As a result of our first suitability tests, UV-stabilized composite films are selected for prototyping a prismatic structure as light-redirecting device with seasonal shading features.

On the basis of the experimental material data, we carry out ray-tracing simulations with different glazing designs integrating the prototype film to determine the solar optical properties of the complete system. Centre-of-glass performance indices (solar heat gain coefficient — SHGC — and thermal transmittance — U-value-) of the corresponding glazings are then calculated taking into account the infrared optical properties and thermal conductivities of the single components.

Figure 4 shows the incident angle dependent SHGC of a triple glazing with 2 highly transmittive low-e coatings: values variable from 0.58 at low solar altitude to less than 0.10 at high solar altitude can be achieved, demonstrating the dynamic properties of the prototype

To evaluate the impact of this angular selectivity on energy savings, solar energy gain for a south facing window is simulated over a year and compared to the performance of conventional multiple glazings, whose angular dependent SHGC are calculated with the software WINDOW 5.2 [10]. Hourly simulations are based on beam and diffuse irradiance data of the German Test Reference Year, Region 3 [11]. The results are shown in Fig.5 and confirm the potential of the prismatic glazing, which performs similar to the triple glazing during the winter months and similar to the

best solar-control double glazing during the summer period, thus providing both heating and cooling energy savings.

Fig.4. Simulated angular

Fig.5. Calculated monthly solar energy gain of the prismatic prototype compared to
commercial double and triple glazings (conventional glazings are labelled
with light transmittance and SHGC evaluated at normal incident angle)

For the use as transparent insulating material, where high energy gain is required, the performance
of the glazing can be further improved by substituting clear with white glass and by applying

antireflective coatings. According to our simulations SHGC up to 0.64 can be achieved, even higher than a common double glazing.

For the use in a glass facade, where visual appearance and comfort play an important role, the implementing of a light scattering glass pane as inner layer is currently investigated. Depending on the optical properties of this pane, it is possible to adjust the visible and solar transmission as well as the solar heat gain coefficient over a wide range in order to meet the application-specific requirements [12]. Detailed results of our investigations will be published at a later date.

Concerning the insulating properties of the glazing, we calculate night-time thermal transmittances between 0.6 and 0.8 W/m2K, depending on gap-width (10-12mm) and gas-fill (Argon or Krypton). These values are comparable to those of common triple glazed systems and demonstrate the negligible effect of partial infrared transparency of plastic films if combined with low emissivity coatings.