Numerical simulation of thermal mass enhanced envelopes for office buildings in subtropical climate zones

Abstract

The morning power overloading for office cooling is often the aftereffect of not using air conditioning on hot summer nights. Integration of phase change material (PCM) into the office building envelope can be a solution, but, prior to usage, the PCM layer has to be optimized to deal with a number of alternating boundary conditions. The diurnal alternation of the heat load level facing the building envelope is very critical in predicting PCM behavior and cooling energy consumption in a real PCM integrated office building in Hong Kong.EnergyPlus numerical simulation of a PCM layer with specific properties and encased in between two layers of a building envelope was adopted to assess PCM efficiency in relation to its placement, thickness and orientation to the sun, causing moderate and elevated heat loads. The optimized PCM envelope with windows was simulated as a part of a realistic office building model to estimate the air conditioning power balancing on the hottest days and during the entire hot season.The results show that the considered PCM layer should be placed at the inner building envelope part, i.e., close to the office interior. The optimization stage revealed the negative effect of the excessive PCM thermal mass bringing indoor overheating in certain combinations of diurnal heat loads. This is an outcome of not using air conditioning at nights and on Sundays. In the realistic office building model, the negative PCM effect diminishes, though the PCM reduces the morning peak power by up to about 5%. The PCM impact is clearly shown in the comprehensive results for the entire cooling season, where effective smoothing of the AC power demand is indicated. The PCM usage, however, considerably prolongs the AC operation in a narrow power mode (200–244kW) that can aid the chiller selection.The study demonstrates that the morning peak power can be effectively managed via the PCM optimized building envelope in subtropical climate zones. For an office building, a combined window and building envelope with PCM can be an excellent alternative to fully glazed curtain walls.