Summary:

The report centers on silver-on-glass (“second-surface”) mirrors as the preferred reflectors for solar-thermal systems because silver’s solar reflectance surpasses aluminum’s (≈97–98% vs. 88–92%). The silver film is bonded to a glass superstrate and backed with copper and paint; this stack must keep silver intact for ~30 years while withstanding temperature cycling, moisture, UV, dust, and mechanical loads.

Optically, even ideal coatings lose a little performance through protective dielectrics: the report notes practical solar reflectance ceilings near 0.97 for silver (vs. ~0.88 for aluminum). It explains how mirror construction (superstrate, metal film, back layers) affects both hemispherical reflectance and specularity. Front-surface silver can reach ~95–98% reflectance with silicone overcoats in lab tests, but environmental exposure revealed adhesion failures and silver’s reactivity to atmospheric gases—hence the emphasis on robust second-surface designs.

Because glass governs how much light reaches (and returns from) the silver, the document highlights low-iron glass to minimize Fe²⁺ absorption and raise transmittance, boosting silvered-mirror output; it cites a 4.7% transmittance gain and ~89.6% calculated reflectance for 3 mm low-iron float glass. It also flags composition (e.g., sodium migration), flatness/specularity, and manufacturing quality as critical to preserving silver’s performance.