Abstract
Climate-Based Daylight Modelling (CBDM) gives designers the possibility to evaluate complex, long-term luminous environment dynamics. This complexity can be challenging to simulate, and even more challenging to communicate effectively through the use of performance metrics. A multiplicity of CBDM techniques and metrics has been developed over the last two decades, but these were rarely assessed against each other. This paper reviews four state-of-the-art techniques based on the Radiance raytracing engine and systematically compares them against a benchmark CBDM method. Four classroom spaces are used to carry out an inter-model comparison between performance metrics commonly used for compliance verification obtained from all analysed techniques. Additional sensitivity analyses assessed how changes in input variables influence such metrics.Results from the inter-model comparison showed that the representation of direct sunlight is markedly different between the various CBDM techniques, and that metrics based on horizontal direct sunlight are very sensitive to the choice of simulation method. This led to differences in predicted Annual Sunlight Exposure up to 39 percentage points. Metrics that consider both direct and inter-reflected light were found to be more robust, with variations from benchmark results within ±15%. The analysis of the input variables showed that sensor grid spacing and time-step interpolation do not significantly affect any of these metrics. Changes in orientation and sky discretisation scheme had different effects depending on the metric and technique considered. The need for authoritative benchmarking systems when introducing new performance metrics for compliance verification or new simulation methods is also discussed.
Highlights
The consideration of daylight in the design of a building was traditionally the preserve of the architect rather than the engineer
The techniques were evaluated by inter-model comparison using the 4component method as the benchmark since that has been shown to be the most rigorously validated of all the techniques
Outcomes were evaluated for four very different real-world spaces using key annual daylight metrics such as Daylight Autonomy (DA), Useful Daylight Illuminance (UDI) and Annual Sunlight Exposure (ASE) since these form the basis of current Climate-Based Daylight Modelling (CBDM) guidelines
Summary
The consideration of daylight in the design of a building was traditionally the preserve of the architect rather than the engineer. Dynamic thermal modelling using computer simulation gradually evolved from analytical/steadystate methods to become a commonplace and routinely applied technique to predict the overall energy and environmental performance of a building [3]. Using standardised climate data (i.e. weather files) to define the prevailing external conditions, dynamic thermal modelling became established in the 1980s as the foundation of Building Performance Simulation (BPS). By the year 2000 the prediction of annual profiles of daylight illumination (using the same climate files as dynamic thermal modelling to generate the sun and sky conditions) became a practical possibility [5,6]. The prediction technique was eventually given the name Climate-Based Daylight Modelling (CBDM) in 2006 [7], and in the last decade it has all but completely replaced traditional daylighting evaluations, i.e. those founded on a single point-in-time condition such as the CIE standard overcast sky
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