Building Energy Modeling Concepts
This pages offers some thoughts on how to set up a variety building energy models (BEM) during the different building design phases and for different purposes.
Massing Building Energy Model
A Massing BEM is the most basic whole building energy model. The overall building volume is divided into a series of touching but not overlapping thermal zones. The division separates different floors with further subdivisions into perimeter and core zones on each floor. Perimeter zones extend about 5m (15’) into the building floor plan with the core taking up all remaining area. The motivation for the perimeter/core division is that a surplus of solar radiation on an equator-facing façade is not “dumped” into a larger zone, making up for temporary radiation deficit on the opposing façade. Division by floor is necessary since occupant, lighting and equipment loads are usually assigned per zone floor area. A typical floorplan division is shown in Figure 1.
Zone Templates
For more expedient modeling, template assignments for all zones in a Massing BEM may be the same and correspond to the “typical” construction and usage pattern of the building. Ideally, the template should be an area-weighted mean of the different usage types that exist in the building such a open office, closed offices, conference room, corridor, etc. ClimateStudio offers area-weighted mean templates for all DOE benchmark buildings call “whole building [benchmark type name].” Note that using area-weighted means is a good first approach but may introduce errors if the mix of different use types varies in your building to that of the underlying DOE benchmark building.
Internal Zone Boundaries
Since the program is the same on both sides of interior partitions, there will be limited conduction heat flow through these surfaces and they can be modeled as walls. Note that this simplified modeling approach causes errors since direct beam solar radiation can never enter a core zone.
Model Uses
Massing BEMs can be used to explore the relative impact of different façade layouts, equipment and electric lighting upgrades as well as HVAC systems on annual and monthly building energy use, making them particularly suitable for initial EUI Studies. Their main advantage is that the generation process required limited effort and can be largely automated.
Figure 1: Floorplan subdivision into perimeter and core zones for the DOE Medium office building
Room-By-Room Building Energy Model
A Room-by-Room BEM expands on the Massing BEM by further subdividing a building into rooms with different programmatic uses. A motivation for introducing explicit use schedules rather than area-weighted means is that the model can be customized to predicted use schedules and loads for each space. This then allows the modeler to study thermal comfort conditions in these spaces. For example, indoor temperatures in a private office with good solar gains control and reduced equipment and electric lighting loads may be adequate while using natural or hybrid ventilation. However, in a cafeteria and/or conference room higher internal loads nay require a different conditioning system. An example floorplan division for the DOE Medium Office is shown in Figure 2.
Zone Templates
As explained above, different space types require different templates in a Room-by-Room BEM. Tools such as ClimateStudio offer a variety of space use templates that are derived from the DOE Benchmark Buildings.
Internal Zone Boundaries
From Room-by-Room BEMs internal partitions should be measured according to the actual interior design, i.e. as air walls if the are no internal walls between different zones.
Model uses
Room level analysis to evaluate thermal comfort conditions, local leak loads etc.
Figure 2: Floorplan subdivision into different thermal zones by space use for the DOE Medium office building