Development of a novel silver reflector on glass substrate for
parabolic trough solar collector applications
Master’s Thesis in Engineering Physics
by
Rasmus Öberg
Department of Physics
Umeå University
Abstract
In the pursuit of shifting away from carbon-based economy the world is in need of green, renewable energy sources; not just for electricity, but all forms of energy consumption. Solar energy has in recent decades grown to be a promising alternative to fossil fuels for heating and cooling processes, with concentrating solar thermal energy being at the forefront of this development. Concentrating solar thermal energy using parabolic trough solar collectors requires a highly reflective material to concentrate sunlight towards a central receiver. As such, making a high quality reflector essential to efficiently harnessing radiation energy from the sun.
For this project, the prototype for a silver reflector was developed using silver film ther-mally evaporated onto a glass substrate as a base. The reflector was developed to have a high reflectance while simultaneously being able to withstand environmental conditions like humidity and extreme outdoor temperatures. To achieve this the reflective silver layer on the substrate was protected using a novel application of dip coated using two oxide layers, set at thicknesses as for incident light to constructively interfere upon reflection. The effects of these layers on the optical properties and durability of the reflector were evaluated. The optical results, found using reflectance spectrophotometry, showed that using oxide lay-ers of the appropriate thickness yielded reflectors with reflectance only marginally smaller than pure silver. A silver reflector with protective two appropriately thick protective layer achieved a total solar-weighted reflectance of 95.3%, and experienced only slight deterio-ration when exposed to climate testing, lowering reflectance to 90.1%. For reference, an unprotected silver reflector displayed a total solar-weighted reflectance of 96.5% but expe-rienced severe deterioration in environmental testing. Thinner protective layers achieved slightly higher reflectance 95.5%, but the reflector was more severely affected by climate testing. The structure of the more promising reflectors is analyzed using SEM, EDS and XPS and discussed in greater detail. Overall, although further development is needed to reach a finished product, the results from this project suggest this is a durable reflector design with good optical properties to be further built upon.
