View factor

Method

Each point up the collector sees the sky from its own plane down to the top edge of the row in front. The fraction of a uniform sky that point sees is its view factor. The factor is averaged over 200 points at the middles of equal steps across the collector width and divided by the view factor of a free plane at the same tilt. For fixed rows the result is constant; for trackers it is evaluated at every timestep.

Settings

Setting	Value
Points across the collector	200, at the middles of equal steps

The math

$$F(x) = \tfrac{1}{2}\big(1 + \cos(\beta + e(x))\big),\qquad e(x) = \arctan\frac{(W - x)\sin\beta}{P - (W - x)\cos\beta},\qquad F_{sky} = \frac{\overline{F(x)}}{\tfrac{1}{2}(1 + \cos\beta)} \tag{1}$$

where

Symbol	Meaning	Unit
$F(x)$	the sky view factor of the point at distance $x$ up the collector from its bottom edge	—
$\beta$	the tilt of the module plane from the horizontal	degrees
$e(x)$	the elevation of the front row's top edge seen from the point $x$; 90° when the denominator is 0 or less	degrees
$W$	the collector width	m
$P$	the row pitch, the distance between the rows	m
$F_{sky}$	the fraction of the sky light the modules receive	—
$\overline{F(x)}$	the average of $F(x)$ over the collector width	—

References

1. Gilman, DiOrio, Freeman, Janzou, Dobos & Ryberg 2018, SAM Photovoltaic Model Technical Reference Update, NREL/TP-6A20-67399

API

array.sky_masking_method = view_factor

Symbols follow the symbol registry.