In order to respond to global warming and natural resources depletion challenges, industrials from the building sector need to propose an adequate offer. Energy simulation tools can support this process. In order to reach high performance level, e.g. primary energy consumption below 50 kWh.m-² per year (including heating, cooling, domestic hot water, lighting and ventilation), various studies and real cases show that, appropriate architecture, high insulation, free cooling and the use of a heat recovery exchanger for ventilation are needed. This last technology will be particularly affected by airflows across the building envelope caused by a low airtightness. Moreover, free cooling ventilation rate will highly depend on temperature difference between outside and inside. Thermal modelling tools need therefore to deal with those two issues precisely.
A multizone model has been developed to compute building airflows in order to evaluate them with a higher degree of precision in the frame of a simplified simulation tool that can be used in early phases of a project. This model is based on well-mixed zones and mass conservation principles. The air flow rate between two zones is expressed as a function of the pressure drop between those two zones. Wind pressure and buoyancy effects are the causes of pressure drops. Several types of connection are implemented: cracks, ventilation inlets, large
openings. More types of connection will be added.
This model has been implemented in the thermal building simulation tool COMFIE. The airflow model uses the temperatures of the zones as an entry and the thermal model uses the airflows as an entry as well. Both thermal and airflow model run at each time step until convergence is reached using a synchronous coupling method. An algorithm has been developed to ensure the convergence for each time step (from 1/10 to 1 hour).
Two case studies are presented. First, the case of a residential building, project of Vinci Construction France where the influence of airtightness on heating loads is being studied. Then the case of a concept building, Effibat, being developed by Vinci Construction France and MINES ParisTech. This building is an urban dwelling building including an atrium. Natural ventilation is used to cool the building at night in summer and the model aims at
evaluating the resulting comfort level.