Solar azimuth angle

Hello,

The calculated solar azimuth angle for tower and trough remains constant for several hours (between sunset and sunrise I think). which is not correct, is there a reason behind this?

Regards

Hello,
The solar azimuth angle is -90 degrees during hours when the elevation angle is zero degrees, or when the sun is below the horizon.
In the graph below, the solar azimuth angle is green, and the elevation angle is blue (for five summer days in Tuscon, AZ at 32 degrees latitude):

Best regards,

Paul.