Low-emissivity (low-E) technology is broadly adopted to develop cost-effective, energy-efficient Low-E glass for buildings and vehicles.
low-e glass
Low-E coating transmits visible light and reflects light with longer wavelength. For the long wavelength light, there are three scenarios: (I) IR part of solar light (0.7-2.5nm), (II) room ambient black body radiation (2.5-50um), and (III) the rest As shown in the following fugue.
Therefore,
(1) For cold weather area, Low-E glass need to allow both visible light and IR (I), but reflects IR(II) and rest (III).
(2) For warm weather area, Low-E glass need to allow visible light, but reflects IR(I), IR(II), and rest (III).
Recently with the develop of telecommunication, it becomes a problem for low-E glass reflecting microwave (or other frequencies in communication) and reducing communication signals of tele devices. New Low-E coating design is needed.
low-e terminology
- Visible light transmittance (Tvis or VT) is the transmitted percentage of visible light (380–780 nm) over total incident solar energy
- Solar heat gain coefficient (SHGC) is the percentage of the solar energy passing through the window over the incident solar energy (including direct solar transmi ance and indirect re-radiation)
- Light-to-solar gain ratio (LSG) is the ratio of VT to SHGC
- U-factor is a measure of air-to-air heat transmission. It is an overall coefficient of heat transfer, the lower the U-factor, the better the insulating properties of the windows
Low-e coating structure
The above are three typical Silver based Low-E coating structures and typical materials for different layers. The key are:
- Silver layer: crystalline structure, size, boundary, microstructure, et.al.
- Other layers’ materials and process
- Optical design and simulation