Transparent phase change hydrogel for energy-efficient thermal regulation in window applications

Improving the thermal performance of building envelopes, especially transparent envelopes, is critical to reducing the significant energy consumption associated with thermal comfort regulation as transparent envelopes are responsible for 25 %-30 % residential heating and cooling energy use. Thermal energy storage utilizing the latent heat of phase change materials has proven effective in reducing and shifting the peak loads when integrated into building envelopes, especially opaque ones. However, the practical application of thermal energy storage is often constrained by factors such as materials' cost, shape stability, and energy density. In this work, a transparent phase change hydrogel with exceptional shape stability and high energy density was fabricated for energy-efficient window applications by incorporating salt hydrates into poly (acrylamide-co-acrylic acid) hydrogel matrices. The excellent shape stability prevents the leakage of salt hydrates upon melting owing to the retention of salt solution by the hydrogel network. By optimizing phase change hydrogel composition and salt hydrates loading, a high melting enthalpy of 133.3 J/g is demonstrated with a melting point of 32.8 C-degrees, while maintaining excellent shape stability with <10 J/g degradations in energy density and minor shifting in melting point after 500 thermal cycles. Furthermore, the phase change hydrogel exhibits 90 % transmittance in the visible range and a total solar transmittance of 78 % at both melt and crystalized states, making it a promising solution for thermal energy storage for practical window applications. The proposed phase change hydrogel/ glass window with a phase change hydrogel dimension of 100 mm x 100 mm x 5 mm can shift the load to offpeak hours by 30 min, enhancing the power grid's resilience.