Hydroxypropyl Cellulose Thermochromic Hydrogel with Efficient Passive Radiative Cooling and Adjustable Visible Light Transmittance

The rising temperature of ground objects under solar irradiation has led people to an increasing dependence on cooling methods that consume huge energy such as air conditioners and fans. Building windows and vehicle windows also have caused huge losses of energy due to allowing sunlight to enter. Therefore, it is of great significance to simultaneously endow outdoor application materials with radiative cooling capacity and adjustable light transmittance. In this study, hydroxypropyl cellulose (HPC) hydrogel radiative cooler was developed with high emissivity (0.96) and excellent visible light modulation ability (Delta Tlum) with a high maximum transmittance (91.2 %) and Delta Tlum (83.8 %). An ultra-low phase change temperature of 45 degrees C is achieved to meet insulation requirements and energy saving targets under hot climatic conditions. In the outdoor experiment, the HPC hydrogel radiative cooler can reduce the temperature by approximately 5 degrees C and 15 degrees C compared with the ordinary glass and aluminum sheet. The HPC hydrogel after phase transformation shows excellent cooling effect on different substrates such as glass and aluminum sheet. After 20 heating/cooling cycles and one month of exposure, the transmittance and Delta Tlum of the cooler have no obvious changes. The significant temperature rise suppression effect and transmittance adjustable performance of the HPC hydrogel radiative cooler opens up a promising avenue for application.