Solar light absorption property of nano-structured silver layer and application to photo-thermal energy conversion coating

摘要

Nano-structured Ag layers with the thickness in a range of d{sub}(Ag) = 0.9-24 nm sandwiched between MgO layers are formed by rf sputtering. The morphology of nano-structured Ag is observed by the transmission electron microscope. Then the optical property is measured in a range of wavelength of 0.24-2.6 μm, which covers the solar energy spectrum. Several characteristics are recognized. For example, the nanostructured Ag layer having the intermediate morphology between the nano-particle and the continuous film shows the effective optical property in absorbing solar spectrum, such as large absorptance not only at visible region but also at near-infrared region. Moreover, it is found that the nano-structured Ag layers with d{sub}(Ag) ≤ 5.4 nm and with d{sub}(Ag) ≥ 24 nm are thermally stable at 673 K, but those with d{sub}(Ag) = 6.3-13.5 nm are thermally unstable. In order to utilize the advantageous characteristics of the nano-structured Ag layers for the solar absorber coating, the optical constant of each layer is experimentally evaluated. Then the selective solar absorber coating with functional graded structure made of all nano-structured Ag layers is designed. The fabricated coating shows the high-performance such as the photo-thermal conversion efficiency of 0.72 for air mass 1.5 at an operating temperature of 373 K, the thermal stability as well.