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Improve schlick_diffuse docstring #1812

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Improve schlick_diffuse docstring #1812

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8b16bb3
docstring improvements
kandersolar Jul 21, 2023
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whatsnew
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correct docstring section order
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Update pvlib/iam.py
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2 changes: 2 additions & 0 deletions docs/sphinx/source/whatsnew/v0.10.2.rst
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Expand Up @@ -27,6 +27,8 @@ Testing

Documentation
~~~~~~~~~~~~~
* Added docstring detail for :py:func:`pvlib.iam.schlick_diffuse`.
(:issue:`1811`, :pull:`1812`)
* Removed Stickler-CI integration as the service has ceased June 2023.
(:issue:`1722`, :pull:`1723`)

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60 changes: 48 additions & 12 deletions pvlib/iam.py
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Expand Up @@ -800,7 +800,7 @@ def schlick(aoi):

In PV contexts, the Schlick approximation has been used as an analytically
integrable alternative to the Fresnel equations for estimating IAM
for diffuse irradiance [2]_.
for diffuse irradiance [2]_ (see :py:func:`schlick_diffuse`).

Parameters
----------
Expand All @@ -813,6 +813,10 @@ def schlick(aoi):
iam : numeric
The incident angle modifier.

See Also
--------
pvlib.iam.schlick_diffuse

References
----------
.. [1] Schlick, C. An inexpensive BRDF model for physically-based
Expand All @@ -822,10 +826,6 @@ def schlick(aoi):
for Diffuse radiation on Inclined photovoltaic Surfaces (FEDIS)",
Renewable and Sustainable Energy Reviews, vol. 161, 112362. June 2022.
:doi:`10.1016/j.rser.2022.112362`

See Also
--------
pvlib.iam.schlick_diffuse
"""
iam = 1 - (1 - cosd(aoi)) ** 5
iam = np.where(np.abs(aoi) >= 90.0, 0.0, iam)
Expand All @@ -845,9 +845,20 @@ def schlick_diffuse(surface_tilt):
ground-reflected irradiance on a tilted surface using the Schlick
incident angle model.

The diffuse iam values are calculated using an analytical integration
of the Schlick equation [1]_ over the portion of an isotropic sky and
isotropic foreground that is visible from the tilted surface [2]_.
The Schlick equation (or "Schlick's approximation") [1]_ is an
approximation to the Fresnel reflection factor which can be recast as
a simple photovoltaic IAM model like so:

.. math::

IAM = 1 - (1 - \cos(aoi))^5

Unlike the Fresnel reflection factor itself, Schlick's approximation can
be integrated analytically to derive a closed-form equation for diffuse
IAM factors for the portions of the sky and ground visible
from a tilted surface. This function implements an integration of the
Schlick approximation provided by Xie et al. [2]_ which assumes isotropic
sky and foreground.
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Parameters
----------
Expand All @@ -863,6 +874,35 @@ def schlick_diffuse(surface_tilt):
iam_ground : numeric
The incident angle modifier for ground-reflected diffuse.

See Also
--------
pvlib.iam.schlick

Notes
-----
The analytical integration of the Schlick approximation was derived
as part of the FEDIS diffuse IAM model [2]_. Compared with the model
implemented in this function, the FEDIS model includes an additional term
to account for reflection off a pyranometer's glass dome. Because that
reflection should already be accounted for in the instrument's calibration,
the pvlib authors believe it is inappropriate to account for pyranometer
reflection again in an IAM model. Thus, this function omits that term and
implements only the integrated Schlick approximation.

Note also that the output of this function (which is an exact integration)
can be compared with the output of :py:func:`marion_diffuse` which numerically
integrates the Schlick approximation:

.. code::

>>> pvlib.iam.marion_diffuse('schlick', surface_tilt=20)
{'sky': 0.9625000227247358,
'horizon': 0.7688174948510073,
'ground': 0.6267861879241405}

>>> pvlib.iam.schlick_diffuse(surface_tilt=20)
(0.9624993421569652, 0.6269387554469255)

References
----------
.. [1] Schlick, C. An inexpensive BRDF model for physically-based
Expand All @@ -872,10 +912,6 @@ def schlick_diffuse(surface_tilt):
for Diffuse radiation on Inclined photovoltaic Surfaces (FEDIS)",
Renewable and Sustainable Energy Reviews, vol. 161, 112362. June 2022.
:doi:`10.1016/j.rser.2022.112362`

See Also
--------
pvlib.iam.schlick
"""
# these calculations are as in [2]_, but with the refractive index
# weighting coefficient w set to 1.0 (so it is omitted)
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