Authors: Chih-Hong Huang, I-Yang Lee, Yu-Lun Chien and Hsin-Yu Tsai
The cycle time in which building envelopes accumulate and release heat is affected by microclimatic conditions such as the strength of solar radiation, outside air temperature fluctuations, wind speed and air temperature, all of which thereby complicate heat transfer on such surfaces and turn it into an unstable state. Delayed heat release from such surfaces is the main reason for the prolonged high temperatures in cities throughout the summer. The purpose of this study is therefore to propose a cost effective and efficient strategy for improving envelope shading performance in subtropical countries that address the characteristics of heat changes in different time periods by discussing an equation for overall heat balance in one stand-alone building and using it to calculate how heat budget evolves on the building envelope throughout the day in the example of perforated plates for shading the building exterior.
In this study, actual measured values of weather conditions were entered into the CFD software without considering evapotranspiration to examine changes in daily energy budgets on the wall surface of the building mass with shading at a distance of 30 cm and in densities of 100%, 80%, 60%, 40%, 20% and 0% and analyze how factors such as solar radiation shading rates on the wall surfaces and microclimate (including wind speed and air temperature on the wall surface) in the wind field, which are affected cover densities, contribute to heat dissipation from the building masses from the perspective of heat balance.
As air temperatures remain high in summer, the conditions in which heat is dissipated are harsher and increase the time of thermal hysteresis impact. Based on the results from the present standard scenario, it is suggested that attention should be focused on controlling the influence of outside air on building envelopes when exterior shading in the 100% cover density is adopted to prevent the negative impact of thermal hysteresis. The adoption of different cover densities or the 20% cover density in particular is most likely to increase load on indoor air conditioning.
Keywords: passive heat dissipation; thermal hysteresis; sunshade strategy; Energy balance; Unstable heat transfer; Convection heat transfer; CFD simulation