If not spaced correctly, tilted rows of PV modules—a common layout for low-slope roofs and ground-mounted arrays—can end up with the tops of one row shading the bottom of the row behind it. In some cases, the entire production of the partially shaded rows can be curtailed.
A common approach is lay out the array to avoid interrow shading between 9 a.m. and 3 p.m. on the winter solstice—the day when the sun is at its lowest angle and has the narrowest range of azimuth angles. The shadows cast on this day are longer than on any other day of the year.
The amount of shading depends on the row spacing. The required spacing can be calculated based on the site’s latitude (the higher the latitude, the longer the shadow); the row height above the ground or the roof surface (the taller the rows, the longer the shadow); and the desired solar window (more space will be required to avoid interrow shading with a wider solar window).
For this example, the PV array is assumed to be facing true south and on a flat surface. If the height (H) of the modules’ top edge above the roof or ground is known, along with the sun’s altitude (y) and azimuth (z) angles at the start (or end) of the desired solar window, then the required distance (D) between the back of one row and the front of the next can be calculated as:
D = [H ÷ tan(y)] × cos(z)
But, let’s face it: Many people prefer to avoid trigonometry, so the formula was used to draw a graph which gives the interrow spacing factor to avoid shading from 9 a.m. to 3 p.m. The graph’s x-axis is the latitude of the site and the y-axis is the row-space factor. Simply multiply the factor for your latitude by the row height to calculate the space required between the back of the one row and the front of the next.
This graph works for arrays facing true south on a flat surface. If the array isn’t facing true south, and/or the mount surface is pitched, then the calculations become more complicated and you’ll have to brush up on your trig!