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What is BEM?

According to the BEM Library:

"Building Energy Modeling (BEM) is the practice of using computer-based simulation software to perform a detailed analysis of a building's energy use and energy-using systems. The simulation software works by enacting a mathematical model that provides an approximate representation of the building. BEM includes whole-building simulation as well as detailed component analysis utilizing specialized software tools that address specific concerns, such as moisture transfer through building materials, daylighting, indoor air quality, natural ventilation, and occupant comfort. BEM offers an alternative approach that encourages customized, integrated design solutions, which offer deeper savings. Using BEM to compare energy-efficiency options directs design decisions prior to construction. It also guides existing building projects to optimize operation or explore retrofit opportunities."

Some other terms that are used to describe the same topic are:

  • Building Simulation
  • Building Performance Simulation
  • Building Energy Simulation
  • Building Performance Modeling

Somewhat confusing, the term "model" can refer to both the specific building being described and analyzed as well as referencing the software for implementing a specific component of the building such as a wall or chiller.

To learn more about building energy modeling consider reviewing the following resources:

Building energy modeling is necessary to understand the overall energy consumption in buildings, because, the energy flows in buildings can be very complicated. The heat flow through walls is determined not just by the area and the temperatures, but the mass characteristics of the walls may result in heat flowing out of a wall several hours after it has flowed in. The constantly changing temperature on the outside of the wall and the less frequent but often large step changes in thermostat setpoints between day and night operation for the inside of the building make the direction of the heat flow vary over time. Solar energy is absorbed on the exterior wall and roof surfaces as well as entering windows. The intensity of solar energy varies by the sun position for each period of time over the day and over the year. Heat sources within the building including people, office and other equipment, and lighting include both heat that immediately impacts the air within (convective heat) and heat that is absorbed by the surfaces and released slowly over time (radiant heat). Lighting may be controlled by a sensor that turns it on or off or changes its intensity based on the amount of natural daylight that is entering the space through windows and skylights making the heat generated from the lighting system change over time. Air conditioning and heating systems come in a huge number of configurations, and each one can be used with many different control configurations based on the temperature or other conditions within each space in the building, and ultimately their operation accounts for a large portion of the energy consumed in a building. For these reasons and more, what might first appear as something that can be calculated with just a few formulas in a spreadsheet is instead a software program and, in the case of EnergyPlus, with over 500,000 lines of code.