Adhesive materials in solar-thermal collectors: Results of seven years field exposure and accelerated aging tests used to simulate 25 years of operation

The adhesive joint takes over multiple functions in the solar thermal collector, such as the structural bonding, the absorption of thermal stress of the transparent cover to the collector housing, and the sealing of the collector with respect to humidity, air and particle entry. Thus, it is a central component in a solar thermal collector. The aging effects occurring in solar collectors within the adhesive material are by now not well understood. They are determined primarily by the temperature level in the collector. This temperature level has significantly increased during the last years due to the enhancement of the collector efficiency and the trend towards solar thermal systems with higher solar fractions resulting in an increase in stagnation time and temperature. Furthermore, durability analyses of novel market products are needed, since only little is known about their long-term behavior. The accelerated aging tests described in this article represent the measured load in the solar collector at the component adhesive joint to a calculated corresponding period of 25 years. We found that all test specimens showed a 100 % cohesive fracture pattern after these stress tests. Thus, the adhesion between the transparent cover and the collector housing can be assumed to be ensured using the framework with the load depicted in the defined test cycle over 25 years. Compared to the reference value before aging, the tensile strength values change only marginally after being degraded. The results of the mechanical properties of the small test specimens before and after seven years of exposure in maritime climate were determined and compiled. A nondestructive characterization method at the test specimens with Raman spectroscopy accompanied before the mechanical tensile testing was done.