Create ivtools (#718)

* initial commit fit_cec_using_sam

* outline ivcurves

* use nrel-pysam

* remove sam_dir

* add Sandia single curve fit

* complete function

* add test, move code to ivtools.py

* remove single files

* change PySCC usage to wrapper

* add nrel-pysam to ci/requirements

* stickler, add pysam version in ci/requirements

* does case matter?

* remove pysam for py27, move import pysam to try/except

* add requires_scipy

* rename variables for style guide

* add test for fit_cec_with_sam

* fix conflict in requirements

* fix typo, raise syntax

* remove ivtools dir

* fix pytest fixture usage

* linter, fix np.allclose

* relax test condition

* remove try/except, add to docstring, use helper functions

* add requires_pysam

* fix import

* fix infinite loop

* update whatsnew, api.rst

* fix lint errors

* lint

* resolve comments

* comment response

* fix function name in test

* more comment responses

* add nan output to fit_cec_sam, comment responses

* lint

* consistent return nan, v_oc arg etc. defaults to None, edit description

* initialize failed

* lint

* implement exception for failed fitting

* remove requirement for sorted i

* update internal variable name for consistency

* add to __init__.py

* code style changes

* lint

* adjust error messaging in _calculate_sde_parameters

* edits to units in docstring

* change error handling in fit_sde_sandia, docstring reformat

* fix tests, docstring edits

* test math formatting

* more edits to docstring

* multiline raw string

* try another approach

* move r

* change function name, docstring edits

* update function name

* add math delimiters

* remove delimiter, use indent

* fix alignment in docstring

* some edits to docstring format

* change name back to sde

* add bad IV curves for coverage test

* separate bad IV test, fix test

* lint

* update api.rst and whatsnew

* hardwire test output rather than read from old CEC file

* test fix

* really fix it this time

* fix raise statement

Author: cwhanse

ci/requirements-py35.yml

@@ -24,4 +24,5 @@ dependencies:
     - shapely  # pvfactors dependency
     - siphon  # conda-forge
     - pip:
+        - nrel-pysam
         - pvfactors==1.0.1

ci/requirements-py36.yml

@@ -24,4 +24,5 @@ dependencies:
     - shapely  # pvfactors dependency
     - siphon  # conda-forge
     - pip:
+        - nrel-pysam
         - pvfactors==1.0.1

ci/requirements-py37.yml

@@ -24,4 +24,5 @@ dependencies:
     - shapely  # pvfactors dependency
     - siphon  # conda-forge
     - pip:
+        - nrel-pysam
         - pvfactors==1.0.1

docs/sphinx/source/api.rst

@@ -294,6 +294,14 @@ PVWatts model
    pvsystem.pvwatts_losses
    pvsystem.pvwatts_losses
 
+Functions for fitting PV models
+-------------------------------
+.. autosummary::
+   :toctree: generated/
+
+    ivtools.fit_sde_sandia
+    ivtools.fit_sdm_cec_sam
+
 Other
 -----
 

docs/sphinx/source/whatsnew/v0.7.0.rst

@@ -104,6 +104,26 @@ Documentation
 ~~~~~~~~~~~~~
 * Corrected docstring for `pvsystem.PVSystem.sapm`
 
+API Changes
+~~~~~~~~~~~
+
+
+Enhancements
+~~~~~~~~~~~~
+* Add `ivtools` module to contain functions for IV model fitting.
+* Add :py:func:`~pvlib.ivtools.fit_sde_sandia`, a simple method to fit
+  the single diode equation to an IV curve.
+* Add :py:func:`~pvlib.ivtools.fit_sdm_cec_sam`, a wrapper for the CEC single
+  diode model fitting function '6parsolve' from NREL's System Advisor Model.
+
+
+Bug fixes
+~~~~~~~~~
+
+
+Testing
+~~~~~~~
+
 Removal of prior version deprecations
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 * Removed `irradiance.extraradiation`.
@@ -119,6 +139,7 @@ Contributors
 ~~~~~~~~~~~~
 * Mark Campanellli (:ghuser:`markcampanelli`)
 * Will Holmgren (:ghuser:`wholmgren`)
+* Cliff Hansen (:ghuser:`cwhanse`)
 * Oscar Dowson (:ghuser:`odow`)
 * Anton Driesse (:ghuser:`adriesse`)
 * Alexander Morgan (:ghuser:`alexandermorgan`)

pvlib/__init__.py

@@ -7,6 +7,7 @@
 from pvlib import location
 from pvlib import solarposition
 from pvlib import iotools
+from pvlib import ivtools
 from pvlib import tracking
 from pvlib import pvsystem
 from pvlib import spa