Calculating Far Shading in SAM

Hello-

My colleague and I would like to expose the far/horizon shading functionality in SAM and had some questions about the best way to go about it.

In my understanding, the shading calculation has two inputs - a shade table of 8760 values that describes when the beam irradiance is being shaded by the horizon for each hour of the year and by how much in that hour, and a factor that describes how much of the diffuse irradiance is shaded by the horizon.

Regarding the diffuse loss factor, how would one calculate this factor? Intuitively, we think it would be the area of the skydome visible to the array/divided by the total area of the skydome. Is our intuition correct in this case? Regardless, it would be quite helpful to have an exact formula of how to calculate the loss factor, which we imagine involves integration.

Since you can only put in one loss factor, we assume that the diffuse loss is only calculated in fixed-tilt systems.

Thanks,
Joe

Joe,

Your intuition is correct as far as the integration of the shaded area of each active surface in the spherical dome.
The actual integration is performed in the function in the s3tool.cpp file at github.com/NREL/SAM/blob/f306e9117325327da0c44275c4c162e71dfefec5/src/s3tool.cpp#L773

The 3d shade scene is processed to give a shading fraction at each 1 degree azimuth and 1 degree zenith angle differential in the spherical dome. The sum of the ratio of the shaded to the active portion of each spherical dome differential is multiplied by the spherical differential to get the entire diffuse shading fraction. This calculation is done in s3tool.cpp at github.com/NREL/SAM/blob/f306e9117325327da0c44275c4c162e71dfefec5/src/s3tool.cpp#L864

You are correct that the diffuse shading fraction is appropriate to only the 3d shading scene which implies a fixed tilt system.

Each active surface in the 3d shading scene then has a calculated diffuse shading fraction as shown in s3tool.cpp at github.com/NREL/SAM/blob/f306e9117325327da0c44275c4c162e71dfefec5/src/s3tool.cpp#L881

Hope this helps,
Steve