22.214.171.124 Building Modelization
Building modelization is the process by which the program’s user represents a building as a number of thermal zones, separated by walls. HVAC systems are connected to these zones. Abstraction is necessary for the definition of the zones and the buildup of walls, and those components available in the code which match the reality best must be selected.
The user must have experience in thermal analysis and HVAC dimensioning and be familiar with the theoretical principles and details upon which such analysis is based in the program used. Engineering judgment will have to be used for the definition of the input parameters.
The information required to run a thermal building-dynamics simulation can be classified on the base of the following items:
• Outdoor climate
• HVAC system
• Internal heat gains in conditioned spaces (casual gains)
• Operation and use of building
In order to run a dynamic simulation, weather data files are needed, providing (usually hourly) data on the following quantities: air temperature, air humidity, ground temperature, global and diffuse irradiation on horizontal surfaces, and wind speed and direction.
Location and Orientation
In order to specify the building location on the surface of the earth, the values of latitude, longitude, and altitude of the site are required. The orientation of the building can be specified as an azimuth angle between the main axis of the building and the north (or south) direction.
Neighbor Buildings and Shading Devices
Position, size, and orientation of external objects must be specified in order to take into account both the shade cast on the exterior of the building and the modification of wind conditions. External objects are nearby buildings, trees, hills, or attached objects, such as overhangs.
Identification of Zones
The identification of zones can be made in different degrees of detail, depending on the specific purpose of thermal building-dynamics simulation. In most cases a zone can include several rooms having the same thermal conditions. Sometimes it is necessary to split a very large room into two or more thermal zones in order to have more accurate results.
• The size (volume, floor area) of each zone has to be specified.
• The envelope characteristics of each zone are to be specified as described later.
• The thermal behavior of a zone is affected by the presence of internal heat sources and by HVAC operating features, which are dealt in the next sections.
The opaque elements of the building are exterior and interior walls, roofs, interior floors, exterior and interior doors, and underground walls and floors, which are all referred to as Walls in this text.
Thermal characteristics of material layers for each type of wall must be specified, including thickness, conductivity, density, and specific heat. Moreover, the features of internal and external surfaces of each wall must be specified, including solar absorptance and roughness, which affect surface heat transfer coefficients.
For the individual wall, the wall element type, size, position, and connections to adjacent zones must be specified.
Both thermal and optical characteristics of each type of window must be specified, including the dependence on the incidence angle. These characteristics normally are precalculated using a specific code (see Section 11.3.3, “Heat
Transfer through Windows”) and made available in the simulation code by means of a database.
For the individual window, the window type, size, and position must be specified. For natural ventilation analysis it may be necessary to specify also the air-tightness characteristics of the windows.
CABD and Databases
Links or interfaces to CABD (computer-aided building design) tools have become a common feature of many building simulation tools. Efforts to standardize the building model format significantly contribute to this trend. CABD can be used for the establishment of input data as well as for the checking of input data. Normally, the geometric information from CABD drawings must be reduced quite substantially for the thermal simulation input. On the other hand, besides geometric data, information such as material properties, etc., must be supplied. This information is normally not included in the CABD data set, but links to database entities may be established.
A graphic representation of the building input data by CABD offers an easy way of checking the geometric input data.
I 126.96.36.199 HVAC Systems
HVAC system parameters include information for the sizing of components. First of all, the type of HVAC system must be specified, and the thermal zones served by the system must be identified. Afterward, information must be specified on the following items:
• Airflow into and out of each zone and HVAC system (supply air, exhaust air, outside air)
• Zone temperature control characteristics (set point, type of thermostat, throttling range)
• Supply air conditions (set points, control strategy, limits)
• Characteristics of HVAC components
188.8.131.52 Internal Heat Gains (Casual Gains)
Maximum specific internal gains must be specified for each zone, including gains from people, lighting, machinery, equipment, and other sources. Some detailed models also require splitting sensible heat into convective and radiative fractions. This split is important when defining the loads from lighting or machinery. Data on this split between convective and radiative heat release for typical heat sources can also be found in the ASHRAE Fundamentals Handbook.1? In Table 11.4, measured internal heat gains in industrial premises are presented.14 Loads from electric motors can be established considering the power efficiency. For some heat sources (e. g., people), it is useful to distinguish between sensible and latent fractions of energy supplied. Data may be found in Design Guide Book Fundamentals Chapter 5, or in VDI Standard 2067.15 For loads from lighting and machines, part load factors, use factors and, also in case of multiple sources, simultaneity factors have to be considered.
184.108.40.206 Operation and Use of Building
The use of the building is characterized by schedules for indoor environmental conditions, ventilation airflow rates, internal heat gains (casual gains),
TABLE 11 .4 Internal Heat Gains for Different Types of Industrial Manufacturing
Internal heat gain, total effective
Industry [W/m1 floor]
TOC o "1-5" h z Manufacturing of dairy products 30-60
Manufacturing of bakery products 80-200
Textile, wearing apparel industries 70-230
Manufacturing of furniture 4.5
Printing and publishing 40-120
Manufacture of glass and glass products (furnace department) 500-1000
Manufacture of concrete and concrete products 20-40
Manufacture of metal fabricated products, machinery, and equipment 40-100
Occupancy, and HVAC system operation. For the casual gains of lighting, machinery, and equipment, use factors and load factors have to be considered. In order to take into account temporal variations of building use, daily, weekly, and yearly schedules should be specified for occupancy, for different kinds of casual gains, and for temperature and humidity set points.
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