Feasibility Study on using Sunlit View Factor to Model Outdoor Thermal Comfort: A case study in Sha Tin, Hong Kong

Abstract

The building facade not only serves as the outer surface of building itself, but also defines the boundaries of outdoor urban environment. This enclosure entraps more short-wave radiation from the sun due to its complex geometry and thermo-physical properties when compared with an unobstructed flat area. Building surface reflects and absorbs short-wave radiation, and is heated up to a higher surface temperature. The heated surface, especially the sunlit one, emits and exchanges more long-wave radiation with surrounding buildings and other outdoor objects. Therefore, to some extent, more long-wave radiant fluxes existing within urban environment could be regarded as one of the consequences of entrapping solar radiation by urban structure. In other words, more sunlit building area casted by urban morphology should lead to more long-wave radiant fluxes within urban environment. With more emitted longwave and reflected short-wave radiant fluxes by sunlit urban fabric, the resulting mean radiant temperature (Tmrt) will be higher causing human thermal discomfort in outdoor environment. Thus, it is important to identify and evaluate the empirical relations between the geometrical composition of sunlit urban surfaces and outdoor thermal comfort. By treating the urban structure as a black box, and using simple regression analysis, the objective of this feasibility study is to identify and evaluate the empirical relation between radiant fluxes (especially long-wave fluxes) from six directions within urban context and corresponding sunlit area of adjacent building facades in daytime. Radiant fluxes of both long-wave and short-wave were measured by net radiometers in outdoor space in Wo Che Estate, Sha Tin, Hong Kong. The measured radiant fluxes were smoothed out using 5-min mean value. View factor of sunlit area was captured by using fish-eye lens camera, and corresponding values were obtained with the aid of RayMan software. The radiant fluxes and mean radiant temperature are regressed on sunlit view factor. The results showed a strong correlation between sunlit view factor and outdoor thermal comfort.