CaCO₃ micro particle-based radiative cooling device without metal reflector for entire day

Conventional cooling systems such as air conditioners should be replaced, because they consume a substantial amount of energy and cause environmental pollution. In this context, radiative cooling systems are emerging as an alternative, as they perform cooling without consuming energy or causing environmental pollution. However, most radiative coolers explored thus far include metals such as silver used as solar reflectors, thereby entailing problems in terms of practicality, mass production, cost, and light pollution. In this study, obtaining clue from seashells, we propose micro-particle-based radiative cooling based on calcium carbonate (CaCO₃), i.e., the main component of seashells. This approach utilizes the high-energy band gap of CaCO₃ for high-performance radiative cooling. As the cooler has only a single layer of a CaCO₃ composite without any metal reflector, it is mass-producible, cheap, and does not cause light pollution. To demonstrate the cooling performance of the CaCO₃-based radiative cooler, its optical properties and temperature changes are measured and compared with those of commercial white paint. As a result, it is demonstrated that the CaCO₃-based radiative cooler has a cooling power of 93.1 W/m² and can lower the temperature by an average of 6.52 ℃ and 3.38 ℃ under ambient temperatures in daytime and nighttime, respectively. Therefore, it can be used as a radiative cooler throughout the day.