The polymeric material of the present absorbers opens the possibility to choose the colour as a property of the bulk material. In the present studies, the absorbers were coloured by a commercial non-selective alkyd-based paint, Bengalack Emaljelack 90 by Jotun A/S, which revealed suitable adhesion and no visible changes during the relatively short test period. The absorptance for the unpainted black absorber and for the colours green, blue, red and white were measured with an spectrophotometer model 746 from Optronic Laboratory, USA owned by the Solar Energy Research Centre at Hogskalan Dalarna in Borlange, Sweden. The absorptance spectra are shown in Fig. 8 for the wavelength range of 350 nm — 2500 nm. The weighted solar absorptance (Duffie and Beckmann, 1991) of the various colours is listed in Table 2. The uncertainty of the absorptance measurements is approximately 1%. Except for the colour red, all colours were applied on the absorbers and the thermal performance was investigated.

The polymeric absorber is part of a drain-back system. A description and comparison of the most conventional solar heating systems in Europe is given in (Weiss, 2003), and for the polymeric material in (Meir, 2003).

For the fapade integration of drain-back collectors, the location of the heat storage tank determines normally to which level over ground level the fapade can be covered by solar collectors. A complete draining of the solar system is — in the most simple system design — secured when the lower end of the collector fapade is above the storage level. This limitation is normally given in one — storey buildings without basement where the technical room is commonly placed.

In the present test set-up, the performance of the collector fapade with and without ventilated cavity was studied separately. The hydraulic scheme is shown in Fig. 9, details are given in Table 3. Further, following properties of the solar thermal system are specific: During solar system standstill, the absorbers are filled with air and the heat carrier (water) is drained to the storage. For the flow of the heat carrier, several collector modules are connected in parallel. The absorber and the collector cover consist of polymeric materials (modified NORYL blend, polycarbonate) and are sheets with an internal twin-/multi-wall structure. Due to the properties of polymeric materials, the system design avoids high operation and stagnation temperatures.

Table 3. Dimensions of the studied test system

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