WINDOW TECHNOLOGIES: Advanced
The most promising switchable window technology today is electrochromic (EC) windows. The electrochromic thin film stack is deposited on a glass substrate and is typically about one micron thick. The stack consists of ceramic metal oxide coatings with three electrochomic layers sandwiched between two transparent electrical conductors.
When a voltage is applied between the transparent electrical conductors, a distributed electrical field is set up. This field moves various coloration ions (most commonly lithium or hydrogen) reversibly between the ion storage film through the ion conductor (electrolyte) and into the electrochromic film. The effect is that the glazing switches between a clear and transparent blue-gray tinted state with no degradation in view, similar in appearance to photochromic sunglasses.
The main advantages of EC windows are that they typically only require low-voltage power (0–10 volts DC), remain transparent across the switching range, and can be modulated to intermediate states between clear and fully colored. In the tinted state solar radiation is absorbed (similar to tinted glass), although some switchable reflective devices are now in research and development. Low-emittance coatings (an inherent property of some EC glazings) and an insulating glass unit configuration can be used to reduce heat transfer from this absorptive glazing layer to the interior. Typical EC windows have an upper visible transmittance range of 0.50–0.70 and a lower range of 0.02–0.25. The SHGC ranges from 0.10–0.50. A low transmission is desirable for privacy during the day and for control of direct sun and glare, potentially eliminating the need for interior shading. A high transmission is desirable for admitting daylight during the time of the day that the sun is not shining directly into the space, during overcast periods and for passive solar heating in winter. Therefore, the greater the range in transmission, the more able the window is to satisfy a wide range of environmental requirements.
For some EC types (polymer laminate), the device is switched to its desired state and then no power is needed to maintain this desired state. This type of device has a long memory once switched (power is not required for three to five days to maintain a given switched state).
Another EC type (all-solid-state) requires minimal low-voltage power to both change and maintain a given state (0.02 W/sf). When powered off, the EC slowly goes to clear. The advantage of the solid state EC is that it is more durable than its polymer counterpart. This second type has been shown through independent tests to be extremely robust under hot and cold conditions and under intense sun. These devices have been cycled (from clear to colored and then back again) numerous times under realistic conditions so that one can expect long-term sustained performance over the typical 20 to 30 year life of the installation.
Switching speed is tied to the size and temperature of the window. Coloring typically takes slightly longer than bleaching. A 3-by-5-foot window can take 5-10 minutes to switch across its 90% of its range in moderate to warm conditions, longer if cold Note that because in cold weather EC panes will likely be operated to control glare, the impact of direct sunlight exposure on the panes will cause the switching times to be faster because the sunlight absorption will cause the coatings to warm. And of course in cold weather without direct sun, it is less likely for occupants to want to tint the glass. In actual building applications, a gradual transmission change is advantageous because it allows the occupant's eyes to adjust to the change in light levels without causing discomfort. In fact, modulating the amount of light in a room slowly and in a measured manner is a technique used widely in lighting control and is favored by lighting designers to prevent the occupant distraction.
Electrochromic glazings are fabricated as insulated glass units using standard or laminated glazing. Wire leads extending from one edge are tied into a low voltage control system provided by the EC manufacturer that is powered by the building's electrical system. The window can also be powered using photovoltaic cells to avoid the cost of routing the wiring to the glass. Once installed, the window or skylight can be operated by a manual switch or remote controller, a simple stand-alone automatic system, or a sophisticated central energy management system that integrates its operation with other building systems, such as the electric lighting and mechanical system.
Controlling and modulating incoming light and solar heat gains leads to lower energy bills and increased occupant comfort. The higher initial price of electrochromic glazing can be partially offset by these factors and can be cost competitive or have a lower initial cost than the complete alternative solution of "static" glass, additional HVAC equipment, plus interior window shades/blinds and exterior sunshades – particularly those that are mechanized and automated. Electrochromic windows give building owners the ability to modulate heat gain through the window, reducing cycling stress on HVAC motors and other equipment. Additional operating costs for the static glazings—shade replacement and cleaning, UV fading of textiles and fabrics, increased HVAC maintenance, and reduced life of the HVAC system can also buy down electrochromic glazings . Electrochromic glazing provides functionality that other types of shading do not—for example, in darker states, it is still possible to provide a view rather than blocking it completely with drawn shades or blinds.
Electrochromic technology has been actively researched throughout the world for over thirty years, and promising laboratory results have led to prototype window development and subsequent product commercialization with installations in both commercial and residential applications. Examples of electrochromic window prototypes have been demonstrated in a number of buildings in Japan, Europe, and the United States. Full scale field tests have been conducted in Berkeley California and at the DOE Headquarters Building in Washington, DC by Lawrence Berkeley National Laboratory(LBNL).
For the Berkeley test, 10-by-15-foot-deep private offices were fitted with a large 10-by-9-foot window wall of EC glass. Each EC pane in the window-wall can be switched independently to control local sources of glare and to admit daylight. The windows were linked electronically to light sensors and dimmable fluorescent office lighting. The glazings were operated so that they admit maximum daylight under dark, overcast conditions and minimum solar heat when sunny skies prevail. Occupants in the offices could override the automated controls if they desired. Overall, this building-scale field test reinforces the capability of switchable windows to provide energy savings while improving comfort and amenity in an office environment. The energy-saving results of the LBNL study were up to 20% reduction in daily cooling load and up to 60% reduction in daily lighting energy. A 19-26% peak demand reduction was also achieved.
Electrochromic glazings have been installed in hundreds of commercial and residential buildings. Currently, flat durable window and skylight products are available in sizes up to 40-by-60 inches. By the end of 2012, 5 ft x 10ft EC windows will be produced in SAGE Electrochromics' high volume manufacturing plant in Faribault, MN.
Millions of small electrochromic mirrors have also been sold for use as rearview mirrors in automobiles and trucks. Electrochromic glazings have also been installed as prototype sunroofs in cars.