The European project SOLABS is developing a novel coloured collector for building facades.

This new absorber will be made of steel, coated with weather resistant, coloured selective layers, and operated as an unglazed collector. It will be designed from start for integration into the building envelope.

The innovative architectural approach is to consider this solar element as an integral part of the building and to design it as a cladding element, taking inspiration from the existing products proposed by the metal cladding market. These claddings can be roughly divided into three basic types, all already accepted and widely used in building fagades:

• Cassettes (rectangular panels with a ratio of 1:1 to 1:4)

• Tongue-and-groove panel planks (20 to 40 cm width, ~4m. max length) • Profiled metal sheets

As profiled sheets involve a complicated technical adaptation of the heat collection system, they are not further considered, but the cassettes and the planks seem both to be reasonable formal options.

The “cassettes”(Fig. 2) are well suited for cladding large wall surfaces. As the choice of a right dimensioning of the module is the base for an appropriate cladding, it is important to define the proper dimensions, or range of dimensions, to offer to the architects. The more flexibility the more freedom you give to the designers, but probably at an increased cost for low standardisation. This point is even more crucial in the case of an element that is technically more complex than a simple cladding, as there are thermal constraints as well as water connexion issues to be addressed.

The trade-off limit between flexibility and standardization is difficult to draw on objective grounds.

The "panel plank” (Fig. 3) shape seems to be more promising in that regard, thanks to the flexibility of use it ensures even though it is a standard element. Due to its very structure, the panel plank offers a good flexibility in the vertical dimension:

With a small plank width (20-30 cm.) and a variable width reveal (Fig. 4), this system allows covering almost any surface height.

Cut-to-length elements should be feasible in a reasonable range and would give the needed flexibility in the horizontal dimension.

Those formal characteristics make the plank element one effective answer also to the problems related to the integration of solar thermal collectors in renovation.

On the colour approach, the new palette will be the result of a complex trade-off between technical feasibility, thermal efficiency, rationalization and architect’s demands (expectations / wishes).

The R&D work underway in the project aims at minimizing the inevitable efficiency losses inherent to the back reflection of a part of the solar spectrum, needed to give the collector a "colour". Architects on the other hand would like to have the usual freedom of choice they enjoy when selecting a "normal" cladding.

The broad range of questions related to the design led us to organise a large survey, including web-based questionnaire and personal discussions with architects of diverse experiences from all Europe. This survey (Fig. 5) is still currently underway to help defining the formal preferences related to the shapes, colours, details and fagade positioning of the collectors, and the consequent possible paths the project could follow. (http://www. solabs. net)

The preliminary results of the survey and the high level of interest received during the personal interviews seems to confirm that the approach taken (cassettes, planks…) is correct and that the project is going in the right direction, opening wide perspectives for the years to come.

Fig. 5 Example of a survey’s webpage