New collector glazing

In the field of glazed collectors, one of the main issues is the aesthetical problem due to the transparency of the glazing: black appearance of the absorber, visible pipes and imperfections of the absorber surface that gleam through the glass. Moreover, the transparent glass just acts as a showcase for these elements, drawing attention to the problematic parts.

The second research project, developed at the Laboratory of Solar Energy (LESO) with the collaboration of the University of Basel (Prof. Oelhafen’s group), aims at solving this problem by modifying the transparency of the glazing used in front of the absorber, without sacrificing the efficiency of the whole collector.

Incident radiation from sun and sky

colored reflection

cover glass

black absorber

coating

The current work aims at designing special filters tailored to this purpose, using thin film interference layers deposited on glass.

Fig. 6 New filter principle.

These newly developed thin film filters reflect only a small part of the sun’s spectrum in the visible range, letting most of the energy pass through, to be collected by the standard absorber placed behind. By selecting carefully the spectral position and weight of the peak reflection, one can freely choose the colour and intensity of the light reflected back by the collector’s glass (Fig 6).

As in the case of unglazed collectors, these new properties open new possibilities to the architectural integration work. Here are a few tracks that these new glazing elements will allow to investigate further:

• Curtain wall facades (Fig. 7)

Fig. 7 Widmer Architekten AG, Bank building in Zurich

The new physical characteristics of this glazing combined with the due attention to the detail’s design can bring solar collectors into curtain facades. For instance, using the transparency quality of the treated glass in the windows openings and its reflective properties to hide the collectors behind in the plain surfaces will make the design of homogeneous active solar facades possible. With made to measure elements, this approach will really become formally interesting.

• Double skin facades (Fig. 8 a and b)

Using the new glazing in double skin facades will bring the question of the transparency level to be selected. A more transparent coating might be used to let perceive the fagade as a double skin while being reflecting enough to make the absorber colour and surface imperfections acceptable. Technical problems related to the thermal collectors use in these facades will have to be addressed as part of the global study.

• Frosted glass look (Fig. 8 b & c)

Another interesting approach is to combine the coloured effect of the thin film interference filters deposited on the internal side of the glass with special treatments on the other side. One promising option is the frosted glass. The combination of the selective coating in the inner side with the frosted treatment in the outer side will create a non-specular, semi-transparent coloured material.

a

c

b

This will offer a real option for the fagade covering, corresponding to both energy and aesthetical trends.

Fig. 8 a) Herzog & de Meuron, Hospital Pharmacy, Basel. b) Peter Zumthor, Kunsthaus, Bregenz. c) Diener & Diener Architekten, Power Tower, Baden.

• Collector glass fixing (9)

Fig. 9 Glass fixing details

One main point in developing new collector concepts for fagade use is the radical change from the "glazed shoebox" of standard collectors. Together with other technical details modifications, the design of the glass fixing will be an essential issue in order to offer an aesthetically attractive alternative to the frame jointing proposed by the traditional glazed solar collectors.

Conclusion

Starting from the "colour" problem, seen at first as the major obstacle to the integration of solar collectors into facades, several developments are bringing both new colours and new questions: how to use this new freedom in the facades? How to make the thermal collector an architectural element?

The answers will be coming out from the cooperation between engineers and architects, keeping in mind construction market trends, and the role renovation is playing and will play in the future.