Natural hazard performance testing of PV modules23. June 2010 | By: Mark Tyrol
Photovoltaics (PV) are often installed on roof tops, allowing the building to generate some or all of its electrical power. The benefits that PV provide are significant. However, there can be unintended consequences that should not be overlooked.
Not just electrical testing is needed
PV modules have been tested for electrical safety and performance for decades. But building owners, insurance companies, investors, etc. need to look beyond the many benefits that PV provide and make sure that unintentional risks are not taken.
By installing PV systems on a roof, the roof may become vulnerable to natural hazards, such as windstorm and hail exposures. Additionally, combustible loading may be added to the roof construction making the roof more vulnerable to fire. PV modules are susceptible to wind, fire, and hail storm damage. Therefore, they must be resistant to these natural hazards in order to prevent property loss.
According to several web sources, the photovoltaic effect was discovered in 1839 and the first solar cell was created in 1883. However, it is only in the late 20th century that PV have become commonplace. Photovoltaics are now often installed on roof tops, allowing the building to generate some or all of its electrical power.
A New Standard for Natural Hazard Testing of Photovoltaic Modules FM Approval Standard 4476 evaluates flexible PV modules as part of a finished roof assembly for their performance in regard to fire from above the structural deck, simulated wind uplift and susceptibility from hail storm damage. Flexible PV modules submitted for FM Approval undergo an examination and tests on production samples to evaluate:
- The suitability of the product for use in a roof assembly;
- The performance of the product as part of a roof assembly as specified by the manufacturer and required by FM Approvals; and as far as practical
- The durability and reliability of the product. A separate Approval Standard is currently being prepared for rigid photovoltaic modules.
The expected date of publication is July 2010. Together, these standards are the first to provide test procedures to evaluate the natural hazard resistance of roof mounted photovoltaic modules.
Performance requirements for natural hazard resistance included in FM Approval Standard 4476 include:
- Combustibility From Above the Roof Deck
- Wind Uplift Resistance
- Hail Damage Resistance
- Compatibility Tests
- A series of electrical safety and performance tests
Combustibility from above the roof deck
Testing for combustibility from above the roof deck shall be in accordance with ASTM E 108, Fire Test of Roof Coverings. Testing is performed using a device that exposes the flexible PV panel to wind blown fire. For Class A and B, the duration of the test is 10 minutes, the flame temperature is 760 C, and the wind speed is 5.4 m/s. For Class C, the duration of the test is four minutes, the flame temperature is 705 C, and the wind speed is the same.
For Class A, the maximum flame spread of the sample materials shall not exceed 1.8 m. For Class B, the maximum flame spread of the sample materials shall not exceed 2.4m. For Class C, the maximum flame spread of the sample materials shall not exceed 3.9m. There shall be no excessive lateral flame spread, which is defined as flames extending to the two lateral edges of the exposed module, roof covering or coating beyond 305 mm from the ignition source.
There shall be no portion of the module or roof covering material blown, or falling, off of the test deck in the form of flaming or glowing brands that continue to glow after reaching the floor, and there shall be no portion of the roof deck that falls in the form of particles that continue to glow after reaching the floor. Wind Uplift Resistance Testing for wind uplift resistance shall be in accordance with ANSI/FM Approvals 4474, Evaluating the Simulated Wind Uplift Resistance of Roof Assemblies Using Static Positive and/or Negative Differential Pressures.
The minimum rating required for FM Approval is Class 1-60. The maximum rating available is Class 1-990. The Class 1 refers to a fire rating, and the -60 refers to wind uplift pressure resistance pressure expressed in psf. Ratings between 1-60 and 1-990 are available in increments of 15 psf (0.72 kPa). The rating assigned to the assembly shall be the maximum simulated uplift resistance pressure which the assembly maintained for one (1) minute without failure.
Two wind uplift tests are used:
1. 12 x 24 ft (3.7 m x 7.3 m)
Simulated wind uplift pressure test
The 12 x 24 ft (3.7 x 7.3 m) simulated wind uplift pressure test procedure is to be used to determine the simulated wind uplift resistance of assemblies that utilize mechanical fasteners, adhesives, hot asphalt, heat welding or combination thereof, to secure the flexible PV module to the roof cover or to the roof deck. The Conditions of Acceptance for 12 x 24 ft Simulated Wind Uplift Pressure Test are: a. Photovoltaic modules shall not tear, puncture, fracture, delaminate or separate from adjacent components; and b. No failure of membranes, fasteners, stress plates, insulations, adhesives, decks, etc.
2. Simulated wind uplift pull test
The simulated wind uplift pull test shall be used to evaluate fully adhered PV modules used with fully adhered roof coverings having substrates (cover board layer, insulation layer(s) vapor retarder layer) either partially, or fully, adhered to monolithic structural concrete roof decks or gypsum or lightweight concrete cast over monolithic structural concrete. The Conditions of Acceptance for 2 x 2 ft Simulated Wind Uplift Pull Test are: a. Photovoltaic modules shall not delaminate or separate from adjacent components; and b. No failure of membranes, insulations, adhesives, etc.
Hail damage resistance test
Testing for hail damage resistance shall be in accordance with Test Procedure, Test Method for Determining the Susceptibility to Hail Damage of Photovoltaic Modules, FM Approvals, LLC. Two test methods are available: severed (Class SH) or Moderate (Class MH). The minimum rating required for Approval is Class 1-MH.
For the Severe Hail Damage Resistance Test, a steel ball 45 mm, 358 g is dropped from 5.4 m onto the sample 10 times. This provides an impact energy of 19 J. For the Moderate Hail Damage Resistance Test, a steel ball 51 mm, 737 g is dropped from 1.5 m onto the sample 10 times. This provides an impact energy of 10.8 J.
The conditions of acceptance are that the voltage output of the PV module after it has been subjected to simulated hail damage must be at least 95 percent of the module voltage output prior to testing. In addition, the PV module shall show no signs of cracking or splitting. This test is also performed after wind tests.
Electrical safety and performance tests
Electrical performance requirements for flexible PV modules include: Electrical Safety per IEC 61730; and Electrical Performance per IEC 61464. Electrical performance requirements for rigid PV modules include: Electrical Safety per IEC 61730; and Electrical Performance per IEC 61215.
Conclusions Installation of Photovoltaic Modules on the roof of a building may introduce new hazards to the building, including Fire, Wind and Hail. Photovoltaic modules are susceptible to wind, fire, and hail storm damage. Photovoltaic Modules must be resistant to these natural hazards in order to prevent property loss.
FM Approvals introduces approval standard 4476 for flexible PV modules
FM Approval Standard for Rigid Photovoltaic Modules is expected to be issued by 4th quarter of 2010. These standards are the first of their kind that evaluate roof mounted photovoltaics for natural hazard resistance. FM Approval Standard 4476 evaluates flexible PV modules as part of a finished roof assembly for their performance in regard to fire from above the structural deck, simulated wind uplift and susceptibility from hail storm damage.
Authorities having jurisdiction (AHJs), architects, consulting engineers, facility managers, and product buyers rely on FM Approved products to identify products and services that will best reduce their property-loss risks.
FM Approvals offers worldwide certification and testing services of industrial and commercial loss prevention products.