Define internal loads (occupants, lighting, equipment)

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Heat gains from lighting, equipment, and occupants. (Source: IBPSA-USA BEM Workshop)
Example lighting schedules for an office building model (data source: COMNET)

Internal loads refer to features in a building that generate heat (or act as heat sinks). These may come from occupants (people), lighting, or equipment ("plug loads"). In the case of lighting and plug loads, these loads also consume energy. Accurate inputs of the internal loads will lead to better estimation of end-use energy consumption, and loads that must be offset by heating and cooling (HVAC) equipment.

The input process to define internal loads are typically in the form of peak values, such as installed lighting power, and fractional schedules that represent the fraction of that peak that occurs at each timestep. In other words, the peak is when all the lights are on, all equipment running, and the space is fully occupied. The schedules represent hour-by-hour changes, such as turning off some lights or people leaving for lunch. Note that weekday and weekend schedules will often differ significantly in many building types.

Typical input properties

The following are common inputs related to internal heat gains.

  • Occupants
    • Peak number of occupants (occupancy density), typically entered as ft2/person
    • Occupancy schedule, a schedule indicating the fraction of the peak number of people that is present at each timestep
  • Lighting
    Daylight dimming - control options
    • Peak lighting power, typically entered in W/ft2
    • Lighting schedule
    • Automatic daylighting control, a capability of many simulation programs.
  • Plug loads
    • Plug load peak power, typically entered in W/ft2
    • Plug load schedule
  • Other internal loads
    • Cooking equipment energy consumption and heat gain
    • Refrigerators

Choosing Appropriate Inputs

Evolution of ASHRAE Standard 90.1 Lighting Power Allowance for an Office Building from 1975 to 2019

Things to keep in mind when choosing internal loads inputs:

  • An important consideration when choosing inputs for a simulation model is that these internal heat gain inputs will typically be lower than the corresponding inputs used for worst-case cooling load calculations. The goal of a simple box model is usually to estimate annual energy consumption, and the appropriate inputs for internal heat gain are those that represent typical loads, not worst case loads.
  • Some default schedules for receptacle power have unrealistically low values for off-hours, typically night and weekend. Monitoring studies show a significant fraction of receptacle loads do not shut off, and nighttime values of 20% to 40% are typical.[1][2] A realistic schedule is important if an accurate energy consumption estimate is desired.
  • Typical interior lighting power has dropped significantly over the years as lighting technology has improved. The adjacent figure shows the change in lighting power allowed for an office building by ASHRAE Standard 90.1 over time. Check to see that your lighting power inputs are up to date.
  • Nameplate values for equipment power such as copiers or computers are usually much higher than the actual energy demand. Find data sources such as tables in the ASHRAE Fundamentals Handbook,[3] which are based on measured energy demand.
  • Cooking equipment energy does not necessarily all end up as heat in the space if it is located under an exhaust hood.

Assumptions for early stage modeling

At any stage of energy model development, some assumptions are needed about likely space usage, internal heat gains and infiltration. At the simple box modeling stage, many of these assumptions will require educated guesses, and this section provides some guidance on choosing appropriate values for occupant density, lighting power, plug load power and other potential sources of internal heat gain.

There are several potential sources of information for model inputs. Simulation software often provides default values, which may depend on building type. When defaults are used, the values should be recorded and provided to the design team for review. Energy codes are also a common source for lighting power inputs; the lighting power density allowance for the relevant building type can be a good choice if the lighting system has not yet been designed. Other reference sources also provide typical values for internal gain inputs and are discussed below.

Comparative analysis vs. EUI estimation

When one of the goals of the simple box modeling exercise is to estimate EUI or energy performance compared to a benchmark, then extra attention to internal load inputs is warranted, because lighting, plug loads and other process loads are often a big part of the total energy consumption. On the other hand, if the goal is to compare relative performance of design alternatives, then using default values for internal loads may be a reasonable choice.

Sources for default internal gain assumptions

As noted above, there are a number of sources that provide information about internal loads that can be used to verify or replace simulation software default inputs.

Read more here: Sources for default internal gain assumptions


  3. "2021 ASHRAE Handbook—Fundamentals".
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