Design of a passive radiative and evaporative cooling mortar coating that achieves sub-ambient temperature in tropical daytime.

• Main Author: Chionh, Daryl Zhenyu
• Other Authors: Hong Li
• Format: Final Year Project
• Language: English
• Published: Nanyang Technological University 2023
• Subjects: Engineering::Mechanical engineering
• Online Access: https://hdl.handle.net/10356/167981
• Institution: Nanyang Technological University

Cooling is a large component of energy usage in buildings, especially in tropical climates such as Singapore. Improving the energy efficiency of buildings through passive cooling is thus a very enticing prospect, especially given the emphasis placed on energy efficiency and green buildings in the Singapore Green Plan. The 2 ways of passive cooling discussed in this paper are radiative and evaporative cooling. Passive radiative cooling has been proven to independently achieve good cooling performances in temperate climates. However, in tropical climates which have high solar irradiance and humidity, it has not been successful in achieving sub-ambient temperatures especially during high solar conditions such as noon to mid noon periods. Evaporative cooling has also been proven to achieve some cooling performance, and this project thus intends to utilise it in conjunction with radiative cooling to achieve sub-ambient temperature cooling in tropical climates under high solar conditions for the first time. The use of both cooling mechanisms in a building exterior coating has the potential to revolutionise cooling and consequently energy efficiency in the building industry. Herein, we document the design of a prototype paste that demonstrates good passive cooling performance, with adequate mechanical properties for application as a coating onto the exteriors of buildings. The resultant paste demonstrated a solar reflectance of 90.5%, infrared emittance of 93% within the 8-13μm atmospheric window, and sustained daylight sub-ambient temperatures of more than 2°C below ambient temperatures during peak solar irradiation conditions. The paste also demonstrates sufficient workability and adhesivity to be applied onto a vertical concrete slab, and thus application onto building exteriors. Its rheological properties were characterised, demonstrating shear thinning and yield stress fluid behaviour, with a maximum shear stress of 445.9Pa, a static yield stress of 10.62Pa, and flow point stress value of 38.65Pa, which under preliminary calculations is sufficient to be applied onto a vertical surface without flowing off. The wear resistance of the paste was also measured, possessing an average abrasion index of 0.000498g/revolution. The performance of alternative cooling paints was also quantified through our rooftop cooling test and wear resistance test, with our paste outperforming the alternative cooling paint in both aspects. The success in designing a cooling coating using both radiative and evaporative cooling mechanisms opens a fresh and exciting avenue for passive cooling technology to be adopted in Singapore. This can be done through further research in the direction of integrating various passive cooling mechanisms together, and also commercial adoption of the prototype developed.