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The newly developed multi-functional facade elements with integrated evacuated-tube collectors enable a view outside (visualisation)
© Universität Stuttgart, IBK 2
Solar-active facade systems
Projektinfo 07/2013

Computer model: The facade component is affixed in front of the glass pane. The curved, perforated reflector panel bundles the rays of the sun to the absorber tube.
© Universität Stuttgart, IBK 2

Visualisation: an office building with facade collectors could look like this from the outside.
© Universität Stuttgart, IBK 2

Measurements: the solar performance was measured with a test facility. Exposition tests were then conducted. At the Stuttgart Institute for Thermodynamics and Thermal Engineering (ITW), the finished facade modules are mounted as a 0 series – with and without cover panel.
© Universität Stuttgart, IBK 2
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Facade collectors with perspective

Scientists from the research and industry fields have developed a facade system which combines several functions: integrated evacuated tube collectors generate solar heat at a high temperature level, and provide even, semi-transparent light and protection against the sun for indoor areas without impairing the view. The system is particularly suitable for offices and other functional buildings with a large number of windows. It can also be used as an architectural design element for facades.

In office and administrative buildings, solar heat is still relatively seldom used, although this would be possible in technical and economic terms for hot water provision, support for the heating system and solar cooling. Often, the lack of roof space and competition with ventilation systems or recoolers, as well as attractive alternative uses such as roof terraces, are an obstacle. There is great potential for using facade surfaces for solar energy if planners succeed in harmonising all architectural and building equipment requirements such as transparency, light for indoor areas, thermal insulation and sun protection.

Scientists have opted for high-performance evacuated tubes for the new development, which are equipped with perforated parabolic mirrors and which are integrated into a unitised facade. The mirror bundles the direct irradiation of the sun and part of the diffuse light onto the evacuated tubes, in so doing reducing the heat input on the glass facade and therefore the cooling requirement of the building by between 70 and 90 %. By varying the hole density and size, the researchers have optimised the interplay between collector yield, sun protection and room lighting. From the inside, the sunlight appears dampened, is evenly distributed and is not blinding.

The collector facade provides temperatures of over 80 °C all year round, which are used for building heating or cooling and for providing hot water. The system has a modular structure and can be installed on different buildings and with any size required (see Fig.). The tube system is integrated into the profiles of the collectors, and is accessible from outside.

For architectural, geometric, technical and visual reasons, the horizontal alignment – as with sun protection blinds – has proven to be the most suitable (see Fig.). No blinds or sun protection glass is required. In a previous research project at the Institute for Building Construction (IBK 2) at the University of Stuttgart, scientists have already studied evacuated tube collectors for use on the facade. Initial ideas for integration have been sketched and functional prototypes have been produced.

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Perforation, glazing and solar yield

Visual transparency is created by the space between the tubes and the perforation in the reflector mirrors. It has been shown that with a low degree of perforation of 19 %, the solar efficiency and visual transparency were hardly affected. This perforated mirror achieves almost the same level of sun protection as with standard solutions, with a g-value of 0.15. 85 % of the light is reflected. Measurements with different versions – with and without a cover panel made of toughened safety glass or laminated safety glass – have shown that an increase in the perforation share in the reflector mirror of around 3 % only reduces the solar yield by around 2 %.

The yield from the collector is reduced by around 10 % by a 20 mm thick laminated safety glass cover panel in contrast to a 10 mm thick toughened safety glass cover panel. Specifically, this corresponds to an increase in the perforation share of around 10 %. The results of the basic measurements and simulation also show that the solar yield with a toughened safety glass panel and a thickness of 10 mm is reduced by around 10 %, and with a laminated safety glass panel with a thickness of 2 x 10 mm, it is reduced by around 20 %. The cover panels therefore have a greater influence on the yield than the perforation of the reflector mirror.

With or without protective front panel

Depending on how they are used, collector facades with or without a cover panel offer specific advantages (see Fig.). The structure without the cover panel is less complex. It has been the preferred option from an architectural and energy perspective. The more complex structure with a protective cover panel in front of the actual thermal facade – similar to a double window – does however offer advantages in terms of safety and contamination from dirt. The heat which is created in the cavity can also be removed via the collector, thus preventing the cavity from being overheated. Even when the system is at a standstill, the low U-values of the evacuated tubes prevent additional heat input compared to a flat plate collector.

Projektinfo 07/2013:
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Addresses

Measurement of the prototypes and 0 series of the facade collector
Universität Stuttgart, ITW

Simulation and yield assessment
Solites

Industrial partner for collectors, test collector facility Langensteinbach
Ritter XL Solar GmbH

Industrial partner for facade
Wicona

Industrial partner for facade construction
FRENER & REIFER GmbH

Industrial partner for facade construction
Metallbau Früh GmbH