The Question

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With large roof-mounted commercial solar systems in many cases the frame is laid FTR ( Flat to the roof) perpendicular to the actual ‘ribs’ of the roofing material used. The distance between the ribs is constant and dictates where the actual rail feet are located.

Another approach is to run the rail parallel to the rib and positioning is only determined by the engineering report in regards to foot spacing.

With large roof-mounted commercial solar systems in many cases the frame is laid FTR ( Flat to the roof) perpendicular to the actual ‘ribs’ of the roofing material used. The distance between the ribs is constant and dictates where the actual rail feet are located.

Another approach is to run the rail parallel to the rib and positioning is only determined by the engineering report in regards to foot spacing.

In this presentation we will ask this question:

Which approach uses more materials?

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The roof profile

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I have selected Stramit Speed Deck Ultra as the roof example. It has the following characteristics:

• 233.3mm centre to centre
• 43mm high
• Allows non penetrative roof clamps to be utilised

The roof and system

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We are talking about a very large roof where the 500 kW or so of solar barely makes a dent in the available area.

Main component list includes:

• 5 x 100 kW inverters, 6 x MPPT’s, 2 inputs per tracker
• Total of 60 strings
• 23 panels per string ( Panel has low Voc)
• 400 watt panels x 1380
• Framing and cable to suit

Let’s get stuck into it.

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Birds eye view of the roof

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In this example I used a Nearmap image of the roof, got the correct dimensions and imported it into Autocad.

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Outer perimeter of panel area

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Have drawn the outer perimeter and then used a gradient fill to show where the system goes. 

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Overall view of feet and rail layout

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Now we can show the overall layout of the feet and framing with specific differences outlined if there are any. In this case the section on the left is different from the right. 

Section to the right consists of 23 panels in a row 

Section below consist of 11 and 12 panels in a row

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Running rail perpendicular to rib

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We are talking the following configuration:

• 60 strings in total
• 23 x panels per string
• 48 x rows of 23 panels
• 12 x rows  of 12 panels connected to 
• 12 x rows of 11 panels
• Using 4000mm rail
• Feet spacing max 1000mm
• Max panel cantilever is 500mm
• Panel dimensions are 2000 x 1000mm

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Running rail perpendicular to rib, the issue

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Now the engineer has said a max feet spacing of 1000mm BUT the distance between the sheet ribs is 233.3 mm so this defines where the feet go.

For example let’s look at the 11 x panel run:

• Would usually require 12 x feet per rail
• So 24 feet in total
• But using perpendicular approach
• Results in 1 + feet per rail
• So 2 x extra 
• Total of 26

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The results

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So running rail perpendicular to the rib results in the following rail and accessory figures:

• 4000mm rail x 744
• Rail joiners x 600
• Mid and end clamps x 2,856
• Feet x 3288
• Total $42,720

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Running rail parallel to the rib

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Now the engineer has said a max feet spacing of 1000mm and with the frame running parallel to the rib we don’t have to worry about feet placement in regards to rib spacing but we do have to consider rows side by side. 

Because of the configuration of the roof we have the following rows:

• 40 x rows  x 23 panels each, 40 strings
• 20 x rows  x 12  panels each, part of 20 strings
• 20 x rows  x 11  panels each, part of 20 strings

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The results

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So running rail parallel to the rib results in the following rail and accessory figures:

• 4000mm rail x 760
• Rail joiners x 600
• Mid and end clamps x 2,888
• Feet x 2920
• Total $42,272

Comparison

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In this case the parallel to the rib method is only slightly better than the perpendicular method for the same array, the difference being $448 for the rail, feet, joiners and mid clamps.

If we change the configuration slightly and reduce the number of individual 11 and 12 panel runs this could help.

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Running rail parallel to the rib, another option

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We now have:

• 44 x rows  x 23 panels each, 40 strings
• 16 x rows  x 12  panels each, part of 20 strings
• 16 x rows  x 11  panels each, part of 20 strings
• So a total of 76 rows. So how do the figures look now?

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The results of the second parallel option

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So running rail parallel to the rib results in the following rail and accessory figures3:

• 4000mm rail x 736
• Rail joiners x 584
• Mid and end clamps x 2,872
• Feet x 2,912
• Total $41,504

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All three options compared

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As mentioned we have to compare all the associated material and instal costs for the three options under consideration.

Parallel (1): cost per kW installed @$0.41/watt and total cost

Total cost of $331,842 and price per kW installed $601.16

Perpendicular  (1): cost per kW installed @$0.41/watt and total cost

Total cost of 331,398 and price per kW installed $600.36

Parallel  (2): cost per kW installed @$0.41/watt and total cost

Total cost of 331,901 and price per kW installed $599

Conclusion 

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When assessing the viability of a particular design the designer must take into account the material: install cost ratio of the various components involved as well as labour fatigue, time spent onsite, the amount of packaging rubbish, roof mount versus ground mount and more.

Running rail parallel to the rib requires less thought in regards to rail feet placement as there is no incremental “step’ as compared to a perpendicular approach.

With any project costing, all has to be taken into account. 

Good luck on your next project.

If you’d like to see what Greenwood Solutions get up to in the real world of renewable energy, solar, battery storage and grid protection check out our industry and commercial pages:

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https://www.greenwoodsolutions.com.au/industry 

https://www.greenwoodsolutions.com.au/commercial

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