Urban Patterns for Improved Thermal Performance.

Abstract

Urban forms produce microclimates different from the macroclimate by influencing solar exposure, wind speed and direction, air and surface temperature, and heat storage capacity of the ground and buildings. The purpose of this study is to investigate the thermal effect of urban geometry on the microclimate and buildings. The study focuses on the thermal performance of urban canyons as formed by streets and buildings. The geometric analysis of shading shows that canyons with east-west oriented axes have rather steady shading patterns. The shading patterns of canyons with north-south axes, however, change more dynamically and strongly over the course of a day. Canyon cross-section, when varied, affects shading patterns and view factors. Two physical models were built to monitor canyon performance. Concrete panels simulated the canyon wall surfaces and an asphalt street simulated the canyon street. The first model was used to evaluate one canyon. The second model was used to evaluate simultaneously three canyons with various cross-sectional ratios. Statistical analyses were performed for comparison. The data show that thermal performance is significantly affected by cross-sectional ratio. A computer model was written to simulate canyon transient multimode heat transfer. Using explicit finite-difference methods the model predicts canyon surface temperatures. The calculations closely correlate with the empirical results. Computer simulations were performed to predict canyon behavior at 33 and 42 degrees north latitude. Comparative results are presented for summer and winter simulations for a wide and a narrow canyon each oriented north-south and east-west. The surface temperatures are higher at the top of canyons than at the bottom due to longer solar exposure. This is reversed at night as lower points have smaller sky view factors and lose less heat via long wave radiation. Wide canyon surface temperatures are higher in the daytime and lower in the nighttime than those of narrow canyons. The diurnal surface temperature swing is larger for wide than for narrow canyons. Recommendations are made regarding urban and architectural canyon design. Recommendations addressing urban design include strategies for providing solar access in cold climates; and shading in hot climates. Recommendations addressing architectural design include strategies for applying shading devices in areas that are not protected by urban shading.