Array

To view the Array page, click Array on the main window's navigation menu. Note that for the Array page to be available, the technology option in the Technology and Market window must be Photovoltaics - SAM Performance Models.

The Array page displays variables and options that describe the array layout, derating factors, array tracking and orientation, and allows you to choose from several radiation model options.

The variables on the Array page specify the properties of the photovoltaic array, number of inverters, and the model options for the solar radiation data processor. Solar Advisor uses the array properties to calculate the array's DC output and the system's AC output.

Note. Before specifying the array, you should specify the module characteristics on the Module page, and the inverter characteristics on the Inverter page.

To specify the photovoltaic array:

1.	Verify that the input variables on the Module and Inverter pages are correct.

2.	Type a number of modules per string that results in a maximum power point voltage (Vmp) level close to the inverter's AC voltage level. Skip this step if you are using the inverter single-point efficiency model.

3.	Type a number of strings in parallel that results in a total array power value close to the system's rated DC capacity.

4.	Type a number of inverters that results in a total inverter capacity close to the total array power value. See Choosing Numbers of Modules and Inverters below for details.

5.	Type values for the derate factors. To use the derate factor calculator, click Edit Detailed Derates. For help calculating a derate factor, click See About Derate Factors below for details.

6.	Choose a tracking option: Fixed, one-axis tracking, or two-axis tracking.

If you use a tracking system, be sure that the Balance of System cost category on the PV System Costs page includes the cost of installing the tracking system, and that the Operation and Maintenance cost includes the cost of maintaining the system.

7.	Type a value for the array tilt angle in degrees from horizontal. Zero degrees is horizontal, 90 degrees is vertical.

If you are unsure of a value, use the location's latitude (displayed in the navigation menu under Climate and on the Climate page).

If the array is oriented away from due south in the northern hemisphere (or oriented away from due north in the southern hemisphere), change the default azimuth angle to the desired value. An azimuth angle of 0 (facing the equator) normally maximizes energy production. For the northern hemisphere, increasing the azimuth angle favors afternoon energy production, and decreasing the azimuth angle favors morning energy production. The opposite is true for the southern hemisphere. In SAM, 90 degrees corresponds to west, and -90 degrees corresponds to east for both hemispheres.

9.	Unless you have a reason to change the radiation model or tilt radiation type options, use the default options (Total and Beam, and Perez Model). See Input Variable Reference below for descriptions of the options.

Contents

►	Input Variable Reference describes the input variables and options on the Array page.

►	Choosing Numbers of Modules and Inverters describes how to choose values for the Modules per String, Strings in Parallel and Number of Inverters variables to avoid mismatching the array and inverter.

►	About Derate Factors describes how Solar Advisor uses the derate factors in calculations, and provides guidelines for choosing appropriate values.

Input Variable Reference

Layout

Variable	Description	Units
Modules per String	The number of modules connected in series in a single string. Solar Advisor assumes that all strings in the array have the same number of modules connected in series. Press the Enter or Tab key after changing a value to update variables that depend on these values. Note. When the module type on the Module page is an array from the Sandia database (indicated by the word "array" in its name), the Modules per String variable represents the number of arrays rather than number of modules.	--
Strings in Parallel	The number of module strings connected in parallel. Press the Enter or Tab key after changing a value to update variables that depend on these values.	--
Total Modules	The number of modules in the array, equal to the product of the number of modules per string and the number of strings.	--
Total Area	The array's total area, not including space between modules, equal to the product of the module area and the number of modules	m2
Total Array Power	The maximum DC power output of the array, equal to the product of the module's rated power on the Module page and the number of modules in the array.	Wdc
Voc (String)	The open circuit DC voltage of each string of modules, equal to the product of module's open circuit voltage displayed on the Module page and the number of modules per string. Solar Advisor displays an open circuit voltage of zero for the single-point efficiency module performance model because the model does not include voltage ratings.	Vdc
Vmp (String)	The maximum power point DC voltage of each string of modules, equal to the product of the module's maximum power point voltage displayed on the Module page and the number of modules per string. Solar Advisor displays a maximum power point voltage of zero for the single-point efficiency module performance model because the model does not include voltage ratings.	Vdc
Number of Inverters	The total number of inverters in the system. Press the Enter or Tab key to recalculate values that depend on the number of inverters.	--
Total Inverter Capacity	The total inverter capacity in AC Watts, equal to the product of the inverter's nominal AC power rating on the Inverter page and the number of inverters.	Wac

System Derates

Variable	Description	Units
Pre-Inverter Derate	Applies to the array DC power output in the hourly simulation. A derate factor of 100% is equivalent to no derating. A derate factor of 75% would reduce the calculated array DC output by 25%.	--
Post-Inverter Derate	Applies to the inverter AC power output in the hourly simulation. A derate factor of 100% is equivalent to no derating. A derate factor of 75% would reduce the calculated inverter AC output by 25%.	--
Total Derate Factor	The product of the pre- and post-inverter derate factors. This value is useful as a reference to compare to hand-calculated performance estimates, but is not used by Solar Advisor.	--

Tracking and Orientation

Variable or Option	Description	Units
Fixed	The array is fixed at the tilt and azimuth angles defined by the Tilt and Azimuth variables.	--
1 Axis	The array is fixed at the tilt angle defined by the Tilt variable and rotates from east in the morning to west in the evening to track the daily movement of the sun across the sky.	--
2 Axis	The array rotates from east in the morning to west in the evening to track the daily movement of the sun across the sky, and north-south to track the sun's seasonal movement throughout the year.	--
Tilt	Applies only to fixed arrays and arrays with one-axis tracking. The array's tilt angle in degrees from horizontal, where zero degrees is horizontal, and 90 degrees is vertical. As a rule of thumb, system designers often use the location's latitude (shown on the Climate page) as the optimal array tilt angle. The actual tilt angle will vary based on project requirements.	degrees
Azimuth	Applies only to fixed arrays with no tracking. The array's east-west orientation in degrees. An azimuth value of zero is facing the equator in both the northern and southern hemispheres. Positive 90 degrees is facing due west and negative 90 degrees is facing due east in both hemispheres. As a rule of thumb, system designers often use an array azimuth of zero, or facing the equator.	degrees
Ground Reflectance	The ground reflectance value for hours when the weather data indicate that there is no snow on the ground. A value of zero means that the ground is completely non-reflective, and a value of 1 means that it is completely reflective. A typical value for grassy ground is 0.2.	--
Ground Reflectance with Snow	The ground reflectance value for hours when the weather data indicate that there is snow on the ground. A value of zero means that the ground is completely non-reflective, and a value of 1 means that it is completely reflective. A typical value for snowy ground is 0.6.	--

Radiation Model

The data in the weather file includes data for global horizontal radiation, direct normal radiation, and diffuse horizontal radiation. The radiation model options determine which data Solar Advisor uses to calculate the solar radiation incident on the array.

Option	Description
Beam and Diffuse	This option tells Solar Advisor to use the direct normal radiation (beam) and diffuse horizontal radiation data, and to ignore the global horizontal radiation data. Solar Advisor calculates the global horizontal radiation as the sum of the direct normal and diffuse horizontal radiation.
Total and Beam	This option tells Solar Advisor to use the global horizontal radiation (total) and direct normal radiation (beam) data, and to ignore the diffuse horizontal radiation.

Tilt Radiation Type

Solar Advisor allows you to choose the method it uses to convert global horizontal solar radiation data to global solar radiation incident on the array. Each method uses information about the global horizontal solar radiation and either the direct normal or diffuse solar radiation, and about the sun's position and orientation of the array. The four methods differ in how they estimate the diffuse radiation incident on the array.

The isotropic sky model tends to under-predict the global radiation on a tilted surface, and is included as an option for analysis wanting to compare Solar Advisor results with those from other models using this approach. The remaining three methods provide comparable estimates of the incident global radiation.

Option	Description
Isotropic Sky Model	Assumes that diffuse radiation is uniformly distributed across the sky, called isotropic diffuse radiation.
Hay and Davies Model	Accounts for the increased intensity of diffuse radiation in the area around the sun, called circumsolar diffuse radiation, in addition to isotropic diffuse radiation.
Reindl Model	Accounts for the effect of horizon brightening, in addition to circumsolar diffuse radiation.
Perez Model	Accounts for horizon brightening, circumsolar and isotropic diffuse radiation using a more complex computational method than the Reindl and Hay and Davies methods.

Choosing Numbers of Modules and Inverters

For analyses using the single-point efficiency model options for photovoltaic modules and inverters, choose a number of modules and inverters that results in the array's rated power (Total Array Power) and inverter's capacity (Total Inverter Capacity) being as close as possible.

For analyses using the Sandia or CEC models, PV array and inverter capacities and voltages are displayed on the Array page for reference. Some recommendations for the array layout are:

•	The PV array capacity (Total Array Power) and inverter total capacity (Total Inverter Capacity) should be as close as possible.

•	The inverter nominal voltage (Vin,nom) shown on the Inverter page should be as close as possible to the array's maximum power point voltage (Vmp String).

•	The PV array maximum power point voltage (Vmp String) should be between the minimum inverter voltage (Vin,min) and the maximum inverter voltage (Vin,max) shown on the Inverter page.

Solar Advisor assumes that multiple inverters are connected in parallel so that the inverter array voltages are equivalent to the single inverter voltages shown on the Inverter page.

If the inverter and array capacities or voltages are mismatched, Solar Advisor displays a warning message during simulations. The message appears under the following conditions:

•	The total inverter capacity is less than the array power.

•	The total inverter capacity is greater than 1.3 times the array power.

•	The string maximum power point voltage (Vmp String) is less than the inverter minimum input voltage (Vin,min) or greater than the inverter maximum input voltage (Vin,max).

•	The array open circuit voltage (Voc String) is greater than the inverter maximum voltage (Vin,max) shown on the Inverter page.

About Derate Factors

A photovoltaic system typically consists of a DC side that includes modules, diodes, and DC wiring and fuses, and an AC side that may include inverters, AC wiring and fuses, and transformers. Solar Advisor allows you to enter two derate factors, a pre-inverter derate factor to account for electrical losses on the DC side of the system, and post-inverter derate factor to account for losses on the AC side.

Solar Advisor uses the derate factors in the hourly simulation calculations to account for reductions in photovoltaic array and system performance that are not accounted for by either the module or inverter performance models.

Note. The total array power shown on the Array page is the array's rated power based on the module's power from the Module page and the number of modules shown on the Array page. Solar Advisor does not apply the derate factor to this rated capacity value.

Derate Factors in Performance Simulation Calculations

During the simulation, Solar Advisor multiplies the array's DC power output by the pre-inverter derate factor to calculate the inverter's DC input power for each hour of the simulation:

Similarly, to calculate the system's gross hourly output, it multiplies the inverter's output by the post-inverter derate factor:

To calculate the system's net annual output, Solar Advisor adds up the 8,760 hourly gross system output values and adjusts this gross annual output value using the degradation and availability factors from the Annual Performance page.

Choosing Derate Factors

One source of information on derate factors is the website for NREL's PVWatts model, which includes a table of derate factor components for various sources of losses. Because Solar Advisor's performance model already accounts for some of the losses listed in the PVWatts table, it is not appropriate to use some of the PVWatts derate factor components in your Solar Advisor Model analysis.

Note. The PVWatts derate factors are described at http://www.nrel.gov/rredc/pvwatts/changing_parameters.html

If you are in doubt about the value to use for the derate factors, you can use the default values supplied with the Solar Advisor sample files. The following information is based on the information provided on the PVWatts website, and can be used as a reference for choosing values for the derate factors in Solar Advisor.

To calculate the pre-inverter derate factor to use in Solar Advisor, multiply the values of all of the pre-inverter derate factor components. Similarly, to calculate the post-inverter derate factor, multiply the values of all of the post-inverter derate factors.

The following derate factor components described on the PVWatts website are accounted for by Solar Advisor and should not be included in the pre- or post-inverter derate factors.

•

PV module nameplate DC rating: The PV module nameplate DC rating is the manufacturers estimate of power production for a module under standard testing conditions. Solar Advisor's module performance models calculate the module output based on solar resource data from the weather file and the parameters defined on the Module page, and not based on a derate factor. (The performance model used to determine module output is defined on the Module page.)

•	Inverter and Transformer: Solar Advisor's inverter performance models calculate the inverter output based on the output of the module (as determined by the performance model) and parameters defined on the Inverter page, and not based on a derate factor.

•	System availability: The system availability is an input variable on the Annual Performance page, and should not be included as a derate factor.

•	Shading: Solar Advisor accounts for shading based on the parameters specified on the PV Array Shading page.

•	Age: Solar Advisor's degradation factor on the Annual Performance page accounts for performance losses over time due to aging of modules.

The following derate factor components described on the PVWatts website are not accounted for by Solar Advisor. The user may wish to include these factors in the pre- or post-inverter derate factors.

Table 11. Pre-inverter (DC) derate factors not accounted for by the module performance model.

Derate Factor Component	Cause of Loss	PVWatts Default Value	PVWatts Range
Mismatch	Slight differences in performance of different modules in the array.	98.0	97.0 - 99.5
Diodes and Connections	Voltage drops across blocking diodes and electrical connections.	99.5	99.0 - 99.7
DC Wiring	Resistive losses in wiring on the DC side of the system.	98.0	97.0 - 99.0
Soiling	Dirt, snow, or other matter on the module surface blocking solar radiation from reaching cells.	95.0	30.0 - 99.5
Sun Tracking	Inaccuracies in the tracking mechanisms ability to keep the array oriented toward the sun. Applies only to systems with one- or two-axis tracking arrays.	100.0	95.0 - 100.0

Table 12. Post-inverter (AC) derate factors not accounted for by the inverter performance model.

Derate Factor Component	Cause of Loss	PVWatts Default Value	PVWatts Range
AC wiring	Resistive losses in wiring on the AC side of the system.	99.0	98.0 - 99.3