Add Space Concept to EnergyPlus Zone Structure, Part 1

design/FY2021/NFP-Spaces.md

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+# Adding "Spaces" to EnergyPlus #
+
+**Michael J. Witte, GARD Analytics, Inc.**  
+**Jason W. DeGraw, ORNL**  
+**With input from many more . . .**  
+
+ -  Original, November 22, 2019
+ -  Revised, April 1, 2020
+    * Keep the current definition of Zone
+    * Add new objects for Space, SpaceType, CompoundSpaceType, and Enclosure
+ -  Revised, November 25, 2020
+    * Condensed and focused on final proposal somewhat agreed to back in April 2020
+    * Revised to reflect simplified Construction:AirBoundary object (radiant and solar enclosures are now the same)
+
+## Justification for New Feature ##
+
+Thermal zones (HVAC zones) are often composed of a variety of spaces grouped together to be served by
+a single thermostat. For example, an office building core zone may include private offices, conference rooms, restrooms
+and corridors. Each of these spaces (rooms) has different internal gains and may or may not be fully enclosed by solid surfaces.
+Currently, the smallest building element is a zone. This requires the user (or interface) to blend these spaces together for surfaces,
+internal gains, and other specifications. The option to specify each space explicitly would greatly simplify input in data managemenent,
+especially for space-based interfaces (such as OpenStudio) and for codes and standards modeling.
+
+There are also computing performance advantages to dividing zones into smaller enclosures for radiant exchange.
+
+## Overview ##
+
+### Current EnergyPlus model ###
+EnergyPlus input currently has the following (very flat) heierarchy:
+
+    Zone
+        Surfaces (references a zone name)
+        People, Lights, etc. (references a zone name or zone list name)
+        Thermostat (one per zone)
+        HVAC equipment (attaches to a zone)
+
+Zones represent an air mass (air node) that exchanges heat with surfaces, internal loads, HVAC equipment, etc. 
+A Zone is essentially the "atomic unit" of the building. By default a zone has a uniform air temperature, but there are room air models 
+that allow modeling of stratification and other non-uniform temperature effects. Thermostats and HVAC equipment are assigned per Zone.
+
+The internal data model adds another layer:
+
+    Enclosure
+        Zone
+            Surfaces
+            People, Lights, etc.
+            Thermostat
+            HVAC equipment
+        Zone
+            Surfaces
+            People, Lights, etc.
+            Thermostat
+            HVAC equipment
+
+Enclosures are used for radiant and solar/daylighting exchange. Enclosures are primarily concerned with Surfaces, but they
+are also related to internal gains (which cast radiant and visible energy into the enclosure). By default there is one Enclosure for each
+Zone.  By using Construction:AirBoundary, multiple zones may be grouped into a single larger Enclosure. 
+Enclosures are assigned automatically based on the zones connected by an air boundary surface. 
+
+
+## New Space Object
+
+### Relationship assumptions ##
+
+If we keep the current definition of "Zone" and add the concept of
+"Space" (<= Zone), then this would add a new layer to the data model:
+
+    Zone
+        Thermostat
+        HVAC equipment
+        Space 2
+            Surfaces
+            Internal Gains (People, Lights, etc.)
+        Space 3
+            Surfaces
+            Internal Gains (People, Lights, etc.)
+
+    Enclosure
+        Space 1
+            Surfaces
+            Internal Gains (People, Lights, etc.)
+        Space 2
+            Surfaces
+            Internal Gains (People, Lights, etc.)
+
+
+### Definitions
+
+ * Surface - A geometric plane which is attached to a Space. 
+   * A Surface can be opaque, transparent, or an air boundary.
+   * Each Surface belongs to one Space.
+
+ * Space - A collection of one or more Surfaces and internal gains.
+   * Each Space belongs to one Zone (explicitly user-assigned).
+   * The Spaces in a Zone are lumped together for the Zone heat balance.
+   * There is no heat balance at the Space level.
+   * Each Space belongs to one Enclosure (implicitly assigned).
+
+ * Zone - 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.
+   * All Surfaces and internal gains associated with the Spaces are included in the Zone.
+   * The Zone heat balance does not change: it has an air node (or a Room Air Model) and includes all Surfaces and 
+   internal gains from its Spaces plus any HVAC which is attached to the Zone.
+
+ * Enclosure - A continuous volume connecting Surfaces for radiant, solar, and daylighting exchange.
+   * Each Enclosure is comprised of one or more Spaces.
+   * There is one Enclosure for each Space unless there are air boundary surfaces 
+   which group multiple Spaces into a single 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.
+
+## Proposed Input Approach ##
+Very minimal changes to current inputs.
+
+ * Old Zone object becomes Space object plus some tags.
+ * New Zone object has a name and a list of Spaces.
+ * Surfaces will reference a Space instead of a Zone.
+ * ZoneHVAC and Thermostats remain as-is and continue to reference a Zone or ZoneList.
+ * New SpaceList object (equivalent to ZoneList, but for Spaces).
+ * Internal gains reference Space or Spacelist (instead of Zone or ZoneList).
+ * ZoneInfiltration and ZoneVentilation remain at the zone level.
+
+### Proposed objects:
+
+    Space,  !- Same fields as old Zone object plus new tags at the end
+      Name,
+      Origin,
+      Multiplier,
+      Ceiling Height,
+      Volume,
+      Floor Area,
+      Convection algorithms,
+      Part of Total Floor Area,
+      User-defined Tags for Space Types (for reporting purposes only at this point)
+
+    Lighting/People/Equipment
+      Name,
+      Space or SpaceList Name, !- Replace current Zone or ZoneList Name
+      Schedule Name,
+      ...
+
+    ZoneVentilation and ZoneInfiltration
+      Name,
+      Zone or ZoneList Name, (or should these more to the Space level and be renamed?)
+      ...
+      Part of Total Floor Area;
+
+    Surface,
+      Name,
+      ...
+      Space Name,  !- Replace current Zone Name
+      (InterZone surfaces becomes InterSpace surfaces, air boundary surfaces will connect spaces in an enclosure)
+
+## Data Structure Considerations ##
+
+ * Add fields to `Surface` to track the Space name and index along with the existing Zone and Enclosure fields.
+
+ * Proposed Surface ordering (same as current): 
+   - All shading surfaces are first
+   - All air boundary surfaces are next (once PR8370 merges).
+   - Group by Zone
+   - Group by surface type within each Zone
+
+e.g., Zone1 contains Space1 and Space3, Zone2 contains Space2.
+
+      Shading Surfaces
+      Air Boundary Surfaces
+      Space1-OpaqueSurface1 (begin surfaces in Zone1)
+      Space1-OpaqueSurface2
+      Space3-OpaqueSurface1
+      Space3-OpaqueSurface2
+      Space1-Window1
+      Space1-Window2
+      Space2-OpaqueSurface1 (begin surfaces in Zone2)
+      Space2-OpaqueSurface2
+      Space2-Window1
+
+ * Alternate Surface ordering: 
+   - All shading surfaces are first
+   - All air boundary surfaces are next
+   - Group by Enclosure (because enclosure radiant exchange is more computationally intensive than zone heat balance?)
+   - Group by Space within each Enclosure.
+   - Group by surface type within each Space
+   - Modify heat balance Zone loops to be Space loops(?)
+
+e.g., Enclosure1 contains Space1 and Space3, Enclosure2 contains Space2.
+
+      Shading Surfaces
+      Air Boundary Surfaces
+      Space1-OpaqueSurface1 (begin surfaces in Enclosure1)
+      Space1-OpaqueSurface2
+      Space1-Window1
+      Space1-Window2
+      Space3-OpaqueSurface1
+      Space3-OpaqueSurface2
+      Space2-OpaqueSurface1 (begin surfaces in Enclosure2)
+      Space2-OpaqueSurface2
+      Space2-Window1
+
+## Options/Questions for Discussion
+ * Keep ZoneVentilation and ZoneInfiltration at the Zone level or move to the Space level (and rename)?
+ * Allow Zone Names to be the same as Space Names or force all names to be unique across Spaces and Zones?
+ * Zone-based or Enclosure-based surface grouping?
+
+## Testing/Validation/Data Sources ##
+
+There should be no substantive diffs (possibly some small diffs due to change in computational order). 
+For the regression tests which are one-to-one Space-To-Zone, all numeric results
+should stay exactly the same, but output variable names and table headings may change.
+
+## Input Output Reference Documentation ##
+
+The I/O Reference section for Zone becomes Space with some explanatory text at the top.
+The new Zone becomes a list of Spaces.
+
+## Outputs Description ##
+
+Output variables which are zone-based will remain the same.
+Some space-level output variables may be added.
+Table reports summarizing inputs at the space level will be added.
+Table reports allocating energy at the space level *may* be added.
+
+## Engineering Reference ##
+
+Calulations won't change, so doc changes will be minimal to clarify when Space, Zone, and Enclosure.
+
+## Example File and Transition Changes ##
+
+* Convert Zone objects to Space objects and add "-Space" to the name.
+* Insert new Zone objects (one for each original Zone, no name change).
+* Surfaces - Change all Zone names to add "-Space"
+* Internal gains - Change all Zone names to add "-Space"