As an answer to the search for truly building integrated solar energy systems, an experimental design was proposed, which combines all useable forms of solar energy into one system; active and passive heating, PV electricity and daylight. The concept also aims at visually exposing the system in a novel and attractive way. The key for this challenge was simply to use a window as the glazing for a solar collector. By using hybrid absorbers and pivoted reflectors behind the window, a multifunctional and responding building skin is achieved. The basic concept of building integration is

hence changed from the notion of the solar energy system being part of the building envelope, to the idea of the building envelope being part of the solar energy system.

The system consists of three main components: the window, the hybrid absorber and the reflector, see figure 1. The combination is intended to give synergy effects by ascribing the components multiple functions

The hybrid absorber is fixed in an angle of 20° to the horizontal plane. A 2 mm thick aluminium absorber has PV cells laminated on the upper side. The thickness

reduces movements due to temperature differences, which otherwise puts the PV cells at risk of cracking. Water pipes are attached to the bottom for distributing active solar gains and for cooling the PV cells and the cavity between the window surface and the reflectors. Building integrated, they also serve as supporting structure for the absorbers and the reflectors, and as the pivot for the reflectors. EPS insulation around the pipes also makes endings for the rotation of the reflectors, and connects the insulation of the reflectors into a continuous convection shield.

The reflector screens are primarily intended for concentrating the solar radiation onto the hybrid absorber. Thus, the need for expensive absorber and PV cell area is reduced, as it is largely replaced by substantially cheaper reflecting material. The resulting distance between the fixed absorbers thus makes it possible to achieve transparency between them when the reflectors are of little use. Hence, daylight may filter through the structure, which also gives passive thermal gains. For passive solar house designs with use of large south facing window areas, risks of overheating and thermal losses are common. The reflectors are intended to reduce these problems, by serving as internal sunshades during daytime and as internal insulation during night time. The reflecting geometry is a two-dimensional parabolic curve, with the optical axis tilted by 15° from the horizontal plane, see figure 2. It has a geometrical concentration factor, i. e. the ratio between the glazed opening and the absorber area, of 2.45. The curve is extruded horizontally as a trough, and the reflector is constructed as a sandwich composition with a 35 mm EPS core between the reflective film on the concave side and a birch veneer on the convex side.

The window serves as the climate shield and as the solar radiation transmitter for the system. After the solar radiation is transmitted through the window, it is distributed as daylight, passive or active heating, or as PV electricity, in proportions
depending on the handling between the closed or open modes. For maximal input for the PV/T absorber through a vertical surface, the transmittance through the window needs to be maximized. Therefore, a highly transparent glass with anti-reflective coating is used. Due to the over-heating precautions by the solar shading and the cooling effect of the absorber, a higher transmittance of the glazing can be tolerated.