Chapter 1
Types, Properties, Applications, and Emerging Technologies of Smart Windows
1.1Outline
Windows can have a substantial effect on a building’s interior design, operating costs, health, productivity, and resident well-being. The purpose of old-fashioned windows was to provide sunlight, view, and fresh air for the residents. However, these days, commercial buildings have become completely sealed, air-conditioned, and electrically lit [1–3]. For this reason, energy consumption has dramatically increased in modern buildings over the past few years. According to the annual report presented by the U.S. Energy Information Administration (EIA), buildings consume nearly 47.6% of the total energy production and emit 38% of the CO2 into the atmosphere [4], resulting in global warming [5]. Thus, windows are the most important components influencing the building’s energy use and highest electricity demand. Approximately 60% of all the energy used in the building sector is consumed for lighting, space heating, and cooling [6]. It is anticipated that this demand for energy will rise by more than 30% by 2020 [7,8].
Improvements in building envelopes and ventilation can play an important role in reducing space heating and cooling consumption levels [9]. Generally, windows can reduce electric lighting loads by allowing natural light to pass through. A proper window design can also cut peak electricity and cooling loads by restricting the heat flow from outside. This can not only reduce the operation cost of the cooling system but also downsize the indoor cooling requirement, thereby saving capital costs. This has limited the role of traditional windows in addressing residents’ needs. However, now the trend is shifting from human-centered designs toward improvements in building energy performance.
Generally, window design is influenced by two factors: (a) the climate and (b) the building type (or space configuration within a building). Once these factors are considered, the orientation of the window, its size and type, and required shading systems are addressed [10]. Designers also explore the wide range of possible window materials and assemblies based on the mechanisms of heat transfer. To achieve an energy-efficient window system, the designer generally adopts and/or combines glazing, glass compositions, assembly techniques, and smart technologies.
There are five properties of windows that are the basis for quantifying energy performance [11–14]:
1.U-factor
2.Solar Heat Gain Coefficient (SHGC) or shading coefficient (SC)
3.Visible Transmittance (VT)
4.Air Leakage
5.Light-to-Solar-Gain (LSG) ratio
U-factor: When there is a difference in temperature between indoor and outdoor, the heat is lost or gained through the window frame and glazing due to the combined effects of conduction, convection, and long-wave radiation. The U-factor of a window characterizes its overall heat transfer rate or insulating value. The higher the U-factor, the more heat is transferred (lost) through the window in winter. The unit of U-value is Watt per square meter (W/m2). U-factors usually range between 1.3 (for a typical aluminum frame single-glazed window) to 0.2 (for a multi-paned, high-performance window with low-emissivity coatings and insulated frames) [15,16]. A window with a U-factor of 0.6 will lose twice as much heat under the same conditions as one with 0.3.
Solar Heat Gain Coefficient (SHGC): Regardless of the outdoor temperature, heat can flow through windows by direct or indirect solar radiation. The ability to regulate this heat gain through windows is characterized in terms of the solar heat gain coefficient (SHGC) or shading coefficient (SC) of the window. The increment in the SHGC ratio is an indication of the increment in the solar gain potential through a given window, and normally the ratio is between 0.0 and 1.0. When SHGC becomes zero, it indicates that none of the incident solar gain is transmitted through the window as heat. All incident solar energy is transmitted through the window as heat when SHGC equals 1.0 [15–17]. For example, a window with an SHGC of 0.6 will admit twice as much solar heat gain as one with 0.3 [18]. Hence, windows with high SHGC values are desirable in buildings where passive solar heating is needed, and low SHGC values are desirable when the loads due to air conditioning is high. The term “SHGC” is rather new and is intended to replace the term “shading coefficient (SC).” The SC of a glass is defined as the ratio of the solar heat gain through a given glazing as compared to that of clear, 1/8 inch single-pane glass [18].
Visible Transmittance (VT): This is an optical property that indicates the amount of visible light transmitted through the glass. It affects energy by providing daylight that creates the opportunity to reduce electric lighting and its associated cooling loads. Sunlight is composed of a range of electromagnetic wavelengths, which are categorized as ultraviolet (UV), visible, and infrared (IR) regions and are collectively referred to as the solar spectrum. The short, UV wavelengths are invisible to the naked eye and are responsible for color fading of fabric and skin damage. Sunlight is made up of 47% of the visible light and 46% of longer IR wavelengths [17,18]. For a given glazing system, the term “Coolness index (Ke),” (also called efficacy factor), is determined by the ratio of the VT to the SC.
Air Leakage: This refers to the amount of air passing through a unit area of window under given pressure conditions. Air leakage can also lead to heat loss and gain through cracks around the frames of the window assembly [18,19].
Light-to-Solar-Gain (LSG) ratio: This is a ratio of VT and SHGC which has been standardized within the glazing industry to allow accurate comparison of windows [18].
A higher LSG ratio means sunlight entering the room is more efficient for daylighting, especially for summer conditions where more light is desired with less solar gain. This ratio is the measurement used to determine whether the glazing is “spectrally selective.” In absolute percentage terms, a ratio greater than 1 signifies that the daylight passing through the glass is more than the sun’s direct heat passing through it. Selective glasses on the market today have a visible transmittance between 34.0% and 69.0% and a solar heat gain coefficient between 24.0% and 56.0%, with a selectivity index LSG between 1.28 and 2.29 (see Table 1.1) [18].
Table 1.1 Commercially Available Glazings with Specification
| No. | Company Name | VT (%) | SHGC | LSG | U (W/m2) |
| 1 | Pilkington Suncool | 60 | 0.32 | 1.87 | 1.00 |
| 2 | AGC Stopray Ultra | 60 | 0.28 | 2.14 | 1.00 |
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