Jensen, Rasmus Lund
Modelling of Natural Ventilation and Night Cooling - by the Loop Equation Method. Ph.D. Thesis. Department of Building Technology and Structural Engineering, Aalborg University, ISSN 1397-7953 R0402 (262 pages). The thesis is in Danish.
The latest decades’ focusing on energy efficiency and indoor climate has meant a rediscovery of natural ventilation. The architectural style of today and the appearance of the computer have meant that there has been, and still is, a great need of computation models which quickly and accurately are able to compute the consequences by use of natural and hybrid ventilation.
Today more than one third of the primary energy consumption in the industrialized countries goes to heating, cooling and ventilation of buildings (ECBCS 2000).
To encourage natural ventilation and by this process reduce the energy consumption two important barriers are identified:
• Lack of accessible non-expert programs for computation of natural ventilation under realistic conditions
• Uncertainty in proportion to the effect of night cooling
in the light of this the following two problems are drawn up
• Develop an easy to use computer model for computation of natural ventilation and the correlation with the thermal conditions of the building
• Develop a model for computation of the effect of night cooling by natural ventilation
Computation of natural ventilation
When this work started only a few building simulation programs existed which were able to consider both natural ventilation and the thermal properties of the building. This has not changed and still today there are only a few building simulation programs that are capable of combining the two things. In Denmark the thermal simulation program BSim, developed by the Danish Building Research Institute (SBI), is used in most large construction projects. The program is widespread and hereby it constitutes a de facto standard in Denmark.
In the light of this it has been chosen to extend BSim with a multi-zone model for computation of natural ventilation.
A review of the literature for computation of natural ventilation was done intending to draw up a multi-zone model. Two different ways for drawing up an equation system were identified, the Nodal method and the Loop Equation Method.
It soon became apparent that regulation methods for multi-zone models, that make it possible to fulfil the requirements in the individual zones in the building, were much limited. The need of a sturdy and flexible regulation resulted in elaboration of a regulation strategy for the openings.
In view of this a multi-zone model was based on the Loop Equation Method including control strategy implemented in the thermal simulation program BSim.
Computation of night cooling
At the computation of the effect of night cooling primarily simple estimate models or thermal simulation programs are used. However, while the simple models are good at giving an estimate in the beginning of the construction phase they lack accuracy to be used in the design phase. Thermal simulation programs are often used here, but these programs suffer from the use of standard convective heat transfer coefficients for free convection and thus they are not considering the air flow pattern and a local temperature at the surfaces.
Because of this it is chosen to extend BSim with a model for computation of night cooling which considers the local velocity and temperature at the ceiling.
An extensive literature review of measuring of convective heat transfer coefficients for buildings showed that most expressions were built on experiments with small plates. The few full-scale experiments which were carried out showed that results from small plates must be handled cautiously in buildings as the velocity conditions are quite different.
A review of the very few models for determination of a convective heat transfer coefficient at thermal building simulation showed two different approaches.
By one of them knowledge from the flow technique was used, including flow elements to compute the air flow pattern in the room and determine the convective heat transfer coefficient on that basis. This approach made use of the large knowledge that exists of flow conditions in buildings.
The other approach consisted in gathering so many terms for convective heat transfer coefficients for different flow patterns as possible and to make routines to choose the best suitable one. This approach was based on the presence of sufficiently enough and exact expressions for convective heat transfer coefficients.
On the basis of the above-mentioned considerations a simple model based on flow elements was worked out. The principle of the model was to divide the ceiling into two parts – one with free convection and one with forced convection.
Because of the division of the ceiling the energy exchange between air and the ceiling was computed on basis of the local temperature and a convective heat transfer coefficient for the flow type in question. Finally, a fictive convective heat transfer coefficient for the use in the thermal simulation program, which caused the same energy exchange as computed for the two sub-surfaces, was computed.
The model was implemented in BSim.
|Translated title of the contribution||Modelling of Natural Ventilation and Night Cooling - by the Loop Equation Method|
- Multizone airflow model
- Loop equations
- Multizone thermal model
- Convective heat transfer coefficient
- Natural ventilation
- Night cooling
- Full-scale mesurements