Hello Paul,
I have read the docs to understand how self-shading is calculated in pvwatts. Here's what I think is the case for the various configuration options (note, "in front" means a "row" between the "row in question" and the sun):
1. For fixed tilt systems, the gcr is used to determine the inter-row spacing from
gcr = L/R
where L = module length (in the not-horizontal direction) and R is the inter-row spacing. Then pvwatts does a geometric calculation of the shading by a row in front for each hour. The rows are assumed to be infinitely wide (i.e., there are no end-effects accounted for).
2. For 1-axis horizontal tracking, the modules are assumed to be butted end-to-end along the tracking axis and the gcr is L/R where R is the spacing between trackers and L is the width of modules perpendicular to the tracking axis. This is used either to calculate self-shading from a tracker in front, or the angle of incidence for zero shading in the case of backtracking. The trackers are assumed to be infinitely long (i.e. there are no end-effects accounted for).
3. For 1-axis tracking with an inclined tracking axis, the trackers are arranged in rows with some spacing (e.g., east-west rows of trackers with north-south tracking axis orientation), but the gcr is used as with horizontal trackers to calculate only the intra-row self-shading or backtracking angle, and inter-row shading is assumed to be zero. Thus the user should input the intra-row gcr for these cases, rather than the actual gcr accounting for inter-row spacing.
4. For 2-axis tracking, self-shading is assumed to be zero and gcr has no impact on power calculations.
Therefore, for the case of inclined 1-axis or two-axis trackers, a manual shading input should be provided via the losses input, or else using an hourly adjustment array of the shading is available from some other calculation software.
Have I got it right? Of if not, can you clarify whatever I got wrong?
Thanks
Russ Jones