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

Introduction

Modern civilization has grown and developed mostly as a result of energy, which has always been important from the perspective of the development of the global economy. However, along with increased efficiency in the energy sector, significant barriers must also be overcome, including the creation of conventional energy sources, reducing the use of fossil-based fuels, as well as reducing greenhouse gas emissions i.e., CO₂ (carbon dioxide), SO_X (oxides of sulfur), CH₄ (methane), NO_X (nitrogen oxides).¹ Researchers around the world have been looking for new technologies in the last 30–40 years that will reduce the use of fossil fuels and, lessen the detrimental effects that energy production has on the climate and environment. A practical way of using alternative energy is through energy storage, in addition to conserving energy, energy storage also improves the stability and quality of the energy supply and reduces the variation between supply and demand. One of the popular techniques of energy storage is thermal energy storage (TES), which can be classified as²: i) Latent heat storage and ii) sensible heat storage.

Latent heat storage is reliant on heat absorption or release phenomenon, during the phase transition of material from solid to liquid or liquid to gas or vice versa. These materials involved in phase transition are known as phase change materials (PCMs). Whereas in sensible heat storage, energy is stored by increasing the temperature of liquid or solid. In the charging and discharging process, a sensible heat storage system makes use of the material's heat capacity and temperature variation. Stored heat in materials depends on the amount of storage material, the specific heat of the medium, and the temperature change.³

Phase change materials (PCMs)

Due to applied thermal energy, some materials change their state by storing some amount of latent heat during the state/phase transition, such materials are known as PCMs. During the transition from solid to liquid or from liquid to solid, thermal energy is transferred. These solid-liquid PCMs function initially like traditional storage materials, as they absorb heat, their temperature increases, but release heat at a practically constant temperature, in contrast to conventional (sensible) storage materials.⁴ PCMs use chemical bonds for energy storage and release. Although in a sense every material is a PCM, the materials are only categorized as PCM if they have some characteristics of energy storage. High thermal conductivity and significant latent heat should be present in phase transition materials used for energy storage. Additionally, the materials melting points should be within a practical application range; materials should melt consistently with the least amount of supercooling and should be chemically stable. The materials should not be toxic, or chemically corrosive, and should be economical for practical applications.⁵ PCMs are often divided into three groups i.e., organic, inorganic, and eutectics, which is the combination of two or more materials.

Methods

Organic non-paraffin compound beeswax (BW) in different compositions mixed with another organic non-paraffin compound, Palmitic (PA), Myristic (MA), and Lauric (LA) acid at a temperature range of 50–60°C with the help of a magnetic stirrer at 200 rpm for 3–4 hrs followed by sonication for 15 minutes using Sonics Vibracell probe sonicator and get eutectic PCMs. Organic non-paraffin compounds are mixed in different compositions to get desired eutectics. A total of 20 wt % of BW mixed was with 80 wt % of SA to prepare BWSA28. Similarly, 40, 10, and 10 wt % of BW were mixed with 60, 90, and 90 wt % of PA, MA, and LA, respectively, to form BWPA46, BWMA19, and BWLA19 eutectic PCMs. Each sample is prepared in a quantity of 10 gm and acids SA, PA, MA, and LA used here were sourced from MOLYCHEM with product codes 19060, 16705, 16392, and 12520, respectively. Whereas BW was sourced from the center of Excellence on Honey Bees (Nalanda College of Horticulture, Nalanda, Bihar).

The PerkinElmer DSC 4000 equipment was used for the differential scanning calorimetry (DSC) study. In order to measure this little amount (mg) of the samples, an analytical digital weighing machine with a precision of 0.00001 g was used. The weighted sample between 10 to 15 mg was filled into an aluminum pan, and the DSC procedure was carried out in a nitrogen environment at a flux of 20 ml/min at a heating rate of 2°C/min. The accuracy of the DSC device was ±2% for enthalpy measurement and ± 0.1°C for temperature measurement. The reference pan and the sample pan are heated at the same rate during the DSC analysis. The latent heat of fusion, peak melting temperature, and other thermophysical parameters was measured. The top point of the curve offers the peak melting temperature, while the area that comes under the curve explicated latent heat of fusion and crystallization, and the tangent of the highest slope explicated the onset melting point.

Results

Eutectic PCM molecules start to oscillate when heated, this oscillation causes the atoms to move farther apart, occupying more space and resulting in an increase in volume. Further raising the temperature caused molecules to move more quickly due to an increase in kinetic energy, quick movement of molecules disrupting the supramolecular link between the individual atoms. The ordered crystal structure transforms into a randomly oriented liquid state as the temperature rises to a specific critical point, DSC is used to measure this critical point of temperature known as phase transition temperature. Therefore, using 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 1. 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. 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. In the DSC plot, the area under the curve provides latent heat of fusion and latent heat of crystallization as shown in Figure 1 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.

Figure 1. DSC curve of BWSA28, BWPA46, BWMA19, and BWLA19.

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

Discussion

Conclusions

Due to its capability to enhance system performance, energy storage is particularly alluring to a wide range of parties. Technology development is more efficient and practical when excess energy is stored for later use rather than being replaced by new power plants. The latent heat of the phase change is associated with prepared eutectic PCMs and has a crucial influence on their ability to store greater amounts of energy. A target-oriented settling temperature is also supported by PCMs due to the fixed phase change temperature. This paper deals with the development of eutectics PCM. The foremost advantages of these prepared eutectic PCMs were their low cost and eco-friendly nature. DSC analysis of this prepared eutectics BWSA28, BWPA46, BWMA19, and BWLA19 showed good thermal energy storage capacity, and it lies between 155–211 kJ/Kg. These eutectics have a temperature range between 36–56°C.