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System modeling: Natural Ventilation


The purpose of the study is to model effect of window opening on the cooling and ventilation energy requirements, and on the thermal comfort for different opening configurations and control strategies. To study the impact of natural ventilation on the IW indoor conditions, analysis tools must be able to integrate thermal modeling with ventilation modeling. This study has been performed with coupling a thermal simulation tool (TRNSYS) to multizone air flow simulation software (COMIS). COMIS (Conjunction Of Multizone Infiltration Specialists) has been developed for the IEA Annex 23, and is maintained by CSTB, EMPA, LBNL.


The TRNSYS / COMIS  model calculates, for any given control strategy, the effect of window opening on the cooling and ventilation energy requirements and on the thermal comfort in the IW.  Different strategies have studied based on temperature or temperature and humidity criteria to meet the space requirements and to improve the thermal comfort in the bays.
The study was performed for 2 bays in IW (North and South), see figure below. The purpose of modeling the two zones is to determine whether the length of the zone (specially the north one) is not too large to induce a sufficient cross ventilation.
natural ventilation in iw 
The climate in Pittsburgh and the situation in the IW is not ideal for natural ventilation with complimentary nighttime cooling due primarily to the low thermal mass of the IW.
Nevertheless, hybrid ventilation in the IW can reduce sensible and latent loads for the building and for its ventilation while maintaining comfortable indoor conditions. Energy saving of 14%, about 1,000 kWh per IWs bay, each summer can be achieved by the system while maintaining the thermal comfort during 84% of the occupied time.
Additional information is enclosed in the following documents:
- Presentation Natural Ventilation (pdf): Impact of the natural ventilation in a bay of the IW on cooling and ventilation energy requirements for a typical summer in Pittsburgh, October 4th 2006, Sophie Masson, Elisabeth Aslanian, Berangere Lartigue 
-Bloc diagram Natural Ventilation: description of the information flows between components in the TRNSYS simulation
Next Steps
The existing TRNSYS / COMIS model will be updated: the bay will have additional operable windows (transom) and operable ridge vents. The new model will be used to assess the impact of enhancing the use of the stack effect on the cooling and ventilation requirements and thermal comfort. 
Additional features will be implemented, to improve the controller, on:
  • Outside conditions:
  • Control on rain: window closed when it’s raining
  • Control on wind speed: wind closed when the wind speed is greater than a threshold
  • Control on outside temperature: The current control logic compares the outside air temperature with a lower limit. No upper limit control exists for outside temperature for window opening.  In a hot and dry climate, the control on the outside air dew point temperature will not be sufficient to limit outside air admission at high temperature. The model needs to be generalized.
  • Air change rate: Control the window and vent opening (between 0 and 100%: open, partially open, close) to avoid draft in the IW

A more detailed component for the Semco unit (based on look up table) will be integrated in the TRNSYS simulation in order to simulate more accurately the energy consumption of this ventilation unit (electricity, gas to regenerate the wheel). The model will be exercised for different thresholds and different simulation time step in order to assess the optimum control based on energy consumption (fan coils and Semco unit) and thermal comfort.


Sophie Masson, Elisabeth Aslanian, Berangere Lartigue,