Photovoltaic - System Design - Terrain slope

Hi,
I am trying to simulate sloped-ground PV plants with single axis trackers.
When I fix "Terrain slope (deg)" value (let's say 8°) and I modify "Terrain azimuth (deg)" values (0°, 90°, 180°, -90°) and I run simulations, I get inconsistent Energy yield output results.
For instance, I get plants with Terrain azimuth = -90° having higher yield compared to South-sloped plants or in other cases I get more or less the same result for each Terrain azimuth I input.
South-sloped plant should have a higher energy yield compared to a flat plant, North-sloped plant should have a lower energy yield compared to a flat plant, East-sloped and West sloped plants should have more or less the same energy yield between them.
Can you clarify azimuth rules for Terrain slope and how should I simulate a sloped-plane plant in a proper way? 
Thank you very much
Best regards
Claudio Pesenti

Hi Claudio,

We did not have time to document the terrain inputs before we release SAM 2021.12.02. We are working on Help revisions for the first update that we plan to release in late January or early February 2022.

Here is the description:

<BEGIN>

Terrain Angles

The terrain slope and azimuth angles describe the inclination of the ground with respect to horizontal, assuming the subarray is installed on uniformly sloped, flat land. Terrain inputs are only enabled for systems with one-axis tracking. Their effect depends on the self shading options on the Shading and Layout page:

• Backtracking enabled: Backtracking algorithm takes the terrain angles into consideration to calculate the tracker rotation angle.
• Linear self shading enabled: Self-shading algorithm accounts for terrain angles to calculate the shaded fraction of the array.
• Non-linear self shading enabled with no backtracking: Terrain angles do not affect the self-shading calculations.

Note. The terrain angles are not available for fixed (no tracking) subarrays.

The terrain slope model is described in Anderson, K.; Mikofski, M. (2020) Slope-aware Backtracking for Single-axis Trackers. National Renewable Energy Laboratory. 24 pp. NREL/TP-5K00-76626. ( PDF 783 KB ), also listed at sam.nrel.gov/photovoltaic/pv-publications.html .

Terrain slope, degrees (0 to 90 degrees)
The grade slope angle, defined as the angle between the slope plane and the horizontal plane. Zero is for horizontal ground with no slope.

Terrain azimuth, degrees (0 to 360 degrees)
Grade azimuth angle, defined as the angle clockwise from north of the horizontal projection of falling slope. Zero is for a north-facing slope, or ground that slopes down toward the north.

<END>

I did some testing and am having trouble getting the slope to affect results. I will follow up with you after I investigate more.

Best regards,
Paul.

Hi Paul,
I am wondering if you have any feedback about issues concerning energy yield output for sloped-terrain plants?
Best regards
Claudio

Hi Claudio,

Could you attach a .sam file and a description of what you are trying to model?

The latest update (SAM 2021.12.02 r1) includes the documentation of the terrain inputs that I showed in my previous post. In your original message, you refer to negative terrain tilt and azimuth values, which are not valid and may explain the results you saw.

Best regards,
Paul.