Optical evaluation of a smart transparent insulation material for window application

Thermotropic (TT) materials can provide dynamic regulation of solar energy and daylight, thus achieving building energy saving and indoor environment improvement. Parallel Slats Transparent Insulation Materials (PS-TIM) when integrated into the cavity of a double-glazed window can increase the thermal resistance of the window system, thus reducing building heating energy consumption. In this study, these two advanced technologies are combined to form a novel TT PS-TIM window system for adaptive daylighting control and enhanced thermal insulation. To achieve this, thermotropic hydrogels such as hydroxypropyl cellulose (HPC) and poly(N-isopropylacrylamide) (PNIPAm) are proposed for integration within polymethyl methacrylate (PMMA) slats between double glass panes. The optical properties of the PMMA-TT slats have been firstly investigated by optical simulations and experiments. Windows integrated with the PMMA-TT slats have been developed and predicted for their daylight performance under different solar incident angles, slat tilt angles and concentrations of HPC/PNIPAm polymer within hydrogel. It was found that the TT PS-TIM window can effectively reduce visible and solar transmittance when the TT material switches from clear (at low temperatures) to translucent (at high temperatures). In addition, the translucent TT PS-TIM window allows less light to be transmitted into the interior space when the slat tilt angle is increased from 0° (horizontal) to 60° (inclined). More uniform daylight distribution in the indoor space can be achieved by increasing the slat tilt angle. The optical investigation results of this research will provide guidance for the further development of this system.