Daylight and sunlight analysis

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William Fawcett
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Sky view factor map of the historic core of Nottingham.

CAR Associates Dr. Nick Baker and Dr. Vicky Cheng have been involved in urban consultancy projects in Leicester and Nottingham with URBED (Urbanism, Environment and Design) Ltd., Manchester.

The projects applied image processing techniques to digital elevation models (DEMs) of the urban tissue in order to obtain various morphological parameters which are useful to the study of daylight and sunlight availability.

One of these parameters is the sky view factor (SVF), the fraction of the sky visible from a point compared to the unobstructed sky. Another one is the passive zone (PZ) ratio. The passive zone is defined as the floor area of the building lying within a distance of twice the floor to ceiling height from the perimeter; and the PZ ratio is the proportion of passive zone to total floor area.

The Leicester project involved the design of a five hectare low carbon mixed development. In this project, SVFs on the facades of buildings at various floor levels were obtained. This information has been used to assess the degree of local overshadowing, and to work out the required glazing area in order to comply with the minimum daylight factor benchmarks. This method of analysis is particularly useful in high density developments where the mutual overshadowing of buildings is complex and variable.

The purpose of this project was to determine if there were identifiable morphological characteristics of different parts of Nottingham, and in particular, if the historic core displayed any unique character that could be regarded as “Nottinghamness”. The project involved DEM modelling of four different areas of the city of Nottingham that represent a diversity of ancient and modern urban forms. SVFs and PZ ratios of the four study areas were modelled. In addition, the parameter SVF x PZ ratio for each area was calculated. This parameter provided an indication of the potential energy saving of daylight.

The results of the analysis suggest that the tall urban form in the modern area performs as well as the medieval buildings in the historic zone. This is because the lower passive ratio of the modern tall buildings has been compensated by the better availability of daylight, as indicated by the higher sky view factor. This work contributed to URBED’s Nottingham City Centre Design Guide.

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