plot_transposition_gain.py

"""
Modeling Transposition Gain
===========================

Calculating the gain in insolation of a tilted module over a flat module.
"""

# %%
# This example shows how to evaluate the transposition gain of a racking
# strategy.  The transposition gain is the additional insolation collected
# by orienting at a tilt instead of horizontal; using PV modeling lingo, it's
# the increase in POA (plane of array) insolation over GHI (global horizontal
# irradiance) insolation.
#
# This example uses a TMY dataset and the
# :py:meth:`pvlib.irradiance.get_total_irradiance` function to transpose
# irradiance components to POA irradiance for various fixed tilts.  It also
# models a single-axis tracking system for comparison. The monthly POA
# insolation is calculated for each strategy to show how orientation affects
# seasonal irradiance collection.

import pvlib
from pvlib import location
from pvlib import irradiance
from pvlib import tracking
from pvlib.iotools import read_tmy3
import pandas as pd
from matplotlib import pyplot as plt
import pathlib

# get full path to the data directory
DATA_DIR = pathlib.Path(pvlib.__file__).parent / 'data'

# get TMY3 dataset
tmy, metadata = read_tmy3(DATA_DIR / '723170TYA.CSV', coerce_year=1990,
                          map_variables=True)
# TMY3 datasets are right-labeled (AKA "end of interval") which means the last
# interval of Dec 31, 23:00 to Jan 1 00:00 is labeled Jan 1 00:00. When rolling
# up hourly irradiance to monthly insolation, a spurious January value is
# calculated from that last row, so we'll just go ahead and drop it here:
tmy = tmy.iloc[:-1, :]

# create location object to store lat, lon, timezone
location = location.Location.from_tmy(metadata)

# calculate the necessary variables to do transposition.  Note that solar
# position doesn't depend on array orientation, so we just calculate it once.
# Note also that TMY datasets are right-labeled hourly intervals, e.g. the
# 10AM to 11AM interval is labeled 11.  We should calculate solar position in
# the middle of the interval (10:30), so we subtract 30 minutes:
times = tmy.index - pd.Timedelta('30min')
solar_position = location.get_solarposition(times)
# but remember to shift the index back to line up with the TMY data:
solar_position.index += pd.Timedelta('30min')


# create a helper function to do the transposition for us
def calculate_poa(tmy, solar_position, surface_tilt, surface_azimuth):
    # Use the get_total_irradiance function to transpose the irradiance
    # components to POA irradiance
    poa = irradiance.get_total_irradiance(
        surface_tilt=surface_tilt,
        surface_azimuth=surface_azimuth,
        dni=tmy['dni'],
        ghi=tmy['ghi'],
        dhi=tmy['dhi'],
        solar_zenith=solar_position['apparent_zenith'],
        solar_azimuth=solar_position['azimuth'],
        model='isotropic')
    return poa['poa_global']  # just return the total in-plane irradiance


# create a dataframe to keep track of our monthly insolations
df_monthly = pd.DataFrame()

# fixed-tilt:
for tilt in range(0, 50, 10):
    # we will hardcode azimuth=180 (south) for all fixed-tilt cases
    poa_irradiance = calculate_poa(tmy, solar_position, tilt, 180)
    column_name = f"FT-{tilt}"
    # TMYs are hourly, so we can just sum up irradiance [W/m^2] to get
    # insolation [Wh/m^2]:
    df_monthly[column_name] = poa_irradiance.resample('m').sum()

# single-axis tracking:
orientation = tracking.singleaxis(solar_position['apparent_zenith'],
                                  solar_position['azimuth'],
                                  axis_slope=0,  # flat array
                                  axis_azimuth=180,  # south-facing azimuth
                                  max_angle=60,  # a common maximum rotation
                                  backtrack=True,  # backtrack for a c-Si array
                                  gcr=0.4)  # a common ground coverage ratio

poa_irradiance = calculate_poa(tmy,
                               solar_position,
                               orientation['surface_tilt'],
                               orientation['surface_azimuth'])
df_monthly['SAT-0.4'] = poa_irradiance.resample('m').sum()

# calculate the percent difference from GHI
ghi_monthly = tmy['ghi'].resample('m').sum()
df_monthly = 100 * (df_monthly.divide(ghi_monthly, axis=0) - 1)

df_monthly.plot()
plt.xlabel('Month of Year')
plt.ylabel('Monthly Transposition Gain [%]')
plt.tight_layout()
plt.show()


# %%
# Note that in summer, steeper tilts actually collect less insolation than
# flatter tilts because the sun is so high in the sky at solar noon.  However,
# the steeper tilts significantly increase insolation capture in winter when
# the sun is lower in the sky.  In contrast to the highly seasonal gain shown
# by fixed tilts, the tracker system shows a much more consistent gain
# year-round.
#
# Because the seasonality of the fixed-tilt transposition gain is driven by
# solar position angles, the relative behavior of different orientations will
# be different for different locations.  For example, a common rule of thumb
# (considered somewhat outdated today) used to be to set tilt equal to the
# latitude of the system location.  At higher latitudes, the sun doesn't get
# as high in the sky, so steeper tilts make more sense.