PLEA2006 - The 23 rd Conference on Passive and Low Energy Architecture, Geneva, Switzerland, 6-8 September 2006 Daylight in Urban Canyons: Planning in Europe Luisa Brotas 1,2 and Mike Wilson 1 1 Low Energy Architecture Research Unit, London Metropolitan University, London; United Kingdom 2 Department of Renewable Energy, National Institute Engineering, Technology and Innovation, I.P., Lisbon, Portugal ABSTRACT: Daylight calculations are commonly based on the daylight factor method regardless of prevailing weather conditions. While this method may be used for overcast sky conditions, it can be argued that it is not appropriate for clear skies. Furthermore, reflected sunlight from obstructions and ground is a major contribution to the illumination of buildings in orientations and at times when the sun is behind the building. A simplified calculation that takes into consideration reflected sunlight may be used for quick calculations in the initial design stages of the project. Two different set of criteria for daylight and urban planning are presented. They apply to predominantly overcast and clear sky conditions. Both, individually or combined, allow for daylight design in European climates. Keywords: daylight, reflected sunlight, urban canyons, planning 1. INTRODUCTION The ultimate source of all daylight is the sun. However, weather conditions and climate, building orientation and time of the day can suppress sunlight access to buildings. In these cases, interiors are dependent on light from the sky and that reflected by surrounding surfaces. In urban canyons, facing buildings provide considerable obstruction to daylight access by reducing the skylight contribution and blocking sunlight. However, reflected sunlight from the obstructions or the ground can play an important role in the illumination of buildings, particularly in orientations and at times of the day where sunlight is not incident on windows. Furthermore, obstructions and ground can redirect the light to other interior surfaces rather than the horizontal plane, and lead to a greater uniformity of the light inside the space. Under sunny conditions, physical measurements collected in an urban canyon in Lisbon showed a linear relationship between the global horizontal illuminance and the total vertical illuminance on the facade when it is not receiving direct sunlight. Further studies carried out with computer simulations with RADIANCE as well as an analytical calculation confirmed this relationship, which is shown to be relatively stable throughout the year, with latitude and orientations and time of day when sunlight is reflected off obstructions and ground. [1] A simplified calculation to apply for urban canyons under sunny climates is presented. The calculation is analogous to the average daylight factor concept but takes into consideration sunlight reflected from obstructions and ground. It is based on the principle of the integrating sphere and the ratio between the total vertical illuminance on the facade of an urban canyon and global horizontal illuminance. [2, 3] 2. ILLUMINANCE IN THE CANYON It is possible to define three scenarios regarding sunlight availability on a vertical facade. In the first scenario, the sun is in front of the building and is providing direct sunlight on the facade. In the second scenario, the sun is still in front of the building but the obstruction is sufficiently high to block direct sunlight. Any point on the facade will receive light from the sun after it is reflected from other parts of the facade and then from the obstruction and ground. As this involves a minimum of two reflections, the sunlight contribution is significantly reduced. Furthermore, it will be mainly dependent on the reflectance of the surfaces. In the last scenario, the sun is behind the building and sunlight reaches the facade after it is reflected from the obstruction and ground. Besides the sunlight contribution, the illuminance on the facade will be a result of direct light from the visible part of the sky and from other parts of the sky vault by reflection at the obstruction and ground. Lastly, the illuminance on the facade includes the contribution from interreflections within the canyon due to reflected sunlight and skylight. 2.1 Fieldwork Real data collected in Lisbon in urban canyons showed a linear relationship between the global horizontal illuminance, E gh , and the total vertical illuminance, E tv , at the building facade when the facade is not receiving direct sunlight. The 'total vertical illuminance' is defined as the sum of sunlight, skylight and the interreflected component that falls on a vertical plane per unit of area. This relationship can be described by a linear equation of the form C E k E gh tv + × = (1) where the slope k mainly depends on the reflectance of the obstruction, the geometry of the canyon and the position on the facade. The constant