Marriage Peril
The Bright Future of 'Smart' Windows

New technologies, competing to make affordable, variable-tint ‘smart’ windows, promise to decrease energy use in some buildings up to 40 percent.

by Douglas Page 2000

In the last 50 years window glass has gone from functional to ornate. Now, with ‘smart’ windows looming, driven by energy-efficiency demands, glass may be about to become obedient.

The Environmental Protection Agency says an average household spends over 40 percent of its annual energy budget on heating and cooling costs. Office buildings now account for about one-third of all the energy used in the U.S., a quarter of which is lost through the inefficiency of standard windows to retain heat in the winter or deflect heat in the summer.

But new ‘smart’ window technology is poised to change that. While regular glass can only allow a constant amount of light, the ‘smart’ window can be tuned, or dimmed, permitting any amount of light to pass. At the turn of a knob, the amount of light that shines through windows can now be controlled, dialing in an estimated $11 billion to $20 billion dollars a year savings in heating, lighting and air-conditioning costs.

According to the EPA, even a $7 billion savings would equate to a reduction in carbon emissions at power generating plants equal to taking 336,000 cars off the road, and the energy savings would be enough to light every home in New York City.

With ‘smart’ windows, homes and office buildings have the potential to recover installation costs through money saved by decreasing energy lost as the air conditioning battles a hot summer sun or by reducing indoor lighting requirements because the shades need no longer be pulled closed to block the sun.

‘Smart’ windows boast other benefits. They increase comfort, light and view and decrease condensation. Users are given control over their privacy and environment, and harmful ultraviolet rays are blocked, thereby eliminating the fading of furniture, carpets, drapes, artwork and other valuables. The cost of blinds, curtains and drapes are also slashed and in many cases eliminated.

By 2020, industry leaders believe windows will become active parts of building climate, engineering, information and structural systems. Scientists working at the Fraunhofer Institute in Germany, for instance, have developed a hybrid system that collects solar energy that warms the air on the glass facade of the building and funnels it through cavities in the walls and floors. The energy stored in this way can be fed into the building's interior heating system over night.

Presently, three distinct ‘smart’ window technologies are positioning themselves for this endeavor, competing for shares of a global architectural glass market that produces an estimated 20 billion square feet of flat glass each year. Domestically, sales of residential window units have grown by about five percent a year since 1992 to over 50 million units. Commercial window sales have increased by approximately 11 percent annually during the same period to nearly 500 million square feet a year.

The new ‘smart’ technologies are liquid crystal, electrochromic, and suspended particle device (SPD).

A few companies are working with liquid crystals, and a larger number are trying electrochromics. Only one has SPD technology. While none of the new technologies have yet established a serious market presence, even a one percent share of the global market would equate to 200 million square feet a year.


Polymer dispersed liquid crystals (PDLCs), invented at Kent State University in 1983, found a major application in switchable windows, that is, windows that change from clear to opaque with the flip of a switch. Using the same voltage as standard household appliances, multiple windows can be controlled from one switch and can be connected to a timer.

Most uses of PDLCs, however, are confined to privacy applications, where popular uses are found in glass walls for offices, conference rooms, lobbies, and store fronts. Privacy glass also provides unique opportunities for use by homebuilders in bathrooms, entryways, family rooms, bedrooms, and skylights.

In the opaque state, the glass diffuses direct sunlight and eliminates 99 percent of the ultraviolet rays responsible for the fading of carpets and curtains, although unfiltered visible light can also fade fabric.

PDLCs operate on the principle of electrically controlled light scattering. They consist of liquid crystal droplets surrounded by a polymer mixture sandwiched between two pieces of conducting glass. When no electricity is applied the liquid crystal droplets are randomly oriented, creating an opaque state. When electricity is applied the liquid crystals align parallel to the electric field and light passes through, creating a transparent state.

Liquid crystal technology has not been a commercial success. The windows are hazy because they scatter rather than absorb light, so there is a fog factor even when the device is in the transparent state. Also, while liquid crystals work well for interior privacy control, the technology is all-or-nothing, on or off - it can’t be used as a shading device. It also tends to be a little expensive for most popular applications, running between $85 and $150 a square foot.


Another ‘smart’ window technology, perhaps with a brighter future than liquid crystals, are electrochromic windows, which also attempt to control the amount of daylight and solar heat gain through the windows of buildings and vehicles. As with liquid crystals, a small voltage is required, although in the case of electrochromics the voltage causes the windows to darken; reversing the voltage causes them to lighten.

Unlike liquid crystals, however, electrochromic windows can be adjusted to control the amount of light and heat passing through them, a characteristic suggesting a variety of applications.

For instance, a small photovoltaic cell can be used to sense the amount of sunlight, darkening the window when the sun is brightest, then gradually lightening the window as the sunlight diminishes, a feature attractive in Sunbelt regions.

Electrochromic windows consist of up to seven layers of material, the central three layers sandwiched between two layers of a transparent conducting oxide material, all of which are further sandwiched between two layers of glass. All five layers are, of course, transparent to visible light.

These windows function as the result of transport of charged ions from an ion storage layer, through an ion conducting layer into an electrochromic layer. The presence of the ions in the electrochromic layer changes its optical properties, causing it to absorb visible light, the result of which is the window darkens.

To reverse the process, the voltage is reversed, driving the ions in the opposite direction, out of the electrochromic layer, through the ion conducting layer, and back into the ion storage layer. As the ions migrate out of the electrochromic layer, it brightens (or "bleaches"), and the window becomes transparent again.

Electrochromic windows can also be used to help keep cars cool. An electrochromic sunroof could darken in the direct sunlight but lighten at other times, providing function while keeping the car cool. Conceivably, electrochromic rear or side windows in a vehicle could darken while the car is parked, keeping the car cool, and then lighten again once the car is started. So far the technology is used only in self-dimming rear-view mirrors that change from light to dark to prevent eyestrain and temporary blindness from the glare of headlights approaching from the rear, then reversing when conditions permit.

Unfortunately, the electrochromic process is slow, especially when compared to the newer SPD technology.

It can take six seconds for something as small as an automobile’s rear-view mirror to go from clear to dark, and it may take 10 seconds to return to clear. But for something the size of a window it may take six to 10 minutes to change.

SPD windows, on the other hand, react in two seconds or less, regardless of the window size.

"In certain areas, such as rear-view mirrors, SPD technology goes a lot faster than that," said Joseph M. Harary, executive vice president, Research Frontiers of Woodbury, NY, the lab that developed and now licenses SPD technology. "Plus, you can use a knob or rheostat to control an SPD window. You can’t do that with electrochromics because there would be a six minute delay - you’d never get the knob right. Most people want instant feedback to adjust their window properly and SPD is the only one that will allow you to do that."

Electrochromic windows are also expensive, costing on the order of $125 per square foot.


Of the three ‘smart’ window technologies, SPD, in which the user can instantaneously control the passage of light through glass or plastic, appears to be the most promising in terms of cost and performance.

SPD, though the newest of the window technologies, is actually the result of decades of research seeking a ‘light valve’ technology. Physicist Robert Saxe, founder and CEO of Research Frontiers, worked for 34 years and spent $28 million perfecting his light-valve glass technology.

Windows in homes, office building windows, skylights and sun roofs - to say nothing of ski goggles and sunglasses, aviation instruments, automobile dashboard displays and bright, high-contrast digital displays for laptop and other electronic instruments - made with this new SPD technology can now be dimmed or brightened with electronic precision to suit individual needs, allowing an infinite range of adjustment between completely dark and completely clear.

SPD, which produces little or no haze in the transparent state, can be controlled either automatically by means of a photocell or other sensing or control device, or adjusted manually with a rheostat or remote control by the user.

When used in conjunction with Low-E (low emissivity) glass, SPD can also be used to block ultraviolet light (U-factor).

Low-E coatings, sometimes called heat-smart, are microscopically thin, virtually invisible, metallic oxide layers deposited on a window or skylight glazing surface primarily to reduce the U-factor by suppressing radiative heat flow. Low-E coatings are transparent to visible light. Different types of Low-E coatings have been designed to allow for high solar gain, moderate solar gain, or low solar gain.

Still, clear insulated glass units (usually called IGUs) dominate commercial and residential glazing technology, although Low-E windows have slowly gained ground, having risen by about one percent a year recently to well over 30 percent residential and 20 percent commercial market share, according to figures from the American Architectural Manufacturers Association, and the National Wood Window and Door Association.


"It is really simple how SPD technology works," Harary said. "Basically there are millions of black, light-absorbing, suspended-particle devices (SPD) within a film placed between the glass layers. When the user applies a moderate voltage of electricity to the film, the SPDs line up and become perpendicular to the window, allowing more light and increased visibility until the window is completely clear. As the amount of voltage is decreased, the window becomes darker until it reaches a bluish-black color that allows no light to pass through it."

In other words, in the "off" state, when no voltage is applied, the particles (whose exact nature is proprietary) are randomly dispersed and therefore absorb light, creating an opaque appearance. Conversely, when in the "on" state the particles orient, or align, changing the character of the glass from opaque to clear. By adjusting the voltage anywhere between "off" and "on", the degree of light can thereby be precisely controlled.

Therefore, the user has complete control over the amount of transmitted light from the glass or plastic walls.

The black particles are a recent improvement. In the past, particles used in SPDs generally looked dark blue when the device was in its "off" state due to the particles’ inability to absorb blue light well. The new particles look nearly black because they absorb light well throughout the entire spectrum. Black or gray colors, more desirable because they’re neutral, are preferred for most applications.

SPD technology actually originated over 100 years ago, when light-absorbing crystals were first discovered, supposedly through an accident with dog urine. According to folklore, an English chemist noticed that a dog who had been fed quinine bisulfate (perhaps for an upset stomach) had urinated in a tray of iodine. From this accident, green crystals (called herapathite) formed in the tray, and the chemist realized that they were able to filter out light. Edwin Land, inventor of the Polaroid camera, was later the first to fashion an SPD device. Failing in an attempt to make large thin sheets of polarizing iodine compounds, Land turned to making submicroscopic crystals by the billions. These he found could be spread on plastic sheets and lined up by electric or magnetic fields. When two such sheets were rotated with respect to each other, a clear view would gradually change to black.

Land’s research resulted in 535 patents (second only to Thomas Edison.) In terms of patent count, Research Frontiers isn’t far behind.

The lab, which currently hold over 350 U.S. and foreign patents and patent applications on the technology, now licenses its technology to such manufacturers as Dai Nippon Ink & Chemicals, General Electric, Hitachi Chemical, Hankuk Glass Industries (the Korean giant glass works), and Material Sciences Corp., San Diego, (a supplier of specialty films).

In March, ThermoView Industries, Louisville, became the first domestic manufacturer specializing in products for the $8 billion replacement window/door industry licensed to produce SPD ‘smart’ windows.

There are actually over 500 fabricators in the fragmented U.S. window industry, although the glass itself comes from only six U.S. flat glass manufacturers, including PPG Industries, LOF (Pilkington Libby-Owens Ford), AFG Industries, Ford, Guardian, and Cardinal.

In spite of all the licensing activity, SPD windows have yet to appear on the market. Developing the technology and manufacturing processes has been long and difficult.

"The technology is perfectly well-suited for window applications and what’s happening now is our licensees are scaling up to make large quantities of the materials needed for production," Harary said. "As you know, there’s a big difference in doing something on a lab scale and doing something on a commercial scale. We’ve successfully developed procedures to scale up the technology and we’ve licensed and trained companies like Hitachi Chemical and Dai Nippon Ink and Chemicals to make the basic emulsions, which are the liquid materials that eventually get turned into a film, the polymers and the particles that we use in our system."

Those companies in turn are licensed to sell the emulsions to film makers, who are also licensees, who take the liquid emulsion, coat it onto a substrate and cure it into a film. The film is what goes into the window.

"Initially you’ll probably see the film imbedded in the glass itself, say, where it gets laminated on the inside of a insulating glass window," Harary said. Eventually, though, retrofitting will be possible by merely laminating the film to an existing window, hooking it up to an electrical connection and, presto, a ‘smart’ window.

"It’s an easy technology to work with because it’s a film," he said.

The retail price of the SPD windows have yet to be determined, although Harary’s educated guess is the addition of the particle film will not increase the cost of windows by more than about 20 percent, adding about $15 per square foot, which is considerable less expensive than either liquid crystal or electrochromic windows.

"But it’s up to the licensees, because they’re the ones setting the price," he said.

The new SPD glass allows blocking of sunlight without curtains or blinds, the particles that block light from the outside would also block light from the inside, so the privacy one expects when bolting out of the shower for the phone or napping on the job would still exist.

Harary compares the blockage to a one-way mirror. "Instead of having a reflective coating, you have something that's blocking the light. It's one of these technologies that's going to make people's lives more comfortable," he said.


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