In order to calculate the total solar energy transmittance (g-value) [5], the secondary heat flux Is must be calculated. The thermal simulation program HEAT2 was used to estimate the heat flow into the room caused by the absorption on the back of the reflective layer and in the glass itself. It was assumed that the glass bars were positioned 1 mm from the outer glass pane. The bars themselves had a diameter of 10 mm, and the distance from the bars to the inner glass pane was 12 mm.

With the secondary heat flux Isec heat, the direct and diffuse intensities (Isec dir, Isec diff) and the incident global radiation Ig given, it is possible to compute the g-value:

Fig. 7 g-value of the system (в = 90°) for days with Idir = 0 (cloudy) and Idjr > 300 W/m2

(sunny).

The g-value was also estimated for the tilted system, with a slope of 30° (Fig. 8). One can see that the g-value for cloudy weather increases slightly while the one for sunny weather decreases. The latter comes from the fact that, especially in summer when radiation is strongest, we don’t get multiple reflections and so the absorptance on the darkened side remains small. This effect will be more pronounced for the secondary heat flow in this case.

As mentioned before (see Fig.4) a considerable part of the direct radiation is absorbed on the blackened side of the reflective layer. This will lead to a secondary heat flux into the room. In order to get a feeling for how much of the g-value is radiation that can be used for illumination and how much is heat radiation depending on the absorbed power, the amount of heat flowing into the room was computed with HEAT2. Figure 9 shows that even for the в = 0 case in summer the maximum heat flux into the room does not exceed 80 W/m2. For the tilted case (в = 30°) the secondary heat flow is even smaller (Fig.10).

40.0

30.0

20.0

10.0

0.0

0 730 1460 2190 2920 3650 4380 5110 5840 6570 7300 8030 8760

Time [h]

Fig. 10 Secondary heat flow into the room arising from the absorption on the blackened
side of the reflective layer, tilted case (в = 30°).

The percentile contribution of the secondary heat flux and the visible diffuse radiation to the g-value is plotted in Figure 10 for the vertical case. As one can see, the secondary heat flux Isec heat contributes only about 30 % to the total g-value, whereas the diffuse radiation transmittance makes up the greater part and should be sufficient to illuminate the room. For the tilted case Isec heat will contribute about 40% in winter and 15% in summer.

Conclusions

As has been shown, the system efficiently shuts out the direct radiation. This reduces glare. Even though the main part of the direct radiation is absorbed by the blackened side of the reflective layer, overheating should not be a problem, if the glass bars are positioned close to the outside glass pane, as the heat will be conducted that way.

Regardless of the system’s properties for direct radiation, the transmission for the diffuse radiation will be around 60% throughout the year, guaranteeing a high illumination level in the room.

Improvement could be made using photochromic layers, which would darken only on the focusing line). This would make a mechanical adjustment superfluous.

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