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How Smart Windows Will Work
by Kevin Bonsor

If you live in the United States, you've probably heard recently about the impending energy crunch that is facing Americans. In the next 20 years, U.S. energy consumption will increase 45 percent for electricity, 62 percent for natural gas and 33 percent for oil, according to the Department of Energy (DOE). The DOE also says that energy supplies will be unable to meet demand for the next two decades. For consumers, this means paying higher prices for electricity, natural gas and oil.

While the country is concentrating on ways to increase its energy supply, a few researchers are working on new low-power-consuming technologies. Among these new technologies that could make our homes more energy efficient are smart windows, and we aren't talking about Microsoft's operating system. With this developing window technology, consumers can block either all light or just some by simply turning a knob. This type of light control could save consumers billions of dollars on heating, cooling and lighting costs.


Smart windows made with suspended particle devices are controlled with electricity. To block out the light, take away electricity by sliding the red knob down.
NOTE: Netscape users may have difficulty using this file.

Several types of smart-window technologies are being developed and each are vying for a share of the estimated 20 billion square feet of flat glass produced worldwide each year. Liquid crystals, electrochromics and suspended particle devices (SPDs) are all being touted as the next great technology for windows. In this edition of How Stuff WILL Work, you will learn how each of these technologies work and how they could save you money.

Suspended Particle Devices
Windows serve an important function in homes and commercial buildings. They not only let light in to cut down on electricity use for lighting, but the light coming through the window also provides heat. However, windows are not something people typically associate with being a cutting edge technology. At least one company is trying to change that by introducing a new type of window that can quickly be changed from clear to opaque and anywhere in between with the flip of a switch.
A Chance Discovery

Although Edwin Land, inventor of the Polaroid camera, was the first to build a device using suspended particle devices (SPDs), the actual discovery of light-absorbing crystals can be credited to a dog. SPD technology was first discovered more than 100 years ago. As the story goes, the dog of an English chemist had been fed some quinine bisulfate to settle an upset stomach. When the dog accidentally urinated on a tray of iodine, the chemist noticed that green crystals formed in the tray. Somehow, the chemist discovered that these crystals had the ability to filter out light. Land later used these light-absorbing crystals to make a pair of glasses that could block out light.

Conventionally, curtains and mini-blinds have been used to block out light and give us privacy, but they don't block out all of the light. Research Frontiers is the only company developing a new type of window that uses small light-absorbing microscopic particles known as suspended particle devices (SPD), or light valves. Here's a breakdown of the parts that make up SPD light-control windows:

  • Two panels of glass or plastic
  • Conductive material - used to coat the panes of glass
  • Suspended particle devices - millions of these black particles are placed between the two panes of glass
  • Liquid suspension or film - allows the particles to float freely between the glass
  • Control device - automatic or manual

How the SPD windows work is very simple if you think of SPDs as light valves. In a SPD window, millions of these SPDs are placed between two panels of glass or plastic, which is coated with a transparent conductive material. When electricity comes into contact with the SPDs via the conductive coating, they line up in a straight line and allow light to flow through. Once the electricity is taken away, they move back into a random pattern and block light. When the amount of voltage is decreased, the window darkens until it's completely dark after all electricity is taken away.


Control the level of light by increasing or decreasing the electricity flowing to the window. Slide the red button down to decrease the amount of electricity.
NOTE: Netscape users may have difficulty using this file.

Users apply a moderate amount of voltage to the conductive material on the window panes through a control device. Several control methods will be offered with the SPD light-control windows, including automatic and remote devices. Photocells and other sensing devices could be used to control the level of light automatically. Or, the windows can be controlled manually with a rheostat or remote.

Research Frontiers holds about 350 worldwide patents on this light-control technology. Suspended particle devices could also be used for a multitude of other consumer products, including sunroofs, sun visors, rearview mirrors, ski goggles and flat panel displays for computers. SPD technology isn't the only smart-window technology being developed. Liquid crystals have been in use for years, and electrochromics has the potential to perform the same functions as suspended particles. In the next section, we will look at these two technologies.

Competing Technologies
Windows are undergoing a change that hasn't been seen since the horizontal mini-blind was developed 50 years ago. Soon, curtains and window blinds could be obsolete. SPD technology is one of the reasons for this window revolution, but let's look at two of the other technologies competing with SPD light control.

Here are the two technologies competing with suspended particle devices in the smart window niche of the glass industry:

  • Liquid crystals
  • Electrochromics

You are probably familiar with liquid crystals, which are found in many of the products you use every day. Portable computers, calculators, digital clocks and watches, and microwave ovens all use liquid crystal displays (LCDs). In these displays, electricity is used to change the shape of the liquid crystals to allow light to pass through, thus forming figures and numbers on the display.

The technology behind an LCD is similar to polymer dispersed liquid crystals (PDLCs) in switchable windows. In these windows, the liquid crystals respond to an electrical charge by aligning parallel and letting light through. When the electrical charge is absent, the liquid crystals in the window are randomly oriented. With liquid crystals, you either let all the light in or block all of the light out. There are no intermediate settings. PDLCs are not a developing technology. In fact, they can already be found in some offices and homes.

Suspended particle devices require power to be transparent. A different smart window technology is in development that would reverse the process. Electrochromic windows darken when voltage is added and are transparent when voltage is taken away. Like suspended particle devices, electrochromic windows can be adjusted to allow varying levels of light in. They are not an all or nothing technology like liquid crystals.

Electrochromic windows are made by sandwiching several layers of materials between two panes of glass. Here are the basic materials inside an electrochromic window and the order you will find them in:

  • Glass or plastic panel
  • Conducting oxide
  • Electrochromic, such as tungsten oxide
  • Ion conductor/electrolyte
  • Ion storage
  • A second layer of conducting oxide
  • A second glass or plastic panel

Ions in the sandwiched electrochromic layer are what allow it to change from opaque to transparent. It's these ions that allow it to absorb light. A power source is wired to the two conducting oxide layers, and a voltage drives the ions from the ion storage layer, through the ion conducting layer and into the electrochromic layer. This makes the glass opaque. By shutting off the voltage, the ions are driven out of the electrochromic layers and into the ion storage layer. When the ions leave the electrochromic layer, the window regains its transparency.

We're surrounded by windows everyday, but we probably don't stop to think about them very often. With advances in smart window technologies, we could soon begin to see windows in a whole new light.

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