Add numpy SPECTRL2 implementation

docs/examples/plot_spectrl2_fig51A.py

@@ -0,0 +1,97 @@
+"""
+Modeling Spectral Irradiance
+============================
+
+Recreating Figure 5-1A from the SPECTRL2 NREL Technical Report.
+"""
+
+# %%
+# This example shows how to model the spectral distribution of irradiance
+# based on atmospheric conditions. The spectral distribution of irradiance is
+# the power content at each wavelength band in the solar spectrum and is
+# affected by various scattering and absorption mechanisms in the atmosphere.
+# This example recreates an example figure from the SPECTRL2 NREL Technical
+# Report [1]_. The figure shows modeled spectra at hourly intervals across
+# a single morning.
+#
+# References
+# ----------
+# .. [1] Bird, R, and Riordan, C., 1984, "Simple solar spectral model for
+#    direct and diffuse irradiance on horizontal and tilted planes at the
+#    earth's surface for cloudless atmospheres", NREL Technical Report
+#    TR-215-2436 doi:10.2172/5986936.
+
+# %%
+# The SPECTRL2 model has several inputs; some can be calculated with pvlib,
+# but other must come from a weather dataset. In this case, these weather
+# parameters are example assumptions taken from the technical report.
+
+from pvlib import spectrum, solarposition, irradiance, atmosphere
+import pandas as pd
+import matplotlib.pyplot as plt
+
+# assumptions from the technical report:
+lat = 37
+lon = -100
+tilt = 37
+azimuth = 180
+pressure = 101300  # sea level, roughly
+water_vapor_content = 0.5  # cm
+tau500 = 0.1
+ozone = 0.31  # atm-cm
+albedo = 0.2
+
+times = pd.date_range('1984-03-20 06:17', freq='h', periods=6, tz='Etc/GMT+7')
+solpos = solarposition.get_solarposition(times, lat, lon)
+aoi = irradiance.aoi(tilt, azimuth, solpos.apparent_zenith, solpos.azimuth)
+
+# The technical report uses the 'kasten1966' airmass model, but later
+# versions of SPECTRL2 use 'kastenyoung1989'.  Here we use 'kasten1966'
+# for consistency with the technical report.
+relative_airmass = atmosphere.get_relative_airmass(solpos.apparent_zenith,
+                                                   model='kasten1966')
+
+# %%
+# With all the necessary inputs in hand we can model spectral irradiance using
+# :py:func:`pvlib.spectrum.spectrl2`.  Note that because we are calculating
+# the spectra for more than one set of conditions, we will get back 2-D
+# arrays (one dimension for wavelength, one for time).
+
+spectra = spectrum.spectrl2(
+    apparent_zenith=solpos.apparent_zenith,
+    aoi=aoi,
+    surface_tilt=tilt,
+    ground_albedo=albedo,
+    surface_pressure=pressure,
+    relative_airmass=relative_airmass,
+    precipitable_water=water_vapor_content,
+    ozone=ozone,
+    aerosol_turbidity_500nm=tau500,
+)
+
+# %%
+# The ``poa_global`` array represents the total spectral irradiance on our
+# hypothetical solar panel. Let's plot it against wavelength to recreate
+# Figure 5-1A:
+
+plt.figure()
+plt.plot(spectra['wavelength'], spectra['poa_global'])
+plt.xlim(200, 2700)
+plt.ylim(0, 1.8)
+plt.title(r"Day 80 1984, $\tau=0.1$, Wv=0.5 cm")
+plt.ylabel(r"Irradiance ($W m^{-2} nm^{-1}$)")
+plt.xlabel(r"Wavelength ($nm$)")
+time_labels = times.strftime("%H:%M %p")
+labels = [
+    "AM {:0.02f}, Z{:0.02f}, {}".format(*vals)
+    for vals in zip(relative_airmass, solpos.apparent_zenith, time_labels)
+]
+plt.legend(labels)
+plt.show()
+
+# %%
+# Note that the airmass and zenith values do not exactly match the values in
+# the technical report; this is because airmass is estimated from solar
+# position and the solar position calculation in the technical report does not
+# exactly match the one used here.  However, the differences are minor enough
+# to not materially change the spectra.

docs/sphinx/source/api.rst

@@ -416,6 +416,13 @@ Shading
    shading.masking_angle_passias
    shading.sky_diffuse_passias
 
+Spectrum
+--------
+
+.. autosummary::
+   :toctree: generated/
+
+   spectrum.spectrl2
 
 Tracking
 ========

docs/sphinx/source/whatsnew.rst

@@ -6,6 +6,7 @@ What's New
 
 These are new features and improvements of note in each release.
 
+.. include:: whatsnew/v0.8.1.rst
 .. include:: whatsnew/v0.8.0.rst
 .. include:: whatsnew/v0.7.2.rst
 .. include:: whatsnew/v0.7.1.rst

docs/sphinx/source/whatsnew/v0.8.1.rst

@@ -13,6 +13,8 @@ Deprecations
 
 Enhancements
 ~~~~~~~~~~~~
+* Add a numpy-based implementation of the SPECTRL2 spectral irradiance model
+  :py:func:`pvlib.spectrum.spectrl2` (:pull:`1062`)
 * Create :py:func:`~pvlib.pvsystem.PVSystem.fuentes_celltemp` and add ``temperature_model='fuentes'``
   option to :py:class:`~pvlib.modelchain.ModelChain`. (:pull:`1042`) (:issue:`1073`)
 * Added :py:func:`pvlib.temperature.ross` for cell temperature modeling using

pvlib/__init__.py

@@ -20,6 +20,7 @@
     soiling,
     solarposition,
     spa,
+    spectrum,
     temperature,
     tools,
     tracking,

pvlib/data/spectrl2_example_spectra.csv

@@ -0,0 +1,123 @@
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