Efficient usage of finite energy resources is a core approach for preventing major blackouts caused by a severe lack of energy. Smart windows, which modulate thermal energy transferred from the incident sunlight, have attracted tremendous interest as an alternative technology for resolving the fast-approaching energy crisis by suppressing unnecessary energy usage such as air conditioning or heating inside buildings. Here, we demonstrate a set of materials and design concepts for doubly responsive smart windows, which efficiently reduce the consumption of our limited energy reserves. The proposed smart windows are based on the concept of combining the lower critical solution temperature of thermoresponsive polymer hydrogels and the electrical actuation of graphene-based flexible heaters; this combination serves to actively control the passive-type moving thermoresponsive smart window. The proposed smart windows exhibit a highly tunable transparency of above 90%, which corresponds to an almost instantaneous change from high transmission of the incident light to the complete blockage of its penetration under thermal or electrical stimulation. In particular, when the windows of a mockup house are replaced with the developed flexible smart windows, the increment rate of the indoor temperature under white light irradiation reduces drastically. This type of active light control system is expected to create a new opportunity for achieving cost savings on heating, cooling, and lighting through management of light energy transmitted into the interior of a house.
Keywords: energy efficiency; lower critical solution temperature; smart window; thermo-responsive; transmittance.