Monday, 15 May 2017

The western sun is strong

There is common wisdom in Japan that the western sun is strong. Of course it is not—the sun delivers just as much radiation in the West as it did in the morning in the East. And it delivers the most when it is highest and due South. In fact, less radiation may get through in the West as the air may be more hazy and dusty in the afternoon than it was in the morning.

Panels on a west-facing roof titled to the South
Some people are retrofitting solar panels onto pyramid roofs that slope four ways, and are encouraged by the wily sellers of panels to add them to the East and West sides as well as the South side. Panels facing due south will generate the most, and you could generate 20% less if the panels point East or West. The installers will charge the same wherever the panels go, and when grants are available they often do not depend on where the panels go. Orienting panels in different directions will give peak generation at different times of day which may be more useful than maximum total generation. Given a choice, people recommend the East side for panels, rather than the West side. If the western sun were stronger, they would be recommending panels on the West side.

One of my favourite local solar installations.
I wish they all had servos and tracked the sun! 
What is happening in both cases is that the temperature in the afternoon is higher, and in the normal state of affairs, the western sun is going to feel a lot hotter as the ambient temperature is already hot. In the morning it is relatively cool.

For the panels, efficiency drops as temperature rises, so the eastern panels will produce more electricity in the cool morning than the geometrically identical ones on the west that have to wait until the hotter afternoon to produce their electricity.