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What input is needed?

Micro Climate

Solar radiation modelling

A key issue in building simulation is the treatment of direct and diffuse solar radiation. Let us follow the main steps in the treatment of sunlight entering the building. If synthetic weather is used, ICE first computes direct and diffuse sunlight intensities based on the data given in the Location object. At the advanced level, the components used for this computation are found in the Schematic tab of the building form under the heading Climate processor.

In the next model in the chain, the solar position in the sky is computed and all the signals computed so far are sent to a set of Face models, where the climatic conditions outside each main building surface (Façade) are computed. In this model, the distribution of diffuse radiation in the sky is also computed, by default using the Perez model.

In the next step, solar radiation on an individual object such as a window is computed. Connected to each window model (See Figure 3.6) is a shading calculation model (Shade), that computes the shading of both direct and diffuse light on the receiving surface. This model puts all shading surfaces in one ´bin´, including building self-shading, shading by neighboring buildings and shading by (possibly movable) objects directly outside of the window (External shading). Diffuse light from the sky is also shaded, but no reflections other than ground reflection are accounted for. Ground reflection for each face can be set from the Property page if the face name is selected in the Floor plan tab. Diffuse radiation from the ground is not shaded by external objects.

The shade model is difficult to interact with directly at the advanced level since each surface has been subjected to several coordinate transformations. The actual shading factors are precomputed for all (plausible) solar locations and are stored as parameters in the shade model for the simulation.

Once in the window model, diffuse and direct light are reflected and transmitted depending on the window model used. The standard window model uses a fixed curve for the angle dependence. Integrated window shading (interior, interpane or exterior shades in the plane of the window) will reduce radiation by multiplying the basic window parameters. It may also convert direct light to diffuse.

The detailed window model makes a layer-by-layer computation of multiple reflections and each layer temperature is computed. For glazing where all the glass panes and the integrated shading have spectral data, the optical calculation in the solar range is made for each wavelength and the values are then integrated to average values according to EN 410.

Once inside the zone, diffuse light is spread diffusely, while the exact target location of the direct light beam is computed. However, the whole surface of the window is considered as the light source, not just the portion of the glass which is actually not shaded by external objects. After the first reflection on a zone surface, the direct beam is spread diffusely in the room. And also here, the whole surface that is hit is regarded to reflect with equal intensity, not just the lit portion of this surface.

Internal windows and open doors transmit light in a similar way as an external window, i.e. the whole opening is regarded as being lit, even if only a small part of the door receives direct sunlight. The light intensity is of course adjusted accordingly. Similarly, light that enters through one window and exits through another external window, for example in a corner room, is treated in a physically reasonable way.

Results for solar radiation may be obtained not only in the solar, but also in the visual range. Average illuminance on room surfaces is obtained when “Daylighting” is chosen from “Requested output” or if the light level in a room is used for control. The type of measuring plane is set under “System parameters” in the General (Building) tab. Calculation of daylighting is useful for e.g. control of electrical lighting in energy simulations.

More advanced daylight calculations may be performed in the Daylight tab, where the software Radiance (external link) is used as a calculation engine. See more about this here.

Solar radiation to the zone is included in "Window and solar" in the Energy balance, but separate measures are given in the following diagrams and tables:

Solar radiation through windows: Direct, diffuse and total (including indirect) solar energy transmission for each window in the zone.

Heat balance: Net direct + diffuse solar radiation to the zone.

Energy – Log detailed sources (transmission table): Net total solar energy transmission (QSOLAR) for each window in the zone.

Summery – Max solar gain (Energy/Custom simulation): Net total solar energy transmission to the zone.

Wind

External wind can have a big inpact on the infiltration of a building. To describe these effects in the model accurately enough, there is a concept of wind pressure coefficients. In IDA ICE, these coefficients are set to 0 by default. This means that unless otherwise specified by the user, wind has no influence to infiltration of ambient air through leaks or open windows. When wind infiltration is switched on and wind pressure coefficients are set, the wind effects are still not described so accurate that, for example, the height-dependent wind profile, which is defined elsewhere, is taken into account. Windows will for example always have the same wind pressure coefficients as long as they are part of the same building body surface. The maximum level of detail that can be taken into account from the IDA ICE Standard Level is the wind pressure coefficients shown in the image below on the left. For a level of detail as shown in the image on the right, these would have to be entered in the Advanced Level - either manually for each window individually or via a self-written script.

Influence of greenery

Another thing that is actually not covered in IDA ICE is the influence of greenery to the local climate. It can have an influence to the diffuse ground radiation, direct radiation reflexion, seasonally varying shading, longwave radiation due to ground surface temperature, as well as cooling through evaporation on the leaf surfaces of plants.

Local ambient air temperature

The degree to which the ground surface is sealed has a major influence on the local ambient air temperature. If solar radiation is not reflected and absorbed solar radiation cannot be converted organically, it is stored close to the surface and can continue to be emitted for a long time at night. The effects of surface texture, greenery and winds combine to have an overall influence on the local air temperature for a building. Strictly speaking, each partial building surface would have to be given its own local climate data as a boundary condition for the building model. However, this accuracy is of course only required in special cases, such as when the temperature of the air that a building takes in via an open window or via an outside air intake for ventilation has a direct influence on the building's energy consumption.

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