# Source code for pvlib.bifacial.utils

"""
The bifacial.utils module contains functions that support bifacial irradiance
modeling.
"""
import numpy as np
from pvlib.tools import sind, cosd, tand
from scipy.integrate import trapezoid

def _solar_projection_tangent(solar_zenith, solar_azimuth, surface_azimuth):
"""
Tangent of the angle between the zenith vector and the sun vector
projected to the plane defined by the zenith vector and surface_azimuth.

.. math::
\\tan \\phi = \\cos\\left(\\text{solar azimuth}-\\text{system azimuth}
\\right)\\tan\\left(\\text{solar zenith}\\right)

Parameters
----------
solar_zenith : numeric
Solar zenith angle. [degree].
solar_azimuth : numeric
Solar azimuth. [degree].
surface_azimuth : numeric
Azimuth of the module surface, i.e., North=0, East=90, South=180,
West=270. [degree]

Returns
-------
tan_phi : numeric
Tangent of the angle between vertical and the projection of the
sun direction onto the YZ plane.
"""
rotation = solar_azimuth - surface_azimuth
tan_phi = cosd(rotation) * tand(solar_zenith)
return tan_phi

solar_azimuth, gcr, max_zenith=87):
r"""
Calculate the fraction of the ground with incident direct irradiance.

.. math::
F_{gnd,sky} = 1 - \min{\left(1, \text{GCR} \left|\cos \beta +
\sin \beta \tan \phi \right|\right)}

where :math:\beta is the surface tilt and :math:\phi is the angle
from vertical of the sun vector projected to a vertical plane that
contains the row azimuth surface_azimuth.

Parameters
----------
surface_tilt : numeric
Surface tilt angle. The tilt angle is defined as
degrees from horizontal, e.g., surface facing up = 0, surface facing
horizon = 90. [degree]
surface_azimuth : numeric
Azimuth of the module surface, i.e., North=0, East=90, South=180,
West=270. [degree]
solar_zenith : numeric
Solar zenith angle. [degree].
solar_azimuth : numeric
Solar azimuth. [degree].
gcr : float
Ground coverage ratio, which is the ratio of row slant length to row
spacing (pitch). [unitless]
max_zenith : numeric, default 87
Maximum zenith angle. For solar_zenith > max_zenith, unshaded ground
fraction is set to 0. [degree]

Returns
-------
f_gnd_beam : numeric
Fraction of distance betwen rows (pitch) with direct irradiance

References
----------
.. [1] Mikofski, M., Darawali, R., Hamer, M., Neubert, A., and Newmiller,
J. "Bifacial Performance Modeling in Large Arrays". 2019 IEEE 46th
Photovoltaic Specialists Conference (PVSC), 2019, pp. 1282-1287.
:doi:10.1109/PVSC40753.2019.8980572.
"""
tan_phi = _solar_projection_tangent(solar_zenith, solar_azimuth,
surface_azimuth)
f_gnd_beam = 1.0 - np.minimum(
1.0, gcr * np.abs(cosd(surface_tilt) + sind(surface_tilt) * tan_phi))
np.where(solar_zenith > max_zenith, 0., f_gnd_beam)  # [1], Eq. 4
return f_gnd_beam  # 1 - min(1, abs()) < 1 always

[docs]def vf_ground_sky_2d(rotation, gcr, x, pitch, height, max_rows=10):
r"""
Calculate the fraction of the sky dome visible from point x on the ground.

The view factor accounts for the obstruction of the sky by array rows that
are assumed to be infinitely long.  View factors are thus calculated in
a 2D geometry. The ground is assumed to be flat and level.

Parameters
----------
rotation : numeric
Rotation angle of the row's right edge relative to row center.
[degree]
gcr : float
Ratio of the row slant length to the row spacing (pitch). [unitless]
x : numeric
Position on the ground between two rows, as a fraction of the pitch.
x = 0 corresponds to the point on the ground directly below the
center point of a row. Positive x is towards the right. [unitless]
height : float
Height of the center point of the row above the ground; must be in the
same units as pitch.
pitch : float
Distance between two rows; must be in the same units as height.
max_rows : int, default 10
Maximum number of rows to consider on either side of the current
row. [unitless]

Returns
-------
vf : array
Fraction of sky dome visible from each point on the ground.
Shape is (len(x), len(rotation)). [unitless]
"""
# This function creates large float64 arrays of size
# (2*len(x)*len(rotation)*len(max_rows)) or ~100 MB for
# typical time series inputs.  This function makes heavy
# use of numpy's out parameter to avoid allocating new
# memory.  Unfortunately that comes at the cost of some
# readability: because arrays get reused to avoid new allocations,
# variable names don't always match what they hold.

# handle floats:
x = np.atleast_1d(x)[:, np.newaxis, np.newaxis]
rotation = np.atleast_1d(rotation)[np.newaxis, :, np.newaxis]
all_k = np.arange(-max_rows, max_rows + 1)
width = gcr * pitch / 2.
distance_to_row_centers = (all_k - x) * pitch
dy = width * sind(rotation)
dx = width * cosd(rotation)

phi = np.empty((2, x.shape[0], rotation.shape[1], len(all_k)))

# angles from x to right edge of each row
a1 = height + dy
# temporarily store one leg of the triangle in phi:
np.arctan2(a1, phi[0], out=phi[0])

# angles from x to left edge of each row
a2 = height - dy
np.subtract(distance_to_row_centers, dx, out=phi[1])
np.arctan2(a2, phi[1], out=phi[1])

# swap angles so that phi[0,:,:,:] is the lesser angle
phi.sort(axis=0)

# now re-use phi's memory again, this time storing cos(phi).
next_edge = phi[1, :, :, 1:]
np.cos(next_edge, out=next_edge)
prev_edge = phi[0, :, :, :-1]
np.cos(prev_edge, out=prev_edge)
# right edge of next row - left edge of previous row, again
# reusing memory so that the difference is stored in next_edge.
# Note that the 0.5 view factor coefficient is applied after summing
# as a minor speed optimization.
np.subtract(next_edge, prev_edge, out=next_edge)
np.clip(next_edge, a_min=0., a_max=None, out=next_edge)
vf = np.sum(next_edge, axis=-1) / 2
return vf

[docs]def vf_ground_sky_2d_integ(surface_tilt, gcr, height, pitch, max_rows=10,
npoints=100, vectorize=False):
"""
Integrated view factor to the sky from the ground underneath
interior rows of the array.

Parameters
----------
surface_tilt : numeric
Surface tilt angle in degrees from horizontal, e.g., surface facing up
= 0, surface facing horizon = 90. [degree]
gcr : float
Ratio of row slant length to row spacing (pitch). [unitless]
height : float
Height of the center point of the row above the ground; must be in the
same units as pitch.
pitch : float
Distance between two rows. Must be in the same units as height.
max_rows : int, default 10
Maximum number of rows to consider in front and behind the current row.
npoints : int, default 100
Number of points used to discretize distance along the ground.
vectorize : bool, default False
If True, vectorize the view factor calculation across surface_tilt.
This increases speed with the cost of increased memory usage.

Returns
-------
fgnd_sky : numeric
Integration of view factor over the length between adjacent, interior
rows.  Shape matches that of surface_tilt. [unitless]
"""
# Abuse vf_ground_sky_2d by supplying surface_tilt in place
# of a signed rotation. This is OK because
# 1) z span the full distance between 2 rows, and
# 2) max_rows is set to be large upstream, and
# 3) _vf_ground_sky_2d considers [-max_rows, +max_rows]
# The VFs to the sky will thus be symmetric around z=0.5
z = np.linspace(0, 1, npoints)
rotation = np.atleast_1d(surface_tilt)
if vectorize:
fz_sky = vf_ground_sky_2d(rotation, gcr, z, pitch, height, max_rows)
else:
fz_sky = np.zeros((npoints, len(rotation)))
for k, r in enumerate(rotation):
vf = vf_ground_sky_2d(r, gcr, z, pitch, height, max_rows)
fz_sky[:, k] = vf[:, 0]  # remove spurious rotation dimension
# calculate the integrated view factor for all of the ground between rows
return trapezoid(fz_sky, z, axis=0)

def _vf_poly(surface_tilt, gcr, x, delta):
r'''
A term common to many 2D view factor calculations

Parameters
----------
surface_tilt : numeric
Surface tilt angle in degrees from horizontal, e.g., surface facing up
= 0, surface facing horizon = 90. [degree]
gcr : numeric
Ratio of the row slant length to the row spacing (pitch). [unitless]
x : numeric
Position on the row's slant length, as a fraction of the slant length.
x=0 corresponds to the bottom of the row. [unitless]
delta : -1 or +1

Returns
-------
numeric
'''
a = 1 / gcr
c = cosd(surface_tilt)
return np.sqrt(a*a + 2*delta*a*c*x + x*x)

[docs]def vf_row_sky_2d(surface_tilt, gcr, x):
r'''
Calculate the view factor to the sky from a point x on a row surface.

Assumes a PV system of infinitely long rows with uniform pitch on
horizontal ground. The view to the sky is restricted by the row's surface
tilt and the top of the adjacent row.

Parameters
----------
surface_tilt : numeric
Surface tilt angle in degrees from horizontal, e.g., surface facing up
= 0, surface facing horizon = 90. [degree]
gcr : numeric
Ratio of the row slant length to the row spacing (pitch). [unitless]
x : numeric
Position on the row's slant length, as a fraction of the slant length.
x=0 corresponds to the bottom of the row. [unitless]

Returns
-------
vf : numeric
Fraction of the sky dome visible from the point x. [unitless]

'''
p = _vf_poly(surface_tilt, gcr, 1 - x, -1)
return 0.5*(1 + (1/gcr * cosd(surface_tilt) - (1 - x)) / p)

[docs]def vf_row_sky_2d_integ(surface_tilt, gcr, x0=0, x1=1):
r'''
Calculate the average view factor to the sky from a segment of the row
surface between x0 and x1.

Assumes a PV system of infinitely long rows with uniform pitch on
horizontal ground. The view to the sky is restricted by the row's surface
tilt and the top of the adjacent row.

Parameters
----------
surface_tilt : numeric
Surface tilt angle in degrees from horizontal, e.g., surface facing up
= 0, surface facing horizon = 90. [degree]
gcr : numeric
Ratio of the row slant length to the row spacing (pitch). [unitless]
x0 : numeric, default 0
Position on the row's slant length, as a fraction of the slant length.
x0=0 corresponds to the bottom of the row. x0 should be less than x1.
[unitless]
x1 : numeric, default 1
Position on the row's slant length, as a fraction of the slant length.
x1 should be greater than x0. [unitless]

Returns
-------
vf : numeric
Average fraction of the sky dome visible from points in the segment
from x0 to x1. [unitless]

'''
u = np.abs(x1 - x0)
p0 = _vf_poly(surface_tilt, gcr, 1 - x0, -1)
p1 = _vf_poly(surface_tilt, gcr, 1 - x1, -1)
with np.errstate(divide='ignore'):
result = np.where(u < 1e-6,
vf_row_sky_2d(surface_tilt, gcr, x0),
0.5*(1 + 1/u * (p1 - p0))
)
return result

[docs]def vf_row_ground_2d(surface_tilt, gcr, x):
r'''
Calculate the view factor to the ground from a point x on a row surface.

Assumes a PV system of infinitely long rows with uniform pitch on
horizontal ground. The view to the ground is restricted by the row's
tilt and the bottom of the facing row.

Parameters
----------
surface_tilt : numeric
Surface tilt angle in degrees from horizontal, e.g., surface facing up
= 0, surface facing horizon = 90. [degree]
gcr : numeric
Ratio of the row slant length to the row spacing (pitch). [unitless]
x : numeric
Position on the row's slant length, as a fraction of the slant length.
x=0 corresponds to the bottom of the row. [unitless]

Returns
-------
vf : numeric
View factor to the visible ground from the point x. [unitless]

'''
p = _vf_poly(surface_tilt, gcr, x, 1)
return 0.5 * (1 - (1/gcr * cosd(surface_tilt) + x)/p)

[docs]def vf_row_ground_2d_integ(surface_tilt, gcr, x0=0, x1=1):
r'''
Calculate the average view factor to the ground from a segment of the row
surface between x0 and x1.

Assumes a PV system of infinitely long rows with uniform pitch on
horizontal ground. The view to the ground is restricted by the row's
tilt and the bottom of the facing row.

Parameters
----------
surface_tilt : numeric
Surface tilt angle in degrees from horizontal, e.g., surface facing up
= 0, surface facing horizon = 90. [degree]
gcr : numeric
Ratio of the row slant length to the row spacing (pitch). [unitless]
x0 : numeric, default 0.
Position on the row's slant length, as a fraction of the slant length.
x0=0 corresponds to the bottom of the row. x0 should be less than x1.
[unitless]
x1 : numeric, default 1.
Position on the row's slant length, as a fraction of the slant length.
x1 should be greater than x0. [unitless]

Returns
-------
vf : numeric
Integrated view factor to the visible ground on the interval (x0, x1).
[unitless]

'''
u = np.abs(x1 - x0)
p0 = _vf_poly(surface_tilt, gcr, x0, 1)
p1 = _vf_poly(surface_tilt, gcr, x1, 1)
with np.errstate(divide='ignore'):
result = np.where(u < 1e-6,
vf_row_ground_2d(surface_tilt, gcr, x0),
0.5*(1 - 1/u * (p1 - p0))
)
return result