Exterior shading comes in many flavors. Shading can be attached to the building skin or designed into the building; for example. windows can be set back deeply into a wall section to provide some shading. Both methods need to address exterior aesthetic concerns. Overall, exterior shading may be more aesthetically pleasing than interior shading which tends to have shades in a variety of positions. Exterior shades can be fixed or adjustable. Automated adjustable shades provide the most effective means for energy savings and occupant comfort.
Automated shades, either interior or exterior, have higher initial and ongoing maintenance costs than nonautomated shades, but, are much more energy efficient. So the financial calculation is really a comparison of additional lifecycle costs of automation versus increased lifecycle energy savings.
Electrically Switchable Glass
Electrically switchable glass goes by many names: smart glass, smart glazing, or smart windows. It is basically glass or coatings that change light transmission properties when voltage is applied. There are a variety of technical means to accomplish this including electrochromic, suspended particles, and liquid crystals.
When voltage is applied to electrically switchable glass the devices or coatings change to tint and absorb light. Depending on the underlying technical means, either a one-time or constant electrical current is needed to activate. The coatings or devices return to clear when current is interrupted or polarity of the voltage is reserved.
The clear to tint or tint to clear change can occur in just seconds or a few minutes depending on the technology. The tint level can be controlled manually or automated via integration into a BAS system. The control options vary with manufacturers; some being able to go from clear to tint back and forth, with others having some intermediate levels of tinting. Much like the motorized shades, the electrically switchable glass can be manually operated via a switch or automated based on light sensors, schedule, occupancy sensors, lighting control or thermostats.
Electrically switchable glass is not new and you may have used or seen it. It's been used in interactive displays in museums, outdoor displays, privacy glass, projection screens, and in windows on planes, trains, and automobiles. The rearview mirror in your car may be using one of the underlying technologies in electrically switchable glass.
The issues with electrically switchable glass include installation cost, the limitations on the type of windows offered by some manufacturers (i.e., not applicable to operative windows), the degree of transparency of the glass, switching speeds, and the ability to control intermediate light transmission states. Prices on some of the electrically switchable windows are coming down as companies' ramp up their manufacturing to meet what they see as a huge potential marketplace segment in energy conservation.