Heat transfer through window frames in EnergyPlus: model evaluation and improvement

Abstract

Window frames can significantly affect the energy demand of buildings. Due to their complex non-planar geometry, two-dimensional, time-consuming simulations are necessary to estimate with precision heat transfer through frames. However, in dynamic heat transfer calculation through computer-based building performance simulation tools, simplifications are made to reduce simulation time. In particular, EnergyPlus models window frames as rectangular profiles, and uses a one-dimensional heat transfer model. In order to evaluate whether this simplification is legitimate, thermal transmittance values were calculated for a selection of frames, on one hand through two-dimensional finite elements simulation, and on the other hand, through a spreadsheet that reproduces the steps of the algorithm currently used by EnergyPlus to calculate heat transfer through window frames. The comparison of both values showed relative errors higher than 20%, including for rectangular frames (without simplifying geometry). Aluminium frames without thermal break exhibited the highest absolute errors, up to 2.98 W/m2K. An alternative algorithm was proposed, which uses compactness factors for film coefficient calculation and reduced emissivities to take into account self-viewing surfaces. The proposed model improved the estimates, reducing the absolute error to less than 0.10 W/m2K in 52% of the occurrences, and to less than 0.47 W/m2K for aluminium windows.