Building Energy Modeling (BEM) is a physics-based computer simulation of a building that can be used to analyze and predict the energy consumption, HVAC component sizing and utility bills. BEM is used in new building construction and retrofit designs to achieve compliance for the energy code standards (ASHRAE 90.1, Title 24, NECB), different green building certifications like LEED v4 & Green Globes. BEM may also be used to get qualified for tax credits and utility incentives.
BEM input parameters that are taken into consideration for running the include but not limited to building geometry, construction material properties, glazing ratio, lighting, HVAC system, refrigeration, service water heating, component efficiencies, control strategies and renewable generation system configurations. It is also important to know about building’s use type and operation detail for occupancy, lighting, plug-loads and thermostat settings. The standard BEM program takes the inputs mentioned above and takes the building through the weather conditions for an entire year utilizing the physics equations to calculate thermal loads, energy usage intensity (EUI) along with the cost for energy.
The practical knowledge of the building’s use type is extremely important to recognize the particularities of specific energy consumption and expenditure. For example, hospitals, schools, warehouses, data centers, offices and religious spaces have vastly different occupancy rates and heating & cooling requirements. An office space will consume energy on daily basis usually during the hours 7am-5pm. Also, a school will consume energy during the same time, but has month-to-month operation differences because of it’s semester and seasonal-break calendar, such as a 3 month summer break where school building would use a fraction of it’s typical peak operational hours.
It requires a lot of attention to detail regarding the inputs as small change in them can make a huge impact on an energy model which reflects in the values for energy usage intensity and overall cost of energy. For example, the small difference in Air Changes per Hour (ACH) value makes a huge impact on heating and cooling of the building and difference in u-values of walls/roof/glazing can make a huge difference in energy usage intensity values as well as the overall cost of the building.
All inputs that are automatically populated in Cove.tool are calibrated with PNNL prototype models of the respective building types which helps the modeler to get a realistic idea about how the design will perform. Also, the option to add different input values for envelope, roof, HVAC systems, lights, appliances, set-points, schedules, sensors and solar panels to perform cost vs optimization analysis is available on Cove.tool web application.
On a whole, it is important to remember the purpose of running a model. No energy model can give the designer, engineer or owner a prediction of the future, but good models can certainly help making the better design choices to reach the envisioned target.