Numerical procedure for predicting annual energy consumption of the under-floor air distribution system

As compared with the mixing system, indoor air temperature stratification in the under-floor air distribution (UFAD) system offers an opportunity for cooling load reduction in the occupied zone. This stratification is a major feature that offers the energy saving potential, but it has not been thoroughly taken into account in most energy simulation programs. In this article, a numerical procedure, based on coupling two types of modeling, i.e., CFD (computational fluid dynamic) simulation and dynamic cooling load simulation, is proposed to predict annual energy consumption. The dimensionless temperature coefficient is first defined in the UFAD system and obtained from CFD simulation, based on the boundary conditions from a cooling load program ACCURACY. According to this coefficient, temperature stratification input to ACCURACY is then revised to calculate the updated supply and exhaust air temperatures for final annual energy prediction. To demonstrate the method, a small office room is investigated using Hong Kong weather data. With the constant air volume (CAV) supply in the UFAD system, it is found that the dimensionless temperature coefficient is almost a constant, when the locations of heat sources are fixed. As compared with the mixing system, the UFAD system derives its energy saving potential from the following three factors: an extended free cooling time, a reduced ventilation load, and increased coefficients of performance (COP) for chillers.