The approach of selecting materials is based on the alignment of waste valorization and its sustainability. In this work, biochar was synthesized from two local biomass waste sources: coconut fiber, a byproduct of domestic use, and water hyacinth, an invasive aquatic weed. The sequential steps involved in the preparation of the nanocomposite PCM-biochar composite are illustrated in Figure 1. These materials are typically discarded, contributing to solid waste and waterway pollution. Their translation into biochar offers a dual benefit: like waste management and thermal energy storage systems, supporting the goals of the circular economy and environmental sustainability [35]. The approximate composition of coconut coir and water hyacinth is presented in Table 1. The eco-friendly composites are synthesized, when blended coconut coir and water hyacinth together with hydroxyl/carbonyl (cellulose/lignin) groups to improve their bonding, absorbency and lightness in the composites.

The cellulose in the composite provided structural strength and a high surface area once processed in the presence of linear, semi-crystalline polysaccharide polymer. The presence of lingocellulosic biomass in the composite improves the permeable structure needed for energy storage capacity. Hemicellulose on the other hand consists of monosaccharides (xylose, mannose, glucose, etc.) is amorphous, removed by pretreatment processes to increase the cellulose’s accessibility and material’s crystallinity. A complex, aromatic biopolymer acts as a binding agent, providing resistance and rigidity to microbial degradation that makes it a pioneer for carbon materials.

Figure 1.Steps of nanocomposite PCM bio-composite preparation.

Table 1.The approximate composition of coconut coir (CC) and water hyacinth (WH).

In raw form of coir and hyacinth are not used unless they are converted into activated carbon through processes involving carbonization and chemical activation. The organic components like cellulose, hemicellulose, lignin act as the carbon precursor. The treatment processes here remove the non-carbon elements to ensure its porosity with a large specific surface area. The high surface area and porous structure of activated carbon are necessary for the adsorption/desorption of ions in the electrolyte that is also very essential for energy storage systems. To enhance the material’s surface chemistry as well as electrochemical performance, chemical treatments are performed. The chemical treatment was performed to extract cellulose from water hyacinth, as it has low lignin and high holocellulose content. Coconut coir, conversely, has very high lignin content, requiring specific treatments to make it suitable for energy storage applications.

As such, a composite phase change material (PCM) was developed by utilizing paraffin wax as the primary base material, and its performance was enhanced through the addition of aluminum oxide (Al₂O₃) nanoparticles and biochar derived from two different biomass sources, coconut fiber and water hyacinth. The steps involved in the preparation of nano-PCM bio-composite are shown in Figure 1. The paraffin wax was selected here due to its excellent properties, including high latent heat of fusion, chemical inertness, thermal reliability, non-toxicity, and economic affordability. However, despite these benefits, paraffin wax suffers from a major drawback: its intrinsically low thermal conductivity (0.2 W/m·K) that restricts the rate of absorption and release of thermal energy [36][37]. This limitation reduces its overall efficiency in applications; it requires rapid heat exchange or cyclic thermal charging and discharging characteristics.

To address this issue, aluminum oxide (Al₂O₃) nanoparticles were selected as a thermal conductivity enhancer due to their high intrinsic conductivity, chemical stability, and compatibility with organic matrices. A concentration of 1 wt% Al₂O₃ was introduced into molten paraffin wax, and the resulting mixture was subjected to a controlled magnetic stirring process. This was conducted at a temperature of 70˚C and a speed of 700 rpm for a two-hour duration. The aim of this step was to achieve homogeneous dispersion of the nanoparticles throughout the parafene wax matrix without the formation of agglomeration that affects negatively on the uniformity and enhancement of thermal conductivity. To further improve the structural integrity of the composite PCM and to mitigate the leakage commonly observed during the melting phase, biochar was incorporated as a porous stabilizing scaffold. Biochar, a carbon-rich material with high porosity, was derived from the biomass through the pyrolysis process is known for its ability to absorb and retain molten materials. Pyrolysis (heating without oxygen) process was performed at temperatures ranging from 400˚C to 800˚C to maintain the properties, surface area, carbon content and porosity for specific applications. As such two different bio-waste materials, coconut fiber and water hyacinth were selected as feed stocks for biochar production, aligning the goals of waste management and its sustainability. The raw biomass was first thoroughly cleaned to eliminate any surface contaminants or residues, and then left to sun-dry for 48 hours to remove moisture. After drying completely, the biomass was chopped into smaller pieces to increase its surface area and made ready for the pyrolysis process that was conducted in sealed traditional clay cooking pots. This pot was heated on a gas stove to simulate an oxygen-limited environment that facilitated carbonization of the biomass toward achieving structurally porous, carbon-rich structure.

Biochar was produced separately from coconut fiber and water hyacinth through slow pyrolysis under oxygen-limited conditions. Prior to pyrolysis, both biomass feedstocks were thoroughly washed to remove adhering impurities and sun-dried for 48 h. The dried materials were subsequently chopped into small pieces to facilitate relatively uniform thermal decomposition. For each feedstock, a measured quantity of dry biomass was placed in a lidded clay pyrolysis vessel and heated using a gas-fired heating source. The temperature was increased from ambient temperature to a target pyrolysis temperature of 500˚C at an average heating rate of approximately 10˚C min⁻¹, followed by a 60 min residence period at 500˚C. The vessels remained tightly covered throughout carbonization, with only a small outlet provided for the release of volatile pyrolysis gases. Consequently, external air ingress was minimized and an oxygen-deficient atmosphere was maintained without the use of externally supplied nitrogen. After completion of pyrolysis, the heat source was removed and the vessels were kept closed until they cooled naturally to room temperature to prevent oxidation of the hot char.

Once it was pyrolyzed, the resulting biochar was manually crushed using a mortar and pestle into fine powder until it became suitable for mixing with the paraffin matrix. This manual grinding approach was selected to avoid nanoparticle contamination and maintain the natural porous structure of the biochar. The previously prepared Al₂O₃ was then added to 10 wt% of biochar and magnetic stirring process at 70˚C and 700 rpm was conducted for the preparation of composite PCM. This process resulted in two distinct biochar-reinforced nanocomposite PCM variants—one containing coconut fiber biochar and the other containing water hyacinth biochar. Finally, the hot nanocomposite mixtures were carefully poured into pre-cleaned aluminum molds and allowed it to cool and solidify under ambient room temperature (25˚C). This natural cooling process helped to ensure the samples are solidified without introducing thermal shock, deformation, or structural imperfections, thereby preserving the physical integrity and homogeneity of the final composite PCM blocks. Biochar, particularly from biomass waste such as coconut fiber and water hyacinth, has shown promise in improving thermal performance and structural stability of PCM composites. Although a combined system of paraffin, Al₂O₃, coconut or hyacinth biochar has not been directly reported, the components have each been individually validated in the literature as effective PCM enhancers. Thus, three samples are selected here as Paraffin + 1 wt% of Al₂O₃ [38], Paraffin + 1 wt% of Al₂O₃ + 10 wt% of Water Hyacinth Biochar, and Paraffin + 1 wt% of Al₂O₃ + 10 wt% of Coconut Fiber Biochar to analyse the thermal performance.

Prior to composite preparation, the coconut-fiber and water-hyacinth biochars were ground and sieved to obtain a uniform particle-size fraction of <150 µm. The sieved biochar powders were subsequently oven-dried at 105˚C for 24 h to remove residual moisture and then stored in airtight containers until use. The Al₂O₃ nanoparticles, with a nominal particle size of approximately 20 - 50 nm, were dried at 80˚C for 12 h before incorporation into the paraffin matrix to minimize moisture-induced agglomeration. These controlled particle-size and drying procedures were adopted to improve additive dispersion and reproducibility and to minimize variations in composite porosity and thermal conductivity. A pure-paraffin sample was included as the control and tested under identical experimental conditions. Its thermal conductivity was used as the baseline for comparison with the Al₂O₃-containing and biochar-containing PCM composites, enabling the individual contributions of Al₂O₃, coconut-fiber biochar, and water-hyacinth biochar to the enhancement of thermal conductivity to be quantified.

A powerful, non-destructive technique, FTIR (Fourier Transform Infrared) was used for the qualitative and quantitative chemical analysis to identify and verify composition, and to analyze complex mixtures of the sample. The FTIR spectra of Nano-PCM samples are shown in Figure 2. This analysis aimed to determine whether these additives introduced any new chemical bonds that altered the molecular structure of the base PCM. The FTIR spectra were collected across a wavenumber range of 4000 - 500 cm⁻¹ for the three samples: i) Paraffin + 1 wt% of Al₂O₃ (Nano-PCM) [38], ii) Paraffin + 1 wt% of Al₂O₃ + 10 wt% of Water Hyacinth Biochar, and iii) Paraffin + 1 wt% of Al₂O₃ + 10 wt% of Coconut Fiber Biochar are presented in Figure 2. This FTIR spectroscopy shows the chemical structure, functional group, integrity and interaction behavior of paraffin wax that was modified with aluminum oxide (Al₂O₃) nanoparticles and two biochars, coconut fiber and water hyacinth. In all samples, strong absorption peaks were observed in the wavenumber 2880 - 2892 cm⁻¹, corresponding to C–H stretching vibrations in –CH₂ and –CH₃ groups. These peaks are the characteristic of aliphatic hydrocarbons that confirm the presence of base paraffin structure. Specifically, the Nano-PCM sample showed a peak at 2892 cm⁻¹, while the composites with coconut coir and water hyacinth biochar exhibited slight shifts to 2888 cm⁻¹ and 2880 cm⁻¹, respectively. These minor shifts are attributed to weak physical interactions such as van der Waals forces or mild hydrogen bonding between paraffin chains and the added nanoparticles or porous carbon structures. Importantly, no new peaks were detected in any of the samples, indicating that no chemical reaction or covalent bonding occurred between paraffin and the additives.

Figure 2.FTIR spectra of Nano-PCM samples.

These findings are consistent with previous research studies of [22][25] demonstrated the persistence of C–H stretching peaks in FTIR spectra and confirmed the chemical inertness and compatibility of paraffin with additives like Al₂O₃ and biochar. Additionally, the presence of small bands between 1000 - 1200 cm⁻¹ allocated to C–O or C=C stretching vibrations originates from oxygenated surface groups in the biochar and supports paraffin encapsulation shaped stability during melting cycles [31][32]. Thus, these FTIR results confirm the structural integrity of paraffin and ensure no undesirable chemical interactions occurred. This validates the safe integration of Al₂O₃ nanoparticles and biochar into paraffin for the enhancement of thermal energy storage capacity without compromising the chemical stability that is suitable for long-term thermal cycling applications.

The surface topography and composition of the samples are observed in scanning electron microscope (SEM) analysis. This provides high-resolution, magnified images by scanning the materials at higher magnification and in larger depth. The SEM was employed to investigate the surface geomorphology and microstructural supply of aluminum oxide (Al₂O₃) nanoparticles and biochar within the paraffin matrix. This analysis was conducted to determine whether the additives were unvaryingly dispersed and to identify any morphological features, such as voids, particle agglomeration, or porosity, that could influence the thermal performance and structural integrity of the composite PCMs. The SEM images at 500× magnification of paraffin-based Nano-PCM with coconut fiber biochar and with water hyacinth biochar are presented in Figure 3(a) and Figure 3(b), respectively.

(a)

(b)

Figure 3.(a) SEM image of Nano-PCM with coconut coir biochar (CC); (b) SEM image of Nano-PCM with water hyacinth biochar (WH).

The SEM image of the paraffin-based Nano-PCM coconut fiber and water hyacinth biochars reveals a relatively rough and non-uniform surface texture, featuring a number of irregular dark regions. The dark areas in the SEM image confirm the presence of a carbon-rich structure in the biochar. The particle sizes of the sample are found within the range of 500 to 700 nm. Similar SEM images were observed in the previous study [23]. The specific surface areas of biochar are also obtained in the range from 100s to 1000s m²/g and the specific volume of empty space (pores) of pore volume, 0.1 - 1.5 cm³/g. As seen from the SEM images, the biochar particle has lots of pores on its surface that are filled with Nano-PCM, and thus increase the surface area. A similar observation in the SEM images was noted for agricultural biomass, where migration and sintering of inorganic elements in biochar were taking place [39]. The presence of visible pores in the biochar suggests that the porous construction of the coconut fiber biochar was largely retained after composite formation, in Figure 3(a). Such porosity is beneficial, as it enables the melted paraffin to be engrossed and held within the matrix, thus enhancing shape stability and reducing leakage during phase transitions. Furthermore, no significant particle deposit was observed in the images, indicating the well-dispersing of Al₂O₃ nanoparticles and biochar within the matrix. The SEM image in Figure 3(b), water hyacinth biochar surface morphology is marked rougher and more splintered. This dispersion of biochar appeared more uneven compared to the coconut fiber biochar. This variation may be due to the difference in cellular structures and pyrolysis behavior of water hyacinth biomass that yields the presence of loosely packed and irregular pores in the biochar [31]. As could be seen in the image, some microvoids and surface fractures were interspersed with lighter zones, paraffin and Al₂O₃ that could locally disrupt as the thermal conductivity pathways. Despite these textural differences, both SEM micrographs confirm the successful integration of Al₂O₃ nanoparticles and biochar into the paraffin matrix. The biochar appears to provide a steadying porous scaffold, while the Al₂O₃ nanoparticles are evenly dispersed without notable agglomeration. Such morphological integrity is essential for safeguarding consistent thermal behavior, minimal leakage, and structural stability during repeated melting-solidification cycles; this analysis also supported by previous investigations on PCM composite structures [27].

Thermal conductivity is a key thermophysical property governing the heat-transfer efficiency, charging/discharging rate, and overall thermal response of phase change materials (PCMs). It is particularly important in applications requiring rapid thermal energy exchange, including solar thermal systems, building-envelope thermal management, and electronic cooling systems. Although paraffin wax is widely used as a PCM because of its high latent heat, chemical stability, and reliability, its inherently low thermal conductivity limits the rate of heat absorption and release during repeated thermal cycles [37].

In the present study, the thermal conductivity of paraffin-based composites was modified through the incorporation of Al₂O₃ nanoparticles and biochar derived from coconut fiber and water hyacinth. As shown in Figure 4, the paraffin/Al₂O₃ nanocomposite (Nano PCM) exhibited a thermal conductivity of 0.17 W/m·K, whereas the addition of coconut-fiber biochar (Nano PCM+CO) increased the thermal conductivity to 0.24 W/m·K. Similarly, the water-hyacinth-biochar composite (Nano PCM+WH) exhibited a thermal conductivity of 0.23 W/m·K. Thus, the biochar-containing composites showed higher thermal conductivity than the paraffin/Al₂O₃ nanocomposite, with the coconut-fiber-biochar composite providing the highest value among the investigated samples.

The experimentally obtained thermal-conductivity values were compared with previously reported data [24][25][32][36], as summarized in Table 2. The values obtained in the present study ranged from 0.17 to 0.24 W/m·K, while literature values for comparable paraffin-based composites ranged approximately from 0.12 to 0.39 W/m·K. Differences among the reported values may result from variations in filler concentration, particle size, porosity, interfacial thermal resistance, sample preparation, and the degree of nanoparticle or biochar dispersion within the paraffin matrix. Agglomeration of Al₂O₃ nanoparticles or non-uniform distribution of biochar can interrupt continuous heat-conduction pathways and consequently reduce the expected enhancement in thermal conductivity. The incorporation of Al₂O₃ nanoparticles can improve heat transfer within paraffin because the inorganic particles provide relatively conductive pathways through the PCM matrix. The simultaneous presence of biochar may further influence the effective thermal-conduction network through its porous carbonaceous structure and interaction with the nanoparticles. However, the addition of solid fillers can reduce the mass fraction of paraffin and therefore may decrease the overall latent heat of the composite. Consequently, enhancement of thermal conductivity must be balanced against possible reductions in latent-heat-storage capacity.

Figure 4.Thermal conductivity analysis.

Table 2.Comparison of thermal conductivity with literature studies.

During phase change, heat transfer is governed by different mechanisms. In the melting process, both conduction and natural convection within the liquid PCM may contribute to heat transfer, whereas solidification is predominantly controlled by conduction. Therefore, the presence of thermally conductive Al₂O₃ nanoparticles and biochar can have a particularly noticeable influence during solidification by facilitating heat removal through the composite matrix. The combined incorporation of Al₂O₃ and biochar therefore provides a potential strategy for improving the thermal response of paraffin-based PCMs, although optimization of filler loading and dispersion is necessary to achieve an appropriate balance between thermal conductivity and latent-heat-storage capacity.

The effectiveness of encapsulation in composite phase change materials significantly influences their performance and feasibility in practical applications. A moisture absorption test measures how much water a material can absorb, usually by weighing a dry sample, soaking it, and then re-weighing it to find the increase in weight, indicating its suitability for specific applications. To assess the moisture resistance of the PCM composite, the weight of the sample was selected here as 1 g, and it was submerged in water for 12 hours at room temperature, as shown in Figure 5. The final weight was measured, and the increased weight was observed as 1.001 g, indicating negligible moisture absorption. This analysis depicts that the incorporation of biochar into the Alumina-paraffin matrix enhances water repellency. The porous carbon structure of biochar likely helps encapsulate the paraffin more effectively, thereby minimizing water penetration. Furthermore, paraffin itself is hydrophobic that ultimately complements the role of biochar in enhancing leakage and moisture resistance.

The biochar of water hyacinth and coconut coir used here as reinforce materials with alumina and paraffin wax to produce carbon-rich porous material derived from biomass waste. Coir fiber and water hyacinth have the advantage related to their renewable nature and their high output in local sourcing. Also, the effect of reinforcing the composite matrix causes a more flexible behavior that can be an advantage to absorb or release energy in lightweight structures. The improved performance is attributed to the porous and carbon-rich structure of biochar, which facilitates enhanced thermal bridging between paraffin domains. SEM imaging confirmed these insights: the coconut coir biochar demonstrated a more uniform, fibrous, and interconnected matrix compared to the less compact structure of water hyacinth biochar that likely contributed to its superior conductivity. These findings collectively demonstrated that dual reinforcement with Al₂O₃ nanoparticles and biochar not only enhances thermal conductivity but also promotes waste valorization, positioning this approach as both a technical and ecological solution for PCM-based thermal energy storage systems.

Figure 5.Water absorption test.