By Nidhi DhullReviewed by Susha Cheriyedath, M.Sc.Jul 23 2024A recent article published in Scientific Reports proposed the use of a lauric acid-palmitic acid/expanded graphite (LA-PA/EG) composite phase change material (CPCM) to create a composite thermal energy storage cement mortar (CTESCM). The study compared the thermal, physical, mechanical, and microstructural properties of the CTESCM to those of ordinary cement mortar (OCM).
Mass loss rates of the CTESCMs with different CPCM mass contents after thermal cycling. Image Credit: https://link.springer.com/article/10.1038/s41598-024-67573-x
Background
Energy shortages are a significant challenge in the 21st century, with building energy needs for cooling and heating representing a substantial portion of global energy consumption. Enhancing the thermal efficiency of buildings through innovative building materials can contribute significantly to energy conservation efforts.
Latent thermal energy storage using phase change materials (PCMs) can effectively regulate indoor temperatures, thereby improving energy efficiency. Fatty acids are particularly promising as PCMs due to their excellent thermal performance for latent energy storage in buildings.
Despite their potential, practical applications of PCMs are constrained by issues such as low thermal conductivity and leakage during phase transitions. To address these limitations, PCMs are often combined with porous support materials like graphite to create form-stable composite phase change materials (CPCMs). Expanded graphite (EG), with its high porosity and thermal conductivity, helps prevent leakage and enhances the heat storage and release rates of PCMs.
In response to these challenges, this study proposes an innovative composite thermal energy storage cement mortar (CTESCM) that integrates fatty acid binary eutectic mixtures with EG CPCMs. This novel CTESCM demonstrates improved heat transfer capabilities and stability compared to ordinary cement mortar (OCM).
Methods
The LA-PA phase change material was composed of lauric acid and palmitic acid in a mass ratio of 69.8:30.2, with LA-PA PCM constituting 92.4 % of the CPCM. CTESCM samples were then prepared by varying the LA-PA/EG CPCM mass content at 0 %, 5 %, 15 %, 20 %, 30 %, and 40 %. These samples were molded into 40×40×40 mm³ cubes and cured for 28 days.
The thermal properties of the CPCMs and CTESCMs were analyzed using differential scanning calorimetry (DSC). The thermal stability of the CPCMs from 25 to 400 °C was assessed through thermogravimetric analysis (TGA). Additionally, the surface morphology of the mortar samples was examined using scanning electron microscopy (SEM).
Thermal cycling tests, which involved repeated DSC measurements after multiple thermal cycles, and accelerated thermal cycling tests, which used high—and low-temperature test chambers, evaluated the long-term stability of the PCM, CPCM, and CTESCMs.
The compressive strengths of the mortar samples were measured with an electronic universal testing machine. Finally, heat storage and release properties of the CTESCM blocks were tested within the 5-55 °C temperature range, with temperature changes recorded every 10 seconds using a multi-channel intelligent temperature inspection instrument.
Results and Discussion
In the SEM images, EG exhibited a network of graphite flakes and numerous irregular pores. The molten LA-PA was readily absorbed into the EG's microporous structure through capillary action. While LA-PA was uniformly adsorbed into the EG structure, it did not completely fill it. The CPCM maintained the EG's initial worm-like morphology, effectively preventing the leakage of the LA-PA PCM.
The LA-PA/EG CPCM demonstrated melting and freezing temperatures of 35.5 °C and 33 °C, respectively, with latent heats of 169.8 J/g and 155.2 J/g. These thermal properties are well-suited for floor radiant heating systems. The CPCM also exhibited good thermal stability at 100 °C, as shown by the thermogravimetric analysis (TGA) curves.
In contrast, the phase change temperature of the CTESCM was lower than that of the CPCM. Additionally, the latent heat of the CTESCM consistently decreased, and its thermal conductivity increased with reduced CPCM content. This indicates that the CPCM significantly influenced the storage and release rate of thermal energy in the CTESCM. Despite these effects, the CTESCM showed good long-term stability, with less than a 4 % mass loss after 200 cycles.
The density of CTESCM blocks decreased almost linearly with increasing CPCM mass content and was notably lower than that of ordinary cement mortar (OCM). The compressive strength of the CTESCM was also lower compared to OCM, a result attributed to the lower density and porous nature of the CPCM incorporated into the CTESCM.
Furthermore, SEM images revealed loose binding between the CPCM and the mortar, as well as an increase in internal pores within the CTESCM compared to OCM. Higher CPCM content led to significantly weakened binding force between mortar particles due to the enhanced porosity.
Conclusion
Overall, the researchers successfully demonstrated the incorporation of LA-PA/EG CPCM into an innovative cement mortar for building energy conservation applications such as floor radiant heating systems.
However, increasing CPCM content negatively influenced the CTESCM’s thermal conductivity, density, and compressive strength due to the former’s low-density porous structure. Thus, CTESCM with a 20 % mass fraction of CPCM was optimal for practical applications in building thermal management systems. The proposed CTESCM exhibited notable potential for building energy-saving systems, economically aiding energy conservation, and reducing carbon emissions.
Journal Reference
Zhou, D., Xiao, S., & Liu, Y. (2024). Preparation and characterization of innovative cement mortar incorporating fatty acid/expanded graphite composite phase change material for thermal energy storage. Scientific Reports, 14(1). DOI: 10.1038/s41598-024-67573-x, https://link.springer.com/article/10.1038/s41598-024-67573-x
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