Step 3: Prepare to Construct the Building Model

Post date: Sep 28, 2017 7:8:9 AM

Working from blueprints or sketches and following the guidelines in Step 2, the building zones were determined. It is recommended that the engineer sketch the building with its zones. Surface dimensions should be included in the sketch. Additional geometric and the surface information is required before an input file describing the building can be constructed. Specifically, the building model must:

1. Determine heat transfer and heat storage surfaces.

2. Define equivalent surfaces.

3. Specify surfaces and subsurfaces (windows, doors, etc.) construction and materials.

4. Compile surface and subsurface information.

By the way, the file for this example, the 1 zone model are contained in your EnergyPlus installation ExampleFiles\BasicFiles folder.

Step 3.1. Determine heat transfer and heat storage surfaces.

The surfaces of the building can be described in any order; grouping surfaces by zone may help you read the input file. Specifics of the describing surfaces help categorize the surface’s heat transfer/storage as well as identify the surface construction information.

The details of input surfaces are described in the Input/Output Reference document. The allowable surface types are shown in the following table:

Table 3. Surface types and categorization

The pieces of the definition that designate BuildingSurface: Detailed surfaces as either heat transfer or heat storage surfaces are:

Note that subsurfaces (windows, doors) on these base surfaces will inherit the base surface properties listed above. The following examples will use a bit more of the Surface definition to give context.

Surfaces that specify “themselves” as the outside boundary condition are ceilings, floors and partitions that divide temperature-controlled spaces. The program assumes that the surface temperatures on both sides of the surface are the same. This means that even though heat may be stored in a partition, ceiling, or floor, no heat flows through it.

Heat Storage Surfaces (Use current Surface name for ExteriorEnvironment), e.g.:

Some surfaces divide the temperature controlled space from the outside environment. Surfaces that are both sun and wind exposed (e.g. exterior walls, exposed floors, roofs) feel the full effect of both solar radiation and outside temperature, and the outside air film resistance for these surfaces changes with wind speed and wind direction. Surfaces that are not sun or wind exposed (a wall to an “uncontrolled” space) are not affected by solar radiation, wind speed or direction and have a constant outside convective air film resistance.

Heat Transfer Surfaces Exposed to the Outside Environment, such as Exterior Walls, Roofs, Exposed Floors:

Surfaces such as basement walls and slab floors separate the space from the earth surrounding the surfaces. Therefore, the outside surface temperatures become the ground temperatures. Heat Transfer Surfaces in Contact with the Ground, such as Basement Walls or Slab Floors:

Other surfaces separate zones that may be at different temperatures. These surface types allow heat transfer (by conduction through the walls) from a zone at a higher temperature to a zone at a lower temperature. The location of the heat storage surface in the zone is not important except in specialized solar studies. The surface above (wall to uncontrolled space) would be more correctly modeled as an interzone surface.

Heat Transfer Surfaces Exposed to Another Zone, such as Interzone walls, ceilings or floors:

Step 3.2. Define equivalent surfaces as desired.

When the building was zoned, our objective was to define as few zones as possible. Now we would like to extend this objective to include defining as few surfaces as possible without significantly compromising the integrity of the simulation. We reduce the number and complexity of surfaces in our input file by defining equivalent surfaces.

Before dealing with equivalent surfaces, it is appropriate to take the concept of a thermal zone one step further. EnergyPlus performs heat balances on individual zone surfaces and on the zone air. For purposes of the heat transfer calculations, a geometrically correct rendering of the zone surfaces is not required. The surfaces do not even have to be connected. As long as the program knows to which thermal zone (mass of air) each surface transfers heat, it will calculate all heat balances correctly. For example, all heat storage surfaces of the same construction within a zone may be defined as a single rectangular surface. The size of this equivalent surface will equal the sum of all the areas of all the heat storage surfaces in the zone. A few

simple rules will further explain what we mean by equivalent surfaces and how these surfaces may be used. Remember that these are guidelines for optional simplification of input. Each simplification must be evaluated to determine if it would significantly impact certain shading, interior solar gains, or daylighting features. The goal is to seek an adequate level of detail to capture the key features of the building envelope without spending excess time describing and computing results for details that are insignificant.

1. Define all roofs and floors as rectangles regardless of the shape of the zone. Each zone may have one rectangular roof and one rectangular floor of a given construction.

2. Define all heat storage surfaces of the same construction within a zone as a single surface . The size of the single surface is obtained by summing the individual surface areas exposed to the zone. Thus, if a partition is completely within a zone (both sides of the partition are exposed to the zone), the area of each side must be added to the area of the equivalent surface. On the other hand, if the partition separates two zones, the area of only one side should be added to the equivalent surface.

3. Combine all windows on a given exterior surface into a single window . Usually each exterior surface should have only one window of each type. Overhangs or other shading devices may require that more windows be specified or combined together. By using the WindowMaterial:Glazing construction for your glass door, they will be correctly modeled in EnergyPlus with sunlight transferring into the zone.

The following figure shows the surfaces and subsurfaces required for a one-zone model, i.e., the education center. Since there were two types of partitions in the building, two heat storage surfaces ("internal mass") of different constructions were defined.

Figure 15. Simplifications Using Equivalent Surfaces

Step3.3. Specify construction elements

BLAST, DOE-2 and other programs often have “libraries” of construction, schedules, and other aspects of simulating the building. In EnergyPlus, we have a special set of files in the DataSets folder that represent many facets of building simulation. Data sets are usually IDF snippets or macro files. For constructions, using the guidelines in the ASHRAE Handbook of Fundamentals (2005), the file ASHRAE_2005_HOF_Materials.idf contains materials and constructions from Chapters 30 and 25. Since Chapter 30 discusses heating and cooling loads, it includes constructions for light, medium and heavy weight buildings – these constructions are represented in the dataset file. For the education center, “medium” constructions are used. For the windows, we will use the Double Pane Window from the previous exercise.

Table 4. Building Elements

Notes:

(1) The surface type is a wall, floor, roof, window or door.

(2) User supplies name for the element. For this example use name from the DataSet: ASHRAE_2005_HOF_Materials.idf. Similarly, the window was constructed from the Windows.idf dataset.

(3) Material's full name is as found in the ASHRAE_2005_HOF_Materials.idf dataset.

Step 3.4. Compile surface and subsurface information.

Building information:

Building North Axis: This syntax simplifies building geometry specification by designating one wall of the building as the building’s north pointing axis. The building model North axis is measured from true (compass) North. Surface facing angles (see surface information below) are then specified relative to the building north axis. The North Axis entry in the Input Output Reference (duplicated here) illustrates specification of the building north axis.

Figure 16. Illustration of Building North Axis

Zone information:

1. Wall height: In a simple model, one should make all the walls the same height. Then, the simple, 1 zone model can entirely enclose the space. In more complex models, you may resize each wall accordingly.

Surface information:

1. Base Surface Type: Heat Transfer/Heat Storage Surfaces may be of the following types: wall, floor, roof, internal mass, or subsurface

2. Construction: The type of construction of the surface (see previous table).

Subsurface information:

1. Subsurfaces are Windows, Doors or GlassDoors

2. Area: Area of the subsurface.

3. Reveal: For windows only, the distance it is inset from the outside surface of a wall. For simplicity, put all the windows in the same physical plane as the wall they are on.

For the single zone model, the following figure is a schematic representation of a one zone representation.

The figure shows the length of all "base" surfaces and the areas of all "subsurfaces" (windows). Doors are shown and may be entered, if desired. In the table (Table 5), the surfaces are numbered counter-clockwise around the zone beginning at the lower left corner of the figure. This table is the minimum required zone information compiled by the user. A few simple conventions should be followed to facilitate the construction of zone information tables:

1. Number all surfaces in order counter-clockwise around the zone.

2. Keep the subsurfaces with the base surface on which they are located.

3. Specify lengths for base surfaces and areas for subsurfaces and internal mass.

4. Specify the roof and floor as rectangles of the correct size.

Figure 17. Schematic of One Zone Model with Exterior Wall length and Window Areas.

Full Building – 1 Zone model

Table 5. Compilation of Surface Information for the One Zone Model

The column headings in the previous table have the following meanings:

Type: A shortened notation for the surface type in EnergyPlus to differentiate between heat storage surfaces and various types of heat transfer surfaces.

Construction: A name for the surface construction types.

Length: The length of base surfaces (i.e. Exterior Walls).

Area: The area of subsurfaces (windows), roofs, floors.