A simple tool to evaluate the effect of the urban canyon on daylight level and energy demand in the early stages of building design

Daylight is a restricted resource in urban contexts. Rooms situated in an urban context often have a significant proportion of the sky and the sun blocked out by the urban building mass. The reduced direct daylight potential makes daylight reflected from outdoor surfaces an important daylight source to the room. It is therefore important to be able to take into account the daylight reflections from the urban environment in early design stage. This paper describes a simplified method that uses a combination of ray-tracing, the luminous exitance method and the concept of the urban canyon to represent daylight levels in rooms situated in an urban setting. The method is implemented in the daylight algorithm of an existing building simulation tool capable of making rapid integrated daylight and thermal simulation. Comparisons with the more sophisticated lighting tool Radiance show a maximum relative error of 17% but it is often much lower. The accuracy of this approach is therefore considered to be adequate for the early stages of the building design process. The results from integrated daylight and thermal simulations are presented to illustrate how the tool can be used to investigate the impact of urban canyon parameters on indoor environment and energy performance.