Modern Building Design by Ricardo Codinhoto

Author:Ricardo Codinhoto
Language: eng
Format: epub
Publisher: Crowood

Fig. 27 A single day’s worth of data in the .epw weather data format (data shown is for the spring equinox). Where data is not required/available, it is replaced by 9s.

In addition to the geographic location and the allocation of suitable weather data, consideration needs to be given to the actual site of the building. The exposure of the site is important not only for solar access, where adjacent buildings, trees or the surrounding landscape can cause shading and prevent lower angle sunlight from entering the building, but also for wind access. The wind speed and direction obtained from the weather data file is not typically modified, as without performing computationally intensive CFD simulations the software has no idea about wind flow around the building. Instead openings (windows and doors) as well as the external building elements are each given an exposure rating. This adjusts the wind velocities (but not direction) and can reduce (or increase for exposed sites or high-rise buildings) the wind flow through openings, infiltration around windows and doors and the convective heat loss from the building envelope. Other site considerations include the reflectance of the surrounding ground. Typically, this is set within the software with the default values for cropped grass for both solar radiation (UV-Visible-NIR) and for thermal radiation (IR). This value can be overwritten in some software packages if the user knows where to look.

Limitations of the software often include an inability to model the 3D heat and moisture flows to fully account for ground heat losses. Most dynamic modelling tools do not take into account the lateral heat losses into the surrounding ground, which can be significant, especially for uninsulated ground floor slabs. In an attempt to overcome this, ground floor constructions typically include soil modelled to a depth of at least 1m and the deep ground static temperature for the building location is allocated to the underside of this construction. This provides a measure of thermal inertia that is considered comparable to that experienced in practice. Another limitation is the fact that the surrounding landscape, while it has a reflectance, has no mass and hence cannot store solar radiation for heat release later on in the day. Likewise, thermal radiation exchange only takes place between active thermal elements in dynamic models (objects defined as buildings); trees, adjacent buildings, the landscape and other shading objects only intercept and reflect solar radiation, they never warm up or store heat. As such, attempting to take account of heat island effects is challenging in dynamic modelling software.

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