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Volume 6 - Issue 4 October - December, 1996 Spectrally Selective Coatings on Windows for Energy Conservation While many of us brace for the sub-zero temperatures of another long winter, others are still enduring unbearable heat. As the mercury dips and soars, it can be taxing to power sources and cause electric bills to skyrocket. This issue takes a look at thin film applications specifically designed to maximize the use of solar energy and keep us from throwing our money out the window.
Introduction:
Solar radiation useful for lighting falls between wavelengths 400 nm and 700 nm, and half of the sun's blackbody radiation is contained below ~2500 nm. Window (float) glass transmits wavelengths between ~350 nm and 2500 nm with an efficiency near 80%. The half of the solar radiance outside the visible region can enter and cause heating. While this is desirable in cold climates, it increases the cooling load in warm weather conditions. Similarly, radiation form the 300 K source that is the interior of a dwelling can escape, requiring a source of heat energy during winter conditions. The glass window itself has a thermal emission near 85%. It is obvious that this window-to-heat flow must be closed to decrease demands for heating and cooling energy.
Spectrally Selective Coatings:
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| TABLE 1 | ||
| Application | Requirement | Spectrally Selective Coating |
| Admit light, Reject solar heat | Transmit 400 nm - 700 nm, Reflect 700 nm ->2500 nm | Dielectric/Metal/Dielectric |
| Solar Heating | Transmit or Absorb <2500 nm, Reflect/emit >2500 nm | Oxide Semiconductor Cermet/Composite |
| Radiative cooling | Emit longwave IR >5000 nm | Cermet/Composite |
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1. Visible-Transmitting, Solar Heat Reflecting Coatings:
The coatings that satisfy this need can be deposited directly on window glass or on polymer film that is later laminated to glass. Sputter deposition is the most common technique and the most economical for coating large rolls of polymer film. Copper, gold, silver, and TiN are common materials that, in thin film form, transmit visible and reflect near to mid-infrared wavelengths. The metal layer is sandwiched between dielectric layers to increase visible transmission and to protect the metal from abrasion and corrosive degradation. Metal layer thickness is below ~100 Å, and the deposition method is important in determining the amount of coalescing that occurs at these thicknesses. A greater degree of coalescence results in higher reflection and therefore lower transmission. A precursor layer is deposited first, which helps adhesion and controls the amount of metal agglomeration. High index dielectrics such as TiO2, Bi2O3, ZnO, etc. in quarter-wave thicknesses (visible wavelengths) are used to enhance transmission of the metals Au, Ag, or Cu without reducing the reflection of IR energy. The design is: dielectric / metal / dielectric (D/M/D). The transition from transmission to reflection can be steepened by doubling the design. Designs based on a Fabry-Perot cavity consist of D/M/DD/M/D, where DD represents a half-wave spacer. A typical result for the design 300 Å TiO2 / 130 Å Ag / 300 Å TiO2 is ~80% transmission in the visible and 80 % reflection from 2000 nm and above. 2. Visible and Solar Heat Transmitting, Heat Barrier Coatings:Windows can be glazed to provide passive solar heating of interiors, while retaining the heat generated. Doped oxide semiconductors having large bandgaps, for example, In2O3:Sn (ITO), have the property of transmitting below ~1000 nm and reflecting above ~2000 nm. Thus, the major heating wavelengths of the sun are admitted, and the radiant heat of the interior is prevented from escaping. When ITO (or an alternates) is coated on window glass, light and some solar heating is admitted, but radiation from warm objects at 300 K is prevented from escaping. This serves to reduce heating requirements during cold weather. ITO transparent conducting coating technology has been discussed in an earlier CMN. Other materials that fall into this group are SnO2:Sb, SnO2:F, etc. The transition wavelength between transmission and reflection is determined by the carrier concentration in these films. ITO is sputtered onto polymer substrates. Transmission of visible wavelengths is increased with a layer of SiO2 or aluminum oxyfluoride by sputtering. 3. Electrochromic and Thermochromic Coatings:The ability to modulate transmission and reflection at will is of benefit in some applications. The optical properties of these materials can be changed by injecting or extracting ions in layers of transition metal oxides, for example Ni, Mo, V, W. An electrochromic coating is built of layers that provide a source of ions and the electrochromic layer, all sandwiched between transparent conducting layers (ITO). Application of voltage produces ion migration into the electrochromic layer. Two materials are in use today: WO3 and Ni(OH) 2; the former is used to modulate visible transmission between ~80% and 10%, the latter to modulate infrared. A practical application of WO3 is to dim reflections from rear view mirrors. Thermochromic materials change their optical properties with temperature. The most well known is VO2. By replacement of some of the V by other metals or the O by F, the transition temperature between IR transmission and reflection is lowered. The visible transmission is low, and VO2 has been proposed for protection from high energy laser hits. Coatings for Heat Generation:This class of coatings is characterized by high absorption of solar energy and low emittance at longer wavelengths. Thus they generate high temperatures. Solar heat concentrators, water heaters, and surface temperature control are common applications. The coatings might consist of a deposited cermet layer, which contains absorbing species in a dielectric matrix, or single or multiple layer designs. An example of the cermet approach is a nickel-silicon oxide black material that can be evaporated. Another is Cr / Cr2O3. One design consists of a thin layer of Ti immersed between TiO2 layers. Multi-layer structures such as Al2O3 / Mo /Al2O3 have been developed for stability at the high temperatures generated. Intrinsically selectively absorbing materials such as ZrB2 exist. Ge and Si layers on aluminum absorb short wavelengths and reflect above 2000 nm and 1000 nm respectively. These same designs are used to make "black mirrors" for reducing stray light in optical systems. In critical applications where black paint cannot be used because of high temperature or vacuum outgassing, this approach is the only one available. Radiative Cooling Coatings:These coatings depend on the property of some materials to absorb energy in the thermal IR, i.e., near 10 µm, near the center of the 300 K blackbody emission curve. They emit this energy efficiently. Silicon oxides and oxynitrides are useful for this application. These same designs are used to make "black mirrors" for reducing stray light in optical systems. In critical applications where black paint cannot be used because of high temperature or vacuum outgassing, this approach is the only one available. Conclusion:Thin film coatings based on the unique properties of materials or combinations of materials have important applications in making the best use of passive solar energy, in conserving energy, and in reducing energy consumption for heating and cooling. Deposition technology such as sputtering is in place for producing energy conserving windows in high volume at low cost.
Reference:
Dr. Ervin Colton, Editor CERAC, inc. P.O. Box 1178 | Milwaukee, WI 53201 Phone: 414-289-9800 | FAX: 414-289-9805 e-mail: marketing@cerac.com
Samuel Pellicori, Principal Contributor |
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