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Functional thin films provide many opportunities for advanced glazing systems; in an energy-intensive world where reducing energy demand is desirable, films and coatings could play a huge role.
What are Thin-Films?
Thin films are single or multiple layers of metal or semiconductor materials ranging in thickness from nanometers up to micrometers, which are applied to the inner surface of windows. It is produced in a process called deposition, a controlled synthesis of the material involving applying a thin film to a surface; for example, a household mirror consists of a thin metal coating on the back of a sheet of glass to form a reflective surface. The thickness can be controlled to within a few tenths of a nanometer, which can be useful in the manufacture of optics like antireflective and reflective coatings, or self-cleaning glass.
Advances in thin-film deposition techniques during the 20th century led to several breakthroughs in magnetic recording media, electronic semiconductor devices, LEDs, optical coatings, energy generation (thin-film solar cells), and storage (thin-film batteries). Thin-films are also significant in the development and study of materials with novel properties, such as multiferroic materials and superlattices which allow quantum phenomena to be studied. Thin-film drug delivery in pharmaceuticals is also a promising research area.
Windows Essential to Design
Windows are an essential part of building design; they form part of the building façade, define the indoor space, and provide natural daylight. They also keep the natural elements at bay and reduce energy costs.
Thin-film coatings can be retrofitted in existing buildings to control the amount of solar radiation that comes into the room. The films are typically thin layers of polyester which is clear, tinted, or reflective; their principal function is to reflect the worst of the sun’s heat and glare.
The primary energy benefit of window films is limiting infrared solar heat gain; the solar heat gain coefficient (SHGC) is the fraction of solar radiation admitted through a window, door, or skylight either directly or absorbed, and later released as heat inside a home. Some films can reduce solar heat gain by as much as two-thirds.
In warm regions, it’s believed that by controlling how much infrared enters through a window in a spectrally selective manner, less energy will be consumed via air conditioning and indoor comfort levels can be enhanced. The amount saved depends on the level of infrared reflectivity of the film, its thermal energy properties, exposure of the window, and the climate of the location.
Thin-films should exhibit the following properties:
- Emissivity: the measure of an object's ability to emit infrared energy.
- Shading coefficient (SC): a measure of thermal performance of a panel or window in a building; it’s the ratio of solar gain due to direct sunlight passing through the glass to the solar energy which passes through 3 mm Clear Float Glass.
- Solar heat gain coefficient (SHGC): the fraction of solar radiation admitted through a window, door, or skylight either transmitted directly or absorbed, and then released as heat inside a home.
- Visible light transmission (VLT): the amount of visible light that can pass through an optical or sun lens.
Generally, lower emissivity films are best for year-round comfort and savings, while lower SC films block more heat. Lower VLT films block more glare, but make the windows darker.
New window technology could improve the comfort and performance of buildings, add value and reduce energy costs for building owners. It can also assist in universal efforts to lower the greenhouse emissions that contribute to global warming.
Today’s films offer good adhesion and are resistant to surface damage; previously bubbles and surface damage made films a less attractive option than today. Advances in quality control, production speed and reproducibility have lowered the cost of this sophisticated technology.
Spectrally selective glazing can reduce energy consumption, control solar gain, prevent loss of interior heat and reduce electric lighting. Research has shown that thin-films can make more of a financial impact in residential buildings, or even hotels, where DIY installation methods are 75% less expensive than professional ones.
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