Building energy modeling is a powerful tool that predicts a school facility’s energy usage. Different types of energy models are developed at various stages of a project to provide data that can verify or disprove suggested energy-efficiency measures. Education institutions should understand what an energy model can do and, more important, what the limitations of the model are at each stage of design and construction.
One of the main sustainability goals for construction projects is to optimize a building’s energy efficiency. The reduction of a building’s energy usage results in an overall reduction in lifetime owning/operating costs. With so much at stake, energy modeling is a useful tool for school facilities as they make decisions on a project.
Building energy modeling enables engineers to model a building’s performance mathematically over a period of time to gain an understanding of the potential building energy usage.
Energy modeling can be carried out during design, pre-construction and post-construction.
During design
Each energy model that is created has specific requirements and raises specific concerns. During design, energy modeling typically is used to compare options presented by the design team. The goal of modeling at this stage is to develop guidelines and baselines for the design of the building.
Every energy model has two key components: the software and the experience of the modeler. ASHRAE (American Society of Heating Refrigerating and Air Conditioning Engineers) has developed an accreditation process for engineers performing energy models: the Building Energy Modeling Professional (BEMP) credential
A professional energy modeling engineer who understands how buildings go together is critical to a school facility's success. A careful review of the building parameters and a good grasp of the systems, interrelated components and the applicable codes are necessary so that the input is logical and a building matching those inputs can be constructed.
Design Phase Modeling
The energy model for a school facility at this phase often includes different HVAC system types, utility rate structures and energy-efficiency measures. Energy-efficiency measures include building feature alternatives.
Although modeling during the design phase is useful, the energy modeler typically is using many assumptions to complete the model. During the design phase, as long as the assumptions are reasonable and applied equally to this baseline and proposed building models, they allow for a reasonable comparison of input options. At this stage, using the model to create comparisons so that design assumptions can be verified has more value than a single detailed model.
Modeling many different energy-efficiency measures is critical to ensuring that a building will be as energy-efficient as possible.
The most common components being modeled are changes to the building envelope, the fenestration, and heating, ventilation and air conditioning (HVAC) systems. Architectural changes can have a profound effect on the energy usage of the building, but they often come with concerns about how they affect aesthetics. The solar load on a building is a major component of the mechanical system size. Modeling different building orientations enables an education institution and its design team to understand how shifts to the buildings direction can affect the lifetime energy usage. The correlation of a building’s orientation to window locations enables a design to optimize lighting levels in the building. Harvesting daylight and reducing lighting loads can have a significant impact on lifetime building energy use, so incorporating daylight options into an early energy model is valuable.
HVAC system options frequently are modeled to determine which systems may be more effective in a building. It is common to compare typical direct expansion systems with ground-coupled systems to central plant systems. This enables an institution to not only understand the potential energy savings among the available options, but also investigate first-cost and life-cycle cost differences.
Pre-Construction Phase
After the initial energy models are completed during the design phase and a design direction has been defined, a pre-construction energy model is developed that serves a different purpose. At this stage, the proposed design energy model is used to compare with a baseline building energy model. The input for the proposed design energy model is taken from the latest design documents so the proposed building energy model is accurately representing the actual envelope components, fenestration, orientation and HVAC system type. The proposed design building model should begin to remove as many of the assumptions that might have been built into the earlier design models as possible. The model will include building shading, self shading, improved insulation and high-performance glass.
The baseline energy model is built to compare with the proposed design energy model. The baseline energy model will be built to be in compliance with minimum energy code compliance, LEED compliance and in particular, with ASHRAE 90.1 compliance. This model sets the energy performance minimum for any similarly constructed building. The baseline model must emulate the proposed design building in glass-to-wall area and orientation, and will use code minimum insulation for all walls, windows and roof areas. What is not required to be modeled in the baseline model is building self-shading and building shading.
With both energy models constructed, a comparison between the baseline model and the proposed design building model is used to compare the energy usage projected by each. This stage of the design generally is the earliest appropriate time to discuss the building’s energy savings, as many of the input variables are solidified in the design drawings.
It is important to note that the energy model at this stage should not be used to predict energy consumption or set energy budgets. Assumptions such as occupancy numbers, schedules, control schemes and final weather data are still in progress at this stage.
The goal of the model at this stage is to compare the baseline model with the proposed building model for a comparison of potential energy savings.
Post-Construction
In post-construction energy modeling, design and construction are to the point where the majority of the assumptions that have been used in previous models can be validated. The energy model at this point can be made specific to the project.
The starting points for this model are the as-built documents. All envelope components, fenestration, HVAC system type, occupancy type and schedule, controls schemes, lighting levels and plug load power densities can be modeled accurately using the design documents and shop drawings for the actual construction components used in the project.
The energy model at this stage often is used to compare with actual energy bills. It is common for energy bills to be different from the completed final energy model. This usually is because of differences in weather data or building usage. An energy model that is within 10 percent of the actual energy bill provides good accuracy. Often, the energy model is manipulated to match trended data from the building control system to allow for precise schedule matching and electrical usage.
The goal of the model at this stage is to verify that the building is performing as expected and if not, to help isolate underperforming systems or components.
Crutchfield, PE, LEED AP, is the principal-in-charge and Spangler, PE, LEED AP, is an ASHRAE-accredited BEMP at RMF Engineering, Charleston, S.C. They can be reached at [email protected] and [email protected].
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