Optimized TI structure for solar wall applications

Fig. 1 shows a sketch and a fotograph of optimized small-celled TI structures based on polymer films. The structures, which consist of a continuously produced small-celled lamella based on a flat film joined to a corrugated film are either stapled or manufactured in the form of rolled structures. Due to the use of high-quality films structural defects on the surface and within the TI structure were avoided. Based on investigations of more than 20 different polymer film types [2, 3], cellulose acetate films (CA and CTA) were identified as outstanding polymer materials for TI wall applications. Using 30 pm thick cellulose acetate films small-celled lamellae with widths between 100 and 135 mm and a heigth of 5 mm were produced. The material fraction of the lamellae and the stapled or rolled structures thereof were about 1.5 v%. For a 100 mm thick stapled structure a hemispherical solar transmittance of 0.80 and a heat conductance 0.85 W/(m2K) were measured. For 135 mm thick structures, which were used for the application demonstration object, an hemispherical solar transmittance of 0.74 and an heat conductance of 0.74 W/(m2K) were calculated using the program GWERT [5] and polymer film properties as input data [2,3]. The remarkable performance property profile of cellulose based materials for TI applications with maximum service temperatures of about 100°C is related to the high solar transmittance of CA films in combination with a high infrared absorptance due to a high density of functional carbon-oxygene single bonds within the molecular structure of CA. However, this high density of carbon oxygene functional groups results in a high moisture uptake of cellulose based materials. For the fully substituted cellulose triacetate (CTA) the maximum moisture uptake is about 3 m%.

Fig. 1. Stapled and rolled small-celled structures based on polymer films

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