Smart windows are next-generation windows that control light passage and increase energy efficiency of the room in which they are a fixture without having to block the view that the windows offer.
Conventional "Light Control"
There is a need to control the light that gets in through windows. For example, clear 'unblocked' windows in the summer means that the sun's rays can penetrate and heat the room. This will cause the air-conditioning system to work harder and thus increase consumption of electricity. On the other hand, 'blocking' these windows in winter time also has cost implications. A darkened room will make a living or working space cold and this will result to greater heating expenses.
Traditional or conventional 'light control' systems made heavy use of curtains, drapes and blinds to control the amount of light (and by extension, the amount of heat) that enters through windows. These systems present their own maintenance and replacement problems, though.
The thrust of research and development for many windows and light control systems is to find an energy- and cost-efficient means that can adapt to changing circumstances and allow in the 'right' amount of light without losing one's view of the outside.
Smart Window Technologies
Various systems have been tried over the years; some which showed promise had to be dropped reluctantly when faced with practical realities. Photochromic systems, which darken windows when a certain amount of light strikes it, were a promising technology until people realized that this was perfect for the summer (photochromic systems automatically darken windows in summer) but were impractical for winter (when sunlight into a room can help in heating the place up. The same holds true for thermotropic windows which react to heat. While perfect for summer, they can also block out the view.
More promising are the so-called suspended particle devices (SPDs) which make use of tiny 'particles' that can act as window blinds or "light valves." The principles of suspended particle devices are straightforward: millions of tiny particles are sandwiched between two panes of glass or plastic which are coated with a transparent conductive material. Pass an electrical current through the conductive material and the particles "line up" in straight lines which allow light to pass through. Turn off the current and the particles revert back to their original random patterns which block out all light. Varying the voltage adjusts the darkness of the window; this can be controlled manually or automatically (using photocells and other sensing devices to adjust the level of light).
Liquid crystal windows follow the same basic approach – applying a low voltage current to the smart window causes it to become transparent; removing the current brings it back to its 'normal' translucent (not opaque) state. Liquid crystal windows have only two states – transparent or translucent; there is no 'in-between' stage.
A newer technology using electrochromic materials is a reverse of the SPD approach – in 'no-voltage' stage, electrochromic windows are transparent but applying a current to the window changes it to dark (opaque).