Different simulations were made to estimate the optical and thermal properties of the system. The forward ray-tracer ASAP [2] was used for the former and the latter were calculated with the explicit finite differences program HEAT2 [3].

The chosen nomenclature for the irradiances can be found in Figure 2. The system was tested with two different slopes в (the angle between the system’s surface and the horizontal), for в = 90°, the panes are positioned vertically, and with в = 30°, so that Idir on a summer noon would be perpendicular to the surface at WQrzburg latitude. In both cases the daylighting element was facing south (azimuth y = 0°).

Optical properties

A reflective layer was assumed to be deposited on the focusing line of the glass bars (see Fig. 1) in order to send the direct radiation back outside. First simulations showed that starting from solar altitudes as of about 40° for a solar azimuth Ys of 0, the transmission of the direct radiation grows as a result of multiple reflections on the back of the reflective layer (see Fig.3).

Fig. 3 Multiple reflections on the back of the reflective layer for solar altitudse as > 40°,

solar azimuth ys = 0°.

04

dive

In order to avoid this problem and still keep the glare to a minimum, the back of the reflective layer was assumed to be black. This resulted in a higher absorptance but suppressed the high transmittance of the system for direct radiation. The results can be seen in Figures 4a and 4b, which also show the high transmittance for the diffuse radiation.

100 n	a) direct	100 n	b) diffuse
80	R	80
60 < 40 H 20	X’ 60 < 40 H 20	T
A
x •• * T	R
0	———- t——————————— T—	0	————- 1———— 1———— 1———— 1—-

0 20 40 60 80 0 20 40 60 80 solar altitude as / degrees solar altitude as / degrees

Fig. 4 Optical properties with the back of the reflective layer painted black for direct (left)
and diffuse radiation (right) at different altitudes and azimuth = 0°(south).

In a next step the optical properties of the system with the reflective layer painted black on the rear were calculated for all the solar angles occurring on a south fagade. Then the test reference year (TRY 05) for WQrzburg [4], which includes the direct radiation on a normal plane (Idir) and the diffuse radiation on the south fagade (Idiff) for every hour in a year starting January 1 at 0:00h, was applied to these data in order to obtain the transmittance, reflectance, and absorptance over a year for direct and diffuse radiation.

Figure 5 shows that for the direct radiation the transmission rarely rises above 20%. This is important in order to reduce glare problems. As one can see, during summer a large part of the radiation is absorbed mainly by the black side. This will lead to the glass bars heating up, which will in turn emit thermal radiation into the room. As we will show later, this effect is not dramatic as there is still the low-e coated glass pane behind the bars and thus the main part of the heat radiation will be blocked.

80

70

60

50

-V" — w «—■Є-

*■* ОТ»

40

•• •’ r

0 730 1460 2190 2920 3650 4380 5110 5840 6570 7300 8030 8760 Time [h]

0

Fig. 5 Optical data of the system for direct radiation plotted over the hours of a year

(starting January 1, 0:00h).

80

0

70

60

■Q 50

40

30

20

10

730 1460 2190 2920 3650 4380 5110 5840 6570 7300 8030 8760 Time [h]

0

Fig. 6 Optical data of the system for diffuse radiation plotted over the hours of a year (starting January 1, 0:00h).

The same simulation was carried out for the diffuse radiation (Fig.6). It is clearly visible that the transmission is high throughout the year, thus supplying the room with sufficient daylight.