Are There Windows That Darken When the Sun Shines on Them?

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As the cheap fossil fuels of the 20th century fade into memory, smart glass windows look better than ever as potential energy savers. Their ability to darken in sunlight could save billions of dollars in cooling costs each year. The technologies behind these windows, however, are in various stages of development and commercialization as of 2011. They vary widely in their speed of transition from light to dark and their suitability for a range of applications.

Photochromic Windows

  • Photochromic windows, under development as of 2011, contain the same glass that's available as self-darkening sunglasses. Like those glasses, however, the degree to which they darken depends on the amount of light that strikes them. During winter, when the sun sits low in the sky, photochromic windows would get darker than when they're exposed to summer's higher sun. Their responsiveness to light, not heat, would limit photochromic window's effectiveness as summer energy savers.

Thermochromic Windows

  • Commercially available thermochromic windows contain a film that tints automatically with exposure to the sun's heat. Their degree of darkening increases on the warmest days. Individual thermochromic windows darken at different rates throughout the day and year depending on the angle of the sun reaching them. Windows with southern exposure take 12 hours to go from clear to fully dark and back as the sun tracks across the sky. Clear thermochromic windows have a blue-green tint and 45 percent light transmittance. At their darkest, they admit 10 percent of the sunlight.

Electrochromic Windows

  • Electrochromic windows incorporate the same light-responsive technology used in millions of self-dimming car mirrors. These windows contain five transparent layers, two of which hold lithium ions. When exposed to a small electrical current, the ions move slowly move from one layer to the other, darkening the window. When the current stops, the ions return to their original positions. While they don't tint in response to sunlight, electrochromic windows can continually interface with sensors to dim or clear automatically. Electrochromic technology works on square, rectangular, triangular and trapezoidal windows measuring up to 3.5 by 5 feet. Transition time between clear and dark states ranges from three to five minutes.

Suspended Particle Device Windows

  • Suspended particle device, or SPD, windows contain a film of particles. Randomly distributed in the dark, off state, the particles align to let light through in response to an electrical charge. Incremental changes in the charge's strength cause incremental changes in the film’s transparency. SPD windows are the only ones that darken or clear within seconds, while letting users determine exactly how much light they want. Like electrochromic windows, they also interface with light sensors to tint as the sun's position changes, but their adjustments are nearly instantaneous. At their darkest, SPD windows block more than 99 percent of the light. Their maximum transparency is 35 percent. SPD technology is in use in automotive, aeronautical, marine and architectural applications. It's suitable for curved or flat windows of any shape and size, and it works on plastic as well as glass.

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