It seems impossible for people in Hong Kong to endure the heat in the humid summers without air conditioning. However, air conditioning, which consumes a massive amount of electricity and can leak potent greenhouse gases into the atmosphere, contributes significantly to thermal pollution and global warming. This creates a vicious cycle.

What if we could keep indoor temperatures low enough such that air conditioners become unnecessary? This is now achievable – keeping the interior temperature of buildings at least 6°C cooler than the ambient temperature with zero electricity consumption – with a novel method developed by a team of PolyU researchers to cool buildings smartly.

Low-cost anti-heat coating for buildings with zero electricity consumption

Professor Dai Jian-Guo, Associate Head (Academic Development), Department of Civil and Environmental Engineering of PolyU, together with his research team, have invented a method to make advanced coating materials for “smart” sub-ambient radiative cooling (SSRC) to cool buildings off while consuming zero electricity.

PolyU’s SSRC coating, called “UmiCool”, is a multifunctional aqueous polymer coating that can be painted on the exterior surface of buildings and infrastructure. It works by scattering sunlight, converting absorbed UV light to fluorescence emissions and re-emitting infrared radiation to outer space.

 

The PolyU invention is able to reduce the temperature of buildings by 6°C as compared to the ambient temperature in the daytime under direct sunlight and by 4°C at night without electricity consumption. The team conducted model room tests in Beijing and Zhuhai during the sweltering summers. Results revealed that UmiCool is capable of maintaining the interior temperature at around 26°C despite an ambient temperature of up to 40°C, saving as much as 80% of energy consumption in air conditioning.

Compared to a similar mechanism adopted in the industry, “UmiCool uses the sky as a temperature regulator for the dynamic exchange of heat, enhancing daytime cooling while suppressing overcooling at night,” Professor Dai explained. Thus, the huge temperature difference between day and night that may compromise the durability of buildings and infrastructure can be avoided.

 

UmiCool is also a low-cost alternative, as the building materials comprising the coating, including titanium dioxide nanoparticles, fluorescent microparticles, and glass microspheres, are all conventional ones.

Further applications

In addition to applications on the exterior surfaces of buildings, UmiCool also works on vehicles and infrastructure such as bridges and paved roads.

Meanwhile, modern office and residential towers in Hong Kong seem to favour the use of glass curtain walling and full-length panoramic windows that let in the sun and the heat. Professor Dai said the team has considered making a semi-transparent version of UmiCool for windows, but they need to find a balance between the cost and market demand.

“There is low-emissivity (low-E) glass in the market that blocks off much of the heat from the sun, yet without blocking visible light. Thus, we need to consider whether the cooling effect of semitransparent UmiCool justifies its cost,” he explained.

That said, he believes that the formulation of UmiCool can be used to make polymer materials with thermal radiative functions. That means, theoretically, window shades and blinds could be made using UmiCool in future, minimising the heat gain from sunlight through windows.

The team is also looking into the possibility of applying the coating to asphalt roads, oil tanks, outdoor pipes, cars and ships besides buildings.

In summary, UmiCool provides unlimited opportunities for energy conservation in buildings and infrastructure, alleviating the urban heat island effect.

Pro-infra: a PolyU academic-led startup

UmiCool was developed with the support of the Environment and Conservation Fund of the Hong Kong SAR Government and the PolyU Postdoctoral Fellowship Scheme. In the Inventions Geneva Evaluation Days – Virtual Event 2022, the PolyU innovation won a Gold Medal.

To commercialise UmiCool, Professor Dai founded Pro-Infra Science & Technology Limited. The PolyU academic-led startup is currently working with the HKSAR government, major property developers in Hong Kong and construction material companies for field-scale demonstrations.