Add model='gueymard2003' to get_relative_airmass (#1655)

* add new equation to get_relative_airmass

* test

* whatsnew

* remove duplicate whatsnew entry

* true -> apparent

* fix merge error

* Apply suggestions from code review

Co-authored-by: Adam R. Jensen <[email protected]>

---------

Co-authored-by: Adam R. Jensen <[email protected]>

Author: kandersolar

docs/sphinx/source/whatsnew/v0.9.5.rst

@@ -20,6 +20,9 @@ Enhancements
   :py:func:`pvlib.snow.loss_townsend` (:issue:`1636`, :pull:`1653`)
 * Added optional ``n_ar`` parameter to :py:func:`pvlib.iam.physical` to
   support an anti-reflective coating. (:issue:`1501`, :pull:`1616`)
+* Add ``model='gueymard2003'``, the airmass model used for REST and REST2,
+  to :py:func:`~pvlib.atmosphere.get_relative_airmass`. (:pull:`1655`)
+
 
 * Added an optional ``model`` parameter to
   :py:func:`pvlib.bifacial.infinite_sheds.get_irradiance` and

pvlib/atmosphere.py

@@ -158,9 +158,11 @@ def get_relative_airmass(zenith, model='kastenyoung1989'):
         * 'gueymard1993' - See reference [4] -
           requires apparent sun zenith
         * 'young1994' - See reference [5] -
-          requries true sun zenith
+          requires true sun zenith
         * 'pickering2002' - See reference [6] -
           requires apparent sun zenith
+        * 'gueymard2003' - See references [7] and [8] -
+          requires apparent sun zenith
 
     Returns
     -------
@@ -196,7 +198,17 @@ def get_relative_airmass(zenith, model='kastenyoung1989'):
 
     .. [6] Keith A. Pickering. "The Ancient Star Catalog". DIO 12:1, 20,
 
-    .. [7] Matthew J. Reno, Clifford W. Hansen and Joshua S. Stein, "Global
+    .. [7] C. Gueymard, "Direct solar transmittance and irradiance
+       predictions with broadband models. Part I: detailed theoretical
+       performance assessment". Solar Energy, vol 74, pp. 355-379, 2003.
+       :doi:`10.1016/S0038-092X(03)00195-6`
+
+    .. [8] C. Gueymard (2019). Clear-Sky Radiation Models and Aerosol Effects.
+       In: Polo, J., Martín-Pomares, L., Sanfilippo, A. (eds) Solar Resources
+       Mapping. Green Energy and Technology. Springer, Cham.
+       :doi:`10.1007/978-3-319-97484-2_5`
+
+    .. [9] Matthew J. Reno, Clifford W. Hansen and Joshua S. Stein, "Global
        Horizontal Irradiance Clear Sky Models: Implementation and Analysis"
        Sandia Report, (2012).
     '''
@@ -229,6 +241,9 @@ def get_relative_airmass(zenith, model='kastenyoung1989'):
     elif 'gueymard1993' == model:
         am = (1.0 / (np.cos(zenith_rad) +
               0.00176759*(z)*((94.37515 - z) ** - 1.21563)))
+    elif 'gueymard2003' == model:
+        am = (1.0 / (np.cos(zenith_rad) +
+              0.48353*(z**0.095846)/(96.741 - z)**1.754))
     else:
         raise ValueError('%s is not a valid model for relativeairmass', model)
 

pvlib/tests/test_atmosphere.py

@@ -35,7 +35,8 @@ def zeniths():
                           ['kastenyoung1989', [nan, 36.467,  5.586,  1.000]],
                           ['gueymard1993', [nan, 36.431,  5.581,  1.000]],
                           ['young1994', [nan, 30.733,  5.541,  1.000]],
-                          ['pickering2002', [nan, 37.064,  5.581,  1.000]]])
+                          ['pickering2002', [nan, 37.064,  5.581,  1.000]],
+                          ['gueymard2003', [nan, 36.676, 5.590, 1.000]]])
 def test_airmass(model, expected, zeniths):
     out = atmosphere.get_relative_airmass(zeniths, model)
     expected = np.array(expected)