added citations paper.md #197 #193

Added citations pvlib, pvfactors, pvsyst and sam citations

paper/paper.bib

@@ -154,3 +154,46 @@ @inproceedings{Ward1994
 	pages = {459--472},
 	title = {{The RADIANCE lighting simulation and rendering system}},
 	year = {1994}}
+
+@article{pvlib,
+	author = {Holmgren, William F. and Hansen, Clifford W. and Mikofski, Mark M.},
+	doi = {https://doi.org/10.21105/joss.00884},
+	journal = {Journal of Open Source Software},
+	pages = {884},
+	title = {{pvlib python: a python package for modeling solar energy systems}},
+	volume = {3(29)},
+	year = {2018}}
+	
+@inproceedings{PVfactors,
+	author = {Anoma, M. and Jacob, D., and Bourne, B.C., and Scholl, J.A., and Riley, D.M. and Hansen, C.W.}, 
+	booktitle = {44th IEEE Photovoltaic Specialists Conference Proceedings, Washington, DC},
+	title = {{View Factor Model and Validation for Bifacial PV and Diffuse Shade on Single-Axis Trackers.}},
+	year = {2017}}
+
+@inproceedings{SAM,
+	author = {Blair, Nate and DiOrio, Nicholas and Freeman, Janine and Gilman, Paul and Janzou, Steven and Neises, Ty and Wagner, Michael},
+	institution = {National Renewable Energy Laboratory, Golden, CO},
+	pages = {1--19},
+	publisher = {NREL/TP-6A20-70414},
+	title = {{System Advisor Model (SAM) General Description (Version 2017.9.5}},
+	year = {2018}}
+
+@inproceedings{Marion2017,
+	author = {Marion, Bill and Macalpine, Sara and Deline, Chris and Asgharzadeh, Amir and Toor, Fatima and Riley, Daniel and Stein, Joshua and Hansen, 	 	Clifford},
+ 	booktitle = {44th IEEE Photovoltaic Specialists Conference (PVSC), Washington, DC.},
+ 	doi = {10.1109/PVSC.2017.8366263},
+	isbn = {9781509056057},
+	pages = {1537--1542},
+	publisher = {IEEE},
+	title = {{A practical irradiance model for bifacial PV modules}},
+	year = {2017}}
+
+@misc{bifacialvf,
+	author = {NREL},
+	mendeley-groups = {Thesis},
+	publisher = {GitHub},
+	title = {{bifacialVF}},
+	url = {http://github.com/NREL/bifacialvf},
+	year = {2019}}
+
+

paper/paper.md

@@ -31,11 +31,11 @@ bifacial_radiance is hosted on Github and PyPi, and it was developed by contribu
     <figcaption> Visualization of a bifacial photovoltaic array generated through bifacial_radiance. Courtesy of J. Alderman. </figcaption>
 </figure>
 
-The bifacial_radiance API and graphical user interface (GUI) were designed to serve the various needs of the many subfields of bifacial solar panel power research and engineering. The intended audience ranges from PV performance researchers, Engineering Procurement Construction (EPC) companies, installers, investors, consumers and analysts of the PV industry interested in predicting and evaluating bifacial photovoltaic systems. It is implemented in three layers: core RADIANCE-interface functions; ``Bifacial-Radiance``, ``Meteorological``, ``Scene``, and ``Analysis`` classes; and the ``GUI`` and ``model-chain`` classes. The core API consists of a collection of functions that implement commands directly to the RADIANCE software. These commands are typical implementations of algorithms and models described in peer-reviewed publications. The functions provide maximum user flexibility; however, some of the function arguments require an unwieldy number of parameters. The next API level contains the ``Bifacial-Radiance``, ``Meteorological``, ``Scene``, and ``Analysis`` classes. These abstractions provide simple methods that wrap the core function API layer and communicate with the RADIANCE software, which provides ray-trace processing capabilities. The method API simplification is achieved by separating the data that represent the object (object attributes) from the data that the object methods operate on (method arguments). For example, a ``Bifacial-Radiance`` object is represented by a ``module`` object, meteorological data, and ``scene`` objects. The ``gendaylit`` method operates on the meteorological data to calculate solar position and generate corresponding sky files, linking them to the ``Bifacial-Radiance`` object. Then the ``makeOct`` method combines the sky files, ``module`` and ``scene`` objects when calling the function layer, returning the results from an ``Analysis`` object to the user. The final level of API is the ``ModelChain`` class, designed to simplify and standardize the process of stitching together the many modeling steps necessary to convert a time series of weather data to AC solar power generation, given a PV system and a location. The ``ModelChain`` also powers the ``GUI``, which provides a cohesive visualization of all the input parameters and options for most common modeling needs.
+The bifacial_radiance API and graphical user interface (GUI) were designed to serve the various needs of the many subfields of bifacial solar panel power research and engineering. The intended audience ranges from PV performance researchers, Engineering Procurement Construction (EPC) companies, installers, investors, consumers and analysts of the PV industry interested in predicting and evaluating bifacial photovoltaic systems. It is implemented in three layers: core RADIANCE-interface functions; ``Bifacial-Radiance``, ``Meteorological``, ``Scene``, and ``Analysis`` classes; and the ``GUI`` and ``model-chain`` classes. The core API consists of a collection of functions that implement commands directly to the RADIANCE software. These commands are typical implementations of algorithms and models described in peer-reviewed publications. The functions provide maximum user flexibility; however, some of the function arguments require an unwieldy number of parameters. The next API level contains the ``Bifacial-Radiance``, ``Meteorological``, ``Scene``, and ``Analysis`` classes. These abstractions provide simple methods that wrap the core function API layer and communicate with the RADIANCE software, which provides ray-trace processing capabilities. The method API simplification is achieved by separating the data that represent the object (object attributes) from the data that the object methods operate on (method arguments). For example, a ``Bifacial-Radiance`` object is represented by a ``module`` object, meteorological data, and ``scene`` objects. The ``gendaylit`` method operates on the meteorological data to calculate solar position with the support of algorithms from pvlib python [@pvlib], and generate corresponding sky files, linking them to the ``Bifacial-Radiance`` object. Then the ``makeOct`` method combines the sky files, ``module`` and ``scene`` objects when calling the function layer, returning the results from an ``Analysis`` object to the user. The final level of API is the ``ModelChain`` class, designed to simplify and standardize the process of stitching together the many modeling steps necessary to convert a time series of weather data to AC solar power generation, given a PV system and a location. The ``ModelChain`` also powers the ``GUI``, which provides a cohesive visualization of all the input parameters and options for most common modeling needs.
 
 bifacial_radiance was first coded in Python and released as a stable version in Github in 2017 [@MacAlpine2017], and it was submitted as a U.S. Department of Energy Code project on December of the same year [@Deline2017]. Efforts to make the project more pythonic were undertaken in 2018 [@Ayala2018]. Additional features continue to be added as described in [@Ayala2019; @Stein2019] and the documentation’s “What’s New” section.
 
-bifacial_radiance has been used in numerous studies, for example, for modeling and validation of rear irradiance for fixed-tilt systems [@Ayala2019b], estimation of energy gain and performance ratio for single-axis-tracked bifacial systems [@Berrian2019; @Ayala2019c], as well as the study of edge effects [@Ayala2019c] and smart tracking algorithms [@Ayala2018b]; estimation of shading factor from racking structures [@Ayala2019d], and parameterization of electrical mismatch power losses due to irradiance nonuniformity in bifacial systems [@Deline2019; @Deline2019b; @Ayala2019e]. Sensitivity studies of installation and simulation parameters [@Asgharzadeh2018] and optimization for bifacial fields with the aid of high-performance computing [@Stein2019; @Stein2019b] have also been performed with bifacial_radiance. Furthermore, benchmarking with other rear-irradiance calculation software has been performed on several occasions [@Ayala2018b; @DiOrio2018; @Capelle2019]. Rear-irradiance calculation software fall into two categories: view-factor and ray-tracing models. View factor models assume isotropic scattering of reflected rays, allowing for calculation of irradiance by integration. Due-diligence software such as PVSyst or SAM use the view-factor model. Ray-tracing models simulate multipath reflection and absorption of individual rays entering a scene. Raytracing software such as bifacial_radiance, which is the only available open-source toolkit, offers the possibility of reproducing complex scenes, including shading or finite-system edge effects. Model agreement for view factor and bifacial_radiance software is better than 2% (absolute) when compared with measured results. [@Ayala2018b]. 
+bifacial_radiance has been used in numerous studies, for example, for modeling and validation of rear irradiance for fixed-tilt systems [@Ayala2019b], estimation of energy gain and performance ratio for single-axis-tracked bifacial systems [@Berrian2019; @Ayala2019c], as well as the study of edge effects [@Ayala2019c] and smart tracking algorithms [@Ayala2018b]; estimation of shading factor from racking structures [@Ayala2019d], and parameterization of electrical mismatch power losses due to irradiance nonuniformity in bifacial systems [@Deline2019; @Deline2019b; @Ayala2019e]. Sensitivity studies of installation and simulation parameters [@Asgharzadeh2018] and optimization for bifacial fields with the aid of high-performance computing [@Stein2019; @Stein2019b] have also been performed with bifacial_radiance. Furthermore, benchmarking with other rear-irradiance calculation software has been performed on several occasions [@Ayala2018b; @DiOrio2018; @Capelle2019]. Rear-irradiance calculation software fall into two categories: view-factor and ray-tracing models. View factor models assume isotropic scattering of reflected rays, allowing for calculation of irradiance by integration [@Marion2017]. Due-diligence software such as PVSyst or SAM use the view-factor model [@PVSyst, @SAM]. There are also some open-source view-factor models, such as bifacialvf, and PVFactors [@bifacialvf, @PVfactors]. Ray-tracing models simulate multipath reflection and absorption of individual rays entering a scene. Raytracing software such as bifacial_radiance, which is the only available open-source toolkit, offers the possibility of reproducing complex scenes, including shading or finite-system edge effects. Model agreement for view factor and bifacial_radiance software is better than 2\% (absolute) when compared with measured results. [@Ayala2018b]. 
 
 Plans for bifacial_radiance development include the implementation of new and existing models, addition of functionality to assist with input/output, and improvements to API consistency.