Geopolymer‐based sub‐ambient daytime radiative cooling coating

Abstract

Sub-ambient daytime radiative cooling coating (SDRCC) is an appealing thermal management technology with great potential for alleviating the global warming and urban heat island effect. Over the past few years, various polymeric SDRCCs have been developed. However, they may face problems of environmental aging under UV and moisture due to their organic nature. In this study, an ambient-cured inorganic geopolymer-based SDRCC was synthesized with the modification of barium sulfate (BaSO 4 ) and nano-silica (SiO 2 ) particles. The optical and physicochemical properties were systematically investigated. The chemical composition, functional groups, surface morphologies of the raw materials, and the formed geopolymer coating were characterized by XRD, FTIR, SEM, and EDS. The developed coating exhibits a high infrared emissivity of 0.9491 and solar reflectance of 97.6%. When exposed to direct sunlight, the coating's surface could cool down up to 8.9 C below the ambient air temperature under Hong Kong's climate. In addition, the coating could retain well its performance under a variety of harsh environments, including high temperature, water immersion, mechanical wearing, and exposure to sunlight.

Eco-Building Materials: Metakaolin-based Geopolymer for CO2 Emission Reduction and Enhanced Energy Efficiency

“…A feature of these geopolymers is their light coloration, which contributes to a high solar reflectance. The solar reflectance of metakaolin-based geopolymer is considerably high, exceeding 70% [12]. This inherent property not only aids in mitigating the urban heat island effect but also reduces the dependency on air conditioning in buildings, making it suitable for sub environmental daytime radiation-cooled coating (SDRCC) applications, consequently improving energy performance.…”

“…This inherent property not only aids in mitigating the urban heat island effect but also reduces the dependency on air conditioning in buildings, making it suitable for sub environmental daytime radiation-cooled coating (SDRCC) applications, consequently improving energy performance. In practical applications [12], this specifically formulated coating showcased an infrared emissivity of 0.9491 coupled with a solar reflectance of 97.6%. Under the climatic conditions of Hong Kong, when subjected to direct sunlight, the coating's surface displayed an ability to cool down by a remarkable 8.9˚C below the surrounding air temperature.…”

Stay healthy under global warming: A review of wearable technology for thermoregulation

“…Among these four pathways, radiative heat exchange is extremely important under static conditions, 33,36,37 making radiative cooling textiles important in personalized thermoregulation. The fundamental principle for radiative cooling textile is that a material surface spontaneously cools down by radiating heat to the cold outer space through the atmosphere's transparent window 38–41 while suppressing all the solar heat gain 42–44 . From the photonic engineering point of view, the solar bands should be reflected as much as possible, 45 while enhancing human body radiative heat loss can be achieved by enabling the textile's mid‐infrared (mid‐IR) property in two ways: high transmittance or high emissivity.…”

“…The fundamental principle for radiative cooling textile is that a material surface spontaneously cools down by radiating heat to the cold outer space through the atmosphere's transparent window [38][39][40][41] while suppressing all the solar heat gain. [42][43][44] From the photonic engineering point of view, the solar bands should be reflected as much as possible, 45 while enhancing human body radiative heat loss can be achieved by enabling the textile's mid-infrared (mid-IR) property in two ways: high transmittance or high emissivity. The former allows the broadband human body radiation to directly transmit and can be achieved by polyethylene (PE) because of its simple chemical bonds and very few resonance peaks in the mid-IR region.…”

Facile and Widely Applicable Route to Self-Adaptive Emissivity Modulation: Energy-Saving Demonstration with Transparent Wood

“…. As a passive route for thermal regulation systems, radiative cooling (RC) has attracted the interests of many researchers and industrialists. − …”