About setting GCR

Hello, 
I have few questions about setting GCR value in SAM.

1. Meaning of GCR

Is it correct that the GCR value becomes 0 when the PV system consists of only one array?
If not, does GCR mean the area of installation divided by the area of the panel? A/B




2. Project type: PVWatts and Detailed

It appears that the current version of SAM(2021.12) has no GCR setting if the roof type is set to fixed roof mount in the PVWatts project.

Is there a method to avoid setting the GCR value in the Detailed Project?

3. Is the GCR calculation in SAM for the Shading calculation or for the estimation of economical cost  for the land use?

Thank you

Hi Jiyoon,

Here is the definition of GCR from SAM's Help system for the Detailed PV model:

Ground coverage ratio (GCR)

The ratio of the photovoltaic array area to the total ground area. For an array configured in rows of modules, the GCR is 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. An array with a low ground coverage ratio (closer to zero) has rows spaced further apart than an array with a high ground coverage ratio (closer to 1).The ground coverage ratio must be a value greater than 0.01 and less than 0.99.SAM uses the GCR to estimate self-shading losses for fixed and one-axis trackers, determine when to backtrack for one-axis trackers with backtracking enabled, and to estimate the array's land requirement for installation cost calculations.To see the effect of the ground coverage ratio on the system's performance, after running a simulation, you can compare the time series results Nominal POA total irradiance (kW/m2) and POA total irradiance after shading only (kW/m2). You can also run a parametric analysis on the ground coverage ratio value to find its optimal value.

The definition is slightly different for the PVWatts model:

Ground coverage ratio (GCR)

The ratio of the photovoltaic array area to the total ground area. PVWatts uses the GCR to estimate self-shading losses for the fixed open rack and 1-axis array types, and to determine when to backtrack for the 1-axis backtracking option. The GCR does not apply to the fixed roof mount and 2-axis tracking array types.

For an array configured in rows of modules, the GCR is 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. An array with a low ground coverage ratio (closer to zero) has rows spaced further apart than an array with a high ground coverage ratio (closer to 1).

The ground coverage ratio must be a value greater than 0.01 and less than 0.99.

Regarding land area, as of SAM 2021.12.02, the GCR should affect SAM's land area estimate, but there is a bug that we will fix for the first update that we plan to release at the end of this month (January 2022): github.com/NREL/SAM/issues/826 . You can set the land area multiplier to 1 / GCR as a workaround until we fix this issue.

Best regards,
Paul.

HI Paul,

By "array configured in rows of modules", do you refer to roof-mounted installations?

Would you say this graphic is representative of what you mean? 

Thanks in anticipation

Hi Daniel,

Please attach the image as a file and then insert it into your message. Copying and pasting images into the editor does not work.

Best regards,
Paul.

Hi Paul,

This is what I mean, I wasn't sure if you took into consideration tilted roofs in your analysis but for calculating the AC power ouptut for a particular peak capacity on a tilted roof, is the GCR always set to 0.99 since the panels are on the same plane and most likely have rows of PV arrays installed close together? Could there be instances where the GCR on tilted roofs is close to 0.5 if for instance the length of array compared to the distance between the bottom of array on top and the bottom of the neighbouring array under is huge? Say panels with length of array 1m and the second row being 2.5m apart from the first row on a tilted roof.

Also, Is there anything like inter-row shading on tilted roofs and if there isn't, does this mean SAM is only relevant for flat rooftops with PV panels at a relative tilt?

Looking forward to a response.

Regards,

Daniel

Hi Daniel,

The GCR is defined as the total module area divided by the total ground (or roof) area occupied by the array.

In SAM, the GCR only affects the production estimate if self shading is enabled on the Shading and Layout page. SAM's self-shading calculations assume modules are arranged in parallel rows, and uses the GCR to estimate the distance between rows.

For an array with modules in the same plane, there is no self-shading, so you should disable self shading so that the GCR doe snot affect the production estimate.

Best regards,
Paul.