r"""
Functions for the infinite sheds bifacial irradiance model.
"""
import numpy as np
import pandas as pd
from pvlib.tools import cosd, sind, tand
from pvlib.bifacial import utils
from pvlib.irradiance import beam_component, aoi, haydavies
def _poa_ground_shadows(poa_ground, f_gnd_beam, df, vf_gnd_sky):
"""
Reduce ground-reflected irradiance to the tilted plane (poa_ground) to
account for shadows on the ground.
Parameters
----------
poa_ground : numeric
Ground reflected irradiance on the tilted surface, assuming full GHI
illumination on all of the ground. [W/m^2]
f_gnd_beam : numeric
Fraction of the distance between rows that is illuminated (unshaded).
[unitless]
df : numeric
Diffuse fraction, the ratio of DHI to GHI. [unitless]
vf_gnd_sky : numeric
View factor from the ground to the sky, integrated along the distance
between rows. [unitless]
Returns
-------
poa_gnd_sky : numeric
Adjusted ground-reflected irradiance accounting for shadows on the
ground. [W/m^2]
"""
return poa_ground * (f_gnd_beam*(1 - df) + df*vf_gnd_sky)
def _poa_sky_diffuse_pv(dhi, gcr, surface_tilt):
r"""
Integrated view factors from the shaded and unshaded parts of
the row slant height to the sky.
Parameters
----------
f_x : numeric
Fraction of row slant height from the bottom that is shaded from
direct irradiance. [unitless]
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]
npoints : int, default 100
Number of points for integration. [unitless]
A detailed calculation would be
dhi * (f_x * vf_shade_sky_integ + (1 - f_x) * vf_noshade_sky_integ)
where vf_shade_sky_integ is the average view factor between 0 and f_x
(the shaded portion). But the average view factor is
1/(f_x - 0) Integral_0^f_x vf(x) dx
so the detailed calculation is equivalent to
dhi * 1/(1 - 0) Integral_0^1 vf(x) dx
Parameters
----------
f_x : numeric
Fraction of row slant height from the bottom that is shaded from
direct irradiance. [unitless]
dhi : numeric
Diffuse horizontal irradiance (DHI). [W/m^2]
gcr : float
ground coverage ratio, ratio of row slant length to row spacing.
[unitless]
surface_tilt : numeric
Surface tilt angle in degrees from horizontal, e.g., surface facing up
= 0, surface facing horizon = 90. [degree]
Returns
-------
poa_sky_diffuse_pv : numeric
Total sky diffuse irradiance incident on the PV surface. [W/m^2]
"""
vf_integ = utils.vf_row_sky_2d_integ(surface_tilt, gcr, 0., 1.)
return dhi * vf_integ
def _poa_ground_pv(poa_ground, gcr, surface_tilt):
"""
Reduce ground-reflected irradiance to account for limited view of the
ground from the row surface.
Parameters
----------
poa_ground : numeric
Ground-reflected irradiance that would reach the row surface if the
full ground was visible. poa_gnd_sky accounts for limited view of the
sky from the ground. [W/m^2]
gcr : float
ground coverage ratio, ratio of row slant length to row spacing.
[unitless]
surface_tilt : numeric
Surface tilt angle in degrees from horizontal, e.g., surface facing up
= 0, surface facing horizon = 90. [degree]
Returns
-------
numeric
Ground diffuse irradiance on the row plane. [W/m^2]
"""
vf_integ = utils.vf_row_ground_2d_integ(surface_tilt, gcr, 0., 1.)
return poa_ground * vf_integ
def _shaded_fraction(solar_zenith, solar_azimuth, surface_tilt,
surface_azimuth, gcr):
"""
Calculate fraction (from the bottom) of row slant height that is shaded
from direct irradiance by the row in front toward the sun.
See [1], Eq. 14 and also [2], Eq. 32.
.. math::
F_x = \\max \\left( 0, \\min \\left(\\frac{\\text{GCR} \\cos \\theta
+ \\left( \\text{GCR} \\sin \\theta - \\tan \\beta_{c} \\right)
\\tan Z - 1}
{\\text{GCR} \\left( \\cos \\theta + \\sin \\theta \\tan Z \\right)},
1 \\right) \\right)
Parameters
----------
solar_zenith : numeric
Apparent (refraction-corrected) solar zenith. [degrees]
solar_azimuth : numeric
Solar azimuth. [degrees]
surface_tilt : numeric
Row tilt from horizontal, e.g. surface facing up = 0, surface facing
horizon = 90. [degrees]
surface_azimuth : numeric
Azimuth angle of the row surface. North=0, East=90, South=180,
West=270. [degrees]
gcr : numeric
Ground coverage ratio, which is the ratio of row slant length to row
spacing (pitch). [unitless]
Returns
-------
f_x : numeric
Fraction of row slant height from the bottom that is shaded from
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`.
.. [2] Kevin Anderson and Mark Mikofski, "Slope-Aware Backtracking for
Single-Axis Trackers", Technical Report NREL/TP-5K00-76626, July 2020.
https://www.nrel.gov/docs/fy20osti/76626.pdf
"""
tan_phi = utils._solar_projection_tangent(
solar_zenith, solar_azimuth, surface_azimuth)
# length of shadow behind a row as a fraction of pitch
x = gcr * (sind(surface_tilt) * tan_phi + cosd(surface_tilt))
f_x = 1 - 1. / x
# set f_x to be 1 when sun is behind the array
ao = aoi(surface_tilt, surface_azimuth, solar_zenith, solar_azimuth)
f_x = np.where(ao < 90, f_x, 1.)
# when x < 1, the shadow is not long enough to fall on the row surface
f_x = np.where(x > 1., f_x, 0.)
return f_x
[docs]
def get_irradiance_poa(surface_tilt, surface_azimuth, solar_zenith,
solar_azimuth, gcr, height, pitch, ghi, dhi, dni,
albedo, model='isotropic', dni_extra=None, iam=1.0,
npoints=100, vectorize=False):
r"""
Calculate plane-of-array (POA) irradiance on one side of a row of modules.
The infinite sheds model [1] assumes the PV system comprises parallel,
evenly spaced rows on a level, horizontal surface. Rows can be on fixed
racking or single axis trackers. The model calculates irradiance at a
location far from the ends of any rows, in effect, assuming that the
rows (sheds) are infinitely long.
POA irradiance components include direct, diffuse and global (total).
Irradiance values are reduced to account for reflection of direct light,
but are not adjusted for solar spectrum or reduced by a module's
bifaciality factor.
Parameters
----------
surface_tilt : numeric
Tilt of the surface from horizontal. Must be between 0 and 180. For
example, for a fixed tilt module mounted at 30 degrees from
horizontal, use ``surface_tilt=30`` to get front-side irradiance and
``surface_tilt=150`` to get rear-side irradiance. [degree]
surface_azimuth : numeric
Surface azimuth in decimal degrees east of north
(e.g. North = 0, South = 180, East = 90, West = 270). [degree]
solar_zenith : numeric
Refraction-corrected solar zenith. [degree]
solar_azimuth : numeric
Solar azimuth. [degree]
gcr : float
Ground coverage ratio, ratio of row slant length to row spacing.
[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``.
ghi : numeric
Global horizontal irradiance. [W/m2]
dhi : numeric
Diffuse horizontal irradiance. [W/m2]
dni : numeric
Direct normal irradiance. [W/m2]
albedo : numeric
Surface albedo. [unitless]
model : str, default 'isotropic'
Irradiance model - can be one of 'isotropic' or 'haydavies'.
dni_extra : numeric, optional
Extraterrestrial direct normal irradiance. Required when
``model='haydavies'``. [W/m2]
iam : numeric, default 1.0
Incidence angle modifier, the fraction of direct irradiance incident
on the surface that is not reflected away. [unitless]
npoints : int, default 100
Number of discretization points for calculating integrated view
factors.
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
-------
output : dict or DataFrame
Output is a DataFrame when input ghi is a Series. See Notes for
descriptions of content.
Notes
-----
Input parameters ``height`` and ``pitch`` must have the same unit.
``output`` always includes:
- ``poa_global`` : total POA irradiance. [W/m^2]
- ``poa_diffuse`` : total diffuse POA irradiance from all sources. [W/m^2]
- ``poa_direct`` : total direct POA irradiance. [W/m^2]
- ``poa_sky_diffuse`` : total sky diffuse irradiance on the plane of array.
[W/m^2]
- ``poa_ground_diffuse`` : total ground-reflected diffuse irradiance on the
plane of array. [W/m^2]
- ``shaded_fraction`` : fraction of row slant height from the bottom that
is shaded from direct irradiance by adjacent rows. [unitless]
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`.
See also
--------
get_irradiance
"""
if model == 'haydavies':
if dni_extra is None:
raise ValueError(f'must supply dni_extra for {model} model')
# Call haydavies first time within the horizontal plane - to subtract
# circumsolar_horizontal from DHI
sky_diffuse_comps_horizontal = haydavies(0, 180, dhi, dni, dni_extra,
solar_zenith, solar_azimuth,
return_components=True)
circumsolar_horizontal = sky_diffuse_comps_horizontal['circumsolar']
# Call haydavies a second time where circumsolar_normal is facing
# directly towards sun, and can be added to DNI
sky_diffuse_comps_normal = haydavies(solar_zenith, solar_azimuth, dhi,
dni, dni_extra, solar_zenith,
solar_azimuth,
return_components=True)
circumsolar_normal = sky_diffuse_comps_normal['circumsolar']
dhi = dhi - circumsolar_horizontal
dni = dni + circumsolar_normal
# Calculate some geometric quantities
# rows to consider in front and behind current row
# ensures that view factors to the sky are computed to within 5 degrees
# of the horizon
max_rows = np.ceil(height / (pitch * tand(5)))
# fraction of ground between rows that is illuminated accounting for
# shade from panels. [1], Eq. 4
f_gnd_beam = utils._unshaded_ground_fraction(
surface_tilt, surface_azimuth, solar_zenith, solar_azimuth, gcr)
# integrated view factor from the ground to the sky, integrated between
# adjacent rows interior to the array
# method differs from [1], Eq. 7 and Eq. 8; height is defined at row
# center rather than at row lower edge as in [1].
vf_gnd_sky = utils.vf_ground_sky_2d_integ(
surface_tilt, gcr, height, pitch, max_rows, npoints,
vectorize)
# fraction of row slant height that is shaded from direct irradiance
f_x = _shaded_fraction(solar_zenith, solar_azimuth, surface_tilt,
surface_azimuth, gcr)
# Total sky diffuse received by both shaded and unshaded portions
poa_sky_pv = _poa_sky_diffuse_pv(dhi, gcr, surface_tilt)
# irradiance reflected from the ground before accounting for shadows
# and restricted views
# this is a deviation from [1], because the row to ground view factor
# is accounted for in a different manner
ground_diffuse = ghi * albedo
# diffuse fraction
diffuse_fraction = np.clip(dhi / ghi, 0., 1.)
# make diffuse fraction 0 when ghi is small
diffuse_fraction = np.where(ghi < 0.0001, 0., diffuse_fraction)
# Reduce ground-reflected irradiance because other rows in the array
# block irradiance from reaching the ground.
# [2], Eq. 9
ground_diffuse = _poa_ground_shadows(
ground_diffuse, f_gnd_beam, diffuse_fraction, vf_gnd_sky)
# Ground-reflected irradiance on the row surface accounting for
# the view to the ground. This deviates from [1], Eq. 10, 11 and
# subsequent. Here, the row to ground view factor is computed. In [1],
# the usual ground-reflected irradiance includes the single row to ground
# view factor (1 - cos(tilt))/2, and Eq. 10, 11 and later multiply
# this quantity by a ratio of view factors.
poa_gnd_pv = _poa_ground_pv(ground_diffuse, gcr, surface_tilt)
# add sky and ground-reflected irradiance on the row by irradiance
# component
poa_diffuse = poa_gnd_pv + poa_sky_pv
# beam on plane, make an array for consistency with poa_diffuse
poa_beam = np.atleast_1d(beam_component(
surface_tilt, surface_azimuth, solar_zenith, solar_azimuth, dni))
poa_direct = poa_beam * (1 - f_x) * iam # direct only on the unshaded part
poa_global = poa_direct + poa_diffuse
output = {
'poa_global': poa_global, 'poa_direct': poa_direct,
'poa_diffuse': poa_diffuse, 'poa_ground_diffuse': poa_gnd_pv,
'poa_sky_diffuse': poa_sky_pv, 'shaded_fraction': f_x}
if isinstance(poa_global, pd.Series):
output = pd.DataFrame(output)
return output
[docs]
def get_irradiance(surface_tilt, surface_azimuth, solar_zenith, solar_azimuth,
gcr, height, pitch, ghi, dhi, dni,
albedo, model='isotropic', dni_extra=None, iam_front=1.0,
iam_back=1.0, bifaciality=0.8, shade_factor=-0.02,
transmission_factor=0, npoints=100, vectorize=False):
"""
Get front and rear irradiance using the infinite sheds model.
The infinite sheds model [1] assumes the PV system comprises parallel,
evenly spaced rows on a level, horizontal surface. Rows can be on fixed
racking or single axis trackers. The model calculates irradiance at a
location far from the ends of any rows, in effect, assuming that the
rows (sheds) are infinitely long.
The model accounts for the following effects:
- restricted view of the sky from module surfaces due to the nearby rows.
- restricted view of the ground from module surfaces due to nearby rows.
- restricted view of the sky from the ground due to rows.
- shading of module surfaces by nearby rows.
- shading of rear cells of a module by mounting structure and by
module features.
The model implicitly assumes that diffuse irradiance from the sky is
isotropic, and that module surfaces do not allow irradiance to transmit
through the module to the ground through gaps between cells.
Parameters
----------
surface_tilt : numeric
Tilt from horizontal of the front-side surface. [degree]
surface_azimuth : numeric
Surface azimuth in decimal degrees east of north
(e.g. North = 0, South = 180, East = 90, West = 270). [degree]
solar_zenith : numeric
Refraction-corrected solar zenith. [degree]
solar_azimuth : numeric
Solar azimuth. [degree]
gcr : float
Ground coverage ratio, ratio of row slant length to row spacing.
[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``.
ghi : numeric
Global horizontal irradiance. [W/m2]
dhi : numeric
Diffuse horizontal irradiance. [W/m2]
dni : numeric
Direct normal irradiance. [W/m2]
albedo : numeric
Surface albedo. [unitless]
model : str, default 'isotropic'
Irradiance model - can be one of 'isotropic' or 'haydavies'.
dni_extra : numeric, optional
Extraterrestrial direct normal irradiance. Required when
``model='haydavies'``. [W/m2]
iam_front : numeric, default 1.0
Incidence angle modifier, the fraction of direct irradiance incident
on the front surface that is not reflected away. [unitless]
iam_back : numeric, default 1.0
Incidence angle modifier, the fraction of direct irradiance incident
on the back surface that is not reflected away. [unitless]
bifaciality : numeric, default 0.8
Ratio of the efficiency of the module's rear surface to the efficiency
of the front surface. [unitless]
shade_factor : numeric, default -0.02
Fraction of back surface irradiance that is blocked by array mounting
structures. Negative value is a reduction in back irradiance.
[unitless]
transmission_factor : numeric, default 0.0
Fraction of irradiance on the back surface that does not reach the
module's cells due to module features such as busbars, junction box,
etc. A negative value is a reduction in back irradiance. [unitless]
npoints : int, default 100
Number of discretization points for calculating integrated view
factors.
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
-------
output : dict or DataFrame
Output is a DataFrame when input ghi is a Series. See Notes for
descriptions of content.
Notes
-----
``output`` includes:
- ``poa_global`` : total irradiance reaching the module cells from both
front and back surfaces. [W/m^2]
- ``poa_front`` : total irradiance reaching the module cells from the front
surface. [W/m^2]
- ``poa_back`` : total irradiance reaching the module cells from the back
surface. [W/m^2]
- ``poa_front_direct`` : direct irradiance reaching the module cells from
the front surface. [W/m^2]
- ``poa_front_diffuse`` : total diffuse irradiance reaching the module
cells from the front surface. [W/m^2]
- ``poa_front_sky_diffuse`` : sky diffuse irradiance reaching the module
cells from the front surface. [W/m^2]
- ``poa_front_ground_diffuse`` : ground-reflected diffuse irradiance
reaching the module cells from the front surface. [W/m^2]
- ``shaded_fraction_front`` : fraction of row slant height from the bottom
that is shaded from direct irradiance on the front surface by adjacent
rows. [unitless]
- ``poa_back_direct`` : direct irradiance reaching the module cells from
the back surface. [W/m^2]
- ``poa_back_diffuse`` : total diffuse irradiance reaching the module
cells from the back surface. [W/m^2]
- ``poa_back_sky_diffuse`` : sky diffuse irradiance reaching the module
cells from the back surface. [W/m^2]
- ``poa_back_ground_diffuse`` : ground-reflected diffuse irradiance
reaching the module cells from the back surface. [W/m^2]
- ``shaded_fraction_back`` : fraction of row slant height from the bottom
that is shaded from direct irradiance on the back surface by adjacent
rows. [unitless]
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`.
See also
--------
get_irradiance_poa
"""
# backside is rotated and flipped relative to front
backside_tilt, backside_sysaz = _backside(surface_tilt, surface_azimuth)
# front side POA irradiance
irrad_front = get_irradiance_poa(
surface_tilt=surface_tilt, surface_azimuth=surface_azimuth,
solar_zenith=solar_zenith, solar_azimuth=solar_azimuth,
gcr=gcr, height=height, pitch=pitch, ghi=ghi, dhi=dhi, dni=dni,
albedo=albedo, model=model, dni_extra=dni_extra, iam=iam_front,
npoints=npoints, vectorize=vectorize)
# back side POA irradiance
irrad_back = get_irradiance_poa(
surface_tilt=backside_tilt, surface_azimuth=backside_sysaz,
solar_zenith=solar_zenith, solar_azimuth=solar_azimuth,
gcr=gcr, height=height, pitch=pitch, ghi=ghi, dhi=dhi, dni=dni,
albedo=albedo, model=model, dni_extra=dni_extra, iam=iam_back,
npoints=npoints, vectorize=vectorize)
colmap_front = {
'poa_global': 'poa_front',
'poa_direct': 'poa_front_direct',
'poa_diffuse': 'poa_front_diffuse',
'poa_sky_diffuse': 'poa_front_sky_diffuse',
'poa_ground_diffuse': 'poa_front_ground_diffuse',
'shaded_fraction': 'shaded_fraction_front',
}
colmap_back = {
'poa_global': 'poa_back',
'poa_direct': 'poa_back_direct',
'poa_diffuse': 'poa_back_diffuse',
'poa_sky_diffuse': 'poa_back_sky_diffuse',
'poa_ground_diffuse': 'poa_back_ground_diffuse',
'shaded_fraction': 'shaded_fraction_back',
}
if isinstance(ghi, pd.Series):
irrad_front = irrad_front.rename(columns=colmap_front)
irrad_back = irrad_back.rename(columns=colmap_back)
output = pd.concat([irrad_front, irrad_back], axis=1)
else:
for old_key, new_key in colmap_front.items():
irrad_front[new_key] = irrad_front.pop(old_key)
for old_key, new_key in colmap_back.items():
irrad_back[new_key] = irrad_back.pop(old_key)
irrad_front.update(irrad_back)
output = irrad_front
effects = (1 + shade_factor) * (1 + transmission_factor)
output['poa_global'] = output['poa_front'] + \
output['poa_back'] * bifaciality * effects
return output
def _backside(tilt, surface_azimuth):
backside_tilt = 180. - tilt
backside_sysaz = (180. + surface_azimuth) % 360.
return backside_tilt, backside_sysaz
