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3..Tracking or a fixed setting for the array?

 

One alternative to elevating your PV array to suit the season is to have your array's horizontal angle set at the mid-angle, that is, at an angle midway between the sunray paths for December the 21st and June the 21st. The horizontal angle for the mid-angle in the Cairns region is 17 degrees.

Local PV installer, Marcus Axton of Platypus Power, is doing this. An example of his work can be seen to the left. The face of the array is facing true north and at the mid-angle. Also note that the array is well off the roof to allow breezes to flow through under the array helping to keep its temperature down.

 

To make a comparison of the effectiveness of an array set at the mid-angle and a single axis tracker we must first look at the angle of incidence of the sunrays and the resultant electrical output.

Neil Clarke is an electrical, electronic and communications engineer from Melbourne. Neil has produced a graph, this can be seen below, and it shows the angle of incidence versus power loss.
(See the NOTE at the bottom of this page).

 

Using the information above, it is possible to make a comparison between a single axis tracker and an array set at the mid-angle for a period of one year in the Cairns region.

Note that the readings shown below are for midday only.


ARRAY WITH TWO SETTINGS PER YEAR

A geometric calculation of the sun's rays hitting this array over a year indicates the following: four times per year the angle of incidence is 12 degrees. Using Neil's graph this converts to a power loss of 2%. Also, four times per year the array directly faces the sun giving a power loss of zero. So overall, at midday an array with two settings per year would average a power loss of 1%.

See below:

 

 

 

ARRAY SET AT THE MID-ANGLE
A geometric calculation of the sun's rays hitting this array over a year indicates the following: twice per year the angle of incidence is 25 degrees. Using Neil's graph this converts to a power loss of 9%. Also, twice per year the array directly faces the sun giving a power loss of zero. So overall, at midday an array set at the mid-angle would average a power loss of 4.5% over a year.

See below:

 

Therefore (roughly speaking because the readings are for midday only) in the Cairns region, a single axis tracker elevated twice per year would have a 3.5% greater efficiency than an array system that is fixed at the mid-angle.

Even if a resident does choose a mid-angle array rather than a single axis tracker, a problem still exists with the array's exposure to strong winds and its highly visible appearance from the street. (Oh dear, what will the neighbours think!) But is there another option? Go to the next section to find out.

 

NOTE

For people who prefer a table layout rather than a graph, please see the table shown on the left.

 

The angles of incidence are at intervals of five degrees and the power loss is shown as a percentage.

 

This table can be handy for situations when solar panels are to be placed and a more general calculation of the power loss is required.

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