High market penetrations have pool absorbers of (a) rigid, extruded sheets of polypropylene (PP) with intrinsic channels, Fig. 6 (a). Common are also flexible absorber mats of ethylene propylene diene monomer (EPDM) rubber pipes, here in red in order to match the colour of roof tiles (b).

Further examples of pool absorbers with pipe structure are plain polyethylene (PE) pipes (d) and ripped PP pipes (e). Fig. 6 (c) and (d) illustrate two examples of rigid, blow-moulded absorbers in high-density polyethylene (HD-PE): (c) flat panels of 200 x 110 x 1.5 cm and (f) pool absorber cassettes with honeycomb structure of 30 x 30 x 3 cm. Both absorbers can be interconnected in a matrix, parallel and in series. Commonly, pool absorbers are placed on the top of flat or slightly tilted roofs, various sophisticated solutions for fixing and preventing the collectors from displace­ment by wind, rain and snow exist.

2.1 Glazing

The glazing has to sustain the temperature gradient between the collector inside and the ambient temperature, solar irradiation, load of weather impacts due to wind, snow, hail and rain. Compre­hensive work on the durability of polymeric glazing has been performed, e. g. [8, 9, 10]. UV-resistant, thermotropic and anti-soiling coatings for polymeric surfaces are e. g. studied in Subtask C of IEA-SHC Task 39.

The aesthetics of the collector glazing is essential from an architectural point of view. Especially when large parts of a building’s facade or roof are covered by solar collectors, the integrated de­sign, size, shape, surface structure, colour and reflectance will contribute to the building’ s appear­ance. Rather common and known from the building and construction sector are twin-wall sheets of polycarbonate (PC) as collector cover (Fig. 7 (a)). Fig. 7 (b) shows a solar air collector with a PC twin-wall sheet as cover. Different collector cover designs are dome structures of PC (c), corru­gated roof coverings with 6 mm twin-wall structure of PC (d), and acrylic glazing of an integrated storage collector (e). The polymeric glazing in the examples (c) and (d) cover conventional, selec­tively coated metal absorbers with pressurised solar loop and water-glycol mixture as heat carrier.