JUN 2022 | ALL ISSUES
Smart coating UmiCool keeps interior of buildings at least 6°C cooler than ambient temperature in summer
According to Hong Kong Observatory, 2021 is the warmest year on record in Hong Kong, with 54 days when the daily maximum temperature reached 33°C.[1] Arguably, air conditioner is a need, not a luxury, in our subtropical city because prolonged exposure to extreme heat may cause heat exhaustion, heat stroke and even death. However, building cooling has posed a heavy burden on our power grid and the refrigerant gases used in air conditioners are one of the major sources of greenhouse gas emissions.
Vicious cycle of air conditioning
In Hong Kong, air conditioning is the single largest end-use of electricity consumption, accounting for 28% of total power consumed in 2019.[2] Residential and commercial buildings also contribute 60% of the city’s greenhouse gas emissions.[3] Thus, air conditioning harms the environment in a vicious cycle: it causes excessive consumption of electricity which is, more likely than not, generated from non-renewable sources with a heavy carbon footprint; and it also emits greenhouse gases that destroy the ozone layer, exacerbating global warming. The creation of heat island and thermal pollution lead to the need for more air conditioning.
Drawbacks of existing SDRC materials
As a result, scientists and experts have been looking for passive solutions to cool buildings without using power and refrigerant. In 2014, researchers in the U.S. came up with a ground-breaking concept known as sub-ambient daytime radiative cooling (SDRC) that cools buildings under the sun by transmitting heat to the cold outer space by means of infrared radiation, while consuming no power at all. Since then, generations of SDRC materials have been developed, but they often rely on complicated photonic crystals, the use of precious metals and painstaking development of metamaterials. That’s why these materials are too costly to be practically feasible for extensive applications. Sometimes they also entail chemical processes that may harm the environment.
Another drawback of SDRC materials is the excessive night-time heat loss. They may cool down buildings by a few degrees under the scorching sun, but radiation cooling works a lot more effectively at night making buildings 10-plus degrees cooler than outside. That means occupants may feel uncomfortably cold at night. On the other hand, that also exposes the building structure to extreme daily range of temperature, undermining its lifespan.
UmiCool: a smart coating for sub-ambient cooling
In light of this, Prof. DAI Jianguo, Department of Civil and Environmental Engineering, led a research team to developed UmiCool – a smart coating for passive building cooling that adopts a similar mechanism as SDRC, but using only common building materials to significantly bring the cost down, namely titanium dioxide nanoparticles, fluorescent microparticles, and glass microspheres. Instead of noble metal mirrors used in existing SDRC coatings, UmiCool uses titanium dioxide nanoparticles that are engineered to reflect most of the sunlight hitting a building by light scattering. Then fluorescent microparticles convert the absorbed sunlight into fluorescence emissions, dissipating the heat further. Finally, the broadband radiation facilitates heat exchange with the sky and re-emits heat to the cool outer space.
According to Prof. Dai, “UmiCool uses the sky as a temperature regulator for dynamic exchange of heat, so that it enhances daytime cooling while suppresses overcooling at night. That’s why it’s smart. Instead of the narrow spectrum of emissivity in typical SDRC coatings, UmiCool uses functional nanofillers that re-emit a broader mid-infrared spectrum to narrow its day-night temperature difference.”
The team conducted model room tests in Beijing and Zhuhai during the sweltering summers. Results revealed that UmiCool is capable of maintaining 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. The low-maintenance coating also has self-cleaning properties with lotus effect, meaning dirt particles on the surface can be washed away when it rains. And for architects and designers who value form as much as function, the creamy yellow smart coating will be available in an array of pastel colours such as light blue and pale pink in future. In addition to applications on exterior surfaces of buildings, UmiCool also works on vehicles and infrastructures such as bridges and paved roads.
Heat-radiative window treatments
Meanwhile, modern office and residential towers in Hong Kong seem to favour the use of glass curtain walling and panoramic full-length windows that let in the sun and the heat. Prof. 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 semi-transparent UmiCool justifies its cost,” he explained. That said, he agrees the formulation of UmiCool can be used to make polymer materials with thermal radiative functions. That means, theoretically, window shades and blinds can be made of UmiCool in future, minimising the heat gain from sunlight through windows.
Academic-led start-up for knowledge-based building solutions
To commercialise UmiCool, Prof Dai co-founded a start-up known as Pro-Infra Science & Tech Ltd with other researchers. “We strive to develop technologies that would facilitate sustainability in infrastructure and buildings. So far, our company has been awarded two consultancy projects by a government department,” said Prof. Dai. As a researcher, he admitted that the business side of the startup is not his strongest suit. “Sales pitching, administrative duties, looking for the right partners and employees are all rather new to me. Luckily, PolyU has been lending us useful entrepreneurial support on business-related matters such as licensing negotiation, marketing and promotion.” The company has obtained an entrepreneurship fund of HK$600,000 from the government.
In March 2022, UmiCool won a Gold Medal in the online edition of the International Exhibition of Inventions of Geneva – Special Edition 2022 Inventions Geneva Evaluation Days – Virtual Event.
[1] “2021 warmest year on record for Hong Kong”. The Government of HKSAR. 7 January 2022. Retrieved from https://www.info.gov.hk/gia/general/202201/07/P2022010700255.htm
[2] “Hong Kong Energy End-use Data 2021”. 1 January 2022. EMSD. p26. Retrieved from https://www.emsd.gov.hk/filemanager/en/content_762/HKEEUD2021.pdf
[3] “Hong Kong’s obsession with air conditioning is bad for us all”. 7 October 2016. SCMP. Retrieved from https://www.scmp.com/comment/insight-opinion/article/2025857/hong-kongs-obsession-air-conditioning-bad-us-all
