Thermal Profiles of a High Density City

Abstract

The high density city is compact with buildings and in-between voids that form narrow street canyons that trap heat and create microclimates within deep troughs of the city. Hong Kong is an exemplary case of tall residential and commercial towers that are situated along a dense linear strip throughout the city where 40-story public housing residential towers are being proposed and built. Trapped ventilation within the city has gotten much attention after the devastating effect of viral contamination in 2001, SARS epidemic, and awareness of health issues related to trapped heat, also known as urban heat island effect. Urban and thermal relief are core issues that have to be considered for sustainable development and energy efficiency in today’s rapidly urbanizing cities. The paper discusses research conducted to investigate how the design of buildings affect the outdoor thermal environment of the city. Field measurements of thermal radiation and temperature were conducted at various locations throughout the city and data analysis was performed to understand the relationship of 3 key urban conditions: urban surfaces, building morphology and organization and urban canyon that describe the city living context, using short-wave, long-wave radiant fluxes, mean radiant temperature (MRT) as primary thermal comfort indices. Field data are compared to assess three urban conditions were (a) building form and layout; (b) urban geometry ; and (c) urban surfaces. The size of the urban canyon and orientation of the buildings had significant effect on whether the solar radiation radiated back into the urban open space and how well the radiated heat could escape back up to the sky. The amount of thermal mass properties of the building surface receiving direct sunlight affected the amount of heat that was radiated back down into the pedestrian open space. Glass surfaces radiated significant short-wave and long-wave radiation when under direct sunlight but under shaded conditions, was able to cool down and have almost minimal effect of radiated heat to the open space. Concrete surfaces similarly radiated significant heat when under direct sun, and continuously under shaded conditions. The orientation of the buildings and the allocation of materials on the surfaces affects the outdoor thermal environment.