"""
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.

# %%
# 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, the spectral irradiance
# components will be returned in a dictionary as 2-D arrays, with one dimension
# for wavelength and 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.

# %%
# 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`