Infer thresholds for detect_clearsky

pvlib/clearsky.py

@@ -644,8 +644,30 @@ def _clear_sample_index(clear_windows, samples_per_window, align, H):
     return clear_samples
 
 
-def detect_clearsky(measured, clearsky, times=None, window_length=10,
-                    mean_diff=75, max_diff=75,
+def _clearsky_get_threshold(sample_interval):
+    """
+    Returns threshold values for kwargs in detect_clearsky
+    """
+    if (sample_interval < 1 or sample_interval > 30):
+        raise ValueError("sample_interval must be larger than 1 and \
+                          smaller than 30")
+
+    data_freq = np.array([1, 5, 15, 30])
+
+    window_length = np.interp(sample_interval, data_freq, [50, 60, 90, 120])
+    mean_diff = np.interp(sample_interval, data_freq, [75, 75, 75, 75])
+    max_diff = np.interp(sample_interval, data_freq, [60, 65, 75, 90])
+    lower_line_length = np.interp(sample_interval, data_freq, [-45,-45,-45,-45])
+    upper_line_length = np.interp(sample_interval, data_freq, [80, 80, 80, 80])
+    var_diff = np.interp(sample_interval, data_freq, [0.005, 0.01, 0.032, 0.07])
+    slope_dev = np.interp(sample_interval, data_freq, [50, 60, 75, 96])
+
+    return (window_length, mean_diff, max_diff, lower_line_length,
+            upper_line_length, var_diff, slope_dev)
+
+
+def detect_clearsky(measured, clearsky, times=None, infer_limits=False,
+                    window_length=10, mean_diff=75, max_diff=75,
                     lower_line_length=-5, upper_line_length=10,
                     var_diff=0.005, slope_dev=8, max_iterations=20,
                     return_components=False):
@@ -676,6 +698,9 @@ def detect_clearsky(measured, clearsky, times=None, window_length=10,
     times : DatetimeIndex or None, default None.
         Times of measured and clearsky values. If None the index of measured
         will be used.
+    infer_limits : bool, default False
+        If True, does not use passed in kwargs (or defaults), but instead
+        interpolates these values from Table 1 in [2].
     window_length : int, default 10
         Length of sliding time window in minutes. Must be greater than 2
         periods.
@@ -731,6 +756,9 @@ def detect_clearsky(measured, clearsky, times=None, window_length=10,
     .. [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.
+    .. [2] Jordan, D.C. and C. Hansen, "Clear-sky detection for PV
+       degradation analysis using multiple regression", Renewable Energy,
+       v209, p. 393-400, 2023.
 
     Notes
     -----
@@ -773,6 +801,17 @@ def detect_clearsky(measured, clearsky, times=None, window_length=10,
     sample_interval, samples_per_window = \
         tools._get_sample_intervals(times, window_length)
 
+    # if infer_limits, find threshold values using the sample interval
+    if infer_limits:
+        window_length, mean_diff, max_diff, lower_line_length, \
+            upper_line_length, var_diff, slope_dev = \
+                _clearsky_get_threshold(sample_interval)
+
+        # recalculate sample_interval, samples_per_window using returned
+        # window_length
+        sample_interval, samples_per_window = \
+            tools._get_sample_intervals(times, window_length)
+
     # generate matrix of integers for creating windows with indexing
     H = hankel(np.arange(samples_per_window),
                np.arange(samples_per_window-1, len(times)))