SUSTAINABLE ENERGY

PV System Effects on Roofing Flammability

UL worked in partnership with Solar ABCs to design and conduct tests to better understand the effects of rooftop PV systems on the flammability of roofing materials and to develop new consensus standard requirements.


WHY PV SYSTEM EFFECTS ON ROOFING FLAMMABILITY MATTERS

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The use of PV systems as a source of renewable energy is growing at an increasing rate.

The use of renewable energy sources globally, including photovoltaic (PV) systems, is growing rapidly1 largely in response to demand for reduced greenhouse gas emissions.2 An important part of the growth of PV use is in rooftop systems for homes and other buildings. In fact, non-utility PV installations have increased 3,300 percent in megawatts generated in the U.S. over the past 10 years, with more than half of the new PV capacity installed in the past two years.3  The rapid growth of rooftop PV systems has created a new set of potential hazards related to the impact of these systems on roofing and firefighter safety during fires. A better understanding of the effects of PV systems on roofing flammability is critical to developing new ways to safeguard occupants, firefighters and buildings.

CONTEXT

The use of PV systems as a source of renewable energy is growing at an increasing rate. Over the past 10 years, the number of rooftop PV installations in the U.S. has grown by a compound annual growth rate of 42 percent.4  Additionally, it is projected that one new PV system will be installed in the U.S. every 83 seconds by 2016, which is 58 times as fast as the rate of one every 80 minutes recorded in 2006.5  Globally, the projection is for solar PV output to increase by 76 percent between 2013 and 2018, when the market will top $155 billion annually.6

 

Although PV panels have rarely been identified as the cause of rooftop fires — a study in Germany found 75 instances out of almost 1.3 million installations — the presence of PV systems in buildings during a fire can create substantial hazards and hamper firefighting efforts.7  For example, just one or two light-exposed PV panels connected in a string can create a dangerous voltage, and most solar installations in residential buildings contain a dozen or more PV panels, while commercial building installations may contain hundreds.8 So as long as the panels remain connected, a PV system can feed electricity to the fire and hinder extinguishing efforts by firefighters.9

 

The rapid growth of rooftop PV systems — particularly over the past two years, when the rooftop solar generation capacity in the U.S. more than doubled10  — has increasingly complicated traditional firefighter tactics for suppression, ventilation and overhaul.11 Though many of the electrical and fire hazards associated with power generation and distribution systems are well known, rooftop PV systems present unique safety considerations that require better understanding.

WHAT DID UL DO?

UL has worked in partnership with the Solar America Board for Codes and Standards (Solar ABCs) since the summer of 2009 to design and conduct tests to characterize the effects of stand-off mounted PV modules on the fire rating of Class A rated roofing systems.* The research did not address Building Integrated Photovoltaic (BIPV) installations because, with these systems, the PV panels are integrated as part of the roof and must comply with the fire classification requirements for roof assemblies as described in UL Standard 790.12  UL conducted and directed all the tests, in close collaboration with representatives of Solar ABCs. The goal was to better understand any effects of rooftop PV systems on the flammability of roofing materials. The following details the tests and results.13

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The goal of the research was to better understand any effects of rooftop PV systems on the flammability of roofing materials.


Effect of Roof-Mounted Photovoltaic Modules on the Flammability of Roofing Assemblies

This initial research measured the surface temperature and incident heat flux of a noncombustible roof with a noncombustible PV module surrogate installed at 10, five, and two and a half inches above the roof. In addition, tests were conducted with actual PV modules to examine limited burning brand effects and the spread of flame. These tests were designed to:

  • Develop baseline data on the fire exposure during standard tests for roof with no PV module according to UL 170
  • Determine the effect of varying selected PV installation parameters
  • Document the impact of lower fire rated PV modules on common roofing assemblies14

Effect of Rack Mounted Photovoltaic Modules on the Flammability of Roofing Assemblies — Demonstration of Mitigation Concepts

In a continuation of the first study, several simple design concepts were developed to assess the effectiveness of different mitigation measures in improving the fire classification rating of the roof with a rack mounted PV module. The specific measures studied include the use of flashing (a sheet of material used to protect joints and angles where a roof comes in contact with another structure, such as a wall or chimney) at the leading edge of the roof with control of separation between the roof and flashing, and the use of a noncombustible back sheet.15

 

*Acknowledgement: This material is based upon work supported by the Department of Energy under Award Number DE-FC36-07GO17034

 

Effect of Rack Mounted Photovoltaic Modules on the Fire Classification Rating of Roofing Assemblies

This research further investigated rack mounted PV modules on roof decks to determine the:

  • Effect of PV modules mounted at angles (positive and negative) to steep and low sloped roofs
  • Impact of PV modules mounted at zero clearance to the roof surface and with the ignition source directed in the plane of the roof or the plane of the PV surface
  • Heat release rate and transfer to roof surface of Class A, B and C roofing brands and common materials such as leaf debris and excelsior (wood wool).16

 

Characterization of Photovoltaic Materials — Critical Flux for Ignition/Propagation

At this stage, the research was designed to examine the critical flux for ignition of roofing and PV products. Although the individual values varied, most were within the range of the flux values measured on the roof in the original experiments with the PV module in place.17

 

Determination of Effectiveness of Minimum Gap and Flashing for Rack Mounted Photovoltaic Modules

The next round of research was undertaken to validate the performance of two approaches believed to mitigate the effect of roof mounted PV modules on the fire ratings of roofs: a minimum separation gap and a sheet metal flashing to block the passage of flames between the PV module and the roof assembly.18

 

Considerations of Module Position on Roof Deck During Spread of Flame Tests

Our research then progressed to a series of experiments conducted to investigate opportunities to enhance the current UL 1703 spread of flame test We exposed a PV module to flames originating from the UL 790 (ASTM E108) ignition source, allowed these flames to generate on a representative roof section, and observed the propagation of the flames underneath the candidate PV module being tested. The repositioning of the PV module was conducted to investigate an application of the first item (roof) / second item (module) ignition sequence. This concept was investigated to refine the understanding of the effect of a rack mounted PV array on the fire rating of a Class A roof.19

 

Validation of 42” PV Module Setback on Low Slope Roof Experiments

The next phase of research included a series of experiments to generate data in support of proposed changes to UL 1703; specifically, a 42-inch setback of the PV module on low slope roofs. The results of this investigation were intended to validate the performance of low slope roof test parameters as contained in a draft of a revised test method for consideration by the UL 1703 Standards Technical Panel (STP).20

 

Validation of Roof Configuration 2 Experiments

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We converted the findings into consensus standard requirements, and are now testing and certifying products that offer a more sophisticated approach to protecting rooftop systems from fire.

The penultimate phase of research included a series of experiments to generate data on a complete PV assembly/roof configuration, including rack and air deflection hardware. The purpose of this investigation was to generate results that would be used to validate the performance of low slope roof test parameters as contained in a draft of a revised test method for consideration by the UL 1703 STP, and to provide quantitative data to support the proposed standard revisions, specifically a PV assembly that includes a module, rack and air deflection hardware mounted on a standardized roof configuration representing roofs with minimal slope.21

 

Report on Spread of Flame and Burning Brand Performance of Generic Installations

The final study in our PV rooftop flammability research examined the fire rating performance of generic PV and racking systems when subjected to the revised UL 1703 PV system fire test. The results of this investigation may be used to provide quantitative data to support future standard revisions proposals for PV assemblies as described in the report. It may also be used to provide the PV industry with a set of installation practices that facilitate compliance with fire performance requirements.22

IMPACT

After several rounds of research conducted under the Solar ABCs grant, we converted our findings into consensus standard requirements, and are now testing and certifying products that offer a more sophisticated approach to protecting rooftop systems from fire. The regulatory community has embraced the benefits of the new requirements and is moving quickly to implement the new systems as an enhancement. We are working closely with the industry and regulatory community to support the most effective implementation. Looking ahead, we will continue working with industry groups such as Solar ABCs to help safely advance the spread of rooftop and other PV systems to benefit residents, firefighters, businesses and the environment.23

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