The Module page allows you to choose a photovoltaic module performance model from four options:

Note. To model a photovoltaic system using the PVWatts model, you must choose PVWatts System Model in the Technology and Market window.

To specify a photovoltaic module model:

For the simple efficiency model and concentrating PV Module, enter module characteristics.

Each of the four module performance models calculates the hourly DC electrical output of a single module based on the hourly incident solar radiation (plane-of-array irradiance) calculated by the Climate model using data in the weather file and array orientation and tracking information from the Array page. 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 appear on the Inverter page.

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

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, you can either choose a similar module, or use the simple efficiency model. The simple efficiency and concentrating PV models are ideal for analyses involving explorations of the relationship between module efficiency and the system's performance and cost of energy. Both models allow you to define a curve of the module's efficiency versus incident radiation. The Sandia and CEC models do not allow you to modify the module parameters, although advanced users with and understanding of the model algorithms can add their own modules to the database.

Table 9. Guidelines for choosing a photovoltaic module performance model.

The Sandia PV Array Performance Model uses an algorithm and a database of commercially available modules developed at Sandia National Laboratories.

The Sandia model is described in King et al, 2004. Photovoltaic Array Performance Model. Sandia National Laboratories. SAND2004-3535. http://photovoltaics.sandia.gov/docs/PDF/King%20SAND.pdf

To use the Sandia photovoltaic model:

When you choose a module from the list, Solar Advisor displays the module characteristics at reference conditions on the Module page. Internally, the model applies a set of coefficients from the Sandia Modules library to the simulation equations.

Note. The current version of the database contains a single concentrating PV module, listed as Entech 22X Concentrator [1994].

The Sandia model consists of a set of equations that provide values for five points on a module's I-V curve and a database of coefficients for the equations whose values are stored in the Sandia Modules library. The coefficients have been empirically determined based on a set of manufacturer specifications and measurements taken from modules installed outdoors in real, operating photovoltaic systems.

Note. If you are a module manufacturer and would like to add your module to the Sandia database, you should contact Sandia National Laboratories directly.

Modeling Arrays

The first several items in the list are arrays instead of single modules. The arrays are indicated by the word "Array" in the name. The array coefficients account for some losses not accounted for in the single module parameters, including module mismatch, diodes and connections, and DC wiring losses. When you use an array from the database, you should be sure that the Pre-Inverter derate factor on the Array page does not include these losses.

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 Sandia model may provide more accurate results than the CEC and simple efficiency models, which do not adequately represent module performance at low-light levels. 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 10. Thin-film module manufacturers and model numbers available in the Sandia module database.

Module Characteristics at Reference Conditions

Solar Advisor displays the module characteristics so that you can compare modules in the database to manufacturer specifications or to different modules in the database.

Reference Conditions

The reference conditions describe the incident solar radiation, air mass, ambient temperature, and wind speed that apply to the module characteristics. The module efficiency, power, current, voltage, and temperature coefficients values are those for the module operating at the reference conditions.

Efficiency (%)

The module's rated efficiency at reference conditions. Solar Advisor displays this value for reference only. During simulations, the model calculates an efficiency value for each hour, which you can see in the hourly output data.

Maximum Power, P_mp (Wdc)

The module rated power in DC Watts. Equal to the product of the maximum power voltage and maximum power current.

Maximum Power Voltage, V_mp (Vdc)

Maximum power voltage in DC Volts under reference conditions.

Maximum Power Current, I_mp (Adc)

Maximum power current in DC Amps under reference conditions. Defines the maximum power point on the module's I-V curve.

Open Circuit Voltage, V_oc (Vdc)

Open circuit voltage under reference conditions. Defines the open circuit point on the module's I-V curve.

Short Circuit Current, I_sc (Adc)

Short circuit current under reference conditions. Defines the short circuit point on the module's I-V curve.

Temperature Coefficients

Solar Advisor displays the temperature coefficients in %/°C and W/°C at the different points on the power curve.

Module Structure and Mounting

This option determines the coefficients that Solar Advisor uses to calculate the cell temperature in each hour of the simulation. The default option is User Database Values, which displays the coefficients from the measured data at reference conditions. See Temperature Correction for details.

Physical Characteristics

Material

A description of the semiconductor technology used in the photovoltaic cells.

Vintage

The year that module coefficients were added to the database. The letter "E" indicates that the coefficients were estimated from a combination of published manufacturer specifications and data from the outdoor testing of a similar module.  Entries without an "E" are for modules whose coefficients were derived entirely from outdoor tests involving one more or more modules of that type. Because the tested modules (listed without an "E") may have had different average power ratings than production versions of the same module, the database typically also includes an "E" entry for each of the tested modules that represents the average power rating specified by the manufacturer.

Module Area (m2)

The total area of the module, including spaces between cells and the frame.

Number of Cells

Total number of cells per module, equal to the product of the number of cells in series and number of cell strings in parallel.

Number of Cells in Series

Number of cells connected in series per cell string.

Number of Cell Strings in Parallel

Number of cell strings connected in parallel per module.

References

A link to King et al, 2004. Photovoltaic Array Performance Model. Sandia National Laboratories. SAND2004-3535 document describing the Sandia PV Array Performance Model in detail. You can find links to more documents in References.

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 for modules from the database of eligible photovoltaic modules maintained by the California Energy Commission (CEC) for the California Solar Initiative.

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:

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

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.

Note. If you are a module manufacturer and would like to add your module to the CEC database, you 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:

Module Characteristics at Reference Conditions

Efficiency (%)

The module's rated efficiency at reference conditions. Solar Advisor displays this value for reference only. During simulations, the model calculates an efficiency value for each hour, which you can see in the hourly output data.

Maximum Power, P_mp (Wdc)

The module rated power. Equal to the product of the maximum power voltage and maximum power current.

Maximum Power Voltage, V_mp (V)

Reference maximum power voltage at the reference conditions.

Maximum Power Current, I_mp (A)

Reference maximum power current at the reference conditions.

Open Circuit Voltage, V_oc_ref (V)

Reference open circuit voltage at the reference conditions.

Short Circuit Current, I_sc_ref (A)

Reference short circuit current at the reference conditions.

Number of Cells

Total number of cells per module.

Temperature Coefficients

Solar Advisor displays the temperature coefficients in %/°C and W/°C at maximum power, open circuit, and short circuit.

Physical Characteristics

Material

A description of the semiconductor technology used in the photovoltaic cells.

Module Area

The total area of the module, including spaces between cells and the frame.

Number of Cells

Number of cells per module.

Reference Conditions

T_Amb_NOCT

Ambient temperature at NOCT

T_REF

Module temperature at reference conditions

FFV_Wind

Correction factor for wind speed (FFV_wind = 0.51 if lowest part of the array is less than 22 feet from the ground FFV_wind = 0.61 if lowest part of the array is 22 feet or more from the ground)

I_NOCT

Irradiance at NOCT

I_REF

Irradiance at reference conditions

Tau_alpha

Transmittance-absorptance product

KL

Extinction coefficient

Eta_MPPT

Maximum power point correction factor

E_G

Band gap at reference temperature

Additional Parameters

T_NOCT

Cell temperature at NOCT

A_REF

Modified ideality factor at reference conditions

I_L_REF

Photocurrent at reference conditions

I_O_REF

Reverse saturation current at reference conditons

R_s

Series resistance (constant)

R_sh_ref

Shunt resistance at reference conditions

References

A link to De Soto W, 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. You can find links to more documents in References.

The flat-plate photovoltaic simple efficiency module model calculates the module's hourly DC output assuming that the module efficiency varies with radiation incident on the module as defined by the radiation level and efficiency table. The model makes an adjustment for cell temperature, See Temperature Correction for details.

To use the simple efficiency module model:

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

Solar Advisor uses the reference value to calculate the module's rated power, displayed as the Power variable on the Module page.

Characteristics

The module characteristics define the module's capacity, efficiency, and thermal characteristics.

Maximum Power, Pmp (Wdc)

The module's rated maximum DC power at the reference radiation indicated in the radiation level and efficiency table. Solar Advisor uses this value to calculate the array cost on the PV System Costs page, but not in simulation calculations. The module power is the product of the reference radiation, reference efficiency, and area.

Temperature Coefficient, Pmp (%/°C)

The rated maximum-power temperature coefficient as specified in the module's technical specifications. See Temperature Correction for details.

Area (m2)

The module collector area in square meters. To calculate the area for a given module power rating at a given reference radiation level, divide the power rating by the module efficiency and radiation level. For example, a module with a 100 W rating and 13.5% efficiency at 1000 W/m2 would requires an area of 100 W / (0.135 × 1000 W/m2) = 0.74074 m2.

Module Structure and Mounting

The module's front and back materials (front material/cell/back material) used in the temperature correction algorithm described below. See Temperature Correction for details.

Radiation Level and Efficiency Table

Radiation (W/m2)

The incident global (beam and diffuse) radiation level at which the given efficiency value applies.

Efficiency (%)

The module conversion efficiency at a given incident global radiation level. Solar Advisor calculates an efficiency value for each hour in the simulation using linear extrapolation to determine the value based on radiation data from the weather file. The efficiency values represent the efficiency of conversion from incident global radiation to DC electrical output.

Reference

Indicates the value to use for the reference calculations. Solar Advisor uses the reference values to calculate the module's rated power on which module costs are based.

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 global radiation, module area, and temperature correction factor:

Where,

The Concentrating PV Module 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 from the weather file by the module's area and efficiency as specified on the Module page, and makes a correction for the cell's temperature.

Note. 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.

To use the Concentrating PV module model:

Concentrating PV Module Characteristics

Maximum Power, Pmp (Wdc)

The module's rated maximum DC power at the reference radiation value indicated in the efficiency table below. Solar Advisor uses this value to calculate the array cost shown on the PV System Costs page, but not in simulation calculations. The rated module power is the product of the reference radiation, reference efficiency and area.

Temperature Coefficient, Pmp (%/°C)

The rated maximum-power temperature coefficient as specified in the module's technical specifications. The default value is -0.15. See Temperature Correction for details.

Area (m2)

The module collector area in square meters.

a, b, and dT

Temperature correction coefficients, a=-3.2, b=-0.09, dT=17 by default. See Temperature Correction for details.

Default Temperature Inputs

Resets the a, b, and dT coefficients to their default values.

Radiation Level and Efficiency Table

Radiation (W/m2)

The incident beam radiation level at which the given efficiency value applies.

Efficiency (%)

The module conversion efficiency at a given incident global radiation level. Solar Advisor calculates an efficiency value for each hour in the simulation using linear extrapolation to determine the value based on radiation data from the weather file. The efficiency values represent the efficiency of conversion from incident global radiation to DC electrical output.

Reference

Indicates which value to use for the reference calculations.

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,

The Sandia, Simple Efficiency, and Concentrating PV models all use the temperature correction algorithm originally developed for the Sandia model to calculate a temperature correction factor that accounts for efficiency losses due to heating of the module during the day when the sun is shining. The algorithm calculates an hourly module temperature as a function of the solar radiation, ambient temperature, and wind speed in a given hour. The algorithm uses a set of three coefficients (a, b, and dT, described below), whose values depend on the module's construction and how air circulates around the module when it is installed in the system.

For more details about the algorithm, see King et al, 2004. Photovoltaic Array Performance Model. Sandia National Laboratories. SAND2004-3535. http://photovoltaics.sandia.gov/docs/PDF/King%20SAND.pdf

The CEC model uses a different temperature correction algorithm. For a description of the CEC temperature correction approach, see 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.

Note. The Solar Advisor temperature correction algorithms do not account for cooling strategies used in some innovative photovoltaic systems.

Temperature Correction for Sandia, Simple Efficiency, and Concentrating PV Models

Solar Advisor uses the method described below to calculate a module and cell temperature and temperature correction factor for the Sandia, Simple Efficiency and Concentrating PV models. The model uses the temperature correction factor to adjust each hour's module efficiency value: The higher the module's temperature in a given hour, the lower the module's efficiency in that hour.

You can explore temperature effects on the array's performance in the hourly output data. The data shows the hourly cell temperature, along with the solar radiation, wind speed, and ambient temperature.

The temperature correction equations use the following input values from the Module page:

The equations use four hourly data sets from the weather file. You can see the hourly data by either viewing the climate data from the Climate page, or viewing the hourly results data after running simulations:

Guidelines for choosing the Module Structure - Mounting option for the Sandia and Simple Efficiency models:

Table 11. Description of the module structure and mounting options.

Table 12. Empirically-determined coefficients from the Sandia database for each of the module structure and mounting options available on the Module page.

Table 13. Sample temperature coefficient values for different cell types based on an informal survey of manufacturer module specifications.

The temperature correction algorithm first calculates the module back temperature based on the incident solar radiation, a and b coefficients, and the ambient temperature and wind speed:

Next, the cell temperature is calculated based on the module back temperature, incident radiation, and dT:

The temperature correction factor FTempCorr is:

Where,