SurfaceGeometry
Before the surface objects are explained in detail, a description of geometric parameters used in EnergyPlus will be given. Since the input of surface vertices is common to most of the surface types, it will also be given a separate discussion. Some flexibility is allowed in specifying surface vertices. This flexibility is embodied in the SurfaceGeometry class/object in the input file. Note that the parameters specified in this statement are used for all surface vertice inputs – there is no further “flexibility” allowed. In order to perform shadowing calculations, the building surfaces must be specified. EnergyPlus uses a three dimensional (3D) Cartesian coordinate system for surface vertex specification. This Right Hand coordinate system has the X-axis pointing east, the Y-axis pointing north, and the Z-axis pointing up.
Information from the EnergyPlus Input Output Reference:
Field: SurfaceStartingPosition
The shadowing algorithms in EnergyPlus rely on surfaces having vertices in a certain order and positional structure. Thus, the surface translator needs to know the starting point for each surface entry. The choices are: UpperLeftCorner (ULC), LowerLeftCorner (LLC), UpperRightCorner (URC), or LowerRightCorner (LRC). Since most surfaces will be 4 sided, the convention will specify this position as though each surface were 4 sided. Extrapolate 3 sided figures to this convention.
Field: VertexEntry
Surfaces are always specified as being viewed from the outside of the zone to which they belong. (Shading surfaces are specified slightly differently and are discussed under the particular types). EnergyPlus needs to know whether the surfaces are being specified in counterclockwise or clockwise order (from the SurfaceStartingPosition). EnergyPlus uses this to determine the outward facing normal for the surface (which is the facing angle of the surface – very important in shading and shadowing calculations.
Field: CoordinateSystem
Vertices can be specified in two ways: using “Absolute”/“World” coordinates, or a relative coordinate specification. Relative coordinates allow flexibility of rapid change to observe changes in building results due to orientation and position. “World” coordinates will facilitate use within a CADD system structure.
Relative coordinates make use of both Building and Zone North Axis values as well as Zone Origin values to locate the surface in 3D coordinate space. World coordinates do not use these values.
Typically, all zone origin values for “World” coordinates will be (0,0,0) but Building and Zone North Axis values may be used in certain instances (namely the Daylighting Coordinate Location entries).