Steve Humphreys, Technical Commercial Manager at NAPIT, provides a guide on how to fix PV arrays to on-roof solar photovoltaics systems.
In this article, we will look at a simplified wind uplift calculation to determine how any fixings would be required for the array mounting system.
One of the most important aspects of installing a solar photovoltaic (PV) system is the mounting of the PV array on the roof. Fortunately, most modern domestic roofs can comfortably withstand the weight of a solar panel array. The weight of a typical domestic array will be lower than the weight carrying capacity of the average roof.
However, all roof structures should still be assessed by a professional. If it can be seen that the roof components are in poor condition or that the property is very old, then guidance should be sought from a roofing professional or structural engineer.
Wind load is more of a concern when mounting a PV array on a roof. It can cause uplift when it makes its way between the roof and the solar panels, causing the panels to rise or break free, see Fig 1.
Wind loads can vary significantly across the UK and is influenced by factors such as altitude, building height and local topography.
In areas where the panels are close to the roof edge, additional consideration should be given to the fixing points as the wind uplift will be greater there.
There are various software applications available that can be used to determine how many fixings are required, however, it’s important to understand the basis of wind uplift calculation.
Wind Force (uplift) = Qp x A x Cp x SF
Where:
Qp is the peak velocity pressure
A is the area of module or array
Cp is the pressure coefficient
SF is the safety factor
Peak velocity pressure (Qp)
Peak velocity pressure is the maximum wind pressure that is to be expected at a particular location over a 50-year period.
The procedure for calculating peak velocity pressure is contained within BS EN 1991 Eurocode 1: Actions on structures, Parts 1-4: General actions – Wind Actions. In order to determine the peak velocity pressure, we need to consider the following site-specific factors:
• Basic mean wind velocity (this can vary according to location and is taken from a map of the UK)
• Altitude correction factor (this accounts for the height above sea level)
• Reference height (the height of structure above ground level)
• Local terrain (the terrain type i.e. sea, town or country)
• Topography (this adds a correction factor where the site is on a hill or escarpment)
• Distance from the sea.
Area of module or array (A)
This is quite simple to work out as the array size will be the metre square (m²) of one panel times the number of panels.
Pressure coefficient (Cp)
The pressure coefficient is the external wind-induced pressure acting on the outside of the building, including the PV array. The pressure coefficients will depend on where the PV array is installed on the roof and on the particular type of PV array.
BRE DG 489 provides information on the selection and use of pressure coefficients for PV arrays mounted on roofs.
In general terms, for PV arrays that are installed in the ‘central zone’ of a Duopitch roof at an angle of 30 ̊ and has a gap of less than 200 mm from the underside of the array to the roof surface, a pressure coefficient of -0.5 can be used. In contrast, for PV arrays installed in the ‘edge zone’ of a similar roof, a pressure coefficient of -0.6 is used, as displayed in Fig 2.
Safety Factor (SF)
A safety factor should be applied to all wind load calculations. For PV systems mounted on roofs, a safety factor of 1.35 can be used.
Let us now look at a worked example assuming the following scenario:
• An on-roof PV array installed in the ‘central zone’ of the roof
• The area of the array is 20 m²
• The array mounting is a rail and fixing bracket system with each fixing bracket having a rated capacity of 500 N
• The site is located in Birmingham, not on a hill, in urban terrain and is more than 20 km from the sea
• The altitude of the site (height above sea level) is 100 m
• The height of the building (from ground level to ridge height) is 10 m.
First, we must determine the peak velocity pressure using a wind zone map for the UK, shown in Fig 3. Birmingham is located in Wind Zone 1, which has a value of 22 metres per second (m/s).
Our next step will be to determine the peak velocity pressure in the PV array by using the information provided in Table 1, along with the assumptions for the PV array.
Peak velocity pressure (Qp) = 763 pascal (Pa)
The site altitude is not applicable to this example as the site is 100 metres above sea level.
In the case of sites located over 100 metres above sea level, the formula shown in Table 2 should be used to calculate the correction factor. Topography is also not applicable in this example.
If the site is on a hill or escarpment, the correction factor derived is shown in Table 3.
Let us now add all our values to the original formula:
Wind force (uplift) = Qp x A x Cp x SF
Qp is 763 Pa
A is 20 m²
Cp is -0.5
SF is 1.35
Therefore: 763 x 20 x -0.5 x 1.35 = 10,300.5 N or 10.30 kN
Finally, we need to establish the number of fixing brackets needed for the imposed total wind uplift force.
• Total wind (uplift) force acting on array = 10,300.5 N
• Each fixing brackets rated capacity = 500 N
Total wind (uplift) force acting on array/each fixing bracket having a rated capacity.
10,300.5/500 = 20.6 so therefore at least 21 fixing brackets would be required.
Conclusion
You can see from the wind uplift calculation above, determining how many fixing brackets are needed for an on-roof mounted PV array can be complex. As mentioned earlier, software applications may be suitable when designing the PV array and mounting system.
It’s also worth pointing out that the roof structure and the distance between the rafters will often dictate the location and number of fixing brackets used. Manufacturer’s instructions will also play a large part in the spacing of fixing brackets. Manufacturer’s instructions will give a maximum spacing between fixings and a maximum cantilever for the end fixings. Whatever method is used, it’s essential that the correct amount of fixings are used to prevent panels, the whole array, or the mounting system from breaking free.
All of this information and more is available from the latest NAPIT publication: Practical Guide: Solar Photovoltaic Systems, available for pre-order at NAPIT Direct.
For more information on NAPIT Scheme registration, click here
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