The photovoltaic module performance models calculate the hourly DC electrical output of a single module based on the hourly total incident solar radiation (plane-of-array irradiance) calculated by the Climate model. The photovoltaic array output depends on the number of modules and the pre-inverter derate factor specified on the Array page. Solar Advisor passes the array's hourly DC power output to the inverter model, whose characteristics are on the Inverter page.

Note. In addition to the number of modules and derate factors, the input variables on the Array page specify the array orientation and tracking, and the method used to calculate total incident radiation.

There are four options available for modeling PV modules:

Each model uses a different algorithm to predict module performance. In general, if you are modeling a system that uses a particular brand and type of flat-plate PV module, you should first look for the module in the Sandia database, and then in the CEC database. If you do not find the module in either database, or are not modeling a particular brand of module, then use the single-point efficiency model.

For concentrating photovoltaic (CPV) modules, use the Concentrating PV Module model unless you are modeling the Entech 22x module, in which case you can choose the module from the database in the Sandia model.

To specify a photovoltaic module model:

For the Simple Efficiency Model and Concentrating PV Module, enter module characteristics.

 

Choosing the Best Flat-plate Model for your Analysis

This section briefly describes each model and provides guidelines for choosing the best model to meet the goals of your analysis. Each model is described in more detail in the "About" sections below.

There are three flat-plate PV performance models available in the current version of Solar Advisor:

Because the Sandia model is the most accurate of the three models, it should be your first choice when choosing between the models. If you are investigating a module that is not included in the Sandia database, you can either choose a different module in the database with similar characteristics, or see if the model is in the CEC database. If you do not find your module in either the Sandia or CEC databases, choose the single-point efficiency model. Some types of analyses that require sensitivity or parametric analyses are best performed with the single-point efficiency model.

Table 13. Summary of guidelines for choosing a photovoltaic module performance model.

Modeling Thin-film Modules

For modules based on thin-film cell technology, including amorphous silicon, copper indium diselenide (CIS), cadmium telluride (CdTe), and heterojunction with intrinsic thin layer (HIT), the CEC and single-point efficiency models do not adequately represent module performance at low-light levels. Because the Sandia model is based on field measurements, it provides a more reliable prediction of thin-film module performance, but its results for thin-film modules have only been validated at the module level, not at the system level.

For best results, if you are modeling a thin-film module, look for the module in the Sandia database. If the module is not available in the Sandia database, you may want to use a module from the database with similar characteristics to the one you are modeling. Use the table below to help identify the thin-film modules in the Sandia database.

Table 14. Thin-film module manufacturers and model numbers available in the Sandia module database.

Modeling Concentrating Photovoltaic (CPV) Modules

For CPV modules, other than the Entech 22X Concentrator module, use the Concentrating PV Module option. This model uses a simple algorithm that calculates the module's hourly DC output by multiplying the hourly direct normal component of the solar radiation data in the weather file by the module's area and efficiency as specified on the Module page.

If you are modeling the Entech 22X Concentrator (c-Si), you can use the Sandia PV Array Performance model instead of the Simple Efficiency model. The Entech module is modeled using a set of coefficients determined by analyzing field test measurements. To use the Entech module, choose the Sandia model option and select the Entech 22X Concentrator [1994] module from the list of available modules. The Entech module is a special case in the Sandia module database, and its coefficients have been selected to allow it to be modeled using global radiation data.

To use the single-point efficiency CPV model:

The module's hourly DC output is the product of the hour's direct normal solar radiation from the weather file as defined on the Climate page, collector area, and module efficiency from the Module page:

Where,

About the Sandia PV Array Performance Model

The Sandia PV Array Performance Model uses an algorithm and a database of commercially available modules developed at Sandia National Laboratory. To use the model, you simply select a module or array from the Module Name list.

The Sandia model is described in King et al, 2004. Photovoltaic Array Performance Model. Sandia National Laboratories. SAND2004-3535. http://www.osti.gov/bridge/product.biblio.jsp?query_id=0&page=0&osti_id=919131

To use the Sandia photovoltaic model:

The first several items in the Sandia database are arrays instead of single modules. The arrays are indicated by the word "Array" in the name, which also includes the number of modules and the module type. When you use an array from the database, you should set the Pre-Inverter derate factor on the Array page to zero.

If you are modeling a module not included in the database and want to use the Sandia model, you can try to find a module with similar characteristics to your module's specifications.

The Sandia model consists of a set of equations that provide values for five points on a module's I-V curve. The equations involve a set of coefficients that have been empirically determined based on field measurements taken from modules installed in real, operating photovoltaic systems. The Sandia database consists of a set of manufacturer specification and measured data for each module and array in the database.

Solar Advisor does not track voltage and current levels in the system and assumes that the array operates at its maximum power point.

The Sandia Module Library Parameters

When you select a module from the Sandia database on the Module page, Solar Advisor displays module's parameters for your reference. You can see the complete set of parameters in the Module library by using Solar Advisor's library editor.

The Sandia module library includes parameters for modules that have been tested by Sandia National Laboratory. Manufacturers wishing to add their modules to the Sandia database should contact Sandia National Laboratory directly. Because the parameters involve data from field measurements, it is not possible to generate parameters from manufacturer specifications.

The tables below describe the parameters in the Sandia module library, which are explained in more detail in the King 2004 reference cited above.

About the CEC Performance Model

The California Energy Commission (CEC) Performance Model uses the University of Wisconsin-Madison Solar Energy Laboratory's five-parameter model with a database of module parameters maintained by the California Energy Commission (CEC) for the California Solar Initiative. To use the model, you simply choose a module name from the CEC Module list.

The five-parameter model is described in brief in De Soto 2003, "Improvement and Validation of a Model for Photovoltaic Array Performance," Solar 2003 Conference Proceedings, American Solar Energy Society.

A more detailed description can be found in De Soto 2004, Improvement and Validation of a Model for Photovoltaic Array Performance, Master of Science Thesis, University of Wisconsin-Madison. http://sel.me.wisc.edu/theses/desoto04.zip.

For information about the CEC list of eligible photovoltaic modules, see http://www.gosolarcalifornia.org/equipment/pvmodule.html.

To use the CEC photovoltaic model:

The modules in the list are from Solar Advisor's CEC module library, which is described in more detail below.

Note. To make sure that you have the latest CEC module library, on the Help menu, click Check for updates. Solar Advisor will connect to NREL servers on the Internet and, if a more recent version of the library is available, automatically update your current library. Updating the library only affects the standard CEC module library, and will not affect any modules you may have added to the library.

The five-parameter model calculates a module's current and voltage under a range of solar resource conditions (represented by an I-V curve) using an equivalent electrical circuit whose electrical properties can be determined from a set of five parameters. These five parameters, in turn, are  determined from standard reference condition data provided by either the module manufacturer or an independent testing laboratory, such as the Arizona State University Photovoltaic Testing Laboratory.

Solar Advisor does not track voltage and current levels in the system and assumes that the array operates at its maximum power point.

The CEC Module Library Parameters

When you select a module from the CEC database on the Module page, Solar Advisor displays module's parameters for your reference. You can see the complete set of parameters in the Module library by using Solar Advisor's library editor.

The CEC module library contains a set of module parameters for modules included in the CEC's database of eligible photovoltaic modules. Manufacturers wishing to add their modules to the CEC database should contact the CEC directly.

If you would like to add a module to the CEC module library in your copy of Solar Advisor, please contact us at solar.advisor.support@nrel.gov. If you can provide us with a list of the module's specifications as shown on the Module page under Module Characteristics, we may be able to create a set of coefficients for you to use. We provide this service on a case-by-case basis, and cannot guarantee the quality of the coefficients we generate.

The parameters in the CEC module library include:

About the Simple Efficiency Model for Flat-plate Modules

The flat-plate photovoltaic single-point efficiency model calculates the module's hourly DC output assuming that the module efficiency is fixed, regardless of the amount of solar radiation incident on the module. The model makes an adjustment for cell temperature, assuming that it is mounted on an open rack that allows air to flow over the back of the module.

The model calculates the DC power output of a single module using the equations described below.

To use the single-point efficiency flat-plate model:

This is the number typically reported on manufacturer specification sheets as the maximum power coefficient. See the table below for sample values for different cell types.

Module manufacturers typically include a description of the front material, and frame or back material in a mechanical characteristics section of module specification sheets.

Table 15. Sample temperature coefficient values for different cell types based on an informal survey of manufacturer module specifications.

For each hour of the year, the flat-plate single-point efficiency model calculates the module DC output as the product of the total incident radiation, module area, and temperature correction factor:

Where,

The temperature correction factor algorithm is the same one used for Solar Advisor's implementation of the Sandia model, and is described in detail in King et al, 2004. Photovoltaic Array Performance Model. Sandia National Laboratories. SAND2004-3535. http://www.osti.gov/bridge/product.biblio.jsp?query_id=0&page=0&osti_id=919131.

First, the model calculates the module back temperature TBack:

The cell temperature Tcell is:

The temperature correction factor FTempCorr is:

Where,

Table 16. Empirically-determined a and b coefficients for each of the three Module Structure options available on the Module page.