a double-pane insulating glass unit (IGU). The panes are proposed to have anti­reflective coatings in order to increase the active thermal and PV performance for a vertically oriented element. The insulating and sun-shading properties of the reflectors and the anti-reflective coatings are objects for evaluation.

A 1 m2 prototype of the solar window has been constructed for evaluation of the thermal properties. Five hybrid absorbers with reflectors were mounted in a wooden frame with a double pane IGU attached in the front. The U value of the window together with the closed reflectors has been calculated from measurements in a guarded hot-box, according to ISO 8990 (Johansson 2004, ISO8990 1994). The Solar Window was placed in a square shaped hole between a cold and a hot space of 21.6 m3 each. The hot space contained a guarded measuring box, covering the hole and the heating device. The U value was calculated according to Eq. (1):

U = —

A — AT

[Eq. (1)]

U is the U-value of the construction (W/m2K), Q is the power input for heating the guarded measuring box, A is the area of the window, and ATn is the environmental temperature difference between the hot and cold space.

Tests were made for the window separately and with the solar window components attached with the reflectors in six different positions, with four intermediate opening angles between the fixed open or closed modes. The window separately represents a U value of 2.80 W/m2K. The U value of the whole Solar Window differs between 2.42 in the fully open mode, to 1.33 for the fully closed mode, see figure 4.

The effect on the U value with the reflectors opened derives from the reduced convection due to interruption of cold downdraught. Hence, the effect of the open reflectors could be regarded as an added internal surface resistance, which varies by opening angle.

The prototype construction was not made sufficiently airtight, why some compensation was made for this by sealing the gaps in its closed position. One measurement was made with the reflectors closed and sealed towards the absorber insulation, and another one also with added sealing between absorber insulation and window, in order to reduce the channel of cold downdraught to one individual module. These two steps made the U value drop from 1.33 to 1.22 and 1.17 W/m2K respectively. Air tightness is hence an important criterion for further design studies.

The visual shading effect of the reflectors as sunshades in a closed position is total, which means that effective solar shading, visible shading and effective insulation is obtained simultaneously. However, the shading effect of the reflectors in an open position needs to be evaluated. Especially the risk of glare due to the concentration

37°

1,73

47°

1,86

56°

2,07

66°

2,25

a: 0°

U: 1,33

95°

2,43

Figure 4: U values for different opening angles, a

Figure 5: Ray-tracing illustrating the distribution of direct radiation with the reflectors opened at a solar height of 30° (left) and 20° (right).