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2..Manual elevation of PV arrays to suit the Cairns Region

Arrays are facing north south

 

Photovoltaics, although getting cheaper, are still expensive and everyone will try to get the best value for their dollar when generating electricity from the sun. One money saving solution may be to manually elevate the PV arrays so as to follow the sun's seasonal movement throughout the year. Brian, an Engineer by profession, who lives in Jullaten in far north Queensland does just that with his arrays. They are shown on the left. Brian changes the elevation of his arrays twice per year. For the "summer" period, he makes the change on the 21st of September and for the "winter" period he makes the change on the 21st of March.

 

 

There are a number of methods to calculate these elevations but the simplest one, I feel, is a geometric calculation as shown in these sketches. If you go back to the previous section dealing with the sun path for Cairns, you will notice that at the middle of the day the sun path for mid-summer is 7 degrees south (pale blue line) and mid-winter is 42 degrees north (dark green line). Using this information the horizontal angles for the Cairns region calculate as follows: for the "summer" period it is 5 degrees and for the "winter" period the horizontal angle calculates as 30 degrees.

 

Also, positioning the panels at a reasonable height above the roof has an additional benefit as well. Let Brian explain:

"Notice that the arrays have a gap between the panels as well as being well raised off the roof. There are two reasons for this: One, in the event of high winds (cyclone in the extreme) the spaces diffuse some of the pressure. In cyclonic conditions the array is laid horizontally.

The other purpose is, under normal conditions, the gap between the panels allows for air circulation [see left & below], thereby helping to keep the temperature of the modules down, thus enhancing the efficiency of the electricity conversion." [Less heat results in less electrical resistance. NOTE: The Australian Government website "Your Home" has this statement in its technical manual : "Crystalline modules need to be cool. Output efficiency of crystalline PV arrays decreases by 0.5 per cent per degree Celcius over the standard test temperature of 25°C"].


 

Brian is critical of some installers who use cheaper grade modules. Although passing all govenmental requirements, these modules don't measure up in efficiency when compared to the more expensive brands. Also, Brian disapproves of the way some panels are placed. Often the installers place them on a roof that just happens to mostly face north, and are often mounted flat with little clearance from the roof. This can trap the heat generated by the sun on the roof.

A tour of housing around the Cairns region reveals that little attention is being paid to placing the PV array such that its electrical output can be maximised. The attitude seems to be, if you want more electricity from the sun then get more panels! Certainly, many residents would feel uncomfortable about having a single axis tracker placed on their roof. So what is an alternative? The answer may be an array placed and fixed on the roof at the mid-angle. The next section compares the electrical output of both systems in tropical regions, such as Cairns.

NOTE: This section and the previous one were the basis of an article published in ReNew magazine in their April-June issue of 2012. Click here.

Behind the array

 

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