Sun timing

What it is

The weather file gives one irradiance value per timestep, and that value is the average over the whole interval. The model reads each row as the interval that starts at its timestamp: the row stamped 12:00 holds the sunlight that fell between 12:00 and 13:00. On upload, a file stamped at the end of each interval, such as a TMY3 file whose hours run from 01:00 to 24:00, is shifted back one step, and stamps at the half hour are moved to the start of their interval. The model needs the sun at one instant of the interval to compute the incidence angle and the shadows, and the options set that instant.

For an interval in the middle of the day both options do the same thing: the sun is placed at the middle of the interval. The 12:00 row uses the sun at 12:30.

The options differ only in the intervals that contain a sunrise or a sunset. Take the 07:00 row on a day when the sun rises at 07:40. The figure draws both placements. Sunlight reached the plane only between 07:40 and 08:00, so the hour's average in the file comes from those twenty minutes.

The sunlit-slice midpoint places the sun at 07:50, the middle of the lit part, where the sun is above the horizon. The interval midpoint places the sun at 07:30, the middle of the whole hour, where the sun is still below the horizon; the row then counts as dark. A sunset works the same way: for the 19:00 row with a sunset at 19:20, the lit part runs from 19:00 to 19:20 and its midpoint is 19:10, while the interval midpoint 19:30 falls after sunset.

Over a year the difference is a fraction of a percent of the plane irradiance. The same rule applies to sub-hourly files.

Options

• Sunlit-slice midpoint: sunrise and sunset intervals evaluated at the middle of their sunlit part
• Interval midpoint: every interval evaluated at its middle

API

site.sun_timing = sunlit_midpoint | interval_midpoint