Microencapsulated phase change composite materials for energy efficient buildings
This study aims to elucidate how phase change material (PCM)-composite materials can be leveraged to reduce the energy consumption of buildings and to provide cost savings to ratepayers. Phase change materials (PCMs) can store thermal energy in the form of latent heat when subjected to temperatures exceeding their melting point by undergoing a phase transition from solid to liquid state. Reversibly, PCMs can release this thermal energy when the system temperature falls below their solidification point. The goal in implementing composite PCM walls is to significantly reduce and time-shift the maximum thermal load on the building in order to reduce and smooth out the electricity demand for heating and cooling. This Ph.D. thesis aims to develop a set of thermal design methods and tools for exploring the use of PCM-composite building envelopes and for providing design rules for their practical implementation.
First, detailed numerical simulations were used to show that the effective thermal conductivity of core-shell-matrix composites depended only on the volume fraction and thermal conductivity of the constituent materials. The effective medium approximation reported by Felske (2004) was in very good agreement with numerical predictions of the effective thermal conductivity. Second, a carefully validated transient thermal model was used to simulate microencapsulated PCM-composite walls subjected to diurnal or annual outdoor temperature and solar radiation flux. It was established that adding microencapsulated PCM to concrete walls both substantially reduced and delayed the thermal load on the building. Several design rules were established, most notably, (i) increasing the volume fraction of microencapsulated PCM within the wall increases the energy savings but at the potential expense of mechanical properties , (ii) the phase change temperature leading to the maximum energy and cost savings should equal the desired indoor temperature regardless of the climate conditions, (iii) microencapsulated PCM-concrete walls have the best energetic performance in climates where the outdoor temperature oscillates around the desired indoor temperature, (iv) microencapsulated PCM offers the largest energy and cost savings when embedded in South- and West-facing walls and during the summer months in San Francisco and Los Angeles, CA.
Third, a novel experimental method was developed to rapidly quantitatively characterize the thermal performance and potential energy savings of composite materials containing phase change materials (PCM) based on a figure of merit termed the energy indicator (EI). The method featured (i) commonly used specimen geometry, (ii) straightforward experimental implementation, and (iii) sensitivity to relevant design parameters including PCM volume fraction, enthalpy of phase change, composite effective thermal conductivity, and specimen dimensions.
Finally, the widely-used admittance method was extended to account for the effects of phase change on the thermal load passing through PCM-composite building walls subjected to realistic outdoor temperature and solar radiation flux. The speed and simplicity of the admittance method could facilitate the design and evaluation of the energy benefits of PCM-composite walls through user-friendly design software for a wide range of users.