Sol-gel based photochromic coating for solar responsive smart window

Abstract

Transparent photochromic (PC) films can be used for smart window applications to partially block the sunlight and provide comfort vision. However, for large area applications, the high cost and delicate manufacturing processes are the major challenges faced by the prior-art technologies. This paper reports a development of cost effective PC coatings with effective reduction of visible light transmission and total energy transmittance at tinted states. The coating material was prepared by embedding organic PC dyes, 1,2-b Naphthopyran, in sol-gel based organic-inorganic mesoporous coating matrix. The degree of crosslinking of coating matrix and types of organic groups can effectively influence the visible light transmittance and bleaching speed. Visible light transmission reduction at tinted state can be tuned from 30% to 60% by varying the dye concentration in coating and coating thickness. The thermal properties of the PC coatings are evaluated by UV–Vis-NIR and FTIR measurements and further calculation to obtain solar heat gain coefficient (G value) and thermal transmittance (U value). For a glass in thickness of 3mm coated with a PC layer in thickness of 12μm, the G value at tinted state was reduced to 0.78 as compared with a G value of 0.87 for an uncoated glass. A double glazed smart window prototype consisting of a commercial low-E glass and a PC coated glass showed G value of 0.26, and U value of 1.58 as compared to the G value of 0.87 and U value of 5.2 of a single uncoated glass. This demonstrates a significant reduction of energy transmittance through the smart window resulting in high potential energy saving for end users especially in tropical climate.