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COMIS-TRNSYS
Application example
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Air Quality and Thermal Comfort in a Naturally Ventilated
School Building with Glazed Double Facade
Purpose of the study
The building
Approach: Ventilation and thermal model
Simulation results: Air flow rates and indoor
air temperatures
Thermal comfort evaluation
Cross contamination risk evaluation
Conclusions
References
Purpose of the study
For a four-storey building of the ZTL engineering school (figure
1) located close to Lucerne in central Switzerland a retrofit is planned
and respective concepts have been worked out. One proposal is a glazed
double face façade, built up over the original structure which remains
practically unchanged. This approach is effective in respect of construction
costs and ecological aspects. Figure 2 shows one of the proposed constructions.
While the potential for reduced transmission losses in winter time
is quite obvious, more concern was related to the overheating risk in summer
and thus to the possibilities of cooling and ventilating the building satisfactorily.
The simulation study should give answers to the following questions:
1. Overheating risk under summer conditions
2. Temperature reduction potential using passive cooling by natural
night-time ventilation
3. Evaluate different ventilation opening control strategies in terms
of compliance with thermal comfort requirements.
4. Check cross-contamination risk from one class-room to another.
The building
Figure 1: The existing building
Figure 2: Sketch of the retrofit proposal evaluated by simulations.
On the side of a room, the double face façade spaces are open in
a vertical direction, acting thus as a ventilation shaft. In the middle
section, the original window is removed and replaced by a window in the
outer face of the façade.
Approach: Ventilation and thermal model
Due to the thermally driven air exchange and the building masses involved,
the problem must be studied using dynamic thermal building model with an
integrated ventilation model.
The study is performed for a representative section of the building
on the southern side. A network model was established for COMIS, considering
doors, the openings to the double facade space, the windoes and the openings
at the bottom and the top the double facade shaft. As a conservative approach,
wind effects are neglected.
The thermal behaviour is modelled with the TRNSYS multizone type, considering
the classrooms, the staircase and the double facade zones, similarly to
the ventilation model.
Figure 3: Sketch of the airflow network with the zonal nodes and
the air flow conductances.
Simulation results
Figure 4: Temperatures and air flow rates to and from the 2nd
floor room for a warm summer three day period. For better understanding
of the complex and transient interaction of air temperature and air flow
distribution, opening schedules for the different openings are also given.
The bottom staircase opening is opened as soon as the top opening is closed
(and vice-versa).
The specific airflow is expressed as volume flow per hour through
the opening divided by the room volume of the 2nd floor. A positive value
represents a flow into the room.
Thermal comfort evaluation
Figure 5: Room air temperatures in the occupied zone.
Plot according to the Swiss standard SIA 382/2
Cross contamination risk evaluation
Figure 6: CO2 levels for the four class rooms with window
airing just during the breaks
Conclusions
Passive stack ventilation with a double face façade does not work
all the time. At the uppermost floor(s), air from the double façade
space may enter the room, leading both to overheating and low air quality.
This may only be solved by raising the level of the neutral pressure plane,
e.g. by increasing the height of the shaft above the building roof level.
References
Case study retrofit ZTL-building, Luzern (in German), Zentralschweizerisches
Technikum Luzern (ZTL), Switzerland, 1995
V. Dorer and A. Weber: Air, contaminant and heat transport models: Integration
and application, Energy and Buildings, 1998
This page was last changed on 28. Mai 1998
Viktor Dorer (viktor.dorer@empa.ch)