Effect of surface albedo of urban fabrics on urban climate in tropical area

• Main Author: Long, Yongping
• Other Authors: Wan Man Pun
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2023
• Subjects: Science::Physics::Meteorology and climatology; Engineering::Mechanical engineering::Energy conservation; Engineering::Environmental engineering::Environmental protection
• Online Access: https://hdl.handle.net/10356/171384
• Institution: Nanyang Technological University

With urbanization, the original rural landscapes have been replaced by artificial infrastructures, which decrease the effective albedo in urban areas. In addition, human activities produce heat in cities. The resulting urban heat island (UHI) effect can be harmful to urban residents. In tropical regions like Singapore, the direct influence would be heat stress in the outdoor environment and higher energy consumption by air-conditioning. The global warming trend in recent decades makes it worse. To combat the adverse effects of UHI, it’s assumed that the UHI is mainly caused by heat absorption during the daytime and heat release during the night-time. If this assumption is valid, the highly reflective coating (cool coating) can be a potential cooling strategy. Although there are many related studies, there is a lack of cool coating applications in tropical climates, particularly the real-scale experiment. This study investigated the potential impacts of cool coatings on urban climate through both field experiments and numerical simulations. The mesoscale Weather Research and Forecasting (WRF) model simulations validated the assumption that the heat absorption during daytime by urban structures and heat release during night-time is the main contributor to the UHI formation in Singapore. Weather conditions like stronger Global Horizontal Irradiance, higher air temperature and weaker wind speed lead to stronger UHI. The maximum nocturnal UHI intensity in Singapore reached 3.2 °C. WRF results showed that cool coatings can reduce air temperature at 2 m height and surface skin temperature by up to 3.1°C and 9.8°C, respectively. Cool fabrics reduce the radiation absorbed by urban structures during the daytime and thus mitigate the UHI effect during night-time due to reduced heat storage. The cool roofs have the most cooling effect, followed by the cool wall, and the cool road has the least cooling effect. Cool coating applied on all urban fabrics can reduce the UHI intensity by up to 30% in residential areas and by 6% in commercial/industrial areas. One real-scale experiment was conducted on four high-rise residential buildings in Singapore. Observations show that the surface temperature on the roof, top-floor corridor ceiling, façade and driveway was reduced, leading to air temperature reduction. The cool roof reduced the energy that stays on the concrete roof by 32.8%. In another test, the cool coating was applied on the roof of one air-conditioned office. It reduced the daytime net radiation on the roof by 59.1%. As a result, the heat flux reduction reached 70.7%. The air-conditioning electricity consumption reduction was estimated to be in the range of 36.6% to 43.0%. The microscale model EnviBatE reproduced the field observations in the residential site. It proved that a larger area of cool coating leads to more cooling effects. Outdoor thermal comfort can be also improved; the most Universal Thermal Climate Index reduction happened on the cool coating paved roads. Cool coating applied on all urban surfaces in the district lead to overall air-conditioning electricity consumption reduction by about 10%, and about 27% on the top floor. Different cool coating application scenarios had a payback period from 1 to 4 years, in consistency with the top floor office experiment. This study validated the principle of cooling a tropical city with highly reflective coatings by reflecting solar radiation into the atmosphere. Different application scenarios provided insights into outdoor thermal comfort improvement and building air-conditioning energy-saving potentials by cool coatings.