "External fire exposure to roofs in combination with photovoltaic (PV) arrays - Test methods" was published and it describes several tests focused on the PV module only and without any rating purposes. The activities subject of this test protocol were aimed to develop other test protocols, based on those contained in the CLC TR 50670 and with some fire rating criteria. Such protocols involve the use of PV module plus Roof samples installed within a testing room with some kind of "building roof type" geometries.

The testing room is already used in the context of construction products European legislation (Single Burning Item - SBI testing room; ref. CEN EN 13823:2010+A1:2014 “Reaction to fire tests for building products - Building products excluding floorings exposed to the thermal attack by a single burning item”). Furthermore, according to the collected test parameters, the fire behavior of the modules can be expressed using the same quantities applied for classifying reaction to the fire rating of conventional construction products. They allow the rating of module coupled with the coverage (roof) package, which, currently, do not exist in the legislation and standards published at European level, where tests are focused on fire spread feature. Fig. 2 illustrates some PV modules samples carrying out tests according to the following test protocols:

• SBI (EN 13823, original version);

• new protocol with a flat roof version;

• new protocol with a sloped roof version.

Mainly, the new tests revealed the significant influence played by some features of the sample (PV plus roof) on the fire behavior characterization, in particular: sample inclination, initial/ignition flame (power, size, duration) and roof fire safety features. Moreover, these results are substantially in agreement with the results of the experimental campaign reported in Refs. [[6], [7], [8]].