The optical efficiency of the collector is dependent on the incidence angle of direct solar irradiation. By
definition, η0 is the optical efficiency at normal incidence (i.e. sun in zenith), and under ideal conditions (i.e. with
100% clean reflectors and receiver glass tube). For the collector described, it had been determined by IS via
raytracing, being:
η0 = 0.635
To calculate the efficiency for any other incidence angle, an incidence angle modifier (IAM) is defined as a
correction factor to be multiplied with η0. The transversal IAM is independent of the field design, whereas due to
end losses along the fixed axis, the longitudinal IAM depends on the number of modules being combined in one
string. Data on a representative IAM are shown in Figure 4.
Because of shading effects, the IAM is greater than 1.0 for some sun positions. Therefore the optical performance
is not at its maximum when the sun is in zenith position. The maximum efficiency will be reached when the sun is at
a transversal zenith angle of 5° and a longitudinal zenith angle of 0°.
ηmax = 0.663 (for sun at 5° transversal zenith angle, 0°longitudinal zenith angle)