9.5. Determination of effective performance parameters

For further in-depth analysis, the simulation results are related to the incident irradiance to calculate relative transmittance values. By dividing the total transmitted power and the power of the solarinduced secondary heat-flows by the incident solar power, the coefficients solar direct transmittance , the secondary internal heat factor qi and their sum, representing the total solar energy transmittance or g-value can be determined. The coefficients are denoted with the subscript sys (system) to distinguish them from the corresponding coefficients as defined in the standard EN 410 (EN 410, 2011). In order to determine the incident power in this “system approach”, a reference area must be defined. This could be either the area of the entire reveal, the area of the window (glazing and frame) or only the externally visible glazing area. Depending on the application and objective of the study, either approach may be appropriate. For the present analysis, the reference provided by the glazing area seems most suitable for comparison with the standard calculations. As pointed out in section 2.6, the standard values are calculated solely for the case of normal incidence and do not contain any shading or reflection of the window reveal. In contrast, the coefficients calculated here consider all relevant “real-world effects”. Therefore, they show a significant deviation from the standard values. In order to illustrate the magnitude of this deviation, average values for the coefficients were computed for the entire reference years on a daily basis.

The results of this inversion approach to derive the effective performance parameters of the glazings using the simulation results are depicted in Figure 153. It can be seen that the solar direct transmittance , and consequently the total solar energy transmittance g, range significantly below the standard values of 0.47 and 0.53, respectively (see Table 25). This reduction was expected and can be attributed to two factors: first, high proportions of diffuse radiation cause a noise-like variation, which can be seen especially well for the Hamburg location. Second, higher incidence angles significantly reduce the transmittance, which can be seen well in the case ‘Sevilla/south/unshaded’, for example. Note that, unlike in the heat balances presented in Fig. 152, the geometric projection effect Application case RadiCal, D. Rüdisser 222 (cosine-law) is already compensated since the external irradiance on the reference plane is used to calculate coefficients. Hence, the observed incidence-angle-dependent reduction of the transmittance is solely a consequence of reveal-shading, increased reflectivity of the glazing surfaces and higher absorption within the window panes.
Interestingly, in contrast to solar direct transmittance, the secondary internal heat factor shows only a limited variance and ranges relatively close to the normal incidence value calculated according to the standard (0.06). A comparison of the polar plot representation of the absorption SIOPs of glass panes vs. the transmittance SIOP for the entire glazing (not depicted here) reveals that relative to transmittance absorption increases with growing incidence angles. The longer optical paths and higher number of internal reflections within the glass panes cause this significantly higher absorption at higher angles. Therefore, the secondary heat flux is less sensitive to the angle of incidence, as the increased reflectivity that occurs at higher angles is partly compensated by the increased absorption.