Thermal Efficiency of Paraffin Nano Container in Presence of Porous Walls Structures

1. Introduction

Phase change materials (PCMs) are capable of storing and releasing significant thermal energy through phase transitions, thereby offering effective means for heating and cooling applications. Typically, these transitions occur between solid and liquid phases, allowing PCMs to serve as thermal buffers in various environments.

PCMs are broadly classified into organic (carbon-based) and inorganic types. Organic PCMs, such as paraffin, are derived from petroleum, plants, or animals, while inorganic PCMs typically consist of salt hydrates. The versatility of PCMs has made them highly applicable in thermal energy storage systems and in enhancing the thermal efficiency of building envelopes.

Previous studies have explored the integration of PCMs into ventilated façade systems. Macro-encapsulated PCM panels, placed within ventilated air chambers, have demonstrated considerable improvements in thermal performance compared to traditional facades [1,2,3,4].

Results indicate that double ventilated facades incorporating PCMs can reduce building energy demands for heating and cooling by approximately 11.5% in colder seasons and 5.6% in warmer seasons. However, the efficacy of PCMs in ventilated facades varies, being more pronounced in cooler climates [3]. In winter conditions, PCMs can elevate indoor temperatures from 9 °C to 18 °C, as experimental findings have shown [5].

The selection criteria and methods for assessing PCM thermal properties, as well as numerical heat transfer modeling techniques, are critical for optimizing PCM applications in building structures [6].

Research has shown that facade designs with macro-encapsulated PCMs can effectively decrease cooling demands, particularly in Mediterranean climates, through overnight cooling effects [7].

Such PCM-enhanced materials contribute to sustainable building design by improving thermal efficiency and minimizing energy consumption [8,9].

The phase change materials (PCMs) application to improve thermophysical characteristics of the building system are investigated and described by authors [10,11,12,13,14,15,16,17,18,19].

The thermophysical characteristics of a PCM-enhanced structure are related to the thermophysical properties of the PCMs. Thus, the thermal properties of PCMs are necessary to obtain reliable data for modeling the thermal behavior of the structure.

Thermophysical properties of PCMs are vital for accurate thermal modeling of building systems. Experimental evaluations of paraffin-based PCMs across ten manufacturers have provided essential data for advancing PCM modeling, validation, and application [20]. The experimental data can be used for upcoming investigation PCMs, for numerical modeling, validation and verification.

The investigation conducted a numerical modeling of convective and radiation heat transfer in a cavity with transparent inner walls, considering the buoyant force and the melting process velocity. Radiation effect increases the melting process of paraffin materials by 31%. The result shows that an increasing heat flow from 50 to 100 and 300 (W/m²) increases the melting process velocity by 19.3% and 27.2% [21].

A comprehensive review of thermal energy storage systems highlights PCMs’ impact on building heating efficiency, recommending combined active and passive heating strategies to maximize energy savings [22].

Simulations using two-dimensional CFD modeling based on the enthalpy-porosity formula confirm that rectangular PCM-filled cavities exhibit faster melting times compared to triangular configurations, providing insights into optimal design geometries (melting time increased from 35 min to 32 min) [23].

In North-Eastern Mediterranean region climates, PCM applications in building walls have achieved energy savings of up to 56% using paraffin materials with a melting temperature point of 26 °C, showcasing their potential for sustainable construction in warm regions [24].

The authors investigated thermal management within building systems, focusing on the practical performance of micro- and macro-encapsulated phase change materials (PCMs) applied in building wall structures [25,26,27].

Recent studies have demonstrated that filling building cavities, such as hollow bricks, with PCMs can substantially reduce peak heat fluxes, thereby enhancing building thermal stability. The results of a numerical study have shown that the PCM application can reduce the peak heat flux from 45.26 W/m² to 19.19 W/m²–21.4 W/m² [28].

Thermal energy storage systems with the application of PCMs have been recognized as one of the most advanced energy technologies in improving energy efficiency and sustainability of the buildings [29,30,31]. An overview of microencapsulation technologies for thermal energy storage using phase change materials (PCMs) in building construction have been demonstrated in the work of [32,33].

The work [34] examined the thermal energy accumulation characteristics of a paraffin-based phase change material (PCM) enhanced with nanographite and charcoal. It was found that the addition of 0.06 wt.% nanographite and 2.0 wt.% oleic acid reduced the melting time by 21% compared to pure paraffin. Additionally, the research [35] assessed various modeling programs for simulating the thermal behavior of building enclosures containing PCMs, utilizing data from independent studies on nanocapsulated (Nano-PCM) and form-stabilized PCMs. Findings indicate that PCM application effectively reduces daytime temperature fluctuations within buildings, thereby maintaining stable indoor temperatures. Further investigation into simultaneous energy storage and recovery in heat exchangers using paraffin-based PCMs revealed that the incorporation of nanoparticles enhances melting speed [36].

This study aims to evaluate the thermophysical properties and thermal efficiency of paraffin-based PCM nanocontainers when applied in porous wall structures, using computational fluid dynamics (CFD) to assess performance variations across different conditions.

2. Materials

2.1. Material Used

Phase change materials (PCMs) are widely applied to address energy-saving and energy storage challenges across various systems and structures. Their use spans high-performance thermal insulation in building materials, cooling and insulating components in electronics, and finishing materials in diverse applications. Paraffins, in particular, show strong potential as PCMs due to their phase transition temperatures, which closely align with typical ambient conditions, making them ideal for maintaining thermal stability and preventing structural damage in PCM-integrated systems.

To enhance stability and performance, PCMs can be encapsulated within shells of silicon dioxide (SiO₂) or copper oxide (CuO), creating granular additives that are adaptable in size. This flexibility supports the development of both micro- and nano-scale containers filled with PCM, which can be incorporated into insulation materials or structural wall compositions. These micro- and nano-containers provide enhanced thermal performance, making them suitable for integration into construction materials where effective thermal regulation is essential.

2.2. Theproblemstatement

This study aims to evaluate the thermal performance of paraffin-based nanocontainers within porous wall systems, utilizing capsules filled with paraffin at varying volume fractions (with filling degrees VF: 0, 10, 20, 40, 60, 80%). The dimensions of the proposed nanocontainer are D = 700 nm/t = 18 nm; D = 200 nm/t = 5 nm. Two types of shell are modeled in the performed calculations—a shell based on copper oxide (CuO) and a shell based on silicon dioxide (SiO2).

The study employs an unsteady Reynolds-averaged Navier-Stokes (RANS) approach in a two-dimensional (2D) laminar formulation to simulate the thermal behavior. The volume of fluid (VOF) method is used to capture the dynamics of the paraffin-air interfacial boundary. Calculations are executed using the finite-volume method, and the enthalpy-porosity model is applied to simulate the melting and solidification processes.

2.3. The Boundary Conditions

To address the thermal behavior of paraffin melting within nanocontainers, the following boundary conditions are applied: an external temperature of 50 °C and an initial temperature of 20 °C for the system. Figure 1a,b illustrate the phase change results in nanocontainers filled with PCM.

For the solidification phase (inverse case), the final field from the melting simulation is used as the initial condition, with the wall temperature maintained at 20 °C.

3. Results

The thermal performance of paraffin nanocontainers in the presence of porous walls was assessed through numerical modeling, focusing on two nanocontainer dimensions: 200 nm and 700 nm. Simulations covered both the melting and solidification processes, providing insights into the thermal behavior of paraffin under these conditions.

High-quality design of such systems necessitates advanced numerical modeling. With the aid of modern computing technology, numerical methods offer robust solutions for both fundamental and applied challenges in building thermal behavior analysis. These methods enable detailed examination of thermal processes, contributing valuable data to optimize PCM applications in building systems [37,38]

Simulation results are presented in graphs showing the relationship between paraffin liquid volume fraction and time during the melting process for the 200 nm nanocontainer, as illustrated in Figure 2.

The numerical simulation results are presented in graphs illustrating the dependence of total heat flux of the paraffin-air mixture (200 nm) on time during the melting process of the paraffin nanocontainer (200 nm), as shown in Figure 3.

The melting process characteristics of the 200 nm paraffin nanocontainer are presented in Table 1, where t represents the melting time and p denotes the total power transferred to the paraffin-air mixture during the melting process.

The graphs showing the dependence of paraffin liquid volume fraction on time during the solidification process of the 200 nm paraffin nanocontainer are presented in Figure 4.

The graphs illustrating the dependence of total heat flux of the paraffin-air mixture (200 nm) on time during the solidification process are presented in Figure 5.

The solidification process characteristics of the 200 nm paraffin nanocontainer are presented in Table 2, where t represents the solidification time and p denotes the total power transferred to the paraffin-air mixture during the solidification process.

To examine the impact of porosity, graphs showing the dependence of paraffin liquid volume fraction on time for both solid and porous walls were produced and are presented in Figure 6.

To assess the impact of porosity, graphs showing the dependence of total heat flux of the paraffin-air mixture on time for both solid and porous walls were generated and are presented in Figure 7.

The characteristics of the paraffin-air mixture for both solid and porous walls are presented in Table 3.

The numerical simulation results showing the dependence of paraffin liquid volume fraction on time during the melting process of the 700 nm paraffin nanocontainer are presented in Figure 8.

Similarly, Figure 9 presents the dependence of total heat flux of the paraffin-air mixture (700 nm) on time during the melting process.

Figure 9. Heat flux (charging) curves of the 700 nm paraffin nanocontainer.

Figure 9. Heat flux (charging) curves of the 700 nm paraffin nanocontainer.

The melting process characteristics of the 700 nm paraffin nanocontainer are presented in Table 4, where t denotes the melting time and p represents the total power transferred to the paraffin-air mixture during melting.

The dependence of paraffin liquid volume fraction on time during the solidification process of the 700 nm paraffin nanocontainer is presented in Figure 9.

Figure 9. Solidification (discharge) curves of the 700 nm paraffin nanocontainer.

Figure 9. Solidification (discharge) curves of the 700 nm paraffin nanocontainer.

Finally, Figure 10 displays the dependence of total heat flux of the paraffin-air mixture (700 nm) on time during the solidification process.

The solidification process characteristics of the 700 nm paraffin nanocontainer are summarized in Table 5, where t is the solidification time and p denotes the total power transferred to the paraffin-air mixture during solidification.

The absence of experimental data in current literature has restricted the direct validation of the proposed. Nonetheless, the gained results align closely with previously established findings, especially regarding the thermal performance of paraffin-based nanocontainers and the impacts of silicon dioxide and copper oxide shells on phase transition dynamics. This agreement suggests that the proposed model reliably simulates key thermal behaviors observed in similar studies, including the role of porous structures in enhancing heat flux and reducing phase change durations.

While these results provide a strong theoretical foundation, experimental validation remains essential to substantiate our model’s accuracy further. Empirical data would not only confirm the model’s predictive capabilities but also allow for refined adjustments, enhancing its applicability for PCM integration in energy-efficient building materials. Future work should prioritize experimental trials to validate and strengthen the model’s utility in practical thermal storage applications.

4. Conclusions

This study has demonstrated that paraffin-based nanocontainers encapsulated within SiO₂ and CuO shells present a viable solution for thermal energy storage in building applications. The numerical simulations reveal that the choice of shell material and the inclusion of porous wall structures substantially impact the thermal efficiency of the PCM systems. Key findings of the research include the following:

• The melting time of paraffin does not depend on the material of the wall (the thermal conductivity differs by an order of magnitude). After the paraffin has melted, some time passes before the entire volume of the now liquid paraffin is heated.
• CuO shells enhanced heat transfer rates compared to SiO₂, reducing both melting and solidification times. This suggests that CuO-based nanocontainers are more effective for rapid thermal response applications.
• Differences in container dimensions (200 nm vs. 700 nm) influenced the heat flux and liquid fraction changes during phase transitions. Smaller dimensions exhibited faster thermal responses, highlighting their suitability for applications where space and weight constraints exist.
• Porous walls significantly improved the heat flux during both melting and solidification processes. This enhancement supports the use of porous PCM structures in building materials to maximize thermal regulation and efficiency.
• The addition of nanographite particles within the PCM led to a 21% reduction in melting time, offering a promising approach for accelerating thermal response in PCM-based systems.

Overall, this research emphasizes the importance of shell material, container size, and wall porosity in optimizing PCM-based nanocontainers for thermal management. By integrating these findings into building design, it is possible to enhance energy efficiency, stabilize indoor temperatures, and contribute to sustainable construction practices. Future work may focus on experimental validation of these findings and exploring other nanoparticle additives to further optimize PCM thermal behavior.