Facts About Electrochromic Windows - Switchable Glazing
Electrochromic windows, commonly referred to as switchable glazing, have been around for almost two decades. When they first came out, they were used for privacy and switched between a transparent state to an opaque, milky white state. Today there are several manufacturers making switchable glazing in various colors and are soon coming out with intermediate tinting which will have a range between clear and fully tinted.
How they Work
There are several methods currently being used to control the tinting. One way is to apply several thin layers of ceramic material that darkens the window by using a small electrical voltage on the window, and to lighten them by reversing, then removing, the voltage. The electrochromic glazing has memory so that once the tint has been activated the voltage doesn't have to be maintained. During operation, light transmission can vary from 5% to 80%, and the film can even be specified to block out select wavelengths, such as infrared energy.
Another method uses something called a liquid crystal suspended particle device (SPD). Particles suspended in a solution between glass plates move around rapidly and collide in their natural state and blocks the passage of about 90% of the light. When the particles are energized, they align swiftly and render the glazing transparent, letting the light in. These panes are typically used for privacy and require a constant voltage source to keep the glass clear.
There are two types of solar-powered electrochromic windows, created by the National Renewable Energy Laboratory and can vary the tinting based on the amount of sunlight shining on them. Controlling the tint controls the amount of heat that enters through a window. Some manufacturers are developing electrochromic windows that are powered by a small solar cell.
Some electrochromic windows can automatically monitor and control the amount of light and solar energy that can pass through a window to automatically save energy and protect interior furnishing from UV damage. Electrochromic windows do this by responding to external triggers such as an electrical control, or to natural stimuli such as sunlight and temperature. Once they receive a signal, they can change from transparent to tinted, just by applying an electrical current.
Photo from Sage Electrochromics, Inc.
The voltage supplied to these types of windows is usually in the range of 5 volts DC and is supplied by a small transformer and controlled by a wall mounted switch or remote control.
Electrochromic windows are also referred to as “Dynamic Glazing” (DG) according to the National Fenestration Rating Council (NFRC). Switchable glazing technologies within an Insulating Glass Unit (IGU) that can change the Solar Heat Gain Coefficient (SHGC) and Visible Transmittance (VT) of a fenestration product. The U-factor for these particular systems remains constant.
Internal shading systems consist of blinds between glazing panes that open and close, changing the SHGC and U-factor of a fenestration product. These products will be tested and rated using the same methods that NFRC currently requires, allowing for the multiple states of performance that dynamic glazing offers.
Switchable glazings can help reduce energy usage by lowering energy costs for cooling a home by reducing direct solar gain in the summer. They can block UV rays and radiant heat, as with skylights, and lower cooling loads.
By decreasing the solar heat gains, electrochromic windows can also help eliminate the need for window coverings such as shades or blinds. That means that homeowners can not only enjoy the outside view on cloudy as well as sunny days without wasting energy.
Switchable glazings can also protect a home's interiors from damaging UV rays, so that carpets, furniture, and artwork don't experience fading from the sun. Electrochromic windows also offer more control than windows that are photochromic or thermochromic which are windows that turn opaque when exposed to light or warm temperatures.
The energy required to run the windows is minimal and they use virtually no energy to maintain a clear state. The energy that is saved by reducing cooling needs far outweighs the energy use by electrochromic windows.
Solar powered windows go a step further, requiring no purchased electricity to operate the windows whatsoever.
Commercial buildings can receive LEED credits by installing windows with switchable glazing.
Electrochromic windows are complex windows that aren't necessarily a do it yourself project. Electrochromic windows that utilize electricity require a non-standard electrical hookup wiring, including a switch. That means that homeowners may have to hire an electrician as well as a window installer, or a general contractor or other tradesman who can handle all facets of installation.
Few major retailers stock these windows and few major window manufacturers are offering them at this time. Some that are for residential are only available through the manufacturer or select vendors.
Electrochromic windows cost significantly more than a standard window, as much as 2 to 3 times more. Liquid crystal glazing can add as much as $90 per square foot to the glazing.
It the glazing is broken, the tinting will not work and the pane must be replaced. Typically the window can be deglazed and the new replacement pane can be placed in the window frame and plugged back in to a snap connector plug for the power supply.
SAGE Electrochromics, Inc.
National Fenestration Rating Council