The integration of solar thermal collector into building facade brings several essential advantages in comparison with solar collectors mounted separately from building envelope (in front of envelope or on supporting structures above the roof). Facade collectors have been investigated in number of research projects [1-3] to reveal advantageous synergy effects of facade integration. Additionally to the basic function of solar collectors, facade collector serves also as protecting shield against atmospheric effects (weather protection) and improves the thermal properties of the building with respect to passive solar gains in winter season. Furthermore, integration of collectors into building facades leads to aesthetically and visually more attractive solution compared with collector fields placed on flat roofs (most of residential buildings in urban housing estates), which create industrial appearance of the buildings. Facade solar collector can be either thermally coupled with envelope (direct integration) or thermally separated by means of ventilated air gap (indirect integration). Advantage of facade direct integration comes in higher thermal efficiency of solar collector due lower front heat loss through glazing (vertical air gap results in lower Nusselt number than inclined) and due to lower back and edge loss (building insulation layers) at normal incidence of solar radiation.

Behaviour of solar systems with facade collectors has been analysed through usability of solar gains to cover energy demands for hot water and space heating systems and interaction of facade solar collectors with building indoor environment (heat gains through envelope in winter, interior overheating in summer) in detail [4-6]. For climatic conditions in central Europe, the maximum annual irradiation is received with 45° sloped surfaces oriented to the south. In the case of facade collectors with a vertical slope, the reduction in the annual irradiation sum is around 30 %. Fig. 1 shows the annual profile of average daily solar irradiation for roof (45°) and facade (90°) collector based on the test reference year for Prague. The comparison shows a large difference between the summer peak and winter season values for the case of roof solar collector and a relatively uniform profile for the facade collector which corresponds closely to the heat demand profile (approx. constant with a decrease in the summer season, in the case of DHW system). Thus, solar DHW systems with facade collectors require approximately 25 % higher collector area to achieve solar fraction 50 — 60 % compared to optimally sloped collectors (45°). Required faqade collector area becomes equal to roof installations in the case of oversized DHW systems with higher solar

fraction (above 65 %) but with much shorter stagnation periods due lower portion of nonutilisable energy gains in summer [4]. For solar combisystems (DHW and space heating) the situation is different. Due to relatively uniform irradiation profiles, facade collectors provide the same yearly energy gains in comparison with standard roof installations but with radically reduced frequency and level of stagnation conditions [5, 6] for usual design parameters.