by Tanguy Timmermans (AGC Glass Europe) and Louis Dellieu
While dealing with the facade design process, questioning on the use of solar shading systems quickly arrives on the table. Solar shading is a form of solar control system that can be used to optimize the amount of solar heat gain and visible light that is admitted into a building. This can have a significant impact on the energy use of a building as well as on the thermal and visual comfort of occupants, protecting against overheating and glare on hot or sunny days. It can also provide privacy.
Solar shading systems can take many forms : canopies, light shelves, electrochromic glass, internal blinds, curtains, shutters …. Depending on the technology, such systems Therefore, enabling facade illumination study within early development stages allows to fastly determine the best shading systems to choose and to define the relevant installation location on the facade.
In the following blog article, we will demonstrate how Ocean™ can support such studies through the use of shadow and mask applications.
The goal of the analysis is to study when a window is exposed to direct sun depending on :
. Window orientation and inclination
. Location of building and associated sun path
. Element in the surrounding of the building that can cast shadows on facades
Here we focused on the method using 3D modelling and ray-tracing. Different methods (ray-tracing, photographic and geometric) can be combined depending on what is the most practical to deploy. These alternatives will not be developed in this article.
3D MODELS FEATURES
The 3D geometry should include everything that can potentially cast a shadow on a window. It doesn’t need to include detailed architectural features : Crude black geometries are enough (and preferred for computation efficiency. In order to retrieve the city outlines around the building, the following tool can be used CadMapper (https://cadmapper.com/).
The example in Fig. 1 will be consider during this study. The building of interest is highlighted in blue while the surrounding building is highlighted in grey. It is noteworthy that the shadowing effect in this case is due to both the neighboring building and the building itself (self-shading effect).
Figure 1 – 3D system under study.
WINDOW GROUPING DEFINITION
In order to limit complexity and reduce needed input data, windows should be gathered in group (Fig.2).
Figure 2 – Windows grouping.
The grouping should fulfill several aspects :
. A group considered as shadowed implies that all windows within the group are shadowed.
. The group should not be too small in order to avoid complexity in the simulation
. The group should not be too large : The more windows we’ve in a group, the less likely we’re to have them all fully shadowed at the same time
IMPORT INTO OCEAN™
While the 3D model as described above is imported into Ocean™, the following features needs to be defined : materials, environment and camera.
In terms of materials, every elements will be assigned as “black” material, i.e. 100% of absorption. Regarding the environment, standard CIE type 5 sky can be chosen with a very high zenith luminance (lz > 106).The camera will be defined as a Fisheye camera (see Fig. 3 for the parameters).
Using the element defined above will generate a circular (fisheye) black/white render. The centre of the picture corresponds to the centre of the field of view of an occupant standing at the camera location. Every pixel in the image correspond to a specific direction in the field of view with the azimuth horizontally and elevation vertically. Such and image capture at once all the possible positions in the sky from where the sun could be directly seen from the camera location (Fig. 3):
. White region = direction in which the sky is visible from the camera location
. Black region = direction where the view to the sky is obstructed (other building or horizon)
In practice the sun can only be in some part of the field of view depending on the building location and the viewing orientation.
Figure 3 – (a) Viewing position used for the renders. (b) Fisheye render in greyscale for illustration. (c) Fisheye render in black/white as calculated.
CAMERA LOCATION SELECTION
Figure 4 – Camera positioning and Fisheye renders.
The goal is to make renders to capture when a window group is fully in the shadow or not. Multiple renders from different viewpoints are needed.
If a specific sky position is not visible from a specific camera location it means that when the sun is in the location, the camera in the shadow. In other words, when the sun is in a location corresponding to a white pixel, the camera used for the render is exposed to direct sun. Conversely if it is a black pixel, it means that the camera is in the shadow.
Typically it is enough to have 2 cameras, one at each upper corner of the window group as displayed on Fig. 4. In some more complex situations the 4 corners or even more locations are needed.
We end up with multiple images telling us when different points in the window group (typically the upper corners) are in the sun or the shadow. For an entire group to be in the shadow all cameras within the group must be in the shadow (this threshold can be define alternatively by the end-user). The grouping of the two renders in Fig. 4 are displayed on Fig. 5.
Practically speaking this corresponds to a logical OR between the different images (white pixel = yes, black pixel = no). When a pixel is white in one of the image, the corresponding pixel in the group image must be white too. Every points within the group must be in the shadow for the group as a whole to be considered in the shadow.
We see here the importance of properly selecting camera locations. We need to make sure that when all cameras are in the shadow, the entire window group is in the shadow.
Figure 5 – Combined renders.
Figure 6 – Enlarged image
So far, we considered the sun as being a point. In reality, sun (as seen from hearth) is a disc with a specific radius and around that disc we have the circumsolar region that can be very bright.
Even if the centre of the sun is hidden behind an obstruction, some part of the sun disc or the circumsolar region could still be visible from the window.
To account for this effect, it is recommended to enlarge the “white region” of the render by a factor corresponding to the sun disc size + the circumsolar region, typically 2.5° (remember the image scale between -90° and +90° in both direction).
A larger factor could be used for additional safety or if we are afraid the 3D geometry is not 100% accurate. The inclusion of the sun radius into the simulation is displayed on Fig. 6.
ORTHONORMAL PSEUDOCYLINDRICAL PROJECTION
In order to be compatible with other mask generation technics, the image projection must be changed from fisheye (here Equidistant) to a new projection specifically developed for this purpose and called “orthonormal pseudocylindrical”.
This new projection has the advantage of keeping horizontal lines straight in the resulting projection. It also allows to create masks from simple obstruction like overhang very easily using geometrical methods. The results are displayed on Fig. 7.
More details about this projection, the geometrical methods and the benefits can be found elsewhere.
Figure 7 – Projected render.
SOLAR PATH ANALYSIS
After the above mentioned procedure, the solar path analysis can be conducted in order to determine the facade illumination over a year. In order to perform the analysis, which as to be conducted for each windows group, the final render displayed in Fig. 7 is considered. From weather data website/data base (e.g. meteonorm or energyplus), the sun path for different period of the hour and period of the year is calculated with respect to the Fig. 7. The result is displayed on Fig. 8.
From Fig. 8, it can be seen that depending on the period and time of the year, the windows group is going to be exposed (red line) or not (green line) to sun glaring. When group of windows is exposed to the extensive sun, it means we might need glare protection provided by a solar shading system (see beginning of the article). If a group of window is always in the shadow it means that solar shading solution is probably not needed there.
Figure 8 – Sun path where red line corresponds to a sun exposition while green line corresponds to shadowing.
In this blog article, it was showed on Ocean™ be used for generating obstruction mask, enabling the assessment of the use of solar shading system. This approach can be used during the early development stage of the façade design for diagnosing the risk of glare and identifying the best solar shading system solution.
The work presented in this article was realized by Tanguy Timmermans from AGC Glass Europe.