Diurnal load reduction through phase-change building components

Phase-change material (PCM) can be added to building envelope components, such as walls or roofs, to reduce peak diurnal cooling and heating loads transmitted through the envelope. This paper describes a simulation study to quantify the effectiveness ofPCMs at reducing thermal loads through building envelope components and to develop a design strategy for the placement of PCMs within building components. The PCMstudied is the paraffin octadecane, with an average melt/ freeze temperature of 25.6°C (78.1 °F). Thermal loads through PCM concrete sandwich walls, frame walls, and steel roofs were simulated using an explicit finite-difference procedure with the indoor air temperature held constant and with two sets of outdoor boundary conditions: a single day in which the outdoor air temperature varied sinusoidally and a year of typical meteorological data from Dayton, Ohio. The simulation technique has been validated in previous experimental and simulation work. The results indicate that the addition of PCM to building components measurably decreases both peak and annual cooling loads, even through high-mass building components such as concrete walls. Moreover, the load reduction ability of a building component with PCM is strongly related to the placement of the PCM within the building component. To understand this effect, diurnal thermal loads through a 10-node wall were simulated, while the location of both sensible and phase-change thermal mass was varied. Based on these results, a design strategy for locating PCM within building components to minimize thermal load transmission is proposed. The strategy matches the PCM melt/freeze temperature to the average temperature in the component and favors interior locations where the diurnal temperature swings are smaller than the melt/freeze temperature range of the PCMs.