The SAM 3D Shade Calculator uses a sun position algorithm and a three-dimensional drawing of a photovoltaic array and nearby shading objects to generate hour-by-month tables of beam irradiance shade loss percentages, and a sky diffuse loss percentage. It has the following features:

•Uses Bing Maps geocoding to automatically find latitude, longitude, and time zone for any street address.

•A three dimensional drawing, or shade scene, represents the photovoltaic array and nearby shading objects.

•The scene consists of active surfaces (photovoltaic subarrays) and shading objects.

•Four three-dimensional shapes are designed to represent most shading objects: Box, cylinder, tree, and roof.

•View and edit the shade scene in three-dimensional, bird's eye, and elevation views.

•Add an optional aerial photograph from Bing Maps or your own image as an underlay for the bird's eye view to help position objects on the scene.

•Drag the three-dimensional view to see shadows on active surfaces and the shade loss at different sun positions.

•Calculate diurnal and hourly beam irradiance shade loss for a single year based on the scene location and relative positions of each active surface and shading objects in the scene.

To use the SAM shade analysis calculator:

1.On the Shading and Layout page (detailed photovoltaic model) or System Design page (PVWatts), click Open 3D shade calculator.

2.Define the scene location (latitude, longitude, time zone).

3.Add shading objects to the scene -- these are trees, buildings, and other structures near the array that may block direct solar irradiance from reaching the array.

4.Add active surface objects to the scene to represent the photovoltaic array or subarrays.

5.Save and close the calculator to apply shading factor values to the SAM shading inputs.

6.Generate shade data.

7.Export shade data to use in SAM or other simulation software.

For more step-by-step instructions, see How-To.

For a description of the 3D shade calculator's features, see Reference.

To model external shading of a subarray in SAM, you provide a set of beam shading loss percentages in one of the beam shade loss tables, and a single sky diffuse shading loss.

To enable the external shading:

1.Click Edit Shading for the subarray for which you want to enable external shading to open the Edit Shading Data window. (Define the subarrays on the System Design page.)

2.If you are working with a shading file from PVsyst, Solmetric Suneye, or Solar Pathfinder software, in the Edit Shading window, click the appropriate button under Import shading data from external tools to import the file.

3.If you are using a beam shade loss table to specify shading factors (you can type, import, or paste values into the table), check the appropriate Enable box in the Edit Shading window to display the table.

Note. SAM does not prevent you from enabling more than one beam shade loss table even if that results in an unrealistic shading model. Be sure to verify that you have enabled the set of shade loss tables you intend before running a simulation.

To model the effects of self shading and snow cover, SAM needs basic information about the arrangement of modules and rows in each subarray. The total number of modules in each subarray is defined on the System Design page.

Notes.
 
The Array Dimensions input are only active for a subarray when you either choose a self-shading option, or check the Snow Losses check box.
 
SAM assumes that modules in each subarray are laid out in rectangles. It cannot calculate self shading or snow coverage losses for subarrays with irregular shapes.
 
The array dimensions for self shading and snow coverage are different from the electrical wiring of strings defined on the System Design page.

Module orientation

The module orientation determines whether the short or long side of the module is parallel to the ground, assuming that all modules in the array are mounted at a fixed angle from the horizontal equal to the tilt angle specified on the System Design page.

Portrait orientation means the short end of the module is parallel to the ground, or at the bottom of the module.

Landscape orientation means the long end of the module is parallel to the ground, or at the bottom of the module.

Number of modules along side of row

The number of modules along the edge of the subarray array perpendicular to the bottom of the array as defined below.

Number of modules along bottom of row

The number of modules along the bottom of a row, which is the edge of the row nearest to the ground.

For fixed arrays, the bottom edge is perpendicular to the azimuth angle. For example, for a fixed array in the northern hemisphere and an azimuth angle of 180 degrees, the bottom of the row is the southernmost edge of the row.

For arrays with one-axis tracking, the bottom edge is parallel to the tracking axis, which is determined by the azimuth angle. For example, for a one-axis tracking array with an azimuth angle of 180 degrees, the bottom of each row is the edge closest to the east in the morning.

Number of rows

The number of rows given the number of modules in the subarray from the System Design page and the numbers of modules along the side and bottom of row that you specify.

Number of Rows = Number of Modules in Subarray ÷ Number of Modules along Side ÷ Number of Modules along Bottom

To model a realistic rectangular arrangement of modules, the number of rows should be an integer: The product of the number of modules along the side and bottom of each subarray should be a divisor of the total number of modules in the subarray.

Notes.
 
A subarray with only one row will not experience any self shading.
 
If the number of rows is one or is not an integer, SAM generates a simulation message but will still run and generate results.

Modules in subarray from System Design page

The total number of modules in each subarray, as defined on the System Design page.

Length of side (m)

The length of the side of a row. The bottom of the row is parallel to the ground, and the side is perpendicular to the bottom.

For portrait module orientation:

length of side = module length × number of modules along side of row

For landscape module orientation:

length of side = module width × number of modules along side of row

GCR from System Design page

Ground coverage ratio from the System Design page, equivalent to the length of the side of one row divided by the distance between the bottom of one row and the bottom of its neighboring row.

Row spacing estimate (m)

The distance in meters between the bottom of any two rows in the subarray. SAM displays this value for reference. The self-shading calculations use the module area and GCR.

row spacing estimate = length of side ÷ GCR

Module aspect ratio

The ratio of the module length to module width.

The built-in module libraries for the CEC and Sandia module models contain data for the area of each module, but not for the module length and width. For this reason, you must provide the aspect ratio as an input for the self-shading calculations. You can use the module dimensions for your module from the manufacturer data sheet to calculate the aspect ratio, and compare SAM's calculated values to the values on the data sheet to check your work.

Module length

The length of the long side of the module.

module length = √( module area × module aspect ratio )

Module width

The width of the short side of the module.

module width = √( module area ÷ module aspect ratio )

Module area

The module area from the Module page.

The self-shading model estimates the reduction in the array's DC output due to row-to-row shading of modules within the subarray, where shadows from modules in adjacent rows of the array block sunlight from parts of other modules in the array during certain times of day.

The response of a real photovoltaic module to shading is complex, and depends on several factors, including photovoltaic cell material, shape and layout of cells in the module, and configuration of bypass diodes in the module. SAM's self-shading model makes the following simplifying assumptions.

For the Standard (non-linear) option:

•The cell material is crystalline silicon, either mono-crystalline or poly-crystalline. The self-shading model does not work for modules with thin film cells. SAM indicates the cell material on the Module page under Physical Characteristics.

•Each module in the array consists of square cells arranged in a rectangular grid with three bypass diodes.

•The array uses the fixed or one-axis tracking option on the System Design page. The self-shading model does not work for two-axis or azimuth-axis tracking.

For the Thin film (linear) option, the subarray's DC output responds linearly to the reduction in plane-of-array irradiance.

Notes.
 
Self shading is only available for fixed subarrays, or for subarrays with one-axis tracking.
 
Self shading is not available with the Simple Efficiency Module Model on the Module page.
 
For one-axis tracking subarrays with backtracking and/or with non-linear self-shading, you can use the terrain inputs on the System Design page to specify the slope of the ground.

Self Shading Inputs

The self-shading inputs consist of the array dimension inputs that apply to both the self shading and snow loss, and the self-shading inputs described below.

Self shading

None uses the approach of versions of SAM before SAM 2014.1.14. Because this option does not account for any self-shading, it tends to overestimate the array's production. We included this option to allow for comparison between the different options to see the effect of the self-shaded and backtracking options, and for comparison between results from this version and older versions of SAM.

Standard (non-linear) is appropriate for most types of modules with crystalline silicon cells. It estimates losses from self-shading caused by shading of modules in one row by modules in neighboring rows based on the GCR value you specify.

Thin film (linear) is for modules with thin-film cells or for specially-designed modules with cells and bypass diodes wired in such a way that the modules output varies linearly with shaded area of the module.

For a description of the snow model, see the following publications available along with other technical documentation from the SAM website:

•Gilman, P.; Dobos, A.; DiOrio, N.; Freeman, J.; Janzou, S.; Ryberg, D. (2018) SAM Photovoltaic Model Technical Reference Update. 93 pp.; NREL/TP-6A20-67399

•Ryberg, D.; Freeman, J. (2017).  Integration, Validation and Application of a PV Snow Coverage Model in SAM. National Renewable Energy Laboratory. 33 pp. TP-6A20-68705 available along with other technical documentation from the SAM website.

Use the following output variables to explore the effect of snow cover (see Results for descriptions of the variables):

•Array DC power loss due to snow (kW)

•Weather file snow depth file (cm)

Note. Snow depth data is not available in the NSRDB PSM V3 dataset. It is available in the NSRDB 1961 - 1990 Archive Data. This older data does not represent the best up-to-date data from the NSRDB, but may be useful for testing SAM's snow loss model.
 
The Ryberg (2017) paper cited above includes a United States map of annual average snow loss values that could be used to estimate snow loss using inputs on the Losses page instead of the snow model when snow depth data is not available.

SAM's snow model for photovoltaic systems estimates the loss in system output during time steps when the array is covered in snow. It uses snow depth data from the weather file, and for time steps with snow, estimates the percentage of the photovoltaic array that is covered with snow based on the array's tilt angle, plane-of-array irradiance, and ambient temperature. The model assumes that the array is completely covered with snow when the snow depth data indicates a snowfall, and that snow slides off the array as the ambient temperature increases.

Estimate snow losses

Check this option to model snow losses if the weather file on the Location and Resource page includes snow depth data.