Sunlight

As everybody knows the amount of sunlight varies seasonally, both in intensity and duration. Summer brings long days and at any given time of day, the sun is higher in the sky and hence more powerful than in winter. The maximum sunlight intensity on a bright summer day may be around 1400 W/m2 whereas it may be less than half that amount in winter.

In any given year, monthly sunshine hours and hence PV output will vary, but on average output dips in winter and rises in summer. A factor to consider is cloudiness. Subtropical areas, like Brisbane, often have more cloud cover in summer than in winter due the development of the monsoon.  This factor can counteract the tendency towards increased sun duration and intensity in summer.  During the first six month’s operation of the Live Data Display, that is, the second half of 2011, these seasonal factors can be seen at work.   The month by month output graph for June to December 2011 is shown in the left most thumb nail screenshot below. Choose “Pick a Year” and “2011” on the RHS of the Live Data Feed to obtain this graph. The sunlight measure shown on the graph is the average intensity for each month (W/m²) on a whole day or 24 hour basis rather than just during daylight hours. It therefore is a proxy for the amount of solar energy potentially available in each month for PV conversion to electricity.

To get the most energy production over a year, solar panels are usually oriented north and tilted at an angle to the horizontal approximately equal to the site’s latitude – in the case of Brisbane this is about 27°30" south – panels are typically tilted at 30° as this angle is easy to set out. In the subtropics, the sun in winter is not as low as say in Sydney or Melbourne. This means that in Brisbane and places further north, panel orientation and tilt is not so critical. Quite often, satisfactory output can be achieved at least cost by simply placing panels flat on low angle roofs, even if this means they do not face north. 

On a clear day, the output from a flat fixed panel will rise and fall in line with the movement of the sun across the sky. This results in a typical bell shaped power curve as shown in the middle thumb nail screenshot below. The screenshot shows the power output from the UQ Centre Array at St Lucia on 26 December 2011. This day was the best energy day for UQ Centre during 2011.  Interestingly, this day was not the best day for peak power production.  The reasons for this are discussed in the section below on temperature.  Even though the 26 December was fairly hot, over 30°C (which tends to reduce PV output), the duration and power of the sun made this a perfect day overall for PV production.

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