Thermal and Optical Analysis of Switchable Window Glazings

Glazing materials with variable optical properties (switchable glazings) offer the ultimate in control over the light and energy entering a building. Products of this kind are in their initial stages of development, and guidelines that relate window energy performance to glazing material properties are needed. Though the use of a computer program for calculating window thermal and optical performance parameters, we evaluated (1) the relative performances of three switchable glazings prototypes with differing solar transmittance spectra; (2) the differences between glazings that switch from transmitting to reflecting and those that switch from transmitting to absorbing; and (3) the effects of positioning the switchable glazing in a window. We focused on design conditions for cooling-dominated buildings, since switchable glazings are expected to reduce cooling and lighting loads. We conclude that the differences in thermal performance between absorbing and reflecting switchable glazings can be eliminated through proper placement of the glazing in a window system and through the use of other spectrally selective glazings.