Modelling the effects on building temperatures of phase change materials

Abstract

In this thesis, three problems motivated by the possibility of use of phase change materials in buildings are presented. First, a model for the response of phase change material (PCM) to a varying room temperature is extended, and applied to realistic situations in a building. For a spatially varying distribution of PCM through a wall, an equation estimating the appropriate depth for PCM inclusion is found. For cooling at the exterior surface of the wall, a wall thickness and a PCM mass fraction that give a low maximum daily interior surface temperature, without redundant PCM, are identified. The hot upper layer for a displacement ventilated space is cooler but deeper when a PCM ceiling interacting with the exhaust from an air heat pump is used. During hot weather, the maximum daily interior surface temperatures are reduced when using two PCMs with different, appropriate, melting ranges. In each extension, PCM reduces temperature fluctuations substantially when used in appropriate quantities and locations. Second, experimental results are presented, demonstrating that, for a buoyancy source vertically distributed over a full wall, detrainment qualitatively changes the shape of the ambient buoyancy profile in a sealed space. Theoretical models with one-way entrainment predict stratifications that are qualitatively different from the stratifications measured in experiments. A peeling plume model, where density and vertical velocity vary linearly across the plume, so that plume fluid “peels” off into the ambient at intermediate heights, more accurately captures the shape of the ambient buoyancy profiles measured in experiments than a one-way-entrainment model does. For a half wall source, however, detrainment is not observed in the experiments, and a one-way-entrainment model is appropriate. Third, experimental results are presented for a time-varying, vertically distributed buoy- ancy source. For a source whose buoyancy flux decreases linearly with time, the peeling plume model is appropriate, as it was for a constant flux source. For a source providing a buoyancy flux that increases linearly with time, however, the one-way-entrainment model is the most appropriate model for predicting the stratification that develops in a sealed space. A PCM wall solidifying provides a buoyancy source which decreases linearly with time, then is constant, then again decreases linearly with time. Experiments show that, in a space with such a buoyancy source, the first front moves quickly through the space, so the whole space x is stratified for most of the experiment. The shape of the stratification changes only slightly after this point, with heat being added uniformly throughout the space, so that the ambient buoyancy profile simply translates, with little change in its shape.