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Commercial solar system design is moulded by the designer’s experience, product knowledge, existing applicable standards, codes of conduct, local and federal regulations in regards to what equipment to use and how to use it. When dealing with standards whatever you do falls under either 'shall' or 'should'.
‘Shall’ means you must, whereas ‘should’ is a recommendation and, in most cases, the best course of action. Panel row shading is a ‘should’. There is nowhere in any Australian standard the stipulation that a row space must be ‘x’ amount based on panel tilt, geographical location or the panel dimensions.
What the CEC (Clean Energy Council) of Australia recommends is that there is no shading on any panel from the row in front on the shortest day of the year (Winter solstice) between the hours of 10 am - 2pm. Many installers design systems around the 9am - 3 pm window so they have a bit of a buffer.
So why is avoiding shading important? Obviously the panel’s output is determined by the amount of light hitting its face and this relationship is linear.
As a commercial solar system is offered as a financial option to negate a certain amount of electricity drawn from the electrical grid it makes no sense to design a system that has its maximum energy produced per time period, compromised.
So how do we calculate the minimum distance between the panels to satisfy the CEC’s recommendation?
Basically it involves trigonometry but the CEC have made it easy provided a table with Cos (azimuth angle)/Tan (altitude angle) ratio for many geographical locations in Australia. So for example, the ratio for Melbourne is 2.091.
Image courtesy of the CEC.
Now we take the length of the panel (will assume a portrait orientation) to be 2000 mm and the tilt to be 30 degrees. Now some maths!
Sin 30 degrees = 0.5 which you multiply by 2000 (length of the panel) and this equals 1000 mm which is the height of the back of the panel from the roof. We’re nearly there!
So we now multiply this height from the roof, 1000 mm x 2.091 = 2091 mm. This is the spacing recommended for a row of panels that are 2000 mm long at a 30 degree tilt, geographical location, Melbourne.
Image courtesy of the CEC.
What about a lower tilt, Let’s say 20 degrees?
Sin 20 degrees x 2000 x 2.091 = 1430 mm. A difference of 661 mm.
Now, in the southern hemisphere as you get closer to the equator row spacing becomes less and less, so the same configuration in Melbourne will occupy a larger overall area than in Brisbane (Brisbane being closer to the equator).
From a commercial solar design perspective we have our physical limitation being the roof itself, its size and orientation. Then we have the actual site’s load profile that must be matched to the system’s output and in addition the cost benefit analysis of going FTR (Flat to the roof) as opposed to tilt.
Material cost of tilt is higher than FTR and there is an increase in labour time to install tilt.
This has to be weighed up against the fact that FTR costs less from a material and labour perspective but may output less.
The peak cost of the site's electricity the system negates with the solar system versus the lower dollar value of the feed in tariff is another consideration in the mix.
Panel array spacing is just one of the many factors of commercial solar design. The spacing requirements are recommendations that in most cases should be followed and remember, changing one aspect of the design has the potential to change everything.