ESP-r: Building form and composition rules

This section sets out the rules governing the form and composition of thermal zones within ESP-r. Being a general purpose tool allows considerable latitude as to the scale and complexity of thermal zones. For some this latitude is essential and for others it is confusing.

Zone topology

The relationships within thermal zones (topology) are derived from the data model used and the capabilities of the underlying solution techniques. The following apply:

A thermal zone is a volume of air assumed to be well mixed. Tracking temperature stratification or temperature changes near perimeters requires creating additional zones or inclusion of CFD domains.

A thermal zone must be fully bounded by surfaces. AND it is possible that some might be reversed if you do not pay attention.

In ESP-r all surfaces participate in the zone energy balance as well as in solar and longwave radiation exchanges. If a window has a frame it needs to be a surface. But is an architrave worth representing as a separate surface?

The full area of the surface has a uniform surface temperature and uniform heat transfer characteristics. And if you expect the floor of a room to have a range of temperatures (e.g. from solar patches) then consider splitting the floor into multiple surfaces.

Internal thermal mass is represented via pairs of surfaces with share edges and which are thermally linked. There are no non-geometric descriptors of internal mass. There are, however options for including pre-defined objects like desks and chairs into a zone.

Surface topology

Surfaces have the following characteristics and support the following solver facilities:

Surfaces must be flat. Polygons of up to 42 edges are allowed. Polygons include shared edges of child surfaces as well as shared edges with adjacent surfaces.

The minimal area of a polygon is ~10 square mm.

Edge ordering of polygons determines the direction of the surface normal (anti-clockwise looking from the outside).

A zone is fully bounded if each edge of a polygon is matched by one other edge defined in the opposite direction associated with another surface.

Surface parent and child relationships

ESP-r reports on surface hierarchy within thermal zones and records it in the model file. If a parent surface boundary condition changes the default is for child and grandchild surfaces to inherit this change. The topology of surface edges allows for parent, child and relationships to be established. For example, a glazed surface can be a child of a frame surface which can be a child of a facade (parent surface). A glazed surface can also be a parent surface.

Longwave radiation exchanges

Grey-body longwave radiant exchanges (area and emissivity weightings) apply unless surface-to-surface view factors have been computed. A utility application is used to calculate view factors between surfaces of arbitrary complexity within rooms of arbitrary complexity. The utility also supports establishing view factors between MRT block sensors and zone surfaces for purposes of position-specific comfort assessments. Its more efficient to delay calculations and the creation of block sensors until the form of the zone is established.

Solar radiation exchanges (outside)

Surfaces are assumed to be non-specular and absorption is cosine adjusted. If there are shading obstructions defined in the zone and shading calculations carried out reductions to direct radiation are imposed during assessments, otherwise no shading is assumed.

Diffuse solar radiation is absorbed when the sun is above the horizon. Diffuse solar can be treated as either XX or YY.

Solar radiation exchanges (inside)

Solar radiation enters a thermal zone via surfaces that are not OPAQUE either on the facade or which are partitions to adjacent thermal zones. This is distributed according to user directives as follows:

The default treatment is to distribute direct solar radiation on an area absorptivity basis to all surfaces in the zone.

The user may specify one or two surfaces to receive direct solar radiation

Calculated insolation patterns for direct solar radiation are to be used.

When direct solar radiation is reflected from a surface it treated as diffuse and is iteratively distributed. When diffuse solar radiation reaches a transparent surface it is transmitted based on the optical characteristics of the surface.

Single purpose entities

In ESP-r entities which have a specific purpose are:

solar obstructions are used in shading analysis and visual assessments but otherwise do not participate

ground topology surfaces are used in visual assessments but otherwise do not participate

visual blocks are used in visual assessments (e.g. are exported to Radiance)

MRT sensors are used sense and report local comfort

Model complexity

Geometric and compositional complexity limits for simulation models are determined via the header files in place when ESP-r is compiled. The Install script asks the level of complexity and your answer depends on the computer you are using (e.g. how much memory and disk space) as well as the complexity of models you are likely to be working with. Typical limits for small/medium/complex simulation projects are:

zones - 32/82/90

surfaces (per zone) - 60/120/200

polygon edges - 42/82/120

flow nodes - 40/100/200

flow connections - 100/200/200

calendar day types - 15

schedules - 24 periods per day