Our ability to store and control energy lags behind our ability to generate it. We are good at generating heat and then converting this heat energy into electricity, but if the energy is not consumed, it is often lost. Converting and storing the sun's heat in the form of hot water can be economical for houses, but recovering heat from industrial processes using thermoelectric materials is still very immature. A previous Nanoelectronics article discussed some new approaches to recovering heat energy to make solar and photovoltaic systems more efficient. This article reviews approaches to control the transmission of heat and light in film and glass, particularly for automotive and construction applications.

Passive Heat Control in Glass

Outside the industrial sector, the key method for controlling heat comes from changing the transmittance of heat or sunlight through glass. The most obvious—and ubiquitous—market for this functionality is in window glazing. An integrated glazing unit takes the form of two sheets of glass enclosing a vacuum space. The presence of a vacuum reduces the thermal conductivity of the composite unit. Improvements to such window glazing include using as the internal-facing panel low-e glass—a glass that is coated with a metal-oxide layer to reflect heat back into the room, further reducing the thermal conductivity. Today, manufacturers produce about 600 million square meters of low-e glass each year, for a market value of $6 billion to $9 billion, and that low-e glass is the de facto window glass in use in many parts of the world.

As well as keeping buildings warm in winter, coated glass can control the throughput of light and heat in the summer in order to keep buildings cooler and reduce expenditure on air-conditioning systems. The aim is to maintain high visible-light transmittance but reduce the solar-heat gain coefficient, a measure of how much heat is transmitted. Some lower-grade coated glass is manufactured inline as part of the regular float-glass-manufacturing process by the addition of compounds like tin dioxide, but higher-specification solar-control window glass is produced by offline sputtering. Opportunities for solar-control window glass are essentially open to glassmaker OEMs and their equipment partners—such as manufacturers of sputtering equipment—only.

Solar-Control Window Film

The use of state-of-the-art window glass to suit the local environment to save energy may seem obvious—assuming it is affordable—but the installed base of standard, nonoptimized window glass in the world is very, very large. This large installed base has created a market for passive solar-control window film, in which coatings of increasing complexity and functionality are deposited onto typically polyester (PET) polymer films and then applied to the external or internal surface of residential, commercial, and automotive windows.

Typical roll-to-roll processes use PET film wound between a drum within a vacuum with active metallic layers deposited via evaporation. Additional processing steps are application specific: Solar-control window film for the aftermarket requires a scratch-resistant hardcoat layer, whereas solar-control window film for use as safety glass for residential and automotive applications is first laminated with polyvinyl butyral (PVB, a standard polymer film in use in safety glass.

Like solar-control glass, solar-control window film has become more complex in recent years. Nonmetallic oxides have begun to replace metal layers, with a view not only to reducing the reflective appearance and increasing visible light transmittance but also to maintaining low solar-heat gain. Advanced sputtering has begun to play a much more important role in the industry, becoming a strong point of differentiation, and the overall growth in the window-film business has attracted investment from several major companies.

• In January 2012, the Eastman Chemical Company acquired Solutia Inc., a leader in performance materials and specialty chemicals. Solutia's Performance Films division is the largest provider of solar-control window film, and in November 2011, Solutia acquired Southall Technologies, a specialty-window-film maker with extensive know-how and production of high-end sputtered window film.
• 3M is one of the largest developers and suppliers of a broad range of films, including films for glass lamination and solar control. In common with other developers, 3M has been developing sputtered nonmetallic multilayer films to reduce reflectivity but still maintain low solar-heat gain. The company states that its Prestige range of windows, based on nonmetallic nanomaterials, gives reflectivity as low as that of normal glass while cutting UV radiation and reducing solar-heat gain to between 36% and 50%.
• Another major US manufacturer of window film is SolarGard. In August 2011, French glassmaker Saint Gobain acquired SolarGard from the Belgian specialty-materials company Bekaert. Saint Gobain stated that it saw window film as being synergistic with its performance plastics and flat-glass businesses.

Though most of the $1 billion market for passive window film is for solar-control products (namely, reducing the solar-heat gain in summer), several companies have developed more advanced low-e window films as insulation products for the aftermarket and for glass lamination. Using sputter-deposition technology, Solutia in January 2012 launched its Enerlogic70 low-e film that maintains 70% visible-light transmission (versus 90% for uncoated glass) but reduces the solar-heat gain to 51% (from 86%) and reduces the transmission of heat by close to half. With this combination of lower solar-heat gain in summer and particularly better insulation in winter, Solutia claims—on the basis of lower energy consumption—a payback time as low as three years.

Active Window Glass

Passive window-film and coated glass is relatively low in cost, but it does not adapt to local climate conditions or seasonal variations, nor is it controllable. The development of active or smart glass was to fulfill this role, with the main value proposition being increased comfort and cost savings on heating and air-conditioning bills. Several methods of producing active window glass exist, but to date, electrochromic (EC) glass is the most commercially advanced.

Electrochromic Glass

Electrochromic devices are materials that can change color under the influence of an electrical potential. This potential or voltage triggers a redox reaction in which the material gains or loses electrons, undergoing a change in the amount of light that it absorbs or reflects and hence in its color. To be useful, this electrochemical reaction needs to be reversible, allowing repeated dark/light cycles. Several types of electrochromic material exist, including inorganic materials (usually tungsten trioxide) and several families of organic EC compounds, including viologens (which become dark blue or violet when they gain electrons) and phenazine dyes (which turn blue when they lose electrons). In almost all cases, the primary electrochromic effect involves a change in absorption.

From a commercial perspective, the most important current electrochromic device is not advanced window glass but a mirror. Market-leader Gentex produces about 15 million interior and exterior autodimming EC mirrors each year—worth about $800 million—for the automotive business. Its proprietary EC technology comprises two organic electronic compounds—a viologen and a phenazine dye—in a crosslinked polymer gel. Recently, Gentex began to supply small-area EC autodimming windows to Boeing's Dreamliner Series of aircraft.

Gentex's applications are very high in value—worth close to $100 per square foot or 50 to 100 times higher than the worth of normal float glass. This success also shows the cost challenge facing EC windows in automotive and residential markets. Other figures of merit for EC windows include the level of transparency in the bleached (clear) state, which should be 60% to 70%; the transparency in the colored (dark) state, which should be 10% to 15%; the switching voltage and energy; and the lifetime of the panel.

For residential and commercial window applications, the most mature EC technologies are based on inorganic tungsten trioxide materials. Though commercialization remains niche after many years, several recent developments suggest the segment may be on the verge of a step change.

• A longtime leader in EC windows is Sage Electrochromics Inc. (Faribault, Minnesota), whose technology is based on sputtering tungsten trioxide as the EC layer with an ion-conducting lithium salt (such as lithium silicon oxide) to provide ion transfer and indium tin oxide for the transparent electrodes. Sage has had a pilot production line producing small quantities of EC glass with prices of $60 to $80 per square foot, but in November 2009, Saint Gobain SA announced an $80 million equity investment into Sage in order construct a new volume-manufacturing facility. In May 2012, Saint Gobain acquired the remaining shares in Sage, which became a wholly owned subsidiary, and announced that the manufacturing plant would begin production in 2013 (initial production was due for mid-2012) with an annual production of 3.2 million square feet of EC windows (about 300 000 square meters). Sage states that the power necessary to maintain the darkened state of its EC panels is 0.4 watt per square meter.
• Founded in 2007, Soladigm Inc. (Milpitas, California) is another company developing electrochromic windows; it licenses sputtered inorganic EC technology from Lawrence Livermore National Laboratories. Like Sage, Soladigm is in the process of constructing a manufacturing plant, and though production was due to begin in the first quarter, it has not yet started. The company closed further funding of $55 million in June 2012—with investors including GE and Khosla Ventures—and expects production to begin before the year's end.

SPD Active Glass

One of the earliest forms of active glass was the suspended-particle device (SPD) developed by US company Research Frontiers Inc. (Woodbury, New York). This electrically controlled technology is based on rod-like particles suspended in droplets in a film between two panes of glass. With no applied voltage, a random orientation of the particles results in a tinted state, but applying an electric field aligns the microrods, increasing the glass's transparency. Research Frontiers Inc. has outsourced production of the SPD film to Hitachi Chemical and has licensed the technology to companies including Asahi Glass, Pilkington, and Daimler. For example, in 2011, Daimler announced it would offer an all-glass panoramic roof using SPD technology on the Mercedes-Benz 2012 SLK.

Though the technology has been around for several years and Research Frontiers has a wide number of licensing partners, its license revenue remains relatively low, albeit growing. In the first quarter of 2012, the company had licensing fee revenue of $0.5 million. Use of a licensing royalty rate of 10% (the company mentions a rate of between 5% and 15%) implies a market for SPD final glass products of about $20 million per year.

Temperature-Controlled Active Glass

The above-mentioned electrically controlled active glass has benefits in terms of control, whether by the user or via integrated sensors. This flexibility comes at the expense of powering requirements and additional wiring. In contrast, some materials can naturally change their optical properties in response to an external influence—specifically heat or temperature—and several types are in development.

Thermochromic materials change their color in response to heat. Pleotint LLC (West Olive, Michigan) has developed a thermochromic film based on a ligand-exchange-based polymer, and it laminates this coloring film with PVB and two sheets of glass to create a laminate. Its sunlight-responsive-thermochromic (SRT) film is neutral in appearance and changes from 50% transmittance to 5% transmittance according to the temperature change. In September 2011, PPG Industries announced a partnership with Pleotint to market its SRT film as part of a triple-layer unit, which includes a low-e glass layer from PPG.

Also developing a thermochromic film is RavenBrick LLC (Denver, Colorado). RavenBrick's thermochromic film works a little like switching of pixels in an LCD, using a thermotropic liquid-crystal depolarizer sandwiched between two orthogonal polarizers. The thermotropic liquid crystal changes the amount of light scattering according to its temperature, thereby blocking 50% of the incoming light in the normal state and greater than 90% in the switched state.

Summary

Markets for advanced glass, particularly low-e glass, are large and continue to grow strongly. Advanced window films to provide solar control and improved insulation address growing opportunities in the aftermarket and for glass lamination, and figures indicate that such window film can have a short payback period in certain climates.

The market for active, smart glass is much less mature, and developers face many of the challenges—in terms of reducing cost—that developers in the thin-film-photovoltaics business face. Though such active glass can provide a high degree of comfort and the controllability will appeal to many high-end customers, unless prices can decrease to nearer $20 per square foot or otherwise demonstrate a short payback period, commercialization is likely to remain relatively limited. Nevertheless, the large potential addressable market is attracting investors, and we may yet see more glassmakers align themselves with smart glass start-ups.