Scalable hydrogel smart windows with moisture retention and low-emissivity for sustainable buildings

Abstract

Thermochromic hydrogels have shown promise for smart windows but face challenges such as high thermal emissivity and rapid water loss. This paper reports a scalable hydrogel-based smart window that simultaneously addresses the key challenges of moisture retention, low long-wave infrared emissivity (ɛ_LWIR), and large-area fabrication. The hydrogel is synthesized by copolymerizing the moisturizing agent sodium pyrrolidone carboxylate (PCANa) with N-isopropylacrylamide (PNIPAM), forming a stable structure that lowers the phase transition temperature to 25.7 °C and improves water retention by 12% compared to the pure PNIPAM hydrogels. This hydrogel is further integrated with an indium tin oxide (ITO) layer for reduced ɛ_LWIR. We produce a large-area smart window demo (40 × 45 cm²), achieving a visible transmittance (Т_lum) of 69.3%, a solar modulation (ΔТ_sol) of 60.3%, and a low ɛ_LWIR of 0.32. To our knowledge, the demo featured with this performance and the water retention character is rarely achieved in previous reports. The demo test proves an indoor temperature reduction of up to 9.0 °C under daytime illumination compared with glass-based window. EnergyPlus simulations further indicate annual Heating, Ventilation, and Air Conditioning (HVAC) energy savings of 44.9%, 46.1%, and 10.1% in Bangkok, Pheonix, and Changsha, respectively, outperforming conventional glazing systems. The integrated design maintains excellent optical performance over 100 cycles and provides a scalable pathway toward practical, energy-efficient building applications.