1.1. Modelling of Materials

For an accurate simulation result it is essential to truthfully reproduce the optical properties of the different materials of the thermal collector. That means to clearly define for example the geometrical dimensions since they correlate directly with the IAM-values. For this purpose the program OptiCAD offers the opportunity to check the defined geometrical, energy-related and optical parameter by a visual 3-D presentation which is based on the entered data. This is very helpful because non-conformances and errors in entered data can be identified very quickly.

The absorber and the glass cover of a flat-plate collector represent the most relevant optical units for a ray-tracing simulation. Figure 1 shows the interaction of a light beam with two transparent covers and an absorber. Effects like refraction of rays and multiple reflections of them within the glass covers as well as between them and the absorber are well to be distinguished.

In OptiCAD, the optical properties of the solar glass can be calculated by declaring the absorption coefficient and the refraction index. The transmission of solar radiation incident under a specific angle is calculated in the program by applying the Fresnel equations.

The optical properties of the absorber are implemented in OptiCAD according to their measured data. In addition to the capability of absorption the fraction of reflection under different incident angles plays an important role since the portion of reflected radiation increases with increasing incidence angle.

The mode of reflection of rays (direct or diffuse) at the reflectors of concentrating collectors is as substantial as the fraction of reflection. Modelling of scattered reflection in OptiCAD has been treated in [1].

Due to polarisation of light after passing one glass cover it is important particularly in case of collectors with multiple covers to investigate the optical characteristics by utilising a polarised light source. The software OptiCAD then is able to detect the state of the light’s polarisation after passing through the transparent covers and incorporates such effects in its simulation calculations.

In Figure 2 the measured transmittance of one antireflective treated solar glass weighted with the solar spectrum AM 1.5 for s-, p — and non-polarised light is plotted over the incidence angle. The transmittances simulated with the ray-tracing program OptiCAD are also shown in the figure for comparison. It can be seen that the curves fit very well, whereby the adequate performance of the software is demonstrated.