Optimising phase change material use for energy-efficient buildings

: (Alternative Format Thesis)

Abstract

Energy efficient buildings are required to reduce anthropomorphic derived carbon emissions. The development of energy efficient materials to help minimise these emissions is essential. A potential contribution to meeting carbon reduction targets in the building sector is the use of phase change materials (PCMs). PCMs capture and release thermal energy when changing from one phase to another, storing heat gains as latent heat within a specific temperature range, allowing them to control temperature fluctuations in buildings.

This study investigates the effects on the mechanical, physical and thermal properties of different construction materials when incorporated with PCMs. The design and manufacture of novel PCM incorporated lightweight aggregate (LWA) granules was conducted. This is the first time aerated concrete granules (ACG) are used as a carrier for PCMs. The PCM impregnation process was optimised to incorporate the maximum amount of PCM within the PCM-LWA composite referred to as a PCM loaded granule. Three commercial PCMs, with melting points between 18 °C and 25 °C, were used in this study. Methods, including vacuum impregnation and immersion, and conditions for optimal absorption of the PCMs into ACG, were developed and evaluated experimentally. Compared to immersion, vacuum impregnation resulted in an increased absorption by up to 15.6 % depending on the type and viscosity of the PCMs. Different Coating materials were analysed to reduce the amount of PCM leaking from the pores of the LWA when in its liquid form were trialled as part of this study. A combination of CEM I 52.5 R powder and sodium silicate was found to be the most effective solution.

Cement mortar and gypsum plaster were selected as the materials for the PCM loaded granules to be incorporated into, due to the wide use of these materials within buildings. The focus of this work was to enhance the thermal properties of these materials without significantly compromising the mechanical or physical properties. The impregnated aggregates were incorporated into cement mortar mixes, at replacement rates between 10% and 50% (by volume). The effects of PCM impregnated particles on both the mechanical and thermal properties of the cement mortars have been evaluated. Increasing the amount of PCM loaded particles decreased both the flexural and compressive strengths of the cement mortars, by up to 38% and 49% respectively. However, the volumetric heat capacity of the materials increased by nearly 60%. PCM loaded granules were also incorporated into gypsum plaster mixes, at replacement percentages between 10% and 50% (by volume). The amount of PCM-loaded granules directly affected the density and mechanical properties of the material. The optimum replacement of PCM-loaded granules in gypsum plaster materials for maximum strength was 20%

A simulated building model using DesignBuilder was also developed to analyse the heat performance of the PCM gypsum and PCM cement materials. Analysis of the energy consumption through simulation showed both PCM gypsum and PCM cement can be used to reduce the energy consumption of new and retrofitted buildings. The maximum energy savings of 293 kWh were achieved when PCMs granules were incorporated to cement blocks. However, maximum thermal comfort was achieved with PCM gypsum plaster.

As PCMs will undergo many phase transitions throughout their normal life cycle, the effects of thermal cycling on their long-term stability and performance are important considerations in selection. The limited understanding on the long-term stability and potential for degradation of PCMs has restricted wider use of these materials in the construction sector.

This study is the first to present the long-term performance of paraffin PCM loaded particles in gypsum materials. Specimens were subjected to 700 accelerated thermal cycles using an environmental chamber. After cycling experimental results revealed a change of enthalpy in the endothermic and exothermic reactions by up to 23% for the pure PCMs. However, once the PCM had been incorporated into either the gypsum plaster or cement mortars, there was no significant reduction in the thermal conductivity or the specific heat capacity of these materials. Thermal cycling did not decrease the effectiveness of PCM composites, and so increasing their potential for wider acceptance and use by the construction industry.

The findings of this research show that PCMs can be used effectively to make building more energy efficient. The studies presented demonstrate that although the mechanical performance of construction materials will be affected, the thermal properties of the material will be enhanced. The optimum location to incorporate PCMs within the building for thermal comfort was identified as the most internal part of the wall. This research has contributed to knowledge by identifying and analysing materials and methods to optimise the use of PCMs for buildings to make them more energy efficient.

Date of Award	22 May 2024
Original language	English
Awarding Institution	University of Bath
Supervisor	Veronica Ferrandiz-Mas (Supervisor), Peter Walker (Supervisor) & Nick McCullen (Supervisor)