Hello,
In some cases, a geographical feature like a nearby mountain range or coast can cause afternoon clouds or morning fog that would shift the optimal azimuth. For example, in the United States, the mountains just west of Boulder, Colorado cause regular afternoon clouds during the summer, which causes the optimal azimuth angle to maximize annual output of a fixed flat solar collector to be east of south. In coastal Los Angeles, California, the effect of morning fog in the hills east of the city causes the optimal azimuth angle to be west of south.
The weather file you are using is for the Cairns, Australia airport, which is at the coast, so perhaps this is a factor in your case.
Here is a link to an article on optimal tilt and azimuth angle that might be helpful:
Christensen, C.; Barker, G. (2001) Effects of Tilt and Azimuth on Annual Incident Solar Radiation for United States Locations. (
PDF 460 KB
)
Best regards,
Paul.