By Georg Hewelt, Product Manager Trimble Nova, Trimble International
Thermal calculations relating to thermal regulations, summer heat loss and summer heat gain form a fundamental basis for the design of the building's technical installations. The choice of heating, air-conditioning and ventilation equipment will depend directly on the results.
Thermal calculations involve parameters, some of which are taken directly from the architectural model, unlike fluid network dimensioning calculations, where the input parameters are entirely specifiable by the engineer. Many users wish to optimize thermal model input by retrieving data from the architect's model, quickly and efficiently.
The emergence of BIM in construction projects makes it possible to determine a thermal surface model from an architectural model. The IFC exchange format offers a standardized exchange of information relating to a model from architectural software to thermal software.
Current status of the architectural model
To know whether an architectural model can be interpreted in a thermal model, it's necessary to understand what the architect wants the model to show.
When architects create their models in an application, criteria such as 3D visualization, 2D printing plans, quantity take-offs and quantity take-offs come to the fore. In practice, this means modeling elements that are not always relevant to the thermal surface model (e.g. furniture, technical cabinets, flooring, etc.).
In addition, the parameters that are important for thermal calculations are not always adequately provided: for example, considering the glazing and blind casing of a window separately can prove problematic when calculating summer heat gain.
What's more, certain rules of practice relating to the implementation of the architectural model run counter to the surface thermal model, for example when several wall objects are used by the architect to model a multi-layer wall.
In everyday use, an architectural model is only partially suitable for interpreting a thermal model.
Definition of the surface thermal model
What data can be used? Basic information, such as room volumes and openings corresponding to joinery, can be used with a high degree of reliability. As the thermal model works with surfaces, it is not necessary to generate these surfaces from the walls of the architectural model, and in many cases an interpretation of the surfaces based on the geometric relationships between room volumes will be the most efficient way of generating the surface model. The openings corresponding to the joinery, and their associated dimensions, are taken as they are.
It is important for the correct use of information relating to premises that they are entered correctly and assigned to the corresponding floor of the building.
Tabular input or CAD input?
Two approaches are commonly accepted for inputting the thermal surface model: tabular input and input from a graphical environment. The advantages of graphical input are :
Significant time savings
Reliable input of quantity take-offs and calculation input parameters
User comfort, good overview
Efficient and easy control of calculation results
Graphical input also has its drawbacks. For example, certain complex geometrical situations can only be correctly interpreted within the limits of the graphic environment's possibilities: if a spherical wall cannot be correctly modeled in the 3D environment, it will not be possible to deduce its real surface for calculation purposes.
In practice, it is necessary to use specific tools to help the user adjust the thermal surface model on a case-by-case basis, in the event of particular difficulties.
Enhancing the thermal model
As part of a BIM work methodology, a model must contain all the necessary parameters. However, as mentioned above, the architect's point of view is very different from that of the engineer: data on the physical characteristics of materials, or on technical data relating to windows, are generally not contained in the components of the architectural model, or are not sufficiently qualified for the purposes of a thermal calculation. The thermal model must therefore be enriched with specialized technical data, which will be stored and saved directly in the model, under the control, responsibility and ownership of the thermal engineer throughout the calculation process.
During the enrichment process, the software used will have to provide the user with a range of tools enabling him to efficiently relate his technical data to the thermal surface model, via element analysis, grouping and filtering techniques.
Managing modifications... but how?
Change management is a daily task for designers and engineers. Most of the time, these modifications come from the project manager. Fluid networks need to be adapted accordingly, as do thermal calculations. In order to work as effectively as possible, the user should be able to judge, for each modification, to what extent it will have an influence on his design.
Providing the model in a graphical environment means you can react quickly to changes. Since the premises and associated components are generated from the model, changes are automatic.
If necessary, when faced with major modifications, an entire floor can be removed and redefined.
Since the room (as information provider) is automatically updated at any time when the thermal model is modified, the energy consumption values can also be updated or re-dimensioned.
It goes without saying that calculation results must be graphically represented. Thanks to the direct link between the thermal model and the calculations, these results can be returned and visualized directly in the thermal model. An important advantage is that these results (calculation data and associated parameters) can then be used for other calculation processes, such as plant sizing from the model.
Balance sheet
In practice, most of today's architectural models cannot be directly interpreted to create a surface thermal model. However, they do provide a basis that can be used by specialized software to generate a thermal model. The IFC interface is a robust, recognized exchange format for this purpose, and ongoing developments in BIM exchanges mean that we can look forward to regular improvements in this area.
The high frequency of modifications to the architectural model also means that the responsiveness and adaptability of the software responsible for interpreting the thermal model becomes a crucial issue.
