ASHRAE Journal - July 2009 - 104

Technologies for Smart Windows By Stephen E. Rudolph, Ph.D.; John Dieckmann, Member ASHRAE; and James Brodrick, Ph.D., Member ASHRAE Metal Oxide Electrochromics, SPD E lectrochromic (EC) glass uses electrical energy to transition between clear and darkened states. This technology has already achieved commercial success through its use in self-dimming automobile mirrors. This article highlights the use of EC glass in smart windows, i.e., windows that can be adjusted to modulate the amount of light entering a room. Smart window technology based on EC glass is entering early commercialization and has significant implications for the future design and outfitting of commercial and residential buildings. The convergence of the architectural trend towards more use of glass (including growing interest in daylighting) and increasing demand for reducing energy use creates a unique opportunity for smart windows. The value of variable light transmission/ absorption can best be understood in the overall context of heat and energy transfer though glazing systems. A single pane of conventional clear window glass transmits a large fraction of the solar spectrum and absorbs (but does not reflect) a large fraction of long wave infrared radiation, tending to let solar radiation in while retaining some of the black body infrared radiation emitted from surfaces inside the building (helpful in the heating season, but adds to air-conditioning loads). However, a single glazing transfers a maximum of heat via convection/ 104 ASHRAE Journal formance of glazing systems, which are covered in Chapter 15, “Fenestration” of the ASHRAE Handbook—Fundamentals1 (although variable transmissivity glazing is not yet covered in this chapter). “EC glass” and “smart windows” are generic terms that encompass discrete technologies that modify glass in different ways. This category includes approaches such as liquid crystal systems that change from clear to translucent and photochromic glass that darkens in sunlight. Two systems are discussed, with a focus on energy savings and advanced functionality. The first is a metal-oxide product, and the second is suspended particle device (SPD) technology. These two technologies differ significantly from each other in terms of composition, performance, control options and progress towards broad commercialization. Metal Oxide Electrochromics conduction (maximizing cooling season heat gain and heating season heat loss). Almost all windows include two or more panes of glass to reduce conduction/ convection. Variable transmission glazing with suitable controls enables the best combination of daylighting levels and of minimum solar heat gain (when cooling, or maximum solar heat gain when heating), for an overall minimization of energy consumption. Both types of variable transmission glazing described in this article trade light transmission for absorption, as opposed to reflection. When applied to the outer glazing of multiple glazings, little difference exists between reflection and absorption in the energy impact on the inside space, because the outer glazing is more closely thermally coupled to the outside air than to the air inside the building. Energy absorbed as heat in the outer glazing is primarily transferred to the outside air. This description oversimplifies many aspects of the energy perashrae.org This technology consists of a five-layer metal oxide coating sandwiched between two sheets of glass. The layers are 1. Conductive layer; 2. Positive ion storage layer-colorless lithium metal oxide; 3. Conductor/electrolyte layer; 4. Electrochromatic layer-negative tungsten oxide; and 5. Conductive layer. All of the layers are applied by vacuum deposition (sputtering). When voltage is applied, lithium ions travel from the positive layer through the conductor/ electrolyte layer and into the electrochromatic layer to react with tungsten oxide to form lithium tungstate. As this occurs, a charge-compensating electron flows through the circuit from the ion storage layer to the electrochromatic layer. Lithium tungstate is light absorbing and as the reaction proceeds, the glass darkens and sunlight is absorbed within the glass as heat. As mentioned earlier, in an outer glazing, this heat is transferred primarily to the outside air. Reversing the polarity causes the reaction to be reversed and the glass reverts to its July 2009

ASHRAE Journal - July 2009

Table of Contents for the Digital Edition of ASHRAE Journal - July 2009

ASHRAE Journal - July 2009
Contents
Commentary
Industry News
Letters
Meetings and Shows
Feature Articles
     Using Direct Evaporative and Chilled Water Cooling
     Solving Kitchen Ventilation Problems
     Case Against Balancing Valves
     Integrated Design and UFAD
     Anniversary Feature: System Design for Compressor Reliability
Building Sciences
     Sustainable Products Capabilities
Special Products Information Section
Emerging Technologies
Washington Report
Products
People
Special Products
Classified Advertising
Advertisers Index
ASHRAE Journal - July 2009 - ASHRAE Journal - July 2009
ASHRAE Journal - July 2009 - Cover2
ASHRAE Journal - July 2009 - 1
ASHRAE Journal - July 2009 - 2
ASHRAE Journal - July 2009 - Contents
ASHRAE Journal - July 2009 - 4
ASHRAE Journal - July 2009 - Commentary
ASHRAE Journal - July 2009 - Industry News
ASHRAE Journal - July 2009 - 7
ASHRAE Journal - July 2009 - Letters
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ASHRAE Journal - July 2009 - Meetings and Shows
ASHRAE Journal - July 2009 - 15
ASHRAE Journal - July 2009 -      Using Direct Evaporative and Chilled Water Cooling
ASHRAE Journal - July 2009 - 17
ASHRAE Journal - July 2009 - 18
ASHRAE Journal - July 2009 - 19
ASHRAE Journal - July 2009 -      Solving Kitchen Ventilation Problems
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ASHRAE Journal - July 2009 - 25
ASHRAE Journal - July 2009 -      Case Against Balancing Valves
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ASHRAE Journal - July 2009 - 29
ASHRAE Journal - July 2009 -      Integrated Design and UFAD
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ASHRAE Journal - July 2009 - BRC1
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ASHRAE Journal - July 2009 -      Anniversary Feature: System Design for Compressor Reliability
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ASHRAE Journal - July 2009 - Building Sciences
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ASHRAE Journal - July 2009 - Special Products Information Section
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ASHRAE Journal - July 2009 - 103
ASHRAE Journal - July 2009 - Emerging Technologies
ASHRAE Journal - July 2009 - 105
ASHRAE Journal - July 2009 - 106
ASHRAE Journal - July 2009 - 107
ASHRAE Journal - July 2009 - Washington Report
ASHRAE Journal - July 2009 - Products
ASHRAE Journal - July 2009 - People
ASHRAE Journal - July 2009 - 111
ASHRAE Journal - July 2009 - Special Products
ASHRAE Journal - July 2009 - 113
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ASHRAE Journal - July 2009 - Classified Advertising
ASHRAE Journal - July 2009 - 118
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ASHRAE Journal - July 2009 - Advertisers Index
ASHRAE Journal - July 2009 - Cover3
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