Figure 1. Electrochromic skylights in an office building in Irvington, N.Y.

Large-area, user-controllable dynamic windows for external applications have long been the desire of architects, building owners and energy-conscious consumers. Today, thanks in part to advancements in vacuum deposition technology, such windows are a reality. Electronically tintable windows and skylights based on an all-ceramic thin-film electrochromic technology are available for commercial and residential building applications.

The dynamic glass* can be user-operated or integrated with a building's energy management system to switch from a highly transmissive clear state (62% light transmitting) to a dark tint (3.5% light transmitting) without becoming opaque (see Figure 1). Concurrently, the glazing's solar heat gain coefficient (SHGC) ranges from 0.48 in the clear state to 0.09 in the tinted state. (The SHGC is the fraction of incident solar radiation admitted through a window. SHGC is expressed as a number between 0 and 1; the lower the number, the less solar heat transmitted and the more energy-efficient the window is.)

* SageGlass^®, manufactured by SAGE Electrochromics, Inc.

Benefits

Enlarge this picture Figure 2. Illustration of how an electrochromic window controls light and heat.

At the push of a button, the glazing can be tinted to keep out unwanted heat and glare, or cleared to allow in as much light as possible (see Figure 2). When tinted, the dynamic thin film stack on Surface 2 of the insulating glass unit absorbs the sun's energy and preferentially re-radiates it to the outside due to the low emissivity properties of the coating.  Additionally, the low-e coating keeps desired heat inside the building in the winter.

By comparison, even the best conventional low-emittance (low-e) windows are static, with a fixed visible transmission and solar heat gain coefficient. In other words, they cannot tint or clear as needed, and consequently do not achieve optimum energy savings. U.S. Department of Energy (DOE) models predict that energy use in buildings could be reduced by 20% with dynamic glass, compared to today's high-performance Energy Star (low-e) windows.¹

Figure 3. These photos show the Lawrence Berkeley National Lab testing facility in Berkeley, Calif., where dynamic windows were evaluated for their energy performance. In the top photo, the right column of windows is allowing too much glare for the man to work comfortably at his desk. In the bottom photo, the glare has been dramatically reduced, resulting in a much more comfortable work environment where the view and connection to the outdoors has been maintained.

Dynamic windows also help create a comfortable environment for people inside buildings, because they block glare and solar heat gain while allowing the building's occupants to maintain the desired view (see Figure 3).

Many studies have borne out the importance of having a connection to the outdoors for people's health and well-being. It has been shown that when people are exposed to more daylight in offices, their productivity goes up and absenteeism drops.² When children are exposed to more daylight in school, their scholastic achievements improve,³ and studies have even demonstrated a correlation between daylighting and increased retail sales.⁴ In addition, dynamic windows almost completely block the radiation-both ultraviolet (UV) and visible-that damages and fades interior furnishings and artwork.

How It Works

Figure 4. Electrochromic device showing the movement of ions under an externally applied electric field, which causes the thin film stack to color.

The all-ceramic electrochromic glazing product consists of a series of five ceramic layers that are vacuum-deposited onto float glass using the same thin-film sputtering technology that is used to fabricate today's static low-e glazings. The electrochromic coating, and how it fits within the context of an insulating glass unit (IGU), is shown in Figure 4. The coating is on Surface 2 of the window, where it rejects or admits incoming solar radiation depending on whether the glass is in the tinted or clear state. In this solid-state device, the electrochromic layer (EC) and a counter electrode layer (CE) are separated by an ion-conducting layer (IC). These three core layers are interleaved between two transparent conductors (TC1 and TC2).

The coating stack has high transparency in the clear state, when the lithium ions reside in the CE ion storage layer, which can be a metal oxide such as V₂O₅, NiO or IrO₂. When a low DC voltage is applied across the transparent conductors (e.g., indium tin oxide, ITO, or aluminum-doped zinc oxide, AZO), Li⁺ ions move through the ion conductor and into the EC material layer, which undergoes a gradual transition from clear to blue/gray as lithium is inserted. The reaction is completely reversible when the polarity of the applied voltage is changed.

Enlarge this picture Figure 5. An overview of the manufacturing process.

Figure 5 provides an overview of the manufacturing process. The glass used for sputter coating is float glass that has been meticulously cleaned and prepared. After these preliminary steps, the glass enters the cleanroom environment, where the electrochromic layers are deposited in the sputter coating machine. Next, these monolithic panes are tested and those that pass are fabricated into IGUs. Outside the cleanroom environment once again, the IGUs are tested for performance and reliability, and are then shipped to either the job site for installation, or to the glazing contractor where they are framed.

Challenges and Advantages

Four key criteria must be met in order for a large-area glazing product to be viable: durability, performance, cost-effective manufacturing and compatibility with existing window systems.

Durability
Durability testing to assure product reliability has been conducted throughout the materials and technology development processes. Samples of the glass have been tested by several accredited third-party organizations, including the DOE. In a series of tests, which were carried out for more than 10 years, the electrochromic glass was subjected to simulated solar light and heat while being continuously switched between the clear and tinted states.

The units successfully completed all tests, and even surpassed the requirements for the ASTM Test Standard E-2141-02, which evaluates "the combined degradative effects of elevated temperature, solar radiation and extended electrical cycling through 50,000 cycles..." The dynamic windows continued switching through 100,000 cycles (clear/tint/clear), which is double the test standard and "equivalent to switching a window nine times per day for 365 days per year across a 30-year lifetime." In addition, the units successfully completed a 24-month test at a DSET facility in the Arizona desert and 36 months in a Minnesota test site, as well as numerous evaluations carried out by leading companies in the glass industry.

Other electrochromic, or "switchable," technologies employ polymers in their systems, but they have had less success in exterior window applications. Usually, the construction of  polymer, or "organic," electrochromic devices consists of a polymer ion conductor sandwiched between two panes of glass that have been coated with the electrodes and transparent conductors. The ion conductor must act not only as a key component in the active device, but must be the adhesive that holds the two panes of glass together, which requires a compromise in performance for both functions.

Polymer materials are inherently susceptible to degradation from UV radiation and have very different coefficients of thermal expansion than the glass and ceramic films to which they are adhered. Mechanical stresses generated by temperature differences across the window and between the panes can cause delamination with resultant non-uniform switching.

In contrast, the thermal expansion coefficients for the materials used in an all-ceramic electrochromic thin film stack are more closely matched to those for glass, reducing thermally generated mechanical stresses. Moreover, the ceramic materials used in the system are much less vulnerable to photochemical reactions and degradation due to UV radiation.

Performance
Performance requirements in particular include uniformity and overall aesthetics. Color and light transmission must be uniform across the entire window area. As glass sizes get larger, film uniformity becomes a greater hurdle. With vacuum deposition, and particularly when incorporating multiple coatings in the stack, the challenge is to control film thickness variation as well as interfaces and interactions between subsequent layers.

Failure to manage layer-to-layer processing can result in point defects and non-coloring areas. In addition, non-uniform coatings can create visible variations in the optical density of the glazing across its surface area, especially in the tinted state.

The rapid growth of the liquid crystal display (LCD) industry, along with continuing demands by architects and building owners for more and larger panes of glass, have accelerated the development of large area thin film coaters. Most importantly, in-line deposition monitors and rapid feedback control systems have enabled excellent control of coating quality across large substrates.

Coating industry advancements have supported the production of increasingly larger sizes of electrochromic panes over the last 15 years. At this point, the width of electrochromic panes is limited only by equipment size. Ultimately, dynamic windows larger than 50 sq ft will be produced.

Cost-Effective Manufacturing
Like many breakthrough products, those incorporating electrochromic technology initially carry a premium price. The vacuum deposition and other processes used to manufacture dynamic glass products are the same as those that have been employed in the manufacture of low-e architectural glass for 20 years. These processes lend themselves to processing efficiencies and economies of scale.

Dynamic glass is in the early stages of growth and cost reduction, not unlike the cost history of low-e, flat panel displays and cell phones. Significant cost reductions will occur over the next 10 years as people become aware of the many benefits of dynamic glass products and volumes increase.

Compatibility with Existing Window Systems
The insulating glass unit is constructed like industry-standard units, and is accommodated by most of the framing systems produced by leading framing manufacturers.

A Bright Future

Architects require durable, large-size panes to satisfy their needs. Looking into the foreseeable future, the trend will be for even more vision area in buildings. People love beautiful views and dramatic vistas. For conventional static windows, however, it all comes at a cost of excessive solar heat gain, glare and fading of interior furnishing and artwork. Removal of heat requires air conditioning (energy consumption), cutting glare requires blinds (loss of view) and fading means premature replacement of items (wasted resources). All of this runs wholly counter to an even bigger construction trend-green, sustainable buildings.

With dynamic windows, building occupants can still enjoy views and vistas while the windows stop the heat, block the glare and almost completely eliminate fading. Electronically switchable windows provide unprecedented functionality and benefits, and will truly change the way we view windows.

For additional information regarding dynamic windows, contact SAGE Electrochromics, Inc. at One Sage Way, Faribault, MN 55021; (507) 331-4848; fax (507) 333-0145; e-mail info@sage-ec.com; or visit www.sage-ec.com.