Spatial Graphs Explanation

Hi, my name is Luis. 
I’m currently trying to run some simulations for my university thesis, but I’ve been having some issues trying to understand some things. (I chose the Detailed PV Model, in case that’s important). Maybe my doubts would seem easy to some of you, but I’ve been wondering around a couple of weeks and didn’t clearly understand them, so here I am.
I have to run various simulations with two configurations: tilted monofacial solar modules and vertical bifacial solar modules. Furthermore, for each case I have to vary the tilt angle (only on the monofacial tilted modules case) and the distance between rows (varying the GCR). 

I picked the modules and inverters in their corresponding tabs. In the System Design tab I used the default number of modules and inverters that SAM selected (they’re like 220k modules and 99 inverters). I put 3 strings of 3 modules each on the shading 3D design tab, but I don’t know if it’s supposed to connect later on with the simulation process. At least I think mine didn’t, so I assumed that the simulation was going to run with the System Design tab data (I didn’t find anywhere how to connect the, so If any of you know let me know, but I think for a larger number of modules it’s better to use the System Design tab data). Then I proceed to run the simulation.


My doubts are around the Spatial tab in the simulation screen. For each case SAM generates two graphs by default: the radiation between rows and the radiation on the rear side of the module. In both cases, the vertical axis corresponds to the 8760 hours in a year. I have the following doubts: 

1 – What module and of which string string are the graphs of simulation showing? I have a 14 modules per string and around 15k strings in parallel. I thought that the graphs showed the values for a module in a central position belonging to the first front string. Maybe you can select which module you want to run simulations of, but I dindn’t find it anywhere.

2 – Regarding the irradiance between rows in the graph, the horizontal axis represents the “meters from the front”. When I changed the GCR, this axis also changed as I expected, because the distance between rows varied too. However I don’t understand the values. I multiplied the GCR by the length of the module to get the distance between rows when varying the GCR, but they never concur. For example, in one case I have GCR=0.3 (the module is 2.03m x 1m), so the distance should be of 3.5m aprox. And the horizontal axis of this distance goes until 7m. Why is this? Or maybe am I understanding something wrong?

3 – Regarding the radiation on the rear side of the module in the graph, the horizontal axis represents the “meters from the front”. I don’t understand what does it mean with meters from front here. Because one fo the sides of the module is 2m long and the graph horizontal axis was also 2m long, I asumed it showed the radiation of one portion of the rear side of the module for each hour of teh year. Nevertheless, when I tried this with the vertical bifacial modules, my thoughts didn’t add up, because the axis was like 7m long, and the module only 2m long. I dind’t make any sense. So, could you tell me how to read or tell me in what way are this grpahs showing the radiation of the rear side of the module?  

4 – In the Shading and Layout tab, there’s an orientation option. You can choose between landscape and portrait option. However, I have this option blocked because I selected None on the Self Shading option because then I didn’t know how to get a integer number below on the Calculated System Layout part. How can I fix this and select the orientation? or is this not really necessary? In case I didn’t/couldn’t specify because the option is blocked, which orientation does SAM select by default?Also, the dimensions of the modules seem to be different here than in the Module tab, I don’t understand why.

Thanks a lot for your help guys,
Luis

Hi Luis,

If you are modeling row-to-row shading of modules in the array, you don't need to use the 3D Shade Calculator. The shade calculator is intended to calculate shading loss data for arrays with shading by external objects like trees, nearby buildings, etc.

For row-to-row shading, called "self shading" in SAM you can choose the appropriate self shading option on the Shading and Layout page: "Standard (non-linear)" for modules with crystalline sillicon cells, or "Thin film (linear)" for modules with thin film cells. The shading options are described in the Help topic for the Shading and Layout page: samrepo.nrelcloud.org/help/pv_shading.html

As for your questions:

1. The two plots on the Spatial tab of the results page for bifacial modules show the irradiance incident on the ground between rows of modules, and the irradiance incident on the rear of modules in the array. The y-axis for both graphs represents time starting with January 1 (Hour 0) at the top and ending with December 31 (Hour 8759) at the bottom. The x-axis represents the distance in the East-West direction (morning to evening) for one-axis tracking (assuming the tracking axis is defined by the azimuth angle), and from the front of the row for fixed arrays (assuming rows face the azimuth angle). For an azimuth of 180 degrees and one-axis tracking, the tracking axis is North-South so that modules rotate from East to West. For an azimuth of 180 degrees and fixed arrays (no tracking), the row bottom is the southern-most edge of the row.

2. The distance between rows is defined on the Shading and Layout page, and depends on the module dimensions, orientation (portrait or landscape), and number of modules along side and bottom of row. The "Row spacing estimate" on the Shading and Layout page is the distance between rows.

3. See my answer to #1 above.

4. For this analysis, I think you should enable self shading.

Best regards,
Paul.

Hello Paul, thanks a lot for your answer, and sorry for the late response.
Okay so I understand how to read these graphs now. 

What I didn't get is the graphs corresponding to which module. I've reduced the module number as my tutors suggested, so now I have 8 rows of 3 modules each (24 in total). Do this graphs show the irradiation of the central module on the first row? The irradiation shouldn't be the exact same for every module, so that's what I need to know. Or maybe is the easternmost or westernmost of the first row.

Also, I have some Time Series graphs regarding the front, rear side and total radiation of the module/s. The options are in the Lifetime Hourly Data part, so is this showing hourly values for a year? And the results are from which module? Or are they global results for the whole system?

Does SAM allow you to obtain the average irradiation values?

Another topic is the height. My modules are supposed to be 1,5m high. In the module tab I selected the "Ground or rack mounted" option, and for the array height the "One story building height or lower" option. Is there any option where I can select the specific height I want?

Thanks again,
Luis.

Hi Luis,

Each graph represents a subarray. You can define up to 4 subarrays on the System Design page.

The "Ground Irradiance Between Rows" graph shows the solar irradiance incident on the ground in the space between rows of modules.

The "Module Rear Irradiance" graph shows the irradiance on the rear of of a row of modules.

The "Lifetime Hourly" time series data shows irriadiance on each subarray for each hour over the life of the system (8760 hours * analysis period).

As for height, the array height input on the Module page is an input to the cell temperature model, which is separate from the bifacial ground clearance height. You can use the ground clearance height to specify the height of the module for the purpose of calculating rear-side irradiance.

Best regards,
Paul.