Thermal building-dynamics simulation can be run basically under two different conditions:
• Zone temperature control: In this case the indoor temperature is controlled to specified temperature levels. These levels can be according to temperature or other schedules, can depend on other parameters, or as the simplest case, can be constant.
• Free floating: In this case the indoor temperature is left free to fluctuate without any mechanical cooling device.
HVAC System Control
Many HVAC system engineering problems focus on the operation and the control of the system. In many cases, the optimization of the system’s control and operation is the objective of the simulation. Therefore, the appropriate modeling of the controllers and the selected control strategies are of crucial importance in the simulation. Once the system is correctly set up, the use of simulation tools is very helpful when dealing with such problems. Dynamic system operation is often approximated by series of quasi-steady-state operating conditions, provided that the time step of the simulation is large compared to the dynamic response time of the HVAC equipment. However, for dynamic systems and plant simulation and, most important, for the realistic simulation
Of control systems, the 1-h time step, normally required when dealing with thermal building-dynamics simulation, is much too large. In these cases, the HVAC system has to be modeled separately with a much smaller time step, using the results of the dynamic building simulation as boundary conditions.
I 220.127.116.11 Limitations
Several limitations stem from the methods just outlined. The most important ones are summarized below; others have already been mentioned in the text,
• Perfect mixing of the room air is assumed in the individual zone, except in room models with more than one air node (displacement ventilation model, atria model).
• Ventilation pattern (plumes, etc.) can be considered only in a very limited way.
• Conduction in walls is often modelled one-dimensionally.
• Most room models are nongeometric, with the radiation exchange between surfaces being calculated solely on an area-weighted basis.
This has to be considered when splitting walls or floors into several elements, because in such models, these elements have a direct radiative exchange, while in reality there is no exchange or only an indirect one via opposite surfaces.
• Solar radiation is distributed in the first room and not transmitted into adjacent rooms through an internal window (more sophisticated modeling may allow for this).
• Radiation from internal heat sources is not directly considered in thermal comfort calculations.
• HVAC components are mostly modeled in a steady state. However, the normally used time step of 1-h in the simulation is too large for the simulation of control processes.
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