Calculate Relative Humidity via Magnus Tetens Equation

pvlib/spectrum/magnus_tetens.py

@@ -0,0 +1,108 @@
+import numpy as np
+
+
+def magnus_tetens_aekr(temperature, dewpoint, A=6.112, B=17.62, C=243.12):
+    """
+    Calculate relative humidity using Magnus equation with AEKR coefficients.
+    This function was used by First Solar in creating their spectral model
+    and is therefore relevant to the first solar spectral model in pvlib.
+    Default magnus equation coefficients are from [2].
+
+    Parameters
+    ----------
+    temperature : pd.Series
+        Air temperature in degrees Celsius
+    dewpoint : pd.Series
+        Dewpoint temperature in degrees Celsius
+    A: float
+        Magnus equation coefficient A
+    B: float
+        Magnus equation coefficient B
+    C: float
+        Magnus equation coefficient C
+
+    Returns
+    -------
+    pd.Series
+        Relative humidity as percentage (0-100)
+
+    Notes
+    -----
+    Uses the AEKR coefficients which minimize errors between -40 and
+    50 degrees C according to reference [1].
+
+    References
+    ----------
+    .. [1] https://www.osti.gov/servlets/purl/548871-PjpxAP/webviewable/
+    .. [2] https://www.schweizerbart.de//papers/metz/detail/3/89544/Advancements_in_the_field_of_hygrometry?af=crossref
+    """
+
+    # Calculate vapor pressure (e) and saturation vapor pressure (es)
+    e = A * np.exp((B * temperature) / (C + temperature))
+    es = A * np.exp((B * dewpoint) / (C + dewpoint))
+
+    # Calculate relative humidity as percentage
+    relative_humidity = 100 * (es / e)
+
+    return relative_humidity
+
+
+def reverse_magnus_tetens_aekr(
+    temperature, relative_humidity, B=17.62, C=243.12
+):
+    """
+    Calculate dewpoint temperature using Magnus equation with
+    AEKR coefficients.  This is just a reversal of the calculation
+    in calculate_relative_humidity.
+
+    Parameters
+    ----------
+    temperature : pd.Series
+        Air temperature in degrees Celsius
+    relative_humidity : pd.Series
+        Relative humidity as percentage (0-100)
+
+    Returns
+    -------
+    pd.Series
+        Dewpoint temperature in degrees Celsius
+
+    Notes
+    -----
+    Derived by solving the Magnus equation for dewpoint given
+    relative humidity.
+    Valid for temperatures between -40 and 50 degrees C.
+
+    References
+    ----------
+    .. [1] https://www.osti.gov/servlets/purl/548871-PjpxAP/webviewable/
+    """
+    # Calculate the term inside the log
+    # From RH = 100 * (es/e), we get es = (RH/100) * e
+    # Substituting the Magnus equation and solving for dewpoint
+
+    # First calculate ln(es/A)
+    ln_term = (
+        (B * temperature) / (C + temperature)
+        + np.log(relative_humidity/100)
+    )
+
+    # Then solve for dewpoint
+    dewpoint = C * ln_term / (B - ln_term)
+
+    return dewpoint
+
+
+if __name__ == "__main__":
+    import pandas as pd
+    rh = magnus_tetens_aekr(
+        temperature=pd.Series([20.0, 25.0, 30.0, 15.0, 10.0]),
+        dewpoint=pd.Series([15.0, 20.0, 25.0, 12.0, 8.0])
+    )
+
+    dewpoint = reverse_magnus_tetens_aekr(
+        temperature=pd.Series([20.0, 25.0, 30.0, 15.0, 10.0]),
+        relative_humidity=rh
+    )
+    print(rh)
+    print(dewpoint)

pvlib/tests/conftest.py

@@ -474,3 +474,22 @@
                   'IXXO': 3.18803,
                   'FD': 1}
     return parameters
+
+
+@pytest.fixture(scope='function')
+def spectrum_temperature():
+    temperature = pd.Series([20.0, 25.0, 30.0, 15.0, 10.0])
+    return temperature
+
+
+@pytest.fixture(scope='function')
+def spectrum_dewpoint():
+    dewpoint = pd.Series([15.0, 20.0, 25.0, 12.0, 8.0])
+    return dewpoint
+
+@pytest.fixture(scope='function')
+def spectrum_relative_humidity():
+    relative_humidity = pd.Series([
+        72.938767, 73.802512, 74.628205, 82.261353, 87.383237
+    ])
+    return relative_humidity

pvlib/tests/spectrum/test_magnus_tetens.py

@@ -0,0 +1,47 @@
+import pandas as pd
+from pvlib.spectrum.magnus_tetens import (
+    magnus_tetens_aekr,
+    reverse_magnus_tetens_aekr
+)
+
+
+# Unit tests
+def test_magnus_tetens_aekr(
+    spectrum_temperature, spectrum_dewpoint, spectrum_relative_humidity
+):
+    """
+    Test the magnus_tetens_aekr function with sample data.
+    """
+
+    # Calculate relative humidity
+    rh = magnus_tetens_aekr(
+        temperature=spectrum_temperature,
+        dewpoint=spectrum_dewpoint
+    )
+
+    # test
+    pd.testing.assert_series_equal(
+        rh,
+        spectrum_relative_humidity,
+        check_names=False
+    )
+
+
+# Unit tests
+def test_reverse_magnus_tetens_aekr(
+    spectrum_temperature, spectrum_dewpoint, spectrum_relative_humidity
+):
+    """
+    Test the reverse_magnus_tetens_aekr function with sample data.
+    """
+
+    # Calculate relative humidity
+    dewpoint = reverse_magnus_tetens_aekr(
+        temperature=spectrum_temperature,
+        relative_humidity=spectrum_relative_humidity
+    )
+
+    # test
+    pd.testing.assert_series_equal(
+        dewpoint, spectrum_dewpoint, check_names=False
+    )

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "setuptools.build_meta"
 
 
 [project]
-name = "pvlib"
+name = "pvlib-branch"
 description = "A set of functions and classes for simulating the performance of photovoltaic energy systems."
 authors = [
     { name = "pvlib python Developers", email = "[email protected]" },
@@ -17,6 +17,7 @@ dependencies = [
     'requests',
     'scipy >= 1.6.0',
     'h5py',
+    "flake8>=7.1.1",
 ]
 license = { text = "BSD-3-Clause" }
 classifiers = [
@@ -56,7 +57,7 @@ optional = [
 ]
 doc = [
     'ipython',
-    'pickleshare',  # required by ipython
+    'pickleshare',                   # required by ipython
     'matplotlib',
     'sphinx == 7.3.7',
     'pydata-sphinx-theme == 0.15.4',