THERMAL MASS AND TIME LAG: CALCULATING HEATING AND COOLING FLUX FROM A BUILDING ROOF/WALL

摘要

Exploiting the thermal mass in a building has been shown to save energy in hot/dry climate zones with large diurnal swings. Essentially the mass introduces a time lag into the heat transfer from the exterior to the interior and vice versa. With proper selection of the materials used, net energy consumption can be reduced. With this motivation, an Excel-based simulation tool was developed to calculate the thermal time lag behavior and energy flow of a wall/roof for different climate zones. It is based on a forward finite element method that handles one-dimensional heat transfer through roofs/walls. Specific material parameters including thickness, specific heat, and conductivity can be specified. The tool outputs the thermal lag and the heat flow and interior wall surface temperature over time. An architect could study different wall thicknesses and materials to gauge the wall/roofs performance in terms of total cooling and heating in kilowatt~* hour/meter~2. Current work focuses on applying outside surface boundary conditions that use weather data files (temperature and solar radiation) for specific climate zones across an entire year to determine the behavior of the thermal mass with regards to heat flux and properties of the materials. Ultimately the goal is to determine if there are additional opportunities for passive design using thermal mass in different climate zones through "tuning"' the choice of the wall materials to achieve energy savings and providing a link to building information modeling (BIM) software. The correlation of thermal time lag with HVAC energy consumption supports the former.