Session: 02-04 Building Envelope, Building Energy, and Data Centers

Paper Number: 105413

105413 - Cellular Inorganic Ceramic for Highly Efficient Daytime Passive Radiative Cooling 

Traditional compression-based cooling systems consume great amounts of energy, causing adverse impacts on the environment by releasing heat and ozone-depleting pollutants. Daytime passive radiative cooling is regarded as an eco-friendly alternative to generate cooling without electricity and coolants. A radiative cooler on the earth emits thermal radiation from its sky-facing surface to the universe through the long-wave transparent atmospheric window, which couples with radiation intensity of a room-temperature blackbody. In order to achieve continuous and effective cooling, the cooler requires high reflection of the solar spectrum so as to keep the solar heating to a minimum in the daytime. Because solar irradiation is especially intensive at noontime, even absorption of a few percent of solar irradiation can easily compensate for the heat dissipated by thermal radiation, which makes achieving a cooling effect in daytime challenging. Metals, such as silver and aluminum, are widely adopted as solar reflecting layers in conventional radiative coolers. Even though metal-based coolers are efficient in thermal emission when topped with a photonic selective emitting layer, these designs are costly and difficult to maintain. The shining reflection caused by metal reflectors is also unwanted in city landscapes. Compared with metal, a diffused reflecting surface is a more applicable solution to create solar reflection in a radiative cooling application. The most commonly used scattering systems are particles doped in a polymer matrix, which greatly simplify the fabrication and reduce the cost. However, the pigments’ ultraviolet absorptance and polymer’s near infrared absorptance still impede solar reflectance. Here we develop a cellular inorganic radiative cooler in the format of ceramic, which possesses advanced optical properties for radiation cooling while exhibiting great outdoor applicability. The cellular ceramic is made of inorganic particles (i.e., aluminum oxide) that undergoes phase inversion and high-temperature sintering. The as-generated hierarchically porous structure of the cellular ceramic results in efficient scattering across the whole solar spectrum. The average solar reflection of the cellular ceramic reached a recorded-high of 99.6% with porosity of 70% and thickness of around 600 µm. With the near-ideal solar reflection and high thermal emission (96%), the cellular ceramic can theoretically obtain cooling power in excess of 120 W/m² under 1000 W/m² solar intensity. Even in extremely unfavorable conditions where the transmittance in the atmospheric window is low, the cellular ceramic is expected to maintain a cooling power over 50 W/m². Moreover, the high bond strength and chemical inertness of alumina protect the cellular ceramic from the adverse effect of UV exposure, which is desired for long-term outdoor application. The outstanding performance of cellular ceramic surpasses that of current daytime radiative coolers. The facile fabrication and promising applicability of cellular ceramic exhibits great potential for large-scale applications, such as buildings, to saving the energy for indoor thermal regulation.

Presenting Author: Kaixin Lin City University of Hong Kong

Presenting Author Biography: A PhD student from the school of energy and environment, City University of Hong Kong.
His research explores the materials/systems that generate cooling power passively and help end users, such as buildings to save energy, and therefore, make energy usage sustainable. He is also one of the co-founders of a startup, which provides applicable solutions for energy saving by employing passive radiative cooling technology.