TECHNICAL NOTE No.5 Cyclic Testing of Solar Panels

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1 School of Engineering and Physical Sciences James Cook University Townsville QLD 4811 Australia Telephone Facsimile jcu.cts@jcu.edu.au TECHNICAL NOTE No.5 Cyclic Testing of Solar Panels 1 INTRODUCTION The Cyclone Testing Station (CTS) is an independent authority on the effects of severe wind and related damage to low-rise building systems in Australia and internationally. CTS provides a service to the building industry for testing the effects of wind forces on buildings and building components. CTS has the equipment and technical expertise to test solar photovoltaic systems in typical residential applications, which is the main focus of this Technical Note. CTS can also assist with the evaluation of other types of solar systems, including solar hot water systems and solar PV arrays to be mounted on a supporting framework at a different slope to that of the surrounding roof. The latter configurations should be discussed in detail with CTS staff prior to quotation. 2 BACKGROUND This document applies to solar photovoltaic panel systems installed on residential roofs, where the panels are typically parallel to the roofing plane. This may include both solar hot water systems and solar photovoltaic systems. Most of the content can also be applied to the evaluation of solar systems mounted on racks at angles different to the surrounding roof. There are many electrical, structural and safety issues that must be considered in designing and installing a solar system. Amongst these considerations, it is important that the system is designed, tested and installed to resist the wind pressures that may be imposed upon it during a severe wind event such as a thunderstorm or cyclone. In many cases the maximum pressures may be uplift and could result in all or part of the panel system becoming detached from the roof. This could result in the need for premature replacement of the system, which would undermine the sustainability of the technology. Worse still, any components that become dislodged could then become wind-borne debris in a severe wind event, thus resulting in a risk to life and other property. In accordance with AS , solar systems to be used in cyclonic areas (Regions C & D) should also be tested cyclically. Metallic and other components can be subject to fatigue. At present there are no pressure coefficients in AS/NZS that apply specifically to solar systems. This makes it difficult for the designer, tester, certifier or regulatory authorities to be confident that the system is fit for purpose. CTS has recently completed a wind tunnel study on behalf of Building Codes Queensland. It is envisaged that the pressure coefficients derived from this study will be put forward for inclusion in AS/NZS in the near future. In the meantime it should be possible to use this information to calculate the appropriate design pressures. This information, along with documented principles on material variability, can be used to establish pressures for testing.

2 3 AIRBOX AND AIRBAG TESTING 3.1 Air Box Testing The CTS normally conducts simulated wind load testing using the air box facility which uses positive static or cyclic pressure to test various building elements. It is an efficient means of testing and is considered to provide a more accurate representation of pressures on some cladding materials than other forms of testing. The demand on the equipment can occasionally result in lead times of 2 months or more for testing. However an overlapping similar time is often needed for the client to gain approval for testing, for discussions to take place with certifying engineers and for test panels to be manufactured and transported. The air box test rig is an open top pressure chamber. The top testing surface of the air box has an internal clearance width of 2.05 m and an adjustable length of up to 10 m. This maximum area can be adjusted to suit any smaller sized sample. Centrifugal fans introduce air pressure inside the air box and simulate the combined effect of suction pressure on the top surface and any positive or negative pressure on the lower surface of a panel system. Cyclic loading is achieved by controlling pressure dump valves. The tested system will be fixed to the supports to become the top horizontal surface. The supporting members will be bolted to the CTS air box. The test panels are normally mounted horizontally and facing upwards, so that the positive pressure on the underside models a combination of negative (suction) pressure on the top surface and any pressure on the lower surface of the panels, this being the larger magnitude wind load case. 3.2 Air Bags and Hydraulics CTS can also provide an alternative test method. This approach may have shorter lead times, while still providing reasonable representation of the cyclic loading on a PV system. The test method is based on dedicated air bags in direct contact with the underside of solar panels. The air bags apply a positive pressure to the underside of the panels, to simulate the nett negative pressure (suction) that applies to the panels on the roof. The air bags are placed between the solar panels and mechanical platens that are supported by hydraulic cylinders. Cyclic loads are then applied to the panels by cycling the hydraulic pressure applied to the cylinders, with the applied load being measured using one or more in-line load cells. While air bags are not considered as accurate as an air box for some types of products (such as flexible cladding systems), they provide a good simulation of the pressures on solar PV panels. An air box may be more appropriate to simulate the wind pressures applied to a solar hot water system because of its more complex construction. 3.2 Test Specimen The tested system normally consists of two solar panels, including side and central brackets and immediate supporting members (rails etc). This configuration allows all components, including brackets and fastenings, to be loaded in a similar manner to what they would be experience in a design wind event. The test load is factored up slightly from the design load, as is the case for any building component or system. This factor depends upon the number of repeat tests done and accounts for any variability that can occur in materials and installation. It is particularly important brackets, fasteners and supporting members are evaluated as part of the system, as these are equally likely to contribute to a failure of integrity of the system. Page 2 of 5

3 4 SIMULATED WIND LOAD TESTING OF SOLAR SYSTEMS 4.1 General The ability of solar systems to resist wind pressures in cyclonic regions should be determined by testing, as it is difficult to evaluate aspects such as fatigue failure of components or disengagement of elements by calculation alone. The test evaluates the performance of all parts of the solar system, including any fastenings and rails. The test does not specifically aim to evaluate the connection of any rails to the roof structure, although this can be included in the test. Where the system may be connected to different structural materials (such as hardwood, softwood and steel), these connections may be best evaluated separately. 4.2 Serviceability Testing Cyclonic and Non-cyclonic Regions The CTS can perform serviceability testing, prior to any strength test. AS and AS specify identical serviceability testing requirements for non-cyclonic and cyclonic regions respectively and therefore any test performed is valid for both situations. While the AS4040 series strictly only applies to cladding tests, it is the opinion of CTS that similar testing should apply to other materials and we have written to Standards Australia accordingly. Applied pressure versus deflection data is required when developing the system s serviceability limit state design capacity tables. The CTS typically place up to six deflection measurement devices to nominated points on the tested system. The locations of these points are determined during discussions with the client and/or the client s engineer. Note that once serviceability testing is completed, the same test panel may then be tested for strength. The main serviceability criteria relate to no excessive deflection, and no observed cracking, loosening or disengagement of any component of the system. 4.3 Static Strength Testing Before conducting a cyclic strength test, it is recommended that a static strength test is conducted. This can be a test to failure, which will identify the limits of the capacity of the system but the sample cannot be used for further testing. Alternatively, the system can be tested to an identified pressure, based on the maximum design pressure for any location where it will be used and an allowance for variation in manufacturing and installation. If the system passes this test without damage it can be re-used for cyclic testing or returned to service. CTS performs static strength testing in accordance with AS , "Methods of Testing Sheet Roof and Wall Cladding, Method 2: Resistance to Wind Pressures for Non-Cyclone Regions. This standard sets out a test method for determining the resistance of roof and wall cladding to wind pressure for non-cyclonic regions. CTS also considers it an appropriate method for conducting static strength testing on solar systems. Static strength test results can be used to determine strength design wind capacities for non-cyclonic regions but in this context they are useful in confirming that the system appears to be capable of resisting the pressures required in a cyclic testing. The static strength test criteria are specified as follows: The test pressure shall be determined by multiplying the strength limit state design pressure (failure pressure) by the factor to allow for variability of structural units (k t ), as defined in Table 5.1 of AS , Design and Installation of Sheet Roof and Wall Cladding, Part 1: Metal. If a number of identical tests are carried out, then each sample is required to support this test pressure. Note that Clause 6.3 of AS requires that the test pressure must be held for 1 minute. However, as the test method is for an ultimate limit state design strength criteria, the test Page 3 of 5

4 specimen can show signs of distortion and permanent deformation and still be considered a successful outcome. Where a design pressure is to be determined from the static strength failure pressure, the CTS approach is similar. For this case, the static limit state strength design pressure is calculated by dividing the failure pressure by 1.5, the value of (k t ), for a single unit tested. 4.4 Cyclic Strength Testing Cyclonic Regions For cyclic strength testing, a target test pressure is required before starting the test. This is often determined from the highest limit state design pressure for any situation where the panels will be installed multiplied by standardised factors to account for manufacturing and material variations. The alternative approach is to estimate a maximum cyclic test pressure by evaluating the system s performance during the static strength test. The CTS performs cyclic strength testing for solar systems in accordance with Specification B1.2: Design of Buildings in Cyclonic Areas of the Building Code of Australia (BCA) Volume One. This specification sets out a loading regime for determining the resistance of roof cladding to wind pressures in cyclonic regions. It is the opinion of CTS that this method is also valid for solar systems. The Low-High-Low (LHL) test regime is mandatory for metal roof cladding and its immediate supporting members. The LHL test sequence consists of cycles at various percentages of the test pressure (P t ) and one cycle to the full test pressure, which must be held for at least 10 seconds. Details of the loading sequence are presented in Table 1. Table 1: Low-High-Low Fatigue Loading Sequence Sequence No. of Cycles Load A to 0.45 P t B to 0.60 P t C 80 0 to 0.80 P t D 1 0 to 1.00 P t E 80 0 to 0.80 P t F to 0.60 P t G to 0.45 P t The test pressure (P t ) for strength limit state shall be equal to the design pressure for the wind load strength limit state multiplied by the appropriate factor for variability (k t ) as defined in AS/NZS This factor depends upon the number of repeat tests conducted. If the system passes the cyclic test, the test pressure is then divided by the material capacity reduction factor to obtain the strength limit state design wind load capacity for cyclonic parts of Australia. The system would be considered to have failed the test if there is disengagement of any element during the cyclic regime. Loosening, cracking or permanent deformation would not be considered a failure in a cyclic strength test of a panel system. Specific assessment would be required where a system was unable to resist the pressure through the full cyclic regime but had not disengaged. 5 NATA ACCREDITED TEST REPORTS After completion of testing, the CTS will issue a commercial in confidence test report describing the test methods, results and if requested, recommended design capacities for the configurations tested. CTS will also issue a test summary sheet referencing the corresponding report number. Test results are valid until the expiry date identified (typically about 4 years until June 30 or December 31 of the relevant year). CTS offers a free reappraisal service at this time. Any further testing required will be at normal commercial rates. Page 4 of 5

5 Note that the CTS test procedures are conducted in accordance with its quality management system which is NATA accredited. 6 FURTHER INFORMATION For further information on this topic please call Cam Leitch on For testing enquiries, please cts.testing@jcu.edu.au or call Bipin Sumant on REFERENCES AS/NZS1170.0:2002 Structural Design Actions General Principles AS/NZS1170.1:2002 Structural Design Actions Permanent, Imposed and Other Actions AS/NZS1170.2:2011 Structural Design Actions Wind Actions AS1562.1:1992 Design and Installation of Sheet Roof and Wall Cladding Metal AS/NZS1562.2:1999 Design and Installation of Sheet Roof and Wall Cladding Corrugated Fibre-Reinforced Cement AS1562.3:2006 Design and Installation of Sheet Roof and Wall Cladding Plastic AS2050:2002 Installation of roof tiles AS4040.0:1992 Methods of Testing Sheet Roof and Wall Cladding Introduction, List of Methods and General Requirements AS4040.2:1992 Methods of Testing Sheet Roof and Wall Cladding Resistance to Wind Pressures for Non-Cyclone Regions AS4040.3:1992 Methods of Testing Sheet Roof and Wall Cladding Resistance to Wind Pressures for Cyclone Regions Building Code of Australia Page 5 of 5