Hi William,
Here's a little more on your question based on input from one of the NREL CSP engineers.
There is a difference between the weather file input DNI and the receiver incident flux. The former is a value that you can set in the weather file, and the latter is a value that SAM calculates and is dependent on solar position, tracking, and optical properties. You can't change the solar position values because those are calculated by the model. That means that you could not force the receiver to run at night simply by assigning daytime DNI values to nighttime hours, for example, because SAM assumes the sun is down during nighttime hours, regardless of the DNI values.
SAM's physical trough model performs some checks on the weather file DNI values to ensure that those inputs make sense relative to the solar position and global solar constant. The linear Fresnel model does not perform those checks, so it will run simulations even when the DNI values are unreasonable. For example, we ran the linear Fresnel model with the TMY3 file for Blythe, CA, but multiplied the DNI values by 10 to create obviously unreasonable data. The linear Fresnel model ran without errors.
In general, the current version SAM is designed to run with "standard" weather files that have realistic values and does not perform systematic test on the weather file data to check for potential errors in the data.
Best regards,
Paul.
