I’ve been watching the solar collectors on my home and also monitoring a commercial system on a dairy. Today, since were up at a high elevation, we’re above the clouds. This is rare, but it’s nice and sunny, warm (55F). Down on the plains its cold and overcast.
Most people are aware that solar systems need sunlight to work. What many people do not think of is the finer points of sunlight. In most of the world, a good rule of thumb is to setup the solar system to get the best sun between 9AM and 3PM. This is when the sunlight is most directly overhead and provides the closest straight line between the sun and the solar collector. Before 9AM and after 3PM the sun is increasingly slicing through much more atmosphere of the Earth before it reaches the solar collectors. As the sun goes through the atmosphere it can strike water vapor, dust or other particulates, smog……Each of these strips away a little bit of the suns energy, meaning less for the collector. Also, if the collectors are fixed in position (non-tracking), which most of them are. Then more of the sun before 9AM and after 3pm skips off of the glass covering the solar collector as well.
The picture to the right is output from my solar electric system on a good sunny day (8/1/11). The part that is highlighted yellow is the energy generated between 9AM and 3PM. Outside of those times there is some, but not a lot of energy generation. This is why eventhough you may have sunlight from 6AM to 5PM (or more). Your real energy generation is only during the peak times.
The amount of energy generation also depends on the type of collector and the micro-climate at the location. For instance, at the commercial dairy installation that we have been monitoring. They frequently have foggy mornings. This translates too extremely low output on the mornings. This is due to two factors. One, the fog (vapor in the air) is dissipating the sunlight. Two, the collectors used for this installation are of a concentrating nature (Ten Energistics Suntrac 4×8’s). Concentrating solar collectors do not work well with the dissipated light (known as ‘diffuse’). They mostly only operate with ‘direct normal’ light. This is the light that is perpendicular to the panels. Which for these panels is whenever they are turned on, since they track the sun to keep their output up.
That’s the basics of sunlight for solar systems. Please feel free to ask me any questions!