initial implementation of pvsyst_celltemp function (#552) (#628)

* initial implementation of pvsyst_celltemp function (#552)

* Reformat for stickler-ci

* Add error tests for pvsyst_celltemp

* Remove unused variables per stickler-ci

* Add pvsystem.pvsyst_celltemp function to docs

* correct api title

* formatting changes per wholmgren and cwhanse

* Additional formatting improvements

* Fix symbol in Reference

Author: CameronTStark

docs/sphinx/source/api.rst

@@ -268,6 +268,15 @@ PVWatts model
    pvsystem.pvwatts_dc
    pvsystem.pvwatts_ac
    pvsystem.pvwatts_losses
+   pvsystem.pvwatts_losses
+
+PVsyst model
+------------
+
+.. autosummary::
+   :toctree: generated/
+
+   pvsystem.pvsyst_celltemp
 
 Other
 -----

docs/sphinx/source/whatsnew/v0.6.1.rst

@@ -57,6 +57,7 @@ Enhancements
 * Add warning message when :py:func:`pvlib.spa` is reloaded. (:issue:`401`)
 * Add option for :py:func:`pvlib.irradiance.disc` to use relative airmass
   by supplying `pressure=None`. (:issue:`449`)
+* Created :py:func:`pvlib.pvsystem.pvsyst_celltemp` to implement PVsyst's cell temperature model. (:issue:`552`)
 
 
 Bug fixes
@@ -86,4 +87,5 @@ Contributors
 * Cliff Hansen (:ghuser:`cwhanse`)
 * Mark Mikofski (:ghuser:`mikofski`)
 * Anton Driesse (:ghuser:`adriesse`)
+* Cameron Stark (:ghuser:`camerontstark`)
 * Jonathan Gaffiot (:ghuser:`jgaffiot`)

pvlib/pvsystem.py

@@ -1887,6 +1887,87 @@ def sapm_celltemp(poa_global, wind_speed, temp_air,
     return pd.DataFrame({'temp_cell': temp_cell, 'temp_module': temp_module})
 
 
+def pvsyst_celltemp(poa_global, wind_speed, temp_air, eta_m=0.1,
+                    alpha_absorption=0.9, temp_model="freestanding"):
+    """
+    Calculate cell temperature using the PVSyst model.
+
+    Parameters
+    ----------
+    poa_global : numeric
+        Total incident irradiance in W/m^2.
+
+    wind_speed : numeric
+        Wind speed in m/s at a height of 10 meters.
+
+    temp_air : numeric
+        Ambient dry bulb temperature in degrees C.
+
+    eta_m : numeric
+        Module external efficiency as a fraction, i.e., DC power / poa_global.
+
+    alpha_absorption : float
+        Absorption coefficient, default is 0.9.
+
+    temp_model : string, tuple, or list, default 'freestanding' (no dict)
+        Model to be used.
+
+        If string, can be:
+
+            * 'freestanding' (default)
+                Modules with rear surfaces exposed to open air (e.g. rack
+                mounted).
+            * 'insulated'
+                Modules with rear surfaces in close proximity to another
+                surface (e.g. roof mounted).
+
+        If tuple/list, supply parameters in the following order:
+
+            * natural_convenction_coeff : float
+                Natural convection coefficient. Freestanding default is 29,
+                fully insulated arrays is 15.
+
+            * forced_convection_coeff : float
+                Forced convection coefficient, default is 0.
+
+    Returns
+    -------
+    temp_cell : numeric or Series
+        Cell temperature in degrees Celsius
+
+    References
+    ----------
+    [1]"PVsyst 6 Help", Files.pvsyst.com, 2018. [Online]. Available:
+    http://files.pvsyst.com/help/index.html. [Accessed: 10- Dec- 2018].
+
+    [2] Faiman, D. (2008). "Assessing the outdoor operating temperature of
+    photovoltaic modules." Progress in Photovoltaics 16(4): 307-315.
+    """
+
+    temp_models = {"freestanding": (29.0, 0), "insulated": (15.0, 0)}
+
+    if isinstance(temp_model, str):
+        natural_convenction_coeff, forced_convection_coeff = temp_models[
+            temp_model.lower()
+        ]
+    elif isinstance(temp_model, (tuple, list)):
+        natural_convenction_coeff, forced_convection_coeff = temp_model
+    else:
+        raise TypeError(
+            "Please format temp_model as a str, or tuple/list."
+        )
+
+    combined_convection_coeff = (
+        forced_convection_coeff * wind_speed
+    ) + natural_convenction_coeff
+
+    absorption_coeff = alpha_absorption * poa_global * (1 - eta_m)
+    temp_difference = absorption_coeff / combined_convection_coeff
+    temp_cell = temp_air + temp_difference
+
+    return temp_cell
+
+
 def sapm_spectral_loss(airmass_absolute, module):
     """
     Calculates the SAPM spectral loss coefficient, F1.

pvlib/test/test_pvsystem.py

@@ -1078,6 +1078,46 @@ def test_PVSystem_sapm_celltemp(mocker):
     assert out.shape == (1, 2)
 
 
+def test_pvsyst_celltemp_default():
+    default = pvsystem.pvsyst_celltemp(900, 5, 20, 0.1)
+    assert_allclose(default, 45.137, 0.001)
+
+
+def test_pvsyst_celltemp_non_model():
+    tup_non_model = pvsystem.pvsyst_celltemp(900, 5, 20, 0.1,
+                                             temp_model=(23.5, 6.25))
+    assert_allclose(tup_non_model, 33.315, 0.001)
+
+    list_non_model = pvsystem.pvsyst_celltemp(900, 5, 20, 0.1,
+                                              temp_model=[26.5, 7.68])
+    assert_allclose(list_non_model, 31.233, 0.001)
+
+
+def test_pvsyst_celltemp_model_wrong_type():
+    with pytest.raises(TypeError):
+        pvsystem.pvsyst_celltemp(
+            900, 5, 20, 0.1,
+            temp_model={"won't": 23.5, "work": 7.68})
+
+
+def test_pvsyst_celltemp_model_non_option():
+    with pytest.raises(KeyError):
+        pvsystem.pvsyst_celltemp(
+            900, 5, 20, 0.1,
+            temp_model="not_an_option")
+
+
+def test_pvsyst_celltemp_with_index():
+    times = pd.DatetimeIndex(start="2015-01-01", end="2015-01-02", freq="12H")
+    temps = pd.Series([0, 10, 5], index=times)
+    irrads = pd.Series([0, 500, 0], index=times)
+    winds = pd.Series([10, 5, 0], index=times)
+
+    pvtemps = pvsystem.pvsyst_celltemp(irrads, winds, temps)
+    expected = pd.Series([0.0, 23.96551, 5.0], index=times)
+    assert_series_equal(expected, pvtemps)
+
+
 def test_adrinverter(sam_data):
     inverters = sam_data['adrinverter']
     testinv = 'Ablerex_Electronics_Co___Ltd___' \