PET and PVC
The PET samples became brittle and degenerated already after 10 years of exposure. In the case of PVC high losses in transmittance were observed, but mechanical destruction occurred only after 20 years. These samples showed a colour change over yellow and brown to a non-transparent black. This effect was accelerated when exposed under a protective glazing with high transmittance, indicating that temperature and not radiation dose is the driving parameter.
PMMA showed the lowest soiling of the investigated polymers and no significant degradation, which resulted in a better overall weathering performance than the tested glasses. Some PMMA - types showed a persistent UV-blocking over the whole 20 years of exposure. They can be recommended to serve as weather resistant UV-protection coating for other materials such as PC for example. The fluorinated polymer films suffered from surprisingly high total losses mainly caused by soiling (FEP and PVF). For the ETFE samples high losses remained even after cleaning with ethanol such that material degradation can’t be excluded. The tested PC types showed yellowing effects and material erosion already after some years of exposure. The UV-protection additives of the tested PC-types were not able to prevent material degradation at the surface. The tested PET, PVC and UP products are unsuitable for the use as collector glazing because of high losses in transmittance or fast mechanical destruction.
High losses in solar transmittance due to soiling were observed (especially in Rapperswil). Structured glass-surfaces for reflex reduction had no effect on soil accumulation. Soiling leads to elevated losses in the solar transmittance of the collector glazing which can be assigned to the collector efficiency within wide limits. For that reason a regular cleaning of solar collectors is recommended, especially for polluted sites or glazing composed of FEP or PVF. PMMA, FEP and PVF have a better cleaning capability than glasses.
The work presented has been supported by the Swiss Federal Office of Energy SFOE. Some results could have been contributed to the IEA SHC Task 39. Many thanks go to Ueli Frei who initiated this project and to Thomas Hauselmann for many hours of sample preparation and measurements.
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