Function to estimate wind speed at different heights (#2124)

* wind_speed_at_different_heights * minor fixed * fixed toc and added test * edit math mode * changed link for reference * Apply suggestions from code review Co-authored-by: RDaxini <[email protected]> Co-authored-by: Echedey Luis <[email protected]> * Update pvlib/tests/test_windspeed.py Co-authored-by: Echedey Luis <[email protected]> * changed function name * Update pvlib/windspeed.py Co-authored-by: Echedey Luis <[email protected]> * change measured height to reference * moved wind speed to atmosphere * minor formatting fixes * update test parameter names * updated error message * fixed error message vol.2 * updated tests * Update v0.11.1.rst * changed nan condition * changed function name * Apply suggestions from code review Co-authored-by: Cliff Hansen <[email protected]> Co-authored-by: Adam R. Jensen <[email protected]> * changed name (again) * condition fix * corrected the Sandia wording --------- Co-authored-by: RDaxini <[email protected]> Co-authored-by: Echedey Luis <[email protected]> Co-authored-by: Cliff Hansen <[email protected]> Co-authored-by: Adam R. Jensen <[email protected]>
pvlib · Aug 5, 2024 · f6b1d2a · f6b1d2a
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diff --git a/docs/sphinx/source/reference/airmass_atmospheric.rst b/docs/sphinx/source/reference/airmass_atmospheric.rst
@@ -17,3 +17,4 @@ Airmass and atmospheric models
    atmosphere.kasten96_lt
    atmosphere.angstrom_aod_at_lambda
    atmosphere.angstrom_alpha
+   atmosphere.windspeed_powerlaw
diff --git a/docs/sphinx/source/whatsnew/v0.11.1.rst b/docs/sphinx/source/whatsnew/v0.11.1.rst
@@ -17,6 +17,9 @@ Enhancements
   :py:func:`pvlib.spectrum.spectral_factor_firstsolar`.
   (:issue:`2086`, :pull:`2100`)
 
+* Added function for calculating wind speed at different heights,
+  :py:func:`pvlib.atmosphere.windspeed_powerlaw`.
+  (:issue:`2118`, :pull:`2124`)
 
 Bug fixes
 ~~~~~~~~~
@@ -45,4 +48,3 @@ Contributors
 * Leonardo Micheli (:ghuser:`lmicheli`)
 * Echedey Luis (:ghuser:`echedey-ls`)
 * Rajiv Daxini (:ghuser:`RDaxini`)
-
diff --git a/pvlib/atmosphere.py b/pvlib/atmosphere.py
@@ -1,6 +1,7 @@
 """
 The ``atmosphere`` module contains methods to calculate relative and
-absolute airmass and to determine pressure from altitude or vice versa.
+absolute airmass, determine pressure from altitude or vice versa, and wind
+speed at different heights.
 """
 
 import numpy as np
@@ -533,3 +534,158 @@ def angstrom_alpha(aod1, lambda1, aod2, lambda2):
     pvlib.atmosphere.angstrom_aod_at_lambda
     """
     return - np.log(aod1 / aod2) / np.log(lambda1 / lambda2)
+
+
+# Values of the Hellmann exponent
+HELLMANN_SURFACE_EXPONENTS = {
+    'unstable_air_above_open_water_surface': 0.06,
+    'neutral_air_above_open_water_surface': 0.10,
+    'stable_air_above_open_water_surface': 0.27,
+    'unstable_air_above_flat_open_coast': 0.11,
+    'neutral_air_above_flat_open_coast': 0.16,
+    'stable_air_above_flat_open_coast': 0.40,
+    'unstable_air_above_human_inhabited_areas': 0.27,
+    'neutral_air_above_human_inhabited_areas': 0.34,
+    'stable_air_above_human_inhabited_areas': 0.60,
+}
+
+
+def windspeed_powerlaw(wind_speed_reference, height_reference,
+                       height_desired, exponent=None,
+                       surface_type=None):
+    r"""
+    Estimate wind speed for different heights.
+
+    The model is based on the power law equation by Hellmann [1]_ [2]_.
+
+    Parameters
+    ----------
+    wind_speed_reference : numeric
+        Measured wind speed. [m/s]
+
+    height_reference : float
+        The height above ground at which the wind speed is measured. [m]
+
+    height_desired : float
+        The height above ground at which the wind speed will be estimated. [m]
+
+    exponent : float, optional
+        Exponent based on the surface type. [unitless]
+
+    surface_type : string, optional
+        If supplied, overrides ``exponent``. Can be one of the following
+        (see [1]_):
+
+        * ``'unstable_air_above_open_water_surface'``
+        * ``'neutral_air_above_open_water_surface'``
+        * ``'stable_air_above_open_water_surface'``
+        * ``'unstable_air_above_flat_open_coast'``
+        * ``'neutral_air_above_flat_open_coast'``
+        * ``'stable_air_above_flat_open_coast'``
+        * ``'unstable_air_above_human_inhabited_areas'``
+        * ``'neutral_air_above_human_inhabited_areas'``
+        * ``'stable_air_above_human_inhabited_areas'``
+
+    Returns
+    -------
+    wind_speed : numeric
+        Adjusted wind speed for the desired height. [m/s]
+
+    Raises
+    ------
+    ValueError
+        If neither of ``exponent`` nor a ``surface_type`` is given.
+        If both ``exponent`` and a ``surface_type`` is given. These parameters
+        are mutually exclusive.
+
+    KeyError
+        If the specified ``surface_type`` is invalid.
+
+    Notes
+    -----
+    Module temperature functions often require wind speeds at a height of 10 m
+    and not the wind speed at the module height.
+
+    For example, the following temperature functions require the input wind
+    speed to be 10 m: :py:func:`~pvlib.temperature.sapm_cell`, and
+    :py:func:`~pvlib.temperature.sapm_module` whereas the
+    :py:func:`~pvlib.temperature.fuentes` model requires wind speed at 9.144 m.
+
+    Additionally, the heat loss coefficients of some models have been developed
+    for wind speed measurements at 10 m (e.g.,
+    :py:func:`~pvlib.temperature.pvsyst_cell`,
+    :py:func:`~pvlib.temperature.faiman`, and
+    :py:func:`~pvlib.temperature.faiman_rad`).
+
+    The equation for calculating the wind speed at a height of :math:`h` is
+    given by the following power law equation [1]_ [2]_:
+
+    .. math::
+       :label: wind speed
+
+        WS_{h} = WS_{ref} \cdot \left( \frac{h}{h_{ref}} \right)^a
+
+    where :math:`h` [m] is the height at which we would like to calculate the
+    wind speed, :math:`h_{ref}` [m] is the reference height at which the wind
+    speed is known, and :math:`WS_{h}` [m/s] and :math:`WS_{ref}`
+    [m/s] are the corresponding wind speeds at these heights. The exponent
+    :math:`a` [unitless] depends on the surface type. Some values found in the
+    literature [1]_ for :math:`a` are:
+
+    .. table:: Values for the Hellmann-exponent
+
+       +-----------+--------------------+------------------+------------------+
+       | Stability | Open water surface | Flat, open coast | Cities, villages |
+       +===========+====================+==================+==================+
+       | Unstable  | 0.06               | 0.10             | 0.27             |
+       +-----------+--------------------+------------------+------------------+
+       | Neutral   | 0.11               | 0.16             | 0.40             |
+       +-----------+--------------------+------------------+------------------+
+       | Stable    | 0.27               | 0.34             | 0.60             |
+       +-----------+--------------------+------------------+------------------+
+
+    In a report by Sandia [3]_, the equation was experimentally tested for a
+    height of 30 ft (:math:`h_{ref} = 9.144` [m]) at their test site in
+    Albuquerque for a period of six weeks where a coefficient of
+    :math:`a = 0.219` was calculated.
+
+    It should be noted that the equation returns a value of NaN if the
+    reference heights or wind speed are negative.
+
+    References
+    ----------
+    .. [1] Kaltschmitt M., Streicher W., Wiese A. (2007). "Renewable Energy:
+       Technology, Economics and Environment." Springer,
+       :doi:`10.1007/3-540-70949-5`.
+
+    .. [2] Hellmann G. (1915). "Über die Bewegung der Luft in den untersten
+       Schichten der Atmosphäre." Meteorologische Zeitschrift, 32
+
+    .. [3] Menicucci D.F., Hall I.J. (1985). "Estimating wind speed as a
+       function of height above ground: An analysis of data obtained at the
+       southwest residential experiment station, Las Cruses, New Mexico."
+       SAND84-2530, Sandia National Laboratories.
+       Accessed at:
+       https://web.archive.org/web/20230418202422/https://www2.jpl.nasa.gov/adv_tech/photovol/2016CTR/SNL%20-%20Est%20Wind%20Speed%20vs%20Height_1985.pdf
+    """  # noqa:E501
+    if surface_type is not None and exponent is None:
+        # use the Hellmann exponent from dictionary
+        exponent = HELLMANN_SURFACE_EXPONENTS[surface_type]
+    elif surface_type is None and exponent is not None:
+        # use the provided exponent
+        pass
+    else:
+        raise ValueError(
+            "Either a 'surface_type' or an 'exponent' parameter must be given")
+
+    wind_speed = wind_speed_reference * (
+        (height_desired / height_reference) ** exponent)
+
+    # if wind speed is negative or complex return NaN
+    wind_speed = np.where(np.iscomplex(wind_speed) | (wind_speed < 0),
+                          np.nan, wind_speed)
+
+    if isinstance(wind_speed_reference, pd.Series):
+        wind_speed = pd.Series(wind_speed, index=wind_speed_reference.index)
+
+    return wind_speed
diff --git a/pvlib/tests/test_atmosphere.py b/pvlib/tests/test_atmosphere.py
@@ -131,3 +131,74 @@ def test_bird_hulstrom80_aod_bb():
     aod380, aod500 = 0.22072480948195175, 0.1614279181106312
     bird_hulstrom = atmosphere.bird_hulstrom80_aod_bb(aod380, aod500)
     assert np.isclose(0.11738229553812768, bird_hulstrom)
+
+
+@pytest.fixture
+def windspeeds_data_powerlaw():
+    data = pd.DataFrame(
+        index=pd.date_range(start="2015-01-01 00:00", end="2015-01-01 05:00",
+                            freq="1h"),
+        columns=["wind_ref", "height_ref", "height_desired", "wind_calc"],
+        data=[
+            (10, -2, 5, np.nan),
+            (-10, 2, 5, np.nan),
+            (5, 4, 5, 5.067393209486324),
+            (7, 6, 10, 7.2178684911195905),
+            (10, 8, 20, 10.565167835216586),
+            (12, 10, 30, 12.817653329393977)
+        ]
+    )
+    return data
+
+
+def test_windspeed_powerlaw_ndarray(windspeeds_data_powerlaw):
+    # test wind speed estimation by passing in surface_type
+    result_surface = atmosphere.windspeed_powerlaw(
+        windspeeds_data_powerlaw["wind_ref"].to_numpy(),
+        windspeeds_data_powerlaw["height_ref"],
+        windspeeds_data_powerlaw["height_desired"],
+        surface_type='unstable_air_above_open_water_surface')
+    assert_allclose(windspeeds_data_powerlaw["wind_calc"].to_numpy(),
+                    result_surface)
+    # test wind speed estimation by passing in the exponent corresponding
+    # to the surface_type above
+    result_exponent = atmosphere.windspeed_powerlaw(
+        windspeeds_data_powerlaw["wind_ref"].to_numpy(),
+        windspeeds_data_powerlaw["height_ref"],
+        windspeeds_data_powerlaw["height_desired"],
+        exponent=0.06)
+    assert_allclose(windspeeds_data_powerlaw["wind_calc"].to_numpy(),
+                    result_exponent)
+
+
+def test_windspeed_powerlaw_series(windspeeds_data_powerlaw):
+    result = atmosphere.windspeed_powerlaw(
+        windspeeds_data_powerlaw["wind_ref"],
+        windspeeds_data_powerlaw["height_ref"],
+        windspeeds_data_powerlaw["height_desired"],
+        surface_type='unstable_air_above_open_water_surface')
+    assert_series_equal(windspeeds_data_powerlaw["wind_calc"],
+                        result, check_names=False)
+
+
+def test_windspeed_powerlaw_invalid():
+    with pytest.raises(ValueError, match="Either a 'surface_type' or an "
+                       "'exponent' parameter must be given"):
+        # no exponent or surface_type given
+        atmosphere.windspeed_powerlaw(wind_speed_reference=10,
+                                      height_reference=5,
+                                      height_desired=10)
+    with pytest.raises(ValueError, match="Either a 'surface_type' or an "
+                       "'exponent' parameter must be given"):
+        # no exponent or surface_type given
+        atmosphere.windspeed_powerlaw(wind_speed_reference=10,
+                                      height_reference=5,
+                                      height_desired=10,
+                                      exponent=1.2,
+                                      surface_type="surf")
+    with pytest.raises(KeyError, match='not_an_exponent'):
+        # invalid surface_type
+        atmosphere.windspeed_powerlaw(wind_speed_reference=10,
+                                      height_reference=5,
+                                      height_desired=10,
+                                      surface_type='not_an_exponent')