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is vf_row_sky correct? #1665

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@mikofski

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@mikofski

y = 0.5 * (cosd(psi_t_shaded) + cst)

I think this should be:

$$\frac{1 + \cos \left( \text{surface tilt} + \psi_{t}\ \text{shaded} \right)}{2}$$

because in the reference frame of the module surface the angle pointing along the slant height to the sky is actually zero, $cos(0) = 1$, and the angle above the slant height to a horizontal line would be the surface_tilt itself, then the angle from the horizontal to the top of the next row is psi_t_shaded so finally this angle from the slant height all the way up to the top of the next row is surface_tilt + psi_t_shaded:

infinite_sheds

For example, this is why if psi_t_shaded is zero, then the view factor should collapse to the isotropic view factor $(1+\cos(\beta))/2$ as given on the PVPMC website modeling reference for POA sky diffuse.

The actual value difference between the two formulas can be quite small when psi_t_shaded is close to zero (eg less than 5°), but it's significant when as the masking angle is larger (eg greater than 5°).

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