Information contained in a BEM

Building energy models are a representation of the building to be modeled and the characteristics of how it was built, the systems within, and how the building is operated. This page provides a summary of information that is generally contained in a BEM.

Depending on the software tool used for the BEM analysis, input data can be represented in simple text files, or using very detailed graphical user interfaces, as illustrated in the image thumbnails.

Much more detail can be found in the Preparing Model Inputs section of this wiki.

• 

  Example of text file input data in the EnergyPlus IDF file format

• 

  Example of graphical user interface for HVAC system inputs using IESVE software

Geometry

A typical starting point when creating a BEM is to define its geometry including the building's shape and size and locations of windows and doors. Depending on the stage of design, the level of detail used to define the geometry may vary. For example, during early phases of design, it may be appropriate to use a simple box model whereas, later in design, geometry that reflects the building floor plans is more appropriate. Targeted studies of specific features may only require the model to include a single room or "slice" of the building and could be represented by a shoebox model.

(Source: IBPSA-USA BEM Workshop)

Envelope constructions

Opaque constructions

Opaque constructions are typically represented as a series of material layers. Each material layer's thermal properties are input and the software will calculate the overall thermal performance of the whole assembly (all layers combined). The following image is a representation of the thermal layers. For more detailed guidance on how to provide the thermal inputs, refer to the page Define opaque envelope constructions.

Layer-by-layer representation of construction assembly. (Source: IBPSA-USA BEM Workshop)

Glazed constructions

Glazing materials (e.g. windows and skylights) have additional input properties to account for not only thermal performance, but also how they perform to radiation gains and visible lighting. The key features of a fenestration product are reported on a certificate from the National Fenestration Rating Council (NFRC), but much more detailed information is available and may be used depending on the level of detail of the analysis. For more detailed guidance on how to provide inputs for glazed constructions, refer to the page Define fenestration.

Thermal properties of fenestration products. (Source: IBPSA-USA BEM Workshop)

Internal heat gains and schedules

Internal heat gains refer to heat generated inside the building by lighting, equipment, and the occupants. The heat gains are generally specified at each space in terms of radiation from the heat source (heat is directly emitted) , and convection (the heat source heats the air around it). Gains from the occupants also include both sensible and latent components (latent heat gains include moisture). Gains are input into the BEM in terms of the amount of heat gain (usually Watts or Watts/area), the split between radiant and convective portions, and sensible and latent components for occupants.

Associated with each gain is a schedule that represents when the gains occur (e.g. accounting for occupants leaving the space at different times of day, lights and equipment being turned on and off). It is important to input the internal gain information as accurately as possible or to use reliable estimations because this heat must be managed by HVAC systems or passive cooling strategies and therefore the design of these systems is largely dependent on the heat gains.

For more detailed information about the science of how heat gains are added to a space refer to the page Basics of internal gains. For more detailed guidance on how to provide the inputs for heat gains and schedules, refer to the page Define internal loads.

Heat gains from lighting, equipment, and occupants. (Source: IBPSA-USA BEM Workshop)

Infiltration and natural ventilation

Infiltration is when outside air enters the building and exfiltration, conversely, is when indoor air escapes to the outside of the building. Infiltration is mostly uncontrolled flow of air through cracks and gaps in the building fabric. This is influenced by indoor and outdoor temperatures, air buoyancy, wind, number of windows and other openings, building height and quality of construction.

Natural Ventilation is the intentional flow of air through open windows, doors, grilles, and other planned building envelope penetrations, and it is driven by natural and/or artificially produced pressure differentials.

The energy model can represent these phenomena either by explicitly modeling openings in the building and allowing the simulation engine to calculate the amount of infiltration and natural ventilation; or, a more simplistic approach can be taken where an assumed air flow rate is entered with an associated schedule (representing variation in the amount of airflow at different times of the day). The first approach may be more accurate, but requires significantly more detailed model inputs and some assumptions must still be made about the leakiness of the building fabric. The second approach is easier to implement, but does not accurately account for temperature and wind impacts on infiltration and natural ventilation (unless a very detailed schedule is developed to approximate these effects).

Heat gains and losses from infiltration and/or natural ventilation. (Source: IBPSA-USA BEM Workshop)

HVAC system description

For many BEM projects, HVAC system inputs represent the largest proportion of information to be defined in a model. The model describes the system types, capacity, efficiency, heating and cooling sources, control strategies, outdoor air quantities (for ventilation), schedules, zones that they provide conditioning to, and how the heating and cooling is distributed to the zones.

Much more detail is available in the HVAC fundamentals section of this wiki.

Overview of HVAC system inputs in a BEM model. (Source: IBPSA-USA BEM Workshop)

Other systems

Other systems, if present in the building, and depending on the scope of the model, may include:

• Service hot water
• Exterior lighting
• Elevators
• Kitchen
• Renewable energy
• Energy storage

Utility rates and energy source characteristics

The BEM model calculates hourly energy consumption. This information can be used to estimate operational costs and expected CO₂ emissions if utility rates and CO₂ factors are (optionally) included as model inputs. Click for more information about utility rate inputs, source energy and carbon emissions.