Researchers develop carbon dots-driven green radiative cooling coating for energy saving in buildings

Buildings account for about 90% of Hong Kong's electricity consumption and over 60% of carbon emissions. Energy conservation in buildings is crucial for achieving climate mitigation goals. Hong Kong Polytechnic University (PolyU) researchers have developed an environmentally friendly solar-driven adaptive radiative cooling (SARC) coating for building roofs and walls.

This coating can reduce a building's surface temperature by up to 25°C and lower indoor temperatures by 2 to 3°C, all without consuming any energy. The non-toxic, metal-free and durable coating can also be produced on a large scale, promoting an eco-friendly and energy-saving method to mitigate urban heat island effects and support the achievement of carbon neutrality.

Coating a building in a reflective material enables the self-regulation of its thermal environment to minimize indoor temperatures. However, traditional passive radiative cooling materials are unable to automatically adjust cooling capacity in response to environmental changes, which limits their applications.

To address this challenge, a research team led by Prof. Lu Lin Vivien, Professor of the Department of Building Environment and Energy Engineering at PolyU, along with key team member Dr. Quan Gong, Postdoctoral Fellow of the same department, has invented a carbon dots (CDs)-driven SARC coating that can adjust cooling capacity based on solar irradiance.

The work is published in the Chemical Engineering Journal.

This new photoluminescent radiative cooling nanocoating can convert solar energy into light energy. As solar intensity increases, the coating's solar reflectance is enhanced, preventing buildings from absorbing excessive heat.

However, traditional photoluminescent cooling materials typically rely on rare earth metals and perovskite materials, which pose environmental risks. To address these issues, the team has introduced groundbreaking, environmentally friendly polymer-based CDs as photoluminescent materials into radiative cooling coating.

Nano-sized CDs were embedded into polymers to create a biologically harmless material. The polymer CDs were uniformly coated onto hollow glass particles to create smart cooling beads, enabling the coating to effectively convert ultraviolet light into visible light photons and increase effective solar reflectance. This water-soluble SARC only requires the evaporation of water to form a coating on building surfaces without releasing any volatile organic compounds, thereby reducing air pollution.

Results have shown that compared to conventional radiative cooling coating, the new SARC coating improved effective daytime solar reflectance from 92.5% to 95% and increased the cooling effect by 10% to 20%. For example, it can reduce the temperature by up to 25°C when applied to concrete rooftops.

In a demonstration project with the HKSAR Government department, the team applied the SARC coating to the roofs of container houses at a construction site in Hong Kong. After approximately two and a half years of continuous outdoor exposure, the coated roofs remained 24°C cooler than concrete roofs under sunlight. The coating proved highly durable, with solar reflectance decreasing by less than 2% over the two-year period. Annual energy savings of 10% were achieved by reducing the air-conditioning load.

By mapping the average annual temperature drop and cooling power across different climatic regions of Mainland China, the team observed that the stronger the radiation, the greater the temperature difference the new SARC coating achieved. Taking as examples Hong Kong and 10 Mainland cities—Beijing, Hangzhou, Guangzhou, Changsha, Hotan in Xinjiang, Shenyang, Guilin, Fuzhou, Chongqing and Lanzhou—the adoption of this novel cooling coating for buildings is projected to save between 97 and 136 kWh/m² of electricity annually in each city.

Prof. Lu remarked, "As global warming intensifies and extreme weather events like heat waves become more frequent, the scientific community is increasingly focused on finding ways to cool buildings while minimizing energy consumption. Our novel SARC coating demonstrates exceptional cooling performance and is suitable for a wide range of climates, thereby accelerating the development of next-generation cooling materials.

"This water-soluble coating can also be produced in various colors, allowing it to be easily applied to building roofs, walls, roadways, and urban surfaces, using paint rollers. It achieves both cooling and aesthetic enhancements and offers a promising solution for sustainable urban development and mitigating the urban heat island effect."

The team has also integrated the photoluminescent coating with bifacial solar photovoltaics (PV) to achieve synergistic enhancement in thermal management and power generation, transforming buildings from energy consumers into energy harvesters. The team is planning to install bifacial PV panels on the rooftops of the under-construction PolyU Kowloon Tong Student Hostel, with new coating applied on the corresponding area under the panels, to enhance power generation while radiatively cooling the buildings.

The team expects this dual-functional system to improve electrical power generation by 30-50% and reduce the carbon emission by 30% compared with conventional uncoated rooftops. Taking this project as an example, installation of PV over an area of approximately 600 m² on hostel rooftops can generate 97,000 kWh of electricity, resulting in annual cost savings of over HK$120,000.

The team is also developing a paraffin-based self-adaptive radiative cooling coating that can maintain appropriate solar reflectivity in response to cold and hot weather, achieving the effect of keeping warm in winter and cooling in summer.

Provided by Hong Kong Polytechnic University