Development and thermophysical analysis of binary eutectics phase change materials for solar drying application

Inorganic PCMs

This group of PCMs is divided into salt hydrates and metallics. Due to their low cost, better thermal conductivity, cost-effectiveness, and minor volumetric changes for storage, salt hydrates are very appealing materials for phase change energy storage. Salt hydrates are a typical crystalline solid that are a mixture of water (H₂O) and inorganic salts and is written as AB.nH₂O. Salt hydrates can change from a solid to a liquid state by dehydrating or hydrating the salt, even though this process thermodynamically mimics melting or freezing. Generally, a salt hydrate melts into water, and salt hydrate⁶ i.e.,

And in anhydrous form

Most salt hydrates have the problem of incongruent melting. The hydrate crystals disintegrate into anhydrous salt and water, or a lower hydrate and water, at the melting point. The fact that the water released during crystallization is insufficient to completely dissolve all of the solid phases present causes incongruent melting, which is one issue with the majority of salt hydrates. The lower hydrate (or anhydrous salt), due to the density difference, descends to the bottom of the container. Many salt hydrates also have weak nucleating capabilities, which causes the liquid to supercool before crystallization starts. The addition of a nucleating agent, which supplies the nuclei on which crystal formation is started, is one approach to solving this issue. Another option is to keep some crystals in a small, cold area so they can act as nuclei. Some of the salt hydrates with their latent heat of fusion and melting point are listed in Table 1.

Most of the metal eutectics and low melting metals come under the inorganic metallic PCM category, but due to their heavy weight, metallics have not been given substantial consideration for PCMs. They are reasonable candidates when the volume is taken into account due to the high latent heat of fusion output per unit volume. The employment of metallics brings forth a variety of peculiar technical issues. The strong heat conductivity of the metallics distinguishes them significantly from other PCMs.⁸

Organic PCMs

Organic PCMs are divided into the paraffin and non-paraffin subgroups. Without any loss in their latent heat of fusion and phase segregation, these materials have the property of congruent melting i.e., repeatedly melting and freezing. It also exhibits the property of non-corrosiveness and self-nucleation.

Paraffin is mostly made up of an alkanes chain (CH₃–CH₂–CH₃…), and the crystallization of these chains generates a significant amount of latent heat. In general, paraffin is stable below 500°C, and there are no significant changes in the volume on their melting; also they have low vapor pressure while melting.⁹ The melt-freeze cycle of paraffin is often relatively long. With more carbon atoms present, alkane has a higher melting point. The fact that paraffin is accessible in a wide range of temperatures is the primary factor in its qualification as an energy storage material. Along with other beneficial traits like consistent melting and good nucleating qualities, paraffin has several other advantages.¹⁰ They have a few unfavorable characteristics, including low thermal conductivity, incompatibility with plastic containers, and considerable flammability. By slightly modifying the wax and the storage unit, all these negative effects can be somewhat removed. Some of the most desirable and moderate desirable paraffin are shown in Table 2.

Of all phase transition materials, non-paraffin organics are the most prevalent and have the widest range of features.¹¹ Unlike paraffin, which has extremely comparable properties, each of these materials will have unique characteristics. This is the broadest group of potential PCMs. After conducting a thorough analysis of organic materials, Buddhi and Sawhney found several esters, fatty acids, alcohols, and glycols that might be useful as energy storage materials.¹² Fatty acids and other non-paraffin organic compounds are other subgroups of these organic molecules. Due to their flammability, fatty acids and non-paraffin organic materials cannot be subjected to extreme heat, flames, or oxidizing agents. Compared to paraffin, fatty acids have high heat of fusion and have repeatable behavior in their melting and freezing. Fatty acids also freeze without supercooling. All fatty acids are described by the chemical formula CH₃(CH₂)_2n.COOH. Their main disadvantage is that they are 2–2.5 times more expensive than technical-grade paraffin. They are also barely corrosive. Some of the non-paraffin compounds are listed in Table 3 with their melting point and latent heat of fusion.

Eutectic PCMs

A minimal-melting composition of at least two or more materials is known as a eutectic, and during crystallization, each of these materials melts and freezes concurrently to form a mixture of the material crystals.¹³ Because they freeze to a close-knit combination of crystals, eutectic materials rarely melt or freeze without the components segregating. Both components simultaneously liquefy when heated, making separation unlikely. Since they are minimum melting, some segregated PCM compositions have been wrongly referred to as eutectics. But it would be more accurate to refer to them as peritectic as they undergo a peritectic reaction during phase change. Some of the compositions of the eutectics are shown in Table 4.¹¹

Now, if there is a discussion on the merits and demerits of the three types of PCMs mentioned above (inorganic, organic, and eutectic), there are many discrepancies, some of which are listed in Table 5.

The rationale for mixing organic non-paraffin compounds such as beeswax (BW) with fatty acids like stearic acid (SA), palmitic acid (PA), myristic acid (MA), and lauric acid (LA) is to tailor the thermal properties particularly the melting point and latent heat of fusion of the resulting eutectic phase change materials (PCMs) for targeted thermal energy storage applications. Eutectic mixtures typically melt at temperatures lower than their individual components, enabling precise tuning of phase change behavior to the desired range of 50–60°C, suitable for applications such as solar water heating, building thermal management, and electronics cooling. Beeswax, a complex blend of long-chain esters, acids, and hydrocarbons, contributes high thermal stability and reduced subcooling, while the fatty acids offer chemical renewability and high latent heat but may suffer from issues like subcooling or phase separation. By carefully selecting compositions near the eutectic point such as BW:SA (20:80) or BW:PA (40:60) the mixtures form homogenous, stable PCMs that melt sharply and perform consistently over thermal cycles.

Other than the listed eutectic PCMs, many more eutectic mixtures were prepared with the good latent heat of fusion and melt-freeze cycle. In this series, beeswax-Stearic (BWSA), beeswax-Palmitic (BWPA), beeswax-Myristic (BWMA), and beeswax-Lauric (BWLA) are prepared and tested. These four prepared eutectic mixtures are a composition of organic non-paraffin PCMs.

Thermogravimetric behavior of eutectic PCMs

The thermogravimetric analysis (TGA) plot illustrates the thermal stability and decomposition behaviour of pure beeswax (BW) and its eutectic mixtures with stearic acid (SA), palmitic acid (PA), myristic acid (MA), and lauric acid (LA) over a temperature range from room temperature to 800 °C (shown in Figure 1). The weight percentage versus temperature profiles reveal distinct degradation patterns for each sample. Pure beeswax exhibits the highest thermal stability, with initial decomposition starting around 250 °C, major weight loss occurring between 250–450 °C, and near-complete decomposition by ~500 °C. This is attributed to the breakdown of long-chain hydrocarbons, esters, and waxy compounds present in BW. The BWSA28 sample begins degrading slightly earlier (~200 °C) and shows significant mass loss between 250–450 °C, continuing up to ~600 °C, indicating reduced stability compared to pure BW but better than pure SA, likely due to the inclusion of BW which stabilizes the blend. For BWPA46, BWMA19, and BWLA19, thermal stability decreases progressively with decreasing fatty acid chain length. BWLA19 exhibits the earliest degradation (~160 °C) and the fastest decomposition, completing by ~350 °C, highlighting its low thermal resistance due to the volatility and lower molecular weight of lauric acid. BWMA19 and BWPA46 show intermediate behaviour, with BWPA46 displaying a broader degradation range due to multistep decomposition from its mixed components. The thermal degradation proceeds via sequential breakdown of molecular constituents starting with volatilization of free fatty acids and followed by the cleavage of ester bonds and fragmentation of hydrocarbon backbones. These reactions are influenced by molecular weight, bonding type, and blend miscibility, where longer chains require higher activation energy to decompose compared to shorter ones. The near-zero residue at 800 °C confirms complete volatilization, which is advantageous for clean decomposition but may necessitate containment considerations in cyclic TES operations. All eutectic mixtures leave minimal residue (~0–5%) at 800 °C, indicating complete decomposition. The observed mass losses are primarily due to thermal evaporation and breakdown of organic constituent’s fatty acids and hydrocarbon chains into volatile products. These results confirm that beeswax improves thermal stability in eutectic formulations, and the degradation temperature systematically decreases with shorter fatty acid chain lengths. This information is essential for selecting eutectic phase change materials (PCMs) in thermal energy storage systems where both thermal stability and phase transition behaviour are crucial.

Figure 1. Thermogravimetric analysis (TGA) curves of pure beeswax (BW) and its eutectic mixtures with stearic acid (BWSA28), palmitic acid (BWPA46), myristic acid (BWMA19), and lauric acid (BWLA19).

Through DSC analysis, the thermal energy storage properties of BWSA28, BWPA46, BWMA19, and BWLA19 were determined. The DSC curves for BWSA28, BWPA46, BWMA19, and BWLA19 are shown in Figure 2. The DSC curves for BWSA28, BWPA46, BWMA19, and BWLA19 displayed comparable patterns and nearly equal forms. For BWSA28, BWPA46, BWMA19, and BWLA19, the onset melting points were 49.59°C, 48.85°C, 50.91°C, and 41.02°C, respectively, which are listed in Table 6. Similarly, the peak melting temperatures for these materials were 55.17°C, 56.85°C, 55.22°C, and 44.96°C, respectively. These values suggest that the materials begin to undergo phase transition within a relatively narrow and moderate temperature range, making them suitable for low-to-intermediate temperature thermal storage applications. Onset and freezing temperatures obtained from BWSA28, BWPA46, BWMA19, and BWLA19 were 50.79°C, 50.22°C, 45.44°C, 36.77°C, and 48.93°C, 49.59°C, 45.34°C, 36.08°C, respectively. The relatively small supercooling gap between the melting and freezing temperatures across all samples suggests good reversibility and stability in repeated thermal cycling, a critical requirement for TES materials. In the DSC plot, the area under the curve provides latent heat of fusion and latent heat of crystallization as shown in Figure 2 and marked with the arrow. So, the obtained latent heat of fusion and latent heat of crystallization for BWSA28, BWPA46, BWMA19, and BWLA19 were 174.52 kJ/kg, 166.03 kJ/kg, 192.85 kJ/kg, 195.73 kJ/kg, and 177.94 kJ/kg, 155.55 kJ/kg, 211.52 kJ/kg, and 201.04 kJ/kg, respectively. These results indicate that BWMA19 and BWLA19 possess superior thermal storage capacity, making them especially attractive for applications requiring higher energy density. DSC analysis confirms that all four eutectic PCMs possess favorable thermal properties, including sharp phase transition, moderate-to-high latent heat, and low supercooling. Among them, BWMA19 and BWLA19 demonstrate the most promising performance due to their higher latent heat values, making them strong candidates for efficient thermal energy storage in solar thermal systems, building climate control, or electronic cooling applications.

Figure 2. DSC curve of BWSA28, BWPA46, BWMA19, and BWLA19.

DSC, differential scanning calorimetry; BWSA, Beeswax-Stearic; BWPA, Beeswax-Palmitic; BWMA, Beeswax-Myristic; BWLA, Beeswax-Lauric.

Application

Drying is a widely used technique in the preservation of food and agricultural products, primarily employed to reduce moisture content while maintaining the quality, nutritional value, and shelf life of the products. Among various drying techniques, solar drying has emerged as a cost-effective and environmentally friendly alternative, particularly well-suited for rural and off-grid regions. The operating temperature range typically required for drying sensitive agricultural products such as fruits and vegetables lies between 40°C and 80°C, a range that ensures adequate moisture removal without causing thermal degradation of essential nutrients. Effective solar drying requires stable temperature regulation and moisture control, which are heavily influenced by ambient solar radiation, air temperature, humidity, and airflow. To address the challenge of fluctuating environmental conditions during solar drying, phase change materials (PCMs) can be integrated into drying systems to store thermal energy during peak sunlight hours and release it when solar input diminishes (e.g., late afternoon or cloudy periods). This enhances drying consistency, improves energy efficiency, and reduces the risk of spoilage.^15–17 The eutectic PCMs developed in this study BWSA28, BWPA46, BWMA19, and BWLA19 exhibited melting and freezing temperatures in the range of 36°C to 56°C, aligning well with the optimal temperature window for solar drying of food and agricultural products. Specifically, BWLA19, with a melting temperature around 44.96°C and a freezing point near 36.08°C, is particularly well suited for low-temperature drying applications where gentle heat is necessary to preserve delicate nutritional compounds. BWMA19 and BWSA28, having melting temperatures of 55.22°C and 55.17°C respectively, can support drying processes that require temperatures closer to the upper end of the safe drying range. The high latent heat capacities of these eutectics (up to 195.73 kJ/kg for BWLA19 and 192.85 kJ/kg for BWMA19) indicate that they can store and release substantial amounts of thermal energy, thus extending the effective drying period beyond peak sunlight hours. In addition to technical compatibility, the use of PCMs in solar drying systems contributes to energy conservation and sustainability. Unlike fossil-fuel-based thermal storage or continuous electrical input, eutectic PCMs offer passive energy storage with no carbon emissions, aligning with ecological and economic goals of the agricultural industry. Thus, the integration of these eutectic PCMs into solar-assisted drying units could significantly enhance the efficiency, reliability, and product quality of food drying processes, while maintaining a low operational cost and minimizing environmental impact. Compared to commercial PCMs, the thermal storage capacity of BWMA19 and BWLA19 (>190 kJ/kg) is comparable or superior to RT50 and fatty acid eutectics.¹⁸ Phase transition temperatures are finely tuned for different drying needs BWLA19 for more sensitive drying (e.g., herbs, berries), and BWMA19 for general fruits/vegetables. These eutectics likely offer better chemical compatibility, biodegradability, and cost-effectiveness for agricultural use, depending on the specific constituents. Moreover, the narrow supercooling range and consistent melting/crystallization behaviour observed in the DSC curves indicate that these PCMs maintain reversible thermal cycling with minimal thermal hysteresis, enhancing their reliability for repeated drying operations.^19,20