Integrated storage collectors and thermosiphon systems are typically installed in climates without freezing during the winter season. The storage is the collector (ICS), Fig. 1 (c), or is closely con­nected to the collector (thermosiphon system), Fig. 1 (b). These systems have compact designs, are normally on-roof mounted and a relatively easy supplement or replacement of a domestic hot water (DHW) boiler. Here the introduction of polymeric materials contributes to considerable reduction of weight and opens for innovative, functional designs and shapes (see examples, Fig. 9).

2.1 Flat plate collectors and favourable system design

Europe has a sophisticated market for different solar thermal applications, as systems for DHW preparation or combined solar heating systems for DHW preparation and space heating (combisys- tems) in single — and multi-family houses, hotels and large-scale plants for district heating [3]. Due to the heat loss such collector systems require a collector cover (glazing).

Already during 1977-1985 substantial R&D was performed in the US on polymeric solar thermal collectors, first of all for solar DHW systems [6, 7]. Presently there exist few commercial glazed collectors with polymeric absorbers. These are mostly designed for low-pressure systems, which are open vented and have pure water without antifreeze additives as heat carrier. Depending on the

application polymeric absorbers have different design criteria to the solar collector system than conventional, metal-based absorbers; some designs have a built-in overheat/freezing protection mechanisms for the solar collectors, e. g. drain-back technology, ventilation or other designs to avoid thermal stagnation or freezing of the heat carrier in the solar loop.

For polymeric absorbers of commoditive or engineering plastics the operative system temperature should be as low as possible in order to minimize thermal load for the polymer, favour long ser­vice-life and high overall system efficiency. Glazed absorbers of high temperature performance plastics with spectrally selective coatings are not in the market yet, but might/will contribute to serve the retrofit market of small solar DHW systems.

Examples of hydraulic system designs, which are favourable for the application of glazed poly­meric collectors, are — among others — solar heating systems with large DHW demand (sport centres, hospitals, nursing homes, etc., see Fig. 2 (a)); further solar combisystem with large heat store (~100 l/m2 collector area), low temperature heating system (floor/wall heating), avoiding intermit­tent heat exchangers between solar loop, storage volume and heat emission system (Fig. 2 (b)).

(a) (b)

Fig. 2. Examples of hydraulic system designs, which are favourable for the application of polymeric collec-
tors; (a) solar DHW system covering large DHW demand and (b) solar combisystem.