Switchable Windows - Spectral Transmission and Switching Times

Abstract

The switching time and spectral transmission in fully clear and fully opaque states of polymer dispersed liquid crystals (PDLC), suspended particle device (SPD) and electrochromic (EC) switchable windows were evaluated to assess their suitability to control solar heat transmission though glazed façade and comfort to building occupants. The transmission measurement on a SPD film showed a very effective absorption modulation in the visible range but fast decrease in the near infrared region. The switching speed was demonstrated to be related strongly to wavelengths. A SPD film with a larger absorption and faster switching speed in the infrared region was shown to be preferable to control solar heat transfer through glazings and thermal comfort. The PDLC window did not modulate transmission but rather modulated scattering. In the opaque state, the PDLC window was highly scattering resulting in spectral transmission varying at short distances from the window while being constant in the far field. The switching speed measurement demonstrated that liquid-based switchable windows can respond as fast as the eye to have the potential to control spontaneous glare. The EC window demonstrated a lower absorbance efficiency than the SPD in the visible range but appeared to have a much higher efficiency in the infrared region measured. EC windows are characterized by their slow switching speed when compared to SPD and PDLC. This makes them unsuitable for spontaneous glare control and are more suited to predictive solar heat gain control over a defined time horizon. With the increasing integration of adaptive technologies into building facades and to predict the energetical and ambient performance of such buildings and the impact on the occupants' comfort, this paper emphasises on the importance to provide spectral performance information of switchable windows.