Development of phase change materials for low-temperature thermal energy storage application

Introduction

When energy output is insufficient to meet demand, efficient use of existing energy is critical.¹ Thermal energy may be stored in three ways, namely: thermochemical energy storage, sensible heat storage, and latent heat storage (LHS). Thermal energy is absorbed or released in a thermochemical heat storage method by using a completely reversible chemical mechanism to break and mend molecular bonds. In sensible heat storage, thermal energy is stored by heating a solid or liquid to the appropirate temperature. The LHS method relies on the absorption or release of heat during the solid-liquid transition, or liquid-gas transition, as a storage medium. Phase change materials (PCMs) are a type of LHS material that can store and release a significant amount of energy per unit mass.² This energy transfer happens whenever there is a transition from solid to liquid or liquid to solid. This whole process is isothermal. Among all the listed methods, the LHS is the most appealing, and it is capable of compensating for the energy mismatch between the production and consumption of heat.^3,4 There are different kinds of PCMs (organic, inorganic, eutectic) that can melt or solidify at a range of temperatures, and they can be used for a range of applications depending on the temperature. PCMs, find their application in energy-efficient buildings, industrial waste heat recovery, central air-conditioning systems, solar thermal energy storage, temperature-adapted greenhouses, and thermoregulating fibers.^3,4

Among the PCMs investigated, fatty acids, paraffin wax (PW), and their mixtures have drawn a lot of attention. The reasons being that fatty acids have high latent heat of fusion (LHF), exhibit consistent melting, and freezing characteristics, as well as freezing without supercooling, low vapor pressure, non-toxic, minimal volume change during solid-liquid/liquid-solid transitions, self-nucleating characteristics, good chemical and thermal stability over an extended duration of usage, and easy availability.^3,5 Animal and plant-derived fatty acids have the extra benefit of providing a constant supply, notwithstanding fuel shortages. Apart from that, fatty acids are abundant due to their bio-origin and are recyclable, eco-friendly due to lower carbon emissions, and essential for long-term growth.⁶ The major challenge of employing fatty acids as PCM is their pungent stench, corrosive nature, and notably their rapid sublimation rate.⁷ On the other hand, the advantage of using PW is that it is non-corrosive, safe, reliable, chemically stable, inert below 500 °C, has low vapor pressure in the melt form, minimal volume change during solid-liquid, and liquid-solid transition, and is less expensive as compared to other PCMs.^3,8 PW has certain undesirable characteristics, such as being mildly flammable and incompatible with plastic containers.^3,8 In his research, Kahwaji et al. looked at the thermophysical properties, thermal stability, and chemical compatibility of paraffin PCMs. PW48, PW52, and PW58 have melting temperature (T_m) values of 48.2 1.5 °C, 51.8 1.5 °C, and 57.7 1.5 °C, respectively. For the same, the LHF were 156 16 kJ/kg, 171 17 kJ/kg, and 197 20 kJ/kg, respectively. The thermal stability of paraffin PCMs was reported to be over 3000 thermal cycles with no notable change in T_m and LHF.⁹ In their solar chimney, Fadaei et al. employed PW as the PCM. The T_m and LHF of the paraffin utilized were recorded to be 44-46 °C and 189 kJ/kg, respectively. And it was discovered that using PW as PCM improved the solar chimney's thermal efficiency.¹⁰ Omara et al. studied the thermal performance of a solar water heating system combined with PCM that used a flat plate as a heat source in their study. The system's total energy and exergy were determined to be 85 and 76 %, respectively, and the recorded T_m of paraffin PCM was found to be 56 °C.¹¹ Agarwal and Sarviya¹² measured the T_m of commercial and LHF paraffin samples as 41-55 °C and 176 kJ/kg, respectively. Commercial PW was determined to be suitable for sun dryer applications based on the findings of the trial. Mahdi et al.¹³ conducted an experimental study on latent heat thermal helical coil thermal energy storage for melting and solidification phases. PW (type P56-58) was chosen for the job. PW was been determined to have T_m and LHF of 48.3-62 °C and 114.5 kJ/kg, respectively. The results demonstrated that coiled tube latent heat energy storage may be effectively utilized in solar thermal applications. To minimize the absolute drying time of crops in typical sun dryers, Vijayrakesh et al. used PW with T_m equal to 53.7 °C and LHF equal to 190 kJ/kg as PCM-packed floor.¹⁴ Bhagyalakshmi et al. developed a binary eutectic mixture of palmitic acid (60%) and PW (40%). The recorded T_m, solidification temperature, latent heat during charging, and latent heat during discharging were 52.6–59.8 °C, 40.9–47.1 °C, 214.7 kJ/kg, and 194.6 kJ/kg respectively.¹⁵

Thermal energy storage (TES) systems still confront some serious challenges including the stability and reliability of PCM/construction material composites. The PCM must remain useful for an extended length of time in the thermal system. Another major concern is that after repeated cycles of heating and cooling, long-term use of PCM leads to leakage from the building material. Another issue is the thermal stress caused by PCMs and corrosion, which leads to the downfall of the building material.^5,16,17 In addition, PCMs for low-temperature applications are scarce. A lot of studies have been carried out during the last few decades, particularly in this direction, however, there is still a need for commercial PCM especially working between 30-60 °C, and should be widely accessible and reasonably priced in the local market.

A eutectic mixture is a mixture of multiple solids that melts or solidifies at a single temperature lower than the constituents' melting points. Generally, the melting point is sharp, and it has a somewhat larger storage density than the parent materials. The authors in their study chose capric acid (CA) and PW because of their high LHF, appropriate heat transfer properties throughout the phase change transition, easy availability, and low cost of the material. Aside from that, there is a scarcity of PCMs operating in the temperature range of 25-32 °C for construction and solar applications, and those that are available are considerably more expensive than other PCMs. The study's overall purpose is to create low-cost PCMs for TES applications. Binary mixes based on 98.5 % purity CA and laboratory grade (LR-grade) PW with varying weight ratios (10/90, 20/80, 30/70, 40/60, 50/50, 60/40, 70/30, 80/20,90/10, 92/08, 94/06, 96/04, and 98/02) were created for this purpose. The differential scanning calorimetry (DSC) method was used to assess their thermal properties. The developed PCM in the present study is cheap to buy (see Table 2) and readily accessible in India and abroad and can be employed for thermal energy applications.

Methods

The CA (purity 98.5%) and LR grade PW were procured from the Avra Synthesis Pvt. Ltd., Hyderabad, India, and SD fine-CHEM Ltd., Mumbai, India respectively, and the same was utilized without any purification for the preparation of mixtures. To determine the composition of the eutectic mixture, a sequence of binary combinations of CA and PW with different weight proportions were produced (10/90, 20/80, 30/70, 40/60, 50/50, 60/40, 70/30, 80/20,90/10, 92/08, 94/06, 96/04, and 98/02) from their liquid mixtures at constant temperature (70 °C), using magnetic stirrer with a hot plate. Thirteen such units (100g each) were made employing a mixture of CA and PW. These mixtures were initially in liquid form and then kept at room temperature for about an hour for gentle cooling and solidification. Throughout the experiment, an extremely precise and dependable semi-analytical digital balance with an accuracy of about ±0.0001 g was used to complete all weights measurements. The key thermal characteristics required to establish any material as a PCM for TES systems are LHF, T_m, and T_f. The T_m, LHF, of CA, PW, and their eutectic mixes were determined using the DSC method. In DSC, a sample and a reference material is heated in separate aluminum pans at a constant rate of temperature rise. Differences in temperature between the two are proportional to the difference in heat flow between them, and the DSC curve serves as the record. PerkinElmer DSC 4000 equipment was used to test the thermal behavior of all produced eutectics. The prepared samples were investigated between 15 °C and 80 °C at a 2 °C min^-1 heating rate. Throughout the experiment, nitrogen was used as a purge gas, and the constant flow rate was maintained at 20 ml min⁻¹. The largest deviation in enthalpy and temperature measurements was ±2% and ±0.1°C respectively. Thermal properties such as T_m, LHF, T_f, and LHC of CA, and PW procured from Avra Synthesis Pvt. Ltd. and SD fine-CHEM Ltd. respectively were measured by DSC with 2 °C scanning heating/cooling rate at 0^th cycle and information provided from the manufacturer is listed in Table 1.

Results and discussion

Base materials for eutectic mixtures

The CA’s DSC curve has a single, sharp peak that represents the solid-to-liquid transition (Figure 1). The PW’s DSC curve (Figure 2) contains two peaks: the first, which is not as sharp and significant suggests a solid-solid transition, and the second, which is sharp and significant, shows a solid-liquid transformation.¹² Figures 1 and 2 show the melting curve of CA and PW respectively, measured using DSC with a 2 °C scanning heating and the cooling rate at 0^th cycle.

Figure 1. Differential scanning calorimetry (DSC) curve of capric acid (CA) sample, measured at 2 °C scanning heating rate at 0^th cycle.

Figure 2. Differential scanning calorimetry (DSC) curve of PW sample, measured at 2 °C scanning heating rate at 0^th cycle.

CA-PW binary eutectic mixtures

The onset melting point, peak melting point, and LHF of the prepared samples are listed in Table 2. Based on the result, it is clear that the melting point of mixes increases with decreasing the mass ratio of PW until it reaches 8%, and then it decreases at 6% and a further decrease in mass percentage again increases the melting point. In the case of LHF, it was discovered that raising the mass percentage of CA in the mixture causes LHF to rise. LHF of developed mixtures grew until the CA amount reached 94%, after which it began to decline. Figure 3 shows the DSC melting thermograph of a binary eutectic mixture of CA and PW (PWCA-0694). Most of the developed mixes have sufficient LHF value, and based on their melting point range (26.14 °C-30.60 °C), they can be used for TES applications in solar absorption chillers, buildings, surgical dress, clinical beds, and photovoltaic systems.^5,16,17

Table 2. Thermophysical properties of the developed paraffin wax/capric acid (PW/CA) eutectic materials.

Sample code	Wt. %		First peak			Second peak			Cost (US$ kg-1)
	PW	CA	Onset pointa (°C)	Melting peaka (°C)	Latent heata (kJ/kg)	Onset pointa (°C)	Melting peaka (°C)	Latent heata (kJ/kg)
PWCA-0298	2	98	30.60	33.70	139.94				20.82
PWCA-0496	4	96	30.42	33.21	157.81				20.69
PWCA-0694	6	94	30.16	33.31	198.62				20.56
PWCA-0892	8	92	30.21	33.65	154.15				20.43
PWCA-1090	10	90	29.86	32.83	162.22				20.30
PWCA-2080	20	80	29.46	32.75	134.38				19.65
PWCA-3070	30	70	29.17	32.89	120.06				19.00
PWCA-4060	40	60	28.57	32.03	113.88				18.35
PWCA-5050	50	50	28.32	31.48	103.86				17.70
PWCA-6040	60	40	27.69	30.79	81.34				17.04
PWCA-7030	70	30	27.35	29.67	46.07				16.39
PWCA-8020	80	20	26.89	28.98	41.20	40.49	55.40	149.11	15.74
PWCA-9010	90	10	26.14	28.14	15.05	46.56	57.84	154.33	15.09

a Measured through Differential scanning calorimetry. PWCA, paraffin wax capric acid.

Figure 3. Differential scanning calorimetry (DSC) curve of paraffin wax capric acid (PWCA)-0694 sample, measured at 2 °C scanning heating rate at 0^th cycle.

Effect of different heating rates on behaviour of CA/PW eutectic

To check the effect of different heating and cooling rates on T_m, different heating and cooling rates were used for the 400^th thermal cycle of the CA/PW eutectic mixture to examine the influence of scan rate on T_m and LHF. The Figure 4 displays the CA/PW eutectic mixture (94/06 wt.%) DSC curve for the 400^th heating and cooling cycle at 1 °C min^-1, 2 °C min^-1, and 5 °C min^-1 scan rates. To investigate whether the heating/cooling pace has any influence on T_m, CA/PW, eutectic mixture (94/06 wt.%) in a cyclic manner was scanned at different heating rates (1, 2, and 5 °C min^-1) under a constant flow of nitrogen at a flow rate of 20 ml min^-1. During solid-liquid transition different scan rates produced different heat flow magnitudes. The DSC curve of the CA/PW eutectic mixture (94/06 wt.%) for various heating and cooling rates exhibited a smaller peak for low scan rate and a bigger peak for high scan rate Figure 4. The onset melting point did not change much with different scan rates because the leading edge slope remained constant. Based on the findings, it is reasonable to say that the onset melting point is independent of heating and cooling rates. Figure 4 shows, with increasing scan rate, the peak melting point shifts towards higher temperature. LHF for different scan rates can be approximated by peak area divided by scan rate.¹⁸ From the Table 3, it can be seen that for different scan rates LHF is nearly the same. And the table depicts T_m and LHF of PWCA-0694 for the 400^th heating/cooling cycle for different scanning rates. Based on the findings, for the 400^th heating/cooling cycle, there is not much difference in T_m and LHF at different scan rates, indicating the thermal behavior of samples to be similar at different heating and cooling rates.

Figure 4. Differential scanning calorimetry (DSC) curve of PWCA-0694 for 400^th heating cycles with a scan rate of 1, 2, and 5 °C min^-1.

Table 3. Onset melting point, peak melting point, and latent heat of fusion (LHF) of paraffin wax capric acid (PWCA)-0694 for the 400^th heating cycle with a scan rate of 1, 2, and 5 °C min^-1.

Scan Rate [°C min-1]	Onset pointa [°C]	Melting peaka [°C]	Latent heat of fusiona [kJ/kg]
1	30.25	32.74	162.18
2	30.20	33.58	158.98
5	30.21	35.83	156.30

a Measured through differential scanning calorimetry.

Accelerated thermal cycle test of PW/CA eutectic mixture

The T_m and LHF of a binary eutectic mixture (PWCA-0694) were measured after every 100^th heat cycle, for a total of 500 cycles. The Figure 5 shows the DSC curves for the 0^th- 500^th thermal cycle at the succession of 100 thermal cycles of eutectic PCM. The maximum deviation in T_m and LHF was found to be +0.36 °C and -39.65 kJ/kg respectively as shown in Figure 6. The negative and positive values of T_m and LHF are solely relative to the 0th cycle, where positive represents an increase in value and negative represents a decrease in value relative to the 0^th cycle. The T_m and LHF variation of the developed eutectic mixture is within acceptable limits. The developed mixture was thermally stable up to 500 cycles.

Figure 5. Differential scanning calorimetry (DSC) curves of paraffin wax capric acid (PWCA)-0694 at 0^th, 100^th, 200^th, 300^th, 400^th, and 500^th thermal cycles, determined using DSC with 2 °C scanning heating rate.

Figure 6. Variation of T_m and latent heat of fusion (LHF) after 0^th, 100^th, 200^th, 300^th, 400^th, and 500^th thermal cycles.

Conclusions

The PCM development for TES applications is presented in this paper, which is based on binary mixes of CA and PW with various weightratios. The DSC technique was used to analyze the thermal characteristics of these binary blends, which revealed that a few of the developed materials had suitable phase change temperatures and high LHF. The most promising mass percentages of PW contained in CA were determined to be 02–10 wt.%, with reduced T_m of 29.86 °C - 30.42 °C and LHF 154.15–198.62 kJ/kg. From DSC analysis, it was found that the T_m of mixes raises with decreasing the mass percentage of PW until it reaches 8%, and then it decreases to 6% and a further decrease in mass percentage again increases the melting point. In the case of LHF, it was discovered that raising the mass percentage of CA in the combination causes LHF to rise. LHF of developed mixtures grew until the CA percentage reached 94%, after which it began to decline. The developed mixes were found to be suitable for TES applications in buildings, solar absorption chillers, surgical dress/clinical beds, and photovoltaic systems. Because of congruent mixing, desired melting range, and high LHF, PWCA0694 (CA 94 wt.% and PW 06 wt.%) was found to be the most promising mixture of all. From DSC analysis, its melting point was found to be 30.16 °C and LHF to be 198.62 kJ/kg and can be used for application in buildings and photovoltaic systems. Per the findings, different heating and cooling pace have virtually negligible influence on PWCA0694's T_m and LHF. It was also found that with increasing scan rate, the peak melting point shifts towards higher temperature. DSC analysis of 400^th thermal cycles with varied scan rates revealed almost identical results, demonstrating that the thermal characteristics of the sample were constant except for the peak melting point, which rose with increasing heating rate. Accelerated thermal cycling test confirmed the structural and thermal stability of our developed PCM up to 500 thermal cycles. From the investigation, it was also found that the maximum percentage difference in T_m and LHF of PWCA0694 was +0.36°C and -39.65 kJ/kg respectively, which were within an acceptable range. The cost of our developed eutectic is cheap and are easily available in Indian or abroad market. If mass production is done then it can further reduce by 30-40%.