Researchers develop green radiative cooling coating based on carbon dots to save energy in buildings

Buildings account for about 90% of Hong Kong’s electricity consumption and more than 60% of carbon emissions. Energy conservation in buildings is essential to meet climate change mitigation goals. Researchers at the Hong Kong Polytechnic University (PolyU) have developed an environmentally friendly solar adaptive radiative cooling (SARC) coating for building roofs and walls.

This coating can reduce the surface temperature of a building by up to 25°C and lower the interior temperature by 2 to 3°C, all without consuming any energy. This non-toxic, metal-free and durable coating can also be produced on a large scale, promoting an environmentally friendly and energy-efficient method to mitigate the effects of urban heat islands and help achieve carbon neutrality.

Cladding a building with a reflective material allows for self-regulation of its thermal environment to minimize indoor temperatures. However, traditional passive radiative cooling materials are not able to automatically adjust cooling capacity in response to environmental changes, which limits their applications.

To address this challenge, a research team led by Professor Lu Lin Vivien, a professor in the Department of Building Environment and Energy Engineering at PolyU, together with key team member Dr Quan Gong, a postdoctoral researcher in the same department, invented a carbon dot (CD) SARC coating that can adjust cooling capacity based on solar irradiance.

The book is published in the Journal of Chemical Engineering.

This new radiatively cooled photoluminescent coating can convert solar energy into light energy. As solar intensity increases, the coating’s solar reflectivity improves, preventing buildings from absorbing excessive heat.

However, traditional photoluminescent cooling materials usually rely on rare earth metals and perovskite materials, which have environmental risks. To address these issues, the team introduced innovative and environmentally friendly polymer-based CDs as photoluminescent materials in radiative cooling coatings.

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

The results showed that compared with the conventional radiative cooling coating, the new SARC coating improves the effective daytime solar reflectance from 92.5% to 95% and increases the cooling effect by 10% to 20%. For example, it can reduce the temperature by up to 25°C when applied on concrete roofs.

As part of a demonstration project with the HKSAR government department, the team applied the SARC coating to container house roofs at a construction site in Hong Kong. After about two and a half years of continuous outdoor exposure, the coated roofs remained 24°C cooler than concrete roofs under sunlight. The coating proved to be highly durable, with solar reflectivity 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 in different climatic regions of mainland China, the team observed that the stronger the radiation, the greater the temperature difference achieved by the new SARC coating. Taking Hong Kong and 10 mainland Chinese cities (Beijing, Hangzhou, Guangzhou, Changsha, Hotan in Xinjiang, Shenyang, Guilin, Fuzhou, Chongqing and Lanzhou) as examples, adopting this new cooling coating for buildings is expected to save between 97 and 136 kWh/m² of electricity per year in each city.

Professor Lu said: “As global warming intensifies and extreme weather events such as heat waves become more frequent, the scientific community is increasingly focusing on finding ways to cool buildings while minimizing energy consumption. Our new SARC coating demonstrates exceptional cooling performance and is suitable for a wide range of climates, accelerating the development of next-generation cooling materials.”

“This water-soluble coating can also be produced in different colours, making it easy to apply to roofs, walls, roads and urban surfaces of buildings using paint rollers. It provides both cooling and aesthetic benefits and offers a promising solution for sustainable urban development and mitigation of the urban heat island effect.”

The team also integrated the photoluminescent coating with bifacial solar photovoltaic (PV) panels to achieve a synergistic improvement in thermal management and energy production, transforming buildings from energy consumers to energy harvesters. The team plans to install bifacial PV panels on the roofs of the PolyU Kowloon Tong student residence under construction, with a new coating applied to the corresponding area below the panels, to improve electricity production while radiantly cooling the buildings.

The team hopes that this dual-function system will improve electricity production by 30 to 50% and reduce carbon emissions by 30% compared to conventional uncoated roofs. Take this project as an example, installing PV on an area of ​​approximately 600 m² The hostel roofs can generate 97,000 kWh of electricity, saving over HKD 120,000 annually.

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, thereby achieving the effect of keeping warm in winter and cooling in summer.