Smart Windows Presented by John Helme 2/27/07

What are Smart Windows? Smart Windows are windows which use a glazing material to control the amount of light that is allowed to pass through the glass panes. The glazing material is often called “switchable glazing”.

Types of Smart Windows Non-Electrical Photochromatic Thermochromatic Electrical Phase Dispersed Liquid Crystal (PDLC) Electrochromatic Suspended Particle Device (SPD) or Dispersed Particle System (DPS)

Non Electrical Thermochromatic – dependent on temperature Photochormatic – dependent on ultraviolet light intensity Both technologies automatically adjust. There is no “user” control. Both processes were discovered in about 1870.

Thermochromatic Transmittance * Visible light spectrum: 380nm to 720nm

Photochromatic Transmission %

Electrical Systems Liquid CrystalElectrochromaticSuspended Particle Power offWhiteSlight yellow or green tint Dark Blue or Black Power appliedClear (slight haze)Dark BlueLight Blue Variable StateNoYes MemoryNoYes (for a few days) No Power1.44 watt/sq.ft0.32 watt/sq.ft.06 watt/sq.ft Response time.1 sec on, 4 sec off1 to 40 min.5 sec on, 1 sec off T v when off58% (diffused)60%.5% to 10% T v when on77%.05%35% to 55% Temperature-4°F to 158°F14°F to 204°F-22°F to 194°F T v – Transmission of visible light

Liquid Crystal Device Two layers of electrically conductive film enclose a layer of tiny liquid crystals. When the power is off, the liquid crystals are randomly scattered. Light entering the glazing does not have a clear path out, and the window is a translucent milky white. When an electric current is applied between the two conductive coatings, the liquid crystals align and you can see through the window.

Electrochromatic Device A bipolar potential is applied to the outer transparent conductors. Lithium ions migrate across the ion-conducting layer from the counter- electrode layer to the electrochromic layer. A reversible electrochemical reaction takes place causing a tinted Prussian blue appearance. Reversing the potential causes the ions to migrate back, causing a bleached clear appearance.

Suspended Particle Device Microscopic light-absorbing particles are dispersed in droplets within a film. In the "off" state when no voltage is applied, the particles are suspended randomly throughout the film and absorb light, making the window dark. When moderate voltage is applied to the glass or plastic wall coatings on both sides of the SPD film, the particles align and permit light to pass through. By varying the amount of voltage, the user can adjust the level of light transmission.

Modified Smart Windows A metal hydride switchable mirror which becomes reflective when injected with hydrogen

Conclusion Smart Windows should not be used to control the maximum temperature of a solar panel because they transmit a maximum of 80% of the light, reducing the solar panel efficiency by at least 20%. Smart Windows may work very well for applications where transmitting the maximum amount of light possible is not important.