The described semi-transparent collector is a multifunctional architectural design element. This offers a new market potential for solar thermal energy use. At a certain distance the structure of the small openings appears as a homogeneous semi-transparent area with the silhouette of the channels. This offers interesting possibilities in design: straight vertical or horizontal channel arrangements as well as curved designs such as in a FracTherm® absorber  can be realized if roll-bonding or another production method with similar flexibility is used.
Office buildings often feature large glazed fa? ade areas and need appropriate devices for solar control and glare protection. Moreover, cooling becomes more and more important. Provided that the collector efficiency and thus the temperatures will be high enough, it will be possible to use them to drive a solar cooling system. This could also be done floor by floor, since the collectors are nearby the room with the cooling load. The collectors can also be used in hotels, hospitals or sanatoria where both an effective solar control and a high amount of domestic hot water as well as heating water are needed. A special application is shown in Fig. 9, where the semi-transparent absorber is integrated into a balcony balustrade with the insulated flow and return pipes covered by a stainless steel handrail.
The multifunctional collector is still in an early stage of research and development. The necessary work will be carried out as part of a large European research project together with industry partners.
The main challenges will be the following:
• maximization of the absorber efficiency (efficiency factor F’ and insulation of the collector, which is also necessary in order to prevent the room from overheating)
• characterization and optimization of the collector as a classical solar collector (incidence angle modifier IAM, collector efficiency curve)
• characterization and optimization of the collector as a window-like element providing solar control, glare protection, transparency and daylighting (angular-dependent g-value, U-value and transparency)
• fa? ade integration regarding dimensions, aesthetics, connection of tubes including insulation
• feasibility of absorber constructions – especially the production of the openings and coating – including economical aspects
This paper describes a multifunctional semi-transparent fa? ade collector which allows using solar energy to heat a fluid and simultaneously acts as a building element which provides solar control, glare protection, transparency and daylighting. It has been shown that the relation between the aforementioned properties can be adjusted to the specifications of an individual application by geometry and coating. The new collector is both an “active” and a “passive” architectural element which needs to be characterized and optimized with respect to its various functions. This work will be done within a European research project together with industry partners.
The contract for the European research project was not yet signed when this paper was written. This is the reason why the project partners are not mentioned by name. The consortium applied for the project in the call FP7-NMP-2007-LARGE-1 within the Seventh Research Framework Programme (FP7) of the European Commission.
 T. E. Kuhn, “Solar control: A general evaluation method for facades with venetian blinds or other solar control systems” Energy and Buildings, Vol 38, Issue 6, pp. 648-660, June 2006. http://dx. doi. ora/10.1016/i. enbuild.2005.10.002
 T. E. Kuhn, “Solar control: Comparison of two new systems with the state of the art on the basis of a new general evaluation method for facades with venetian blinds or other solar control systems”, Energy and Buildings, Vol 38, Issue 6, pp. 661-672, 2006. http://dx. doi. ora/10.1016/i. enbuild.2005.10.001
 I. Bergmann, W. Weifl, Fassadenintegration von thermischen Sonnenkollektoren ohne Hinterluftung, Berichte aus Energie – und Umweltforschung, 13/2002
Facade-Integrated Thermal Solar Installations (2001), System and building physics fundamentals and implementation of results within the subprogram "Building of Tomorrow", Forschungsforum 3/2001, Publisher: BMVIT, http://www. nachhaltigwirtschaften. at/nw pdf/fofo/fofo3 01 en. pdf
M. Hermann, FracTherm – Fractal hydraulic structures for energy efficient solar absorbers and other heat exchangers. Proceedings, EuroSun 2004, Freiburg, Germany, 20-23 June 2004, Volume 1, pp. 332-338
M. Hermann, (2005). Bionische Ansatze zur Entwicklung energieeffizienter Fluidsysteme fur den Warmetransport. Dissertation, Faculty of Mechanical Engineering, Universitat Karlsruhe (TH)
M. Rommel, V. Wittwer, R. Blessing (1993), The Window-Technology Collector: A new type of flat-plate collector, Proceedings of the ISES Solar World Congress in Budapest, 23-27 August, 1993.