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Energy simulation tools for buildings: Improving airflow parameterizations using CFD

This study found a flaw in the popular EnergyPlus software for estimating airflow through open windows in naturally ventilated buildings. And, using computational fluid dynamics, the project developed a fix to the problem.
Night purge timescale
Year Started: 

Researchers developed a building energy model using the current best practices in computational fluid dynamics, coupled with heat transfer information from an existing energy simulation tool. The project also applied the tool to Stanford’s Y2E2 building’s nighttime air purge, which was underperforming expectations. Initially, researchers found that cost savings for cooling Y2E2 could be increased from 15 percent to 20 percent savings by modifying the night purge algorithm.

Ultimately the study found that detailed computational fluid dynamics simulations can provide a flexible tool to parameterize geometry and flow-specific coefficients, specifically those for discharge though openings. This study found that the popular EnergyPlus software used a formula to calculate the effective area of pivoted windows that tended to underestimate the flow rate through such openings by up to 50 percent. A modification to this expression was proposed that takes into account the airflow through the sides as well as the bottom of an open, horizontally-pivoted window. Both computational fluid dynamics results and data from window manufacturers agree well with the modified formula.

The work on the Y2E2 building continued under a 2013 PEEEC seed grant.

Publication: Using CFD simulations to improve the modeling of window discharge coefficients Fifth National Conference of IBPSA-USA Hult, E., Iaccarino, G., Fischer, M. (2012)

Project update presentation: Improving airflow parameterizations within energy simulation using CFD (797KB PDF)