First of all, the collector can be regarded as a fa? ade collector with its typical advantages, disadvantages and special aspects to be considered in order to be designed properly. One of these important aspects is the minimum backside insulation [3, 4] to prevent the room from overheating. In the case of the semi-transparent collector these minimum insulation requirements have to be fulfilled by the inner glass pane(s). Depending on the efficiency of the collector and thus its temperature level, it will be necessary to use a double glazing with Low-E coating as “backside insulation” or multi­layers of transparent foils [7]. Another important topic will be the effect of the openings and the position of the absorber on the natural convection within the collector and thus on its efficiency. In order to have a high flexibility concerning the absorber channel design as well as an aesthetic appearance, roll-bond absorbers (Fig. 4) seem to be a good choice for this type of collector. In conventional absorbers, the heat transfer direction from the absorber surface to the next channel is basically perpendicular to the channel direction. In the case of an absorber with a three-dimensional structure it has to be considered that the heat transfer distance to the fluid is increased and the cross section is reduced: The sides of the lamellae (bent to outside) are not in contact with the channel; therefore the heat transfer is only possible via the remaining flat areas of the absorber which are connected to the channels. The advantage of roll-bond absorbers is the fact that their channel distance can be reduced without additional costs. It can be expected that smaller channel distances will compensate for the lower heat transfer of the three-dimensional structure mentioned above and thus lead to high collector efficiency factors F’ anyway. Moreover, for large sun elevation angles the incidence angle modifier (IAM) of the lamellae should be higher than for a flat vertical absorber. This could be beneficial for solar gains in summer in order to drive a solar cooling system. Roll-bond technology also offers the possibility to realize optimized channel arrangements such as fractal-like, multiply branched FracTherm® structures which lead to a low pressure drop as well as a uniform flow distribution [5, 6]. Even non-rectangular shapes can be used, which might be interesting for architectural reasons.

Regarded as a window-like building component, the three-dimensional structure of the absorber leads to an angular-selective behaviour. Depending on the geometry of the openings and their coating the properties concerning absorption, transparency, daylighting, solar control and glare protection can be adjusted according to the specifications of the particular application. Fig. 5 shows some principal variations of geometry and coating. Variant a) mainly focuses on absorption, transparency, solar control and glare protection, but it will have a poor performance with respect to daylighting since there are no reflecting surfaces leading the sunlight to the inside. In contrast to a), the diffuse reflector of

Variant b) features better daylighting behaviour, but poor absorption, solar control and glare protection. Variants c) and d) can be regarded as compromises of a) and b) with different focuses.

In Fig. 6 different views of a detail of a possible semi-transparent absorber with lamellae and fluid channel are shown. The size of this small part is only about 128 mm x 150 mm. The exterior view in a) gives an impression of the absorber seen from outside, whereas the internal view in b) shows the appearance from inside. It can be seen that for the assumed sun elevation angle a homogeneous shadow is formed (without light stripes from the openings). View c) and d) show how the effective transparency changes with the angle: only small slits appear if one looks through the absorber horizontally, but they seem to become bigger if one looks downwards. This effect is especially interesting for multi-storey buildings.

Fig. 7 shows the absorber detail at different incidence angles of the sun. In the raytracing simulations the absorber was oriented towards south and the solar azimuth angle was always 0. It can be seen that even for small incidence angles the shadow is homogeneous.

Fig. 8 gives an impression of the appearance of an absorber with an area of 1 m x 2 m and its shadow. In this case parallel, vertically oriented channels were assumed. However, other channel designs are also possible.