The Question

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Being involved with solar renewable energy implies a high level of sustainability and waste reduction is a large part of this approach so with the big commercial solar projects how do you ensure minimum waste in regards to aluminium framing and is there a spreadsheet solution?

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Aluminium is energy intensive

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Aluminium is usually produced from extracted bauxite, an ore made from a mixture of aluminium hydroxide, iron oxide, titanium dioxide and kaolinite. 

The uses a huge amount of electricity and, according to Alcoa, the world’s largest producer of aluminium, the best smelters use about 13 kilowatt hours (46.8 megajoules) of electrical energy to produce one kilogram of aluminium; the worldwide average is closer to 15 kWh/kg (54 MJ/kg).

 Modern iron and steelmaking practice requires around 20 GJ per tonne which is 5.55 kWh/kg.

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Solar panel framing

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The  rail used for the vast majority of solar panel systems is aluminium and is usually a construction grade such as 6005 - T5 or 6106 - T6.

These alloys are combined with both magnesium and silicon (forming magnesium silicide) and are extremely common alloys for general purpose uses in a huge variety of industries such as construction, architecture, automobile and more.

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A spreadsheet approach

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In this presentation we will look at how to automatically select the best length rail for the particular row length under consideration.

But first we will revisit how to calculate the row length required.

I will be using examples in Excel but most spreadsheet applications will suffice.

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Information needed

First off the block is the solar panels being used:

• Brand, model, wattage: ACME 400
• Length: 2000 mm
• Width: 1000 mm
• Row number: Say 1
• How many in a row*: 25
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*number of panels in a row doesn’t necessarily mean string length but more max number due to thermal expansion considerations or physical space limitations and will assume panels in portrait.

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Set up as columns

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Ideally set up in columns so as you add rows these can be dragged down, see to the right.

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Mid clamps, end clamps and more

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So now we have to look at the spacing data:

• Mid clamp spacing: 25 mm
• End clamp spacing: 30 mm
• *End buffer: 50 mm

*This amount is allowed for at both ends as a buffer and can be increased so the number of rail cuts can be reduced.

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Set up as columns 

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Set up in columns like with the panel specs:  

• Mids 25mm
• Ends 30 mm
• Rail buffer 50 mm

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So what is the final length?

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We have all the information to calculate the final length which in the case of Row 1 through to Row 10 is 23,710 mm and the calculation is:

=(Y6*AA6+(AA6-1)*AK6+2*AL6+2*AN6+AT6) where:

• Y6 is the panel width and is multiplied by
• AA6 which is the number of panels in the row
• (AA6-1) number of spaces between the panels and is multiplied by
• AK6 which is the 25mm gap between the panels and then add
• AL6 x 2 which is the end clamp spaces accounted for and then finally add
• AN6 x 2 which is the buffer of 50mm both ends

*The AT6 reference is for additional top or bottom rail length to attach DC isolators but in this case DC isolators may be installed on cable tray or not used at all due to new AS5033:2021 standard.

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Final rail length 

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So you can see all the fields required to calculate the frame length needed.

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Now what about the frame?

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All panel rail manufacturers offer different lengths and what the prudent solar system designer must do is try to reduce the amount of rail used overall, reduce the amount of wastage and keep in mind the important material:install cost ratio. 

In this presentation we will assume that waste lengths from one row are not used on the next. Time spent connecting small sections of rail, cost of extra rail joiner, potential issues of structural integrity are all eliminated.

So how do we calculate this?

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Rail frame sizes offered

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In this example the following rail lengths are options:

• 3600 mm
• 4000 mm
• 4200 mm
• 4400 mm
• 6000 mm

The reality is there may only be two rail lengths on offer.

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What the spreadsheet needs to do

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Effectively the spreadsheet will allow the designer to enter the rail length option, it will look at this option, select the best option and show how much waste is left over.

So the first thing is to list the rail options.

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Next on the list

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Then we have to analyse each row in regards to the frame selected and calculate how much frame is used.

We do this by dividing the length of the row , frame wise by the rail selected.

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And we get these results.

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And then .  .  .

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We use these results in another formula that calculates the amount of wastage, the number nearest to zero is the winner.

In this case both Rail 2 ( 4000 mm) and Rail 5 ( 600 mm) fit the bill.

The calculation is =$AY6-(ROUNDUP($G6,0.5)*$B6)

• $AY6 is total frame length
• $G6 is 6.586111111 from rail used
• Rounded up and multiplied by rail size selected

*Cell reference L6 is the 3600 mm rail

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And the best rail is .  .  .  .  .  .  .

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Now the calculation gets a bit tricky as we use a combination of Offset, Index,Match and Max functions.

• Offset returns a reference to a range constructed around (1) a starting point, (2) a row offset, (3) a column offset
• Index returns the value at a given location in a range or array
• Match searches for a specified item in a range of cells,returns the relative position of that item
• Max  returns the largest numeric value in the data provided.

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Let’s see how this works

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Now with my initial design I selected a 4200 mm rail so there must be a way that the spreadsheet tells you not only if your choice is good or ‘bad’ but also how much waste is involved with your choice.

Now cell AP6 references the result in cell Q6 that says 4000 mm rail is the best option but we have selected 4200 mm rail and the actual waste per rail, top and bottom is 1490 mm x 2. 

So maybe we should go with the 4000 mm rail.

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Now you can see that the waste per row top and bottom is 290 mm x 2.

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No waste maybe?

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With the particular example given there is a possibility that by selecting the 4000 mm rail instead of the 4200 mm rail that there would be no waste and this could be achieved by increasing the buffer amount by adding an additional 145 mm to each end so no cutting at all!

Let’s assume that we maintain the original buffer amount so how much waste have we saved by selecting the 4000 mm rail compared to the 4200 mm rail?

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Waste comparison

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Let’s add some extra rows in fact let's make it a total of 56 rows, system size of 515.2 kW:

1. Using 4200 mm rail total wastage in metres is 166.8 metres @ $7/m = $1,167 uses 2,822.4 metres
2. Using 4000 mm rail total wastage in metres is 32.5   metres @ $7/m = $ 227 uses 2,688 metres
3. Saved $940 worth of rail

To calculate the waste you simply sum up all the waste from top and bottom row and divide by 1000 to get a metre figure and then attach a $value per metre.

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Conclusion

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The example given was fairly basic as all the rows had an equal number of panels therefore if ideal rail length was applicable to Row 1 then it’s applicable to all rows.

The question has to be asked;  what minimum length do  you use to add to the next rail? Is adding rail worth it? There is more labour, more materials ( rail joiner) so at what point do we say, ‘Not worth it’.

This is something we will look at in a future presentation.

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 

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