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Clarify statistics for detect_clearsky #1243

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cwhanse merged 3 commits into from
Jun 9, 2021
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Clarify statistics for detect_clearsky #1243

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9975abb
add comments to _calc_stats
cwhanse Jun 4, 2021
ba438a7
lint, whatsnew
cwhanse Jun 4, 2021
e7fe0d8
Update docs/sphinx/source/whatsnew/v0.9.0.rst
cwhanse Jun 9, 2021
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2 changes: 2 additions & 0 deletions docs/sphinx/source/whatsnew/v0.9.0.rst
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Expand Up @@ -181,6 +181,8 @@ Documentation
* Add a gallery example showing how to appropriately use interval-averaged
weather data for modeling. (:pull:`1152`)
* Update documentation links in :py:func:`pvlib.iotools.get_psm3`
* Clarified how statistics are calculated for :py:func:`pvlib.clearsky.detect_clearsky`
(:issue:`1070`, :pull:`1243`)

Requirements
~~~~~~~~~~~~
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21 changes: 20 additions & 1 deletion pvlib/clearsky.py
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Expand Up @@ -602,7 +602,20 @@ def _calc_d(aod700, p):
def _calc_stats(data, samples_per_window, sample_interval, H):
""" Calculates statistics for each window, used by Reno-style clear
sky detection functions. Does not return the line length statistic
which is provided by _calc_windowed_stat and _line_length
which is provided by _calc_windowed_stat and _line_length.

Calculations are done on a sliding window defined by the Hankel matrix H.
Columns in H define the indices for each window. Each window contains
samples_per_window index values. The first window starts with index 0;
the last window ends at the last index position in data.

In the calculation of data_slope_nstd, a choice is made here where [1]_ is
ambiguous. data_slope_nstd is the standard deviation of slopes divided by
the mean GHI for each interval; see [1]_ Eq. 11. For intervals containing
e.g. 10 values, there are 9 slope values in the standard deviation, and the
mean is calculated using all 10 values. Eq. 11 in [1]_ is ambiguous if
the mean should be calculated using 9 points (left ends of each slope)
or all 10 points.

Parameters
----------
Expand All @@ -624,6 +637,12 @@ def _calc_stats(data, samples_per_window, sample_interval, H):
standard deviation of difference between data points in each window
data_slope : Series
difference between successive data points

References
----------
.. [1] Reno, M.J. and C.W. Hansen, "Identification of periods of clear
sky irradiance in time series of GHI measurements" Renewable Energy,
v90, p. 520-531, 2016.
"""

data_mean = data.values[H].mean(axis=0)
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