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Heat Transfer in Buildings HTB2
HTB2 is a computer program designed to simulate the thermal performance of buildings. Form, fabric, climate, occupants’ activities, and ventilation rates govern the heat transfer in buildings. HTB2 simulates all these factors and will predict the buildings thermal performance and can thus help in the assessment and evaluation of a building’s indoor environment.
A building is a series of spaces linked together by walls and to the outdoor by windows and doors with isolated ventilation paths. It is exposed to thermal loads externally and internally which will generate heat fluxes and air movement in each space. Buildings usually have a complex set of interactions and thermal behaviour, as represented in Figure 8.1. HTB2 uses a ‘finite difference’ technique which divides time into intervals and assumes that in each time interval, the heat transport mechanisms remain constant and independent, as shown in Figure 8.2 (1). In each time interval HTB2 can predict the following:
1. new space temperatures;
2. radiative transfers between internal surfaces;
3. an update of external conditions at the time determined by the met data;
4. radiative flux due to insolation;
5. fabric heat transport;
6. response of controls and heating systems;
7. ventilation heat and moisture transport;
8. incidental gains of heat and moisture;
9. refer to building operation schedule and modify parameters as required;
10.
output
data as required.
Figure 8.1: Fundamental building processes and interactions (1)
Figure 8.2: Partitioning of time and processes by HTB2 (1)
For HTB2 to operate and simulate the required thermal performance of a building, data input is required at three levels:
1. top level file, which contains the necessary information for the simulation, such as, length of time step, the subsystem to be used such as the ventilation and the data file name and the required output;
2. the second level files contain general data about the building, type of services, the daily events (diary information) and meteorological data;
3. the third level files contain the problem definition data, for wall constructions and materials, heating and ventilation, and any other parameters used in the simulation.
Figure 8.3 shows the associated files with that input to HTB2 in a hierarchical order showing firstly the top level then the second then the third.
Figure 8.3: Files associated with the simulation in HTB2 (1)
Running HTB2 the model must consist of the following stages:
1. Defining the building through the scheme, which is a thermal network, viewed by HTB2. It will define the building file and the layout file and will be joined by the construction file, which will contain the material library. The second and third level files are also defined.
2. The second stage is running the model and getting the results and the report files.
3. The third stage is viewing the data through spreadsheet programs.