Engineering Reference — EnergyPlus 22.1

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General Modeling Overview[LINK]

The EnergyPlus program is a collection of many program modules that work together to calculate the energy required for heating and cooling a building using a variety of systems and energy sources. It does this by simulating the building and associated energy systems when they are exposed to different environmental and operating conditions. The core of the simulation is a model of the building that is based on fundamental heat balance principles. Since it is relatively meaningless to state: “based on fundamental heat balance principles”, the model will be described in greater detail in later sections of this document in concert with the C++ code which is used to describe the model. It turns out that the model itself is relatively simple compared with the data organization and control that is needed to simulate the great many combinations of system types, primary energy plant arrangements, schedules, and environments. Figure 1 shows this overall organization in schematic form. Later sections will expand on the details within the blocks of the schematic.

EnergyPlus Program Schematic

EnergyPlus Program Schematic

Building Surfaces, Spaces, Zones, and Enclosures[LINK]

The EnergyPlus building model consists of four main constructs: surfaces, spaces, zones, and enclosures. These are defined below along with some assumptions and relationship rules.

  • a geometric plane which is attached to a Zone and a Space.

  • A Surface can be opaque, transparent, or an air boundary.

  • Surfaces may store and transfer heat and moisture.

  • The inside and outside face of each Surface has a single uniform surface temperature calculated from the surface heat balance.

  • Each Surface belongs to one Zone and one Space.

  • Inter-Space and inter-Zone Surfaces are modeled as two linked surfaces.

  • Air boundary surfaces combine one or more spaces into a common enclosure.

  • Inter-Space surfaces connecting spaces that are part of the same Zone will see the same air temperature. They may or may not be in the same enclosure.

  • Inter-Space surfaces connecting spaces that are in different Zones may see different air temperatures. They may or may not be in the same enclosure.

  • A collection of one or more Surfaces and internal gains.

  • Each Space belongs to one Zone.

  • Spaces may be user-specified or generated by default so that every surface belongs to a Space and every Zone has at least one Space.

  • A Space may have only floor surface(s) or may be fully enclosed (floors, walls, ceilings, etc).

  • Each Space belongs to one Enclosure (implicitly assigned). See Enclosure below for more details.

  • There is no heat balance at the Space level; the heat balance is done at the Zone level.

  • Internal gains are modeled at the Space level and combined for the Zone heat balance.

  • Each Space is assigned a Space Type which is used for output reports and submeters.

  • An air mass connecting Surfaces, internal gains, and HVAC equipment for heat balance and HVAC control.

  • Each Zone is comprised of one or more Spaces.

  • If no Spaces are specified for a Zone, a Space will automatically be created.

  • If any surfaces in a Zone do not have a user-assigned Space, a Space will be automatically created for those surfaces.

  • The Zone heat balance solves for the zone air temperature and humidity (or a Room Air Model with multiple air nodes) and includes all Surfaces and internal gains from its Spaces.

  • HVAC systems are connected at the Zone level.

  • A continuous volume connecting Surfaces for radiant, solar, and daylighting exchange.

  • Each Enclosure is comprised of one or more Spaces.

  • If a Space has a floor surface(s) plus any other surfaces (walls, ceiling, roof, etc.), then it is its own enclosure.

  • If a Space has only a floor surface(s), then it is part of the zone enclosure.

  • Any surfaces with a blank Space is part of the zone enclosure.

  • Air boundaries can be used to combine Spaces into a common enclosure.

  • All Surfaces and internal radiant gains associated with the Spaces are included in the Enclosure.

  • Enclosures only distribute radiant (thermal), solar, and visible energy to and from the Surfaces.

  • There is no full heat balance at the Enclosure level. Each Enclosure only balances the radiant/solar flux on each Surface. These fluxes then become part of the Surface inside heat balance.