Paraffin wax–Cs0.33WO3 composite windows with excellent near infrared shielding and thermal energy storage abilities

Cesium tungsten bronze (Cs0.33WO3) coatings can effectively depress solar heat gain and air-conditioning energy consumption of buildings, because of their highly selective near infrared (NIR) absorbing ability. However, the absorbed solar energy may overheat the window glasses due to the high photothermal conversion efficiency of Cs0.33WO3 coatings, discounting their transparent heat insulating performance. To solve this issue, we design a new energy-efficient composite window by integrating NIR-shielding Cs0.33WO3 with phase change materials (PCMs, paraffin wax in our case). In the composite window, the hexagonal-phase Cs0.33WO3 is prepared by a solid-state reaction method, and then coated onto commercial float glass as Cs0.33WO3–SiO2 composite films via a sol–gel process. Afterwards, a homemade double-plane window consisting of a float glass and a Cs0.33WO3–SiO2-coated glass is filled by molten commercial paraffin wax to obtain the paraffin wax–Cs0.33WO3 composite window. Cs0.33WO3 in the window can reduce solar heat gain of the buildings. Meanwhile, the paraffin wax component can accumulate the Cs0.33WO3-captured thermal energy by its endothermic molting processes, protecting the window from overheating. When the ambient temperature drops low enough, the stored thermal energy in the paraffin wax can be gradually released back by reverse solidifying processes. Therefore, this paraffin wax–Cs0.33WO3 composite window should be very effective to keep indoor temperature stable and comfortable. Performed on a homemade house model, the paraffin wax–Cs0.33WO3 composite window can depress indoor temperature variation from 24.2 down to 7.3 °C, compared with a bare glass window. This composite window is expected to improve energy efficiency of buildings locating in areas with high day–night or summer–winter temperature fluctuations.