Use of wind pressure coefficients to simulate natural ventilation and building energy for isolated and surrounded buildings

Abstract

Wind pressure coefficients (Cp) are critical inputs to building energy simulations. Given differences in the free stream wind speed height two categories exist: (1) Cpr (reference height) and (2) Cpl (local opening height). Additionally, Cp data are influenced by the vertical wind profile which is modified by surrounding buildings. However, these dependencies are often overlooked in building energy simulations (BES). We identified three potential biases from the incorrect use of Cp: (1) Cpr is used alongside the wind speed at opening height rather than reference height (where Cpr is defined); (2) Cpr is used with wind profiles that are different from the wind tunnel experiment or CFD simulation used to derive Cpr; and, (3) Cp is used along with the ‘disturbed’ urbanised wind speed instead of the ‘undisturbed’ free stream wind speed. In this study, we quantify the resulting biases from using Cp data incorrectly by assessing impacts on the resulting ventilation rate, indoor overheating risks and cooling energy saving with EnergyPlus for Shanghai's climate. Modifications to the use of Cp are proposed to improve simulation accuracy. Results show biases mostly exceeding the ±10% limit of ASHRAE-14 in all scenarios analysed. Differences are up to −19.0% for natural ventilation rate, 13.2% for indoor overheating degree hours and −14.0% for cooling energy saving, with such errors being larger during heatwave periods. Our study could provide useful guidance for researchers to carry out wind-driven natural ventilation study and estimate indoor overheating risk and energy consumption with better accuracy.