Energy-efficient glazing

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1 Energy-efficient glazing Low-E solar reflective coatings Float glass is traditionally used in the building and automotive industries to provide both clear and tinted transparent barriers to the elements. While glass by itself has no significant insulation properties, a variety of coatings applied to the glass surface can satisfy both the aesthetic and energy efficient requirements of architects and automotive designers. However, reducing solar heat load does result in the reduction of visible transmittance. James J. Finley* PPG INDUSTRIES Introduction Over the past 15 years, coatings commonly known as hight/lowe (high transmittance/low emissivity) and solar reflective have extended the performance of glass as a transparent barrier. By acting as highly effective reflectors of both solar and thermal infrared radiation, these coatings manage the interaction of both the sun s energy and radiant thermal energy with the enclosed environment. Glass with a low-e coating can be designed to appear indistinguishable from uncoated glass; yet, it can provide significantly better thermal insulation and rejection of solar infrared energy. This means cost savings for heating and air conditioning, as well as added comfort. A silver-based, multistack coating, produced commercially by the dc magnetron sputtering process, increases the performance of glass for the building and automotive industries. This low-e coated surface reduces heat loss due to thermal radiation in an insulating glass (IG) unit. The solar reflective properties of the coatings provide a more effective alternative to heat load reduction in automobiles than standard tinted glass. This article looks into the properties of the low-e coating in the solar and thermal regions of the spectrum. Emissivity is defined and explained in the context of the coating applications. Requirements for coatings are given, which are described in terms of a multilayer thin film filter; the function of the layers on performance and aesthetics are 123

2 analysed. The manufacturing method is described, and the hight/lowe coating technology for products in the window and automotive glazing industries is discussed, pointing out the advantages over traditional products. Solar and thermal properties The total energy of the sun that reaches the earth s surface consists of 5 per cent ultraviolet ( nm), 45 per cent visible ( nm), and 50 per cent solar infrared (780 nm-2100 nm). This solar energy is either reflected, transmitted or absorbed. The conservation of energy dictates that: r + t + α = 1 where r, t and α are the reflectance, transmittance and absorptance, respectively, over the solar wavelength range. Low-E coatings, which act as a selective filter of solar energy, can be designed to attain high transmittance and low reflectance in the visible, and high reflectance in the solar infrared region of the spectrum. Solar energy absorptance is low. Thermal energy is radiated or emitted by all objects, depending on their temperature and material properties. At room temperature, objects radiate in the thermal region of the spectrum ranging from 3 µm-50 µm with a maximum at 10 µm. This emitted energy is related to the absorbed energy by Kirchoff s law, which states that ε = α where ε is the emissivity. As a result, a surface which is a good absorber will be a good emitter of thermal radiation. Substituting α for ε in the above equation, with t = 0, since most materials are opaque to thermal radiation, gives: r + ε = 1 over the thermal wavelength range. Therefore, a poor (low-e) emitter is a good reflector of thermal radiation. Emissivity is defined as the ratio of the energy emitted by a surface (W) to that emitted by a blackbody (W b ), ε = W/W b where a blackbody absorbs and emits all of the incident thermal energy. The emissivity of a blackbody is, by definition, equal to one (1). A float glass surface which has an emissivity of 0.84, is an effective absorber and emitter of thermal radiation. Applying a low-e coating to the glass surface will reduce the emittance. Coating types Basically, two types of low-e coatings are used in large scale industrial applications: a transparent, conductive, doped metal oxide (TCO) coating and a multilayer stack coating. Because of its more general use in both the window and automobile glazing industries, only the multilayer stack coating is considered here, while the TCO coating is only mentioned briefly. The TCO coating is typically made up of fluorine-doped tin oxide, which is about 300 nm thick, and is deposited pyrolytically on the float line. As durable as the glass surface, it allows for easy handling by glass fabricators. Since the reflectance of the TCO coating in the solar infrared is not as high as that of the stacked coating, use of this coating is limited to the window glazing industry. Although both the building and automotive industries use the basic stacked coating, different product applications require unique solutions to achieve the correct balance of visible, solar, and infrared energy desired in an enclosed space. For example, in colder climates a coating allowing more of the sun s energy and visible light to enter the window is preferable; the opposite is true in warmer climates. And a low-e surface, which is important for improving thermal insulation for windows, is not required for windshields in automobiles, where high solar reflectance and high visible light transmittance are important. In general, neutral transmitted and reflected colour with low reflectance in the visible are desirable. In addition, the coatings must be durable enough to survive storage for extended periods, and handling for fabrication. Some applications even demand high temperature processing such as tempering, bending, and annealing in a lehr. 124

3 The multilayer stacked coating consists of heat-reflective, or low-e metal layers sandwiched between transparent dielectric layers (see Figures 1a and b) to both antireflect and protect the metal. The heat-reflective metal is typically silver, which in a double stack coating (Figure 1b) has an emissivity as low as Silver is also a very effective reflector of solar infrared radiation. The dielectric layers are preferably non-absorbing in the visible region of the spectrum with refractive indices of 2 or more. They should also possess good chemical Fig. 1 diagram of the (a) single and (b) double stack coatings with silver Fig. 2 Spectral transmittance (a) and reflectance (b) curves of the coated single silver stack ( ) and double silver stack ( ) coatings and mechanical durability. The higher index dielectric layers will then antireflect the low index silver layer more effectively, decreasing visible reflectance and increasing visible transmittance. Typical metal oxides that meet requirements for these dielectric materials are titanium dioxide, tin oxide, zinc oxide or zinc stannate. To prevent film degradation in the fabricated product, even though the silver layers are surrounded by dielectric layers, the multilayer stacked coating must be located in a protective environment. Fortunately, the product applications provide this. The single stack coating on glass is basically a thin film filter that can generally be described by the layer sequence, Glass/D 1 /M/D 2, where D 1, D 2 and M denote the dielectric and silver layers, respectively. Typically, D 1, D 2 are nm thick, and M (silver layer) is 8-15 nm thick (Figure 1a). To further enhance the performance of the coating in the solar infrared and maintain visible transmittance along with low visible reflectance and neutral reflected colour, the above sequence is repeated to give the double stack coating. This can be described by the layer sequence Glass/D 1 /M 1 /D 2 /M 2 /D 3, where D 2 is 2 x D 1, and D 1 D 3, and M 1, M nm (Figure 1b). Transmittance and reflectance of the single and double stack coatings are compared in Figures 2a and 2b, which illustrate the effect 125

4 that the increase in layer stacking has on sharpening the solar filter. At a wavelength of around 800 nm, the double stack coating has a pronounced increase in reflectance, rising steeply in the solar infrared range, compared to the single stack coating. Reflectance in the visible range is maintained at a low level. Transmittance is also sharply reduced in the solar infrared with only a slight drop in the visible range. This layer sequence can be repeated indefinitely, but transmittance will further decrease because of absorption. Complexity and cost of manufacturing will also increase with each coating stack. Currently, products with one or two stacks are sold commercially. The thickness of each individual layer, D or M, strongly influences the aesthetic qualities of the coating, such as colour. The silver layers, and to a lesser extent the dielectric layers, strongly influence solar performance, emissivity, and transmittance of the coating. By adjusting combinations of layers, the coating can be tuned to meet different product requirements. With a well controlled manufacturing process, all the variables involved in producing these coated products can be managed. Manufacturing process For commercial use, these coatings are economically produced by the dc magnetron sputtering process, an off-line physical vapour deposition process carried out in a vacuum chamber. The sputtering assembly basically consists of a metal plate of the material to be sputtered, called the target. This forms the cathode, or negative electrode, of an electrical circuit. Either an adjacent conductor, or the chamber itself, completes the circuit as the anode, or positive electrode. Fig. 3 diagram of (a) cross section of dc magnetron cathode, and (b) target face with erosion groove or racetrack pattern. E and B denote the electric and magnetic fields, respectively When a voltage is applied ( 400 V) to the target, with the process gas in the chamber at a pressure of about three microns, a plasma, similar to what is seen as the glow in a neon sign, forms at the target surface. Positive ions, formed by collisions with energetic electrons in the plasma, collide with the negatively charged target plate and dislodge atoms. These atoms then coat the glass as it passes under the target. If the process gas is inert, such as argon, the resulting sputtered material will be a metal. If the gas contains oxygen, nitrogen, or a combination of these gases, the resulting film will be a metal oxide, nitride or oxynitride, respectively. The sputtering process is enhanced when the electric field lines cross magnetic field lines that emerge from magnets placed behind the target plate, as shown in Figure 3a. This E x B field forms a bottle which contains electrons that would normally be accelerated away from the cathode. These electrons further contribute to gas ionization, thus increasing the sputtering rate before they lose energy and drift off to the anode. The area of maximum sputtering, where the concentration of plasma is the greatest, 126

5 forms an erosion groove or racetrack pattern (see Figure 3b). Since the electrons are not accelerated away from the cathode at high energy to strike and heat the substrate, the temperature of the substrate typically reaches only tens of degrees above ambient temperature during the coating process. In a coater, several cathodes, usually two or three, are arranged together in separate compartments or zones as shown in Figure 4. In each zone, one gas combination is used to sputter a particular coating composition. Several of these zones are arranged in line, separated by stages that use diffusion or turbomolecular pumps to both isolate the zones and pump the system. To produce the stacked coating, for example, zones are arranged to sputter alternate layers of metal oxides and metals (see Figure 4). Both the entry and exit end of the coater have load lock and hold sections (not shown), which assure a continuous flow of glass through the coating zones. By adding more zones, coatings can be produced at higher throughput. Product applications In the window glazing industry, energy conservation has drawn attention to the need of producing a more energy-efficient window. A significant amount of heat energy literally goes out the window of a home. For better energy conservation, a low-e coating is applied to the inside surface of the IG unit (see Figure 5). The stacked coating in this Fig. 4 diagram of coater showing cathodes in metal and oxide zones Fig. 5 drawing of the cross section of an IG unit position, protected from the outside environment, improves the insulation value, or U-value, of the unit. The low-e coated surface prevents radiative heat loss from the warmer to the cooler glass surface and the unit could be thought of as a transparent Dewar flask. To minimize heat loss by thermal conductivity, the unit is filled with a low thermal conducting gas, such as argon. Spacers designed to minimize heat transfer and prevent condensation are used to separate and frame the glass lights. The unit is sealed with a thermoplastic or other sealant to form a leaktight seal. Factors such as the width of the unit and glass thickness also influence U-value. Table 1 shows the U-value at the centre of the glass, i.e., ignoring effects from the spacer, along with solar properties and visible transmittance for IG units glazed in both 3 mm clear/uncoated, and clear/double stack-coated glass. The data in Table 1 point out that the uncoated glass unit allows twice the heat flow, and transmits twice the solar energy as the double stacked coated glass unit. A single, 5 mm clear glass light (U-value = 1.1) allows more than four times the heat flow. In the automotive glazing industry, the need to reduce the solar heat load has become more critical with the introduction of sloped windshields with large glass areas ( 2 m 2 ). Traditionally, the solar heat load is reduced by using tinted glass. The standard laminated tinted windshield is comprised of inner and outer lights of 2.3 mm tinted glass, which sandwich polyvinyl butyral (PVB) as shown in Figure 6a. Solar infrared radiation is absorbed by Fe +2 in the glass, which re-radiates in the thermal infrared. This leads to higher temperatures in the vehicle and a greater load on the 127

6 air conditioning system to maintain passenger comfort, particularly when the vehicle is not moving. The more effective approach to reducing solar heat load in a vehicle is to use double stacked coatings to reflect the solar infrared. The coating is deposited on clear glass located on the inside surface of the outer light of the windshield (see Figure 6b). In this position, it is protected from the environment and needs only reflect back through the outer (clear glass) light, thus minimizing absorption. The low-e surface is inside the laminate and the emissivity of the windshield is that of the glass surface. The U-value is approximately equal to that of a 5 mm glass light. A low U-value, however, is not required for a windshield application. The double stack coating is designed to reflect the maximum and transmit the minimum amount of solar infrared radiation, while maintaining the legal visible transmittance. Figures 2a and b also illustrate this. Table 1 compares the standard, tinted/uncoated windshield (Figure 6a) with the clear/coated combination windshield (Figure 6b). The visible light transmittance with a clear/coated combination is maintained at 76 per cent, which is designed for the European market, where the minimum legal transmittance limit is 75 per cent. The three per cent reduction in visible transmittance is more than compensated by a 12 per cent reduction in the total solar energy transmitted. The total solar energy reflected is 37 per cent and accounts for a significant Table 1 Solar properties and U-values of coated and uncoated IG units and windshields Product Glass/Coating Visible TSET Visible TSER U-value Reflectance Reflectance IG unit clear/uncoated IG unit clear/coated Windshield tinted/uncoated Windshield clear/coated Windshield tinted/coated The total solar energy transmitted and reflected, TSET and TSER, respectively, are defined as the ratio, expressed as a percentage of the solar energy transmitted or reflected to the total solar incident energy. Transmittance and reflectance data were obtained using a Perkin-Elmer Lambda 9 spectrophotometer. Data were calculated based on NFRC methodology, using LBL s Window 4.1 software. Winter nighttime U- values are expressed in BTU/hr-ft 2 -F. Fig. 6 diagram of (a) standard tinted windshield, and (b) coated windshield with a double stack coating on the inside surface of the outer glass light performance increase for this windshield that is not attainable by standard tinted glass windshields. Replacing the inner clear light with a tinted light reduces the visible light transmittance to 71 per cent, which meets the requirement of 70 per cent or greater for the US market, and reduces the total solar energy transmittance to 34 per cent. *Staff Scientist and manager of Vacuum Coatings Group PPG Industries Inc. - USA First published in the GlassResearcher: Bulletin of Glass Science and Engineering, NSF Industry - University Center for Glass Research, Alfred, NY (Vol. 7 No. 1), USA. Information Service n. 15 See coupon on the last page 128