CONTROLLING THE SUN'S ENERGY
The sun delivers 1.36 KW/m2 at the Earth’s surface. This “free” clean energy powers solar and wind electricity generators, sea waves, and solar water heaters. Chlorophyl uses blue wavelengths to produce plant nutrients while reflecting near-IR energy that produces unwanted heating. Millions of square meters of architectural glass control the heating of indoor space - and this is where thin film coatings play an important role. This article will discuss the use of coatings that selectively absorb and reflect different portions of the solar irradiance spectrum, and the deposition processes used for their manufacture.
Figure 1. Spectrally selective coatings are used to make optical art. Plate with graticulated multi-layer oxide compounds coating resulting in a stress-textured coating (by T. Ives). Squares are multilayer green reflectors on textured glass. Bottom of “Jack in the Pulpet” blown glass vase with chemical thin film, in the style of Louis Comfort Tiffany (Lundberg Studios). Photo by Samuel Pellicori, 2012.
Spectrally Selective Coating Functions
Visible light has wavelengths between ~400 nm and ~700 nm. Window glass transmits visible and IR energy to wavelengths as long as ~2.7µm (2700 nm). This spectral range includes about half of the solar radiation, the other half is at longer IR wavelengths. A significant portion of solar IR energy at wavelengths >700 nm is transmitted by glass and is responsible for heating the interior of a building or automobile. While the transmission of IR energy is desirable in colder climates, in warm climates it imposes the need for interior cooling. At wavelengths where glass does not transmit, it absorbs. Thus, at wavelengths longer than 2.7 µm, high absorption causes the glass to heat up and become a black body radiator at 300 K, with maximum radiation neat 10 µm.
Thin-film coatings have been developed for architetural and automotive glass that selectively transmit visible light to interior space while reflecting IR energy that will either escape from, or be transmitted to, the interior space. The control of the flow of solar energy in both directions enables the reduction of heating demands for cold climates and of cooling demands for warm climates, thereby reducing reliance on non-solar based energy sources.
Three function-related types of spectrally selective solar control coatings have been developed and are in use world-wide. They include passive and active constructions:
• Visible-transmitting, IR heat reflecting
• High- / low-emissivity coatings
• Electrochromic and thermochromic coatings that modulate the reflection of IR energy