Ciarán and Sarah,
the incidence angle is indeed the angle between the beam solar and the normal to the surface. At night, the incidence angle is set to an arbitrary value. However, when the sun is up we do not clip the incidence angle to force it to be between 0 and 90. During summer months, the sun comes up well to the north of the east/west axis (in the northern hemisphere) so the incidence angle can be greater than 90, which simply indicates that the sun is behind the surface in question.2) If I'm not mistaken, you're right, the incidence angle is indeed the angle between the incident beam and the normal of the surface. However for me also the incidence angle profiles do not seem realistic for surfaces which are not horizontal (for horizontal surfaces the incidence angle profile is good, between 90 and 0ish degrees; but for example on a 45 degree slope facing south, the incidence angle jumps from 90 to approx. 120 degrees at dawn and dusk...). Maybe someone else can help you with that one (?)
Best, David -- *************************** David BRADLEY Principal Thermal Energy Systems Specialists, LLC 22 North Carroll Street - suite 370 Madison, WI 53703 USA P:+1.608.274.2577 F:+1.608.278.1475 d.bradley@tess-inc.com http://www.tess-inc.com http://www.trnsys.com