ABSTRACT
During the first half of 1992, SP inspected 50 solar heating installations, with particular attention being paid to the collectors themselves. More than 3000 solar collectors were examined in respect of strength, reliability and durability. 27 of the systems incorporated FEP film convection barriers, which were degraded, or had failed, to a greater or lesser degree in 16 of the systems.
Dismantling the collectors and taking samples from eight solar collectors with damaged convection barriers, from the same number of installations, revealed that the origin of the damage can be explained by mechanical damage, caused during installation or during operation. The age of the collectors has varied from one to ten years. Perforating fasteners, such as pop rivets, screws etc., cause actual or incipient tears, from which tears then grow at a rate of about a millimetre a day. Sharp comers and edges of aluminium strips also damage the films, again causing them to tear. With time, spacers on the absorbers or the insides of the cover sheetes can also damage the films, again causing tears to start.
DMTA and DSC laboratory investigations have been carried out on damaged films, and the results have been compared with those given by brand-new films. It has not been possible reliably to point to any general ageing mechanism on these damaged films, which have been exposed in solar collectors for periods of up to ten years.
From data in the literature, and the manufacturers’ own technical specifications of the materials, it is clear that FEP films with potential tear points or cuts generally have a very low propagating tear strength, of only about 2.5 N.
