Use of hollow silica and titanium dioxide microparticles in solar reflective paints for daytime radiative cooling applications in a tropical region

Passive radiative cooling provides an energy-saving method for heat management in buildings. Solar reflective paints are versatile and scalable formats that can be readily applied for this cooling technology. We investigate solar reflective paints consisting of hollow silica (SiO2) and hollow titanium dioxide (TiO2) microparticles as pigments and methyltrimethoxysilane as a binder. The hollow microparticles are synthesized via sol-gel and etching methods to control their diameter. Paint formulation is optimized for high total solar reflectance, measured by a UV-visible-near infrared photospectrometer, without exceeding the critical particle volume content. By introducing air volume into the particles and selecting optimal particle size, solar reflectance of the dry paints is significantly improved. Compared to TiO2 microparticle samples, paints with hollow SiO2 microparticles are more effective at rejecting solar irradiation on average while also requiring less particle volume content. Thermal and spectral emissivity measurement also indicates that the hollow SiO2 microparticle paint has the highest broadband infrared emission, followed by a commercial paint and a TiO2 sample. Outdoor experiment in a tropical climate demonstrates that the paint with hollow SiO2 microparticles has better cooling performance than a commercial cooling paint product. The results suggest the potential of hollow SiO2 microparticles for improving radiative cooling paints.