3 minute read
Climate-Based Daylight Modelling – A Real World Approach
Cieran Towse of MBS Software Ltd asks: In a world with growing concerns about climate, why not take a real world approach to assessing the quality and quantity of natural daylight and sunlight rooms within a building will receive?
As we carry out daylight and sunlight analysis, it is time to embrace climatebased daylight modelling (CBDM). The results provide a more detailed picture of how light interacts with the built environment which we inhabit and provides crucial information to aid designers.
Whilst not a new concept, the industry has been slow to adopt this methodology, preferring instead to use ‘static’ metrics which, whilst not inaccurate, can perhaps be best described as sufficient. More importantly, they can be bettered, particularly as the tools to do so are readily available. This article attempts to breakdown and simplify CBDM.
What is Climate-Based Daylight Modelling?
CBDM is the prediction of various radiant or luminous quantities (e.g. radiance, irradiance, luminance and illuminance) using realistic sun and sky conditions captured annually from standardised climate data. These data sets therefore create a realistic representation for an analysis environment, reflecting location and orientation.
A climate file contains data for a specific geographic location for many weather variables, such as Illuminance, radiation, temperature and wind direction. Data is collected at every hour of every day for the year.
A ‘typical year ‘is a climate file which contains data selected over a number of years (normally 10 years or more). For each month, the data is selected from the year considered most ‘typical’. For example, June might be from 2007 and July from 2012.
Alternatively, a ‘historic year’ is data collected through a single continuous year rather than a composite one made up from average months.
Standardised climate data is a readily available online resource, however not all files contain the relevant information required and can be biased towards particular variables.
How does Climate-Based Daylight Modelling work?
The method used when carrying out CBDM is ray tracing, in particular backwards ray tracing, since it uses rays moving in the opposite direction to which photons actually travel. Backwards ray tracing is the process of shooting rays from the individual room grid points to the light source and is used for reasons of efficiency, eliminating any superfluous rays.
Ray tracing allows for indirect light as well as direct light and relies heavily upon materials and their properties. The more accurate the reflective properties given to those materials in an analysis model, the more representative of the ‘real world’ the results will be.
The ray tracing technique can produce realistic lighting effects and simulates the way light interacts (i.e. reflection, refraction and scattering) with the materials.
When constructing a 3D model it is important to determine the materials not only for the room/building you are analysing but also the surrounding landscape.
Common materials to consider are: • Interior Walls • Interior Ceilings • Interior Floors • Exterior Walls • Glazing • Landscape (Including surrounding buildings)
Consider the colour, finish and transmittance as they will all have an impact on how the light interacts with the material.
Which Building Standards refer to Climate-Based Daylight Modelling?
Rating systems such as LEED v4, BREEAM and WELL Building Standard all drive the CBDM approach. Most recently, the revised BS EN 17037 also specifies CBDM and it is expected that the revised BRE: Site Layout Planning for Daylight and Sunlight will also incorporate this approach.
How does Climate-Based Daylight Modelling vary from other Daylighting Approaches?
CBDM overall paints a more detailed picture, providing greater detail with regards to light distribution and intensity. It takes into account the buildings configuration, orientation and composition. In comparison, Daylight Factor assumes an overcast sky at a static period of time, which is not a true reflection of the overall annual light availability and performance.
CBDM is a more complex calculation compared to other daylight approaches, however does not require much more user input, the complexity is performed behind the scenes within the ray tracing method.
Due to the complexity of the calculation, ray tracing comes at a greater computational cost when compared with other rendering methods. However, MBS are able to process calculations within seconds using the new CUDA computing platform.
Cieran Towse can be contacted at: ct@mbs-software.co.uk
