Evaluation of wind-driven ventilation in building energy simulation: sensitivity to pressure coefficients

Building Energy Simulation (BES) tools integrate wind-driven ventilation in buildings either by imposing the airflow rate in a Zone-Airflow module or by calculating it through Airflow Network (AFN) models. When the AFN models are used, pressure coefficients are crucial for obtaining accurate results. This is particularly important in case of complex geometries or buildings with large openings, when the uncertainties related with the use of simplified models are higher and can affect the results. This paper describes a preliminary study on the effects of the pressure coefficients on the BES simulations of a cross-ventilated building with large openings. The single-zone building has a high thermal mass and is subjected to wind-induced night ventilation. Simulations are performed with the Zone-Airflow module and the AFN model within the code EnergyPlus. The airflow rate imposed in the Zone-Airflow module is obtained by performing Computational Fluid Dynamics (CFD) simulations of the opened-building (i.e. the building with ventilation openings). The pressure coefficients used for the AFN simulations are obtained in five different ways: (i) the surface-averaged calculation in EnergyPlus, (ii) the Air Ventilation and Infiltration centre (AIVC) database, (iii) the Tokyo Polytechnic University (TPU) database, (iv) wind-tunnel measurements and (v) CFD simulations performed on a closed (sealed) building (i.e. the building with ventilation openings closed). Results are compared in terms of airflow rate and indoor air temperature during a Design Day characterized by a constant wind speed. Differences in airflow rates are up to the 15% among the values predicted with the AFN models and increase up to the 24% when the results of the Zone-Airflow module are compared with the ones obtained from the AFN model simulations. For the case under study, the airflow rates are very high and the indoor air temperatures are not affected by the variation of the pressure coefficients. The results indicate the importance of the pressure coefficients in predicting the airflow rates through large openings. Other ranges of airflow rates might be investigated to see the effects on the indoor air temperature for more realistic building configurations