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, twodimensional, 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/m 2 K. 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/m 2 K in 52% of the occurrences, and to less than 0.47 W/ m 2 K for aluminium windows.

Energy Evaluation and Energy Savings Analysis with the 2 Selection of AC Systems in an Educational Building

“…Buildings have a significant 40% share of thetotal energy use worldwide. Since the buildings' internal environment must be conditioned to human comfort, a considerable amount of its energy consumption, approximately 55%, is spent on heating, ventilation, AC, and refrigeration systems [1][2][3]. In Colombia the energy consumption of buildings is an estimated 22.8% of the total [4] but there is a significant energy saving potential estimated at approximately 25% by The Mining and Energy Planning Unit (UPME) [5].…”

“…Chiller demand has increased in recent years due to its high efficiency compared with other AC systems. [1].…”

Multi-objective optimization of school building envelope for two distinct geometric designs in southern Brazil

“…14 Opaque and transparent elements' thermal properties are able to adjust the building's heat gain exchanges. 15,16 For the transparent envelope, the glazing solar heat gain coefficient (SHGC) 17 and the window-to-wall ratio (WWR) 18 can favour the building's heat gains. For the opaque envelope, materials may significantly influence the final thermal conditions.…”