Lightweight concrete (LWC) panels are becoming popular in buildings because of being lightweight, which allows easy transportation, handling, and installation. They provide the opportunity for modular construction. They also have insulating properties with thermal conductivity of 0.026-1.0 W/m-K. However, they have low thermal mass, which causes overheating during the heatwave period in Mediterranean and temperate climates. Therefore, Phase Change Materials (PCM) are integrated into LWC panels to increase thermal storage, mitigate overheating, and increase energy efficiency. However, the integration of PCM in LWC panels also increases their thermal conductivity, which is not favorable for a sustainable building. There is a need to reduce the thermal conductivity of PCM-integrated LWC panels. Thus, this study aimed to develop new lightweight heat-resistive and thermal storage panels (HRSPs) using porous fillers and PCM for energy-efficient building applications.
First, the optimum PCM melting point for cool temperate climates of Melbourne was identified through parametric analysis considering a typical Victorian house as a case study. The result showed that the optimum PCM melting point for free-running and air-conditioned houses is 30°C and 25°C, respectively. Based on these findings, capric acid (CA) PCM with a 29-32°C melting point was selected. The integration of PCM in cementitious composites may suffer from leakage issues during mixing with cement and other aggregates. The leaked PCM, such as fatty acids, may acidify concrete and reduce its compressive strength. The traditional leakage test proposed by previous researchers was insufficient to identify the microscopic leakage of PCM and its potential acid attack on concrete. Moreover, the conventional Form Stable PCM (FSPCM) synthesis procedures are energy-intensive, increasing the embodied energy of FSPCM. This study proposed a new FSPCM synthesis procedure to reduce energy use, eliminate acid attacks and increase PCM absorption capacity in porous material for energy-efficient buildings. The proposed method was energy-efficient, with CA absorption of 75% in porous hydrophobic expanded perlite (HEP). However, due to acid attack, the compressive strength and thermal conductivity at 75% CA absorption were lower than one with 60% CA.
Hence, the absorption of PCM should not be the only criterion for developing FSPCM. More indicators should be considered to develop an optimum FSPCM.
This study proposed six indicators, including absorption, thermal conductivity, strength, thermal inertia, latent heat storage, and thermal storage, to select the best porous materials to absorb PCM. American Society for Testing and Materials (ASTM) standards were adopted to measure proposed indicators. A comparative study was conducted to select the best porous materials amongst Silica Aerogel Granules (SAG), Hydrophobic Expanded Perlite (HEP), Nano-clay (NC), Recycled Expanded Glass (REG), and Silica Fume (SF) to absorb CA and develop FSPCM. In this study, the FSPCM-integrated concrete panels were named thermal energy storage panels (TESP). The comparative analysis revealed that SAG-based TESP was meeting all five indicators accept compressive strength (3.66 MPa), which was lower than the
minimum compressive strength (4.14 MPa) criteria for non-structural applications. However, HEP-based TESP had acceptable compressive strength and thermal conductivity with the second-best thermal inertia and heat storage, making it a suitable porous material for absorbing polar PCM for buildings. However, the thermal conductivity of TESPs was still higher than LWC because of the higher thermal conductivity of PCM and concrete, although the TESPs have higher thermal storage. Thus, there is a need to develop a TESP with high latent heat storage, low thermal conductivity, and acceptable mechanical properties.
To reduce the thermal conductivity of TESP, sand was volumetrically replaced with SAG to prepare Heat Resistive and Storage Panels (HRSP) using the proposed particle-density-based approach. The developed SAG-based HRSP had lower thermal conductivity than TESP with similar thermal storage. Although HRSP had higher thermal conductivity than SAG-based LWC, it resulted in higher energy savings (9%), emission (24%), and comfort than SAG-based LWC because of higher thermal inertia and storage. Moreover, the HRSPs had lower embodied energy and carbon than SAG-based LWC. However, the SAG-based HRSP still had higher embodied energy than normal concrete panels. Consequently, the SAG was replaced entirely with REG particles to develop eco-friendly HRSP for buildings. Results revealed that REG based HRSP had 27% lower thermal storage then SAG-based HRSP due to high thermal conductivity and slightly lower latent heat storage. The compressive strength of REG-based HRSP (17.77 MPa) was very close to the minimum compressive strength for the structural application of concrete. Applying REG-based HRSP in a building envelope had slightly higher discomfort hours than SAG-based HRSP, and it also reduced annual energy use and CO2 emission by 8.24% and 20% lower than SAG-based HRSP in a typical Victorian house, respectively.
In conclusion, the REG-based HRSP was the best eco-friendly material for buildings with acceptable structural properties and moderate energy savings potential. However, the SAG based HRSP was the most energy-efficient material with acceptable mechanical and thermal properties of the non-load-bearing structure.