7.2.9 Switchable glazing

Switchable glazing can change the light transmittance, transparency, or shading of windows in response to an environmental signal such as sunlight, temperature or an electrical control. Technologies such as electrochromic and liquid crystal windows change from transparent to darkened by applying an electrical current to the window. Once the change in tint has been initiated, the glazing does not need constant voltage to maintain the tinting, so electrical usage is negligible for energy modeling. In addition, the film can be tuned to block certain wavelengths of light, such as infrared energy.

In modeling switchable glazing that includes an automatic control such as a heat sensor, a control set-point, usually in Btu/h, will be used in the model. The basic window will be modeled with the U-Factor, SHGC and VT (visible transmittance) of the glazing with no tinting applied. The energy model will then include modifiers to each of these values when the control set-point has been reached. Thus, the energy model will reduce the energy gain through the windows in response to solar gains on the particular window. 

Another approach is to apply the same modifiers on an hourly basis using a series of schedules. This approach might be used in a circumstance where the glazing is being controlled with a building energy management system and would be applied at a consistent time of day, or seasonally. In this case, the hourly schedule would dictate the multiplier to be applied to the U-Factor, SHGC and/or visible transmittance.

This is but one way to model switchable glazing; other engines such as EnergyPlus have more advanced features and the use of alternative approaches is acceptable.

The baseline building does not have switchable glazing.