The capric-myristic acid (CA-MA) binary eutectic mixture phase change material (PCM) was prepared for low-temperature latent heat thermal energy storage (LHTES). The thermal properties, thermal stability, and long-term cycling reliability of the PCMs were measured. Differential scanning calorimetry results showed that the CA-MA binary eutectic mixture at the mass ratio (72/28 wt%) indicated a high-performance PCM for its suitable phase change temperature (
Thermal energy storage technologies have elicited increasing attention due to its broad application prospects in the fields of solar energy utilization, electric power peak-load shifting, industrial waste heat recovery, building heating, and air conditioning [
The core technology of LHTES is phase change material (PCM). Various inorganic, organic, and mixed PCMs, such as paraffin [
Numerous recent studies have been conducted on the performance of fatty acids, such as on their thermal property, thermal stability, and long-term cycling reliability [
Most of the aforementioned studies focus on single and binary or ternary eutectic fatty acids in the temperature range of 20°C–60°C, mainly used in building energy saving and solar energy utilization. However, as the backfill material in the ground source heat pump (GSHP) system, PCM with a phase change temperature of approximately 19°C has yet to be reported. GSHP is a heat pump technology that utilizes shallow geothermal energy. The heat transfer performance between the buried pipes and around the soil plays a decisive role in the operational stability and operating efficiency of the GSHP. The underground soil temperature is unchanged below 15 m. For example, the temperature is approximately 19°C in Shanghai, China [
In this study, the capric-myristic acid (CA-MA) binary eutectic mixture PCM was prepared for LHTES and as backfill materials around the buried pipe of a GSHP system. The thermal properties and thermal cycling reliability of the materials were tested via DSC, and the thermal decomposition stability was investigated via thermal gravimetric analysis (TGA). In addition, Fourier transform infrared (FTIR) spectroscopy was used to investigate whether the chemical composition of the PCMs changed before and after preparation and determine the possible reason that caused the change of the thermal properties of the materials with the increase in the thermal cycling number.
Capric acid (CA, ≥98.5% purity) and myristic acid (MA, ≥98% purity) were purchased from Shanghai Zhunyun Chemical Co. Ltd.
The solid CA and MA were weighed separately at different weight ratios from 0 wt% to 100 wt%, and the sample weight errors were controlled within 0.1 mg. Then, the CA and MA were mixed in a beaker. Then, the beaker was stored in a vacuum drying oven at a constant temperature of 80°C for 2 h. After completely melting, the fatty acid mixtures were stirred for 30 min at 60°C and 500 r/min in a magnetic stirrer. The beaker of the molten liquid fatty acid mixtures was then placed in an ultrasonic water bath; the temperature was controlled at 60°C to ensure that the fatty acids were constantly in the melting state. Furthermore, the time of ultrasonic vibration was approximately 2 min to ensure that the two types of fatty acids were sufficiently mixed to form binary eutectic mixtures. With these methods, a set of CA-MA binary eutectic mixtures was prepared.
The phase change temperature (melting temperature (
To investigate the effect of the thermal cycling number on thermal properties, the CA-MA eutectic mixtures were heated from solid to liquid state and then cooled from liquid to solid state by a heating controller. The above thermal cycling process was conducted continuously until the values were 500, 1000, and 2000. The changes in the performance of the mixtures were measured by DSC and FT-IR.
The thermal stability of the CA-MA PCM was analyzed by TGA (TA Q50, USA) in the temperature range of 20°C–450°C with a 10°C/min heating rate under nitrogen gas atmosphere and an accuracy of ±0.2%.
The samples of CA, MA, and CA-MA eutectic mixtures were analyzed by FTIR (Thermo Scientific Nicolet iS5, USA). The uncycled and cycled samples were measured by FTIR to explore the reason of the variation of thermal properties of the CA-MA mixtures after thermal cycling.
The CA-MA PCM was prepared by mixing CA and MA at different mass ratios. The phase change temperatures and latent heat of the CA and MA in some references [
Thermal properties of pure CA, pure MA, and CA-MA binary eutectic mixture.
PCM | Thermal properties | References | |||
---|---|---|---|---|---|
CA | 31.53 | 165.21 | 32.05 | 168.43 | Sarı et al. [ |
MA | 53.51 | 192.68 | 53.24 | 195.36 | |
CA | 27.69 | 164.7 | 32.06 | 163.5 | Fu et al. [ |
MA | 50.78 | 203.7 | 55.17 | 201.0 | |
CA | 31.5 | 155.5 | — | — | Gao and Qian [ |
MA | 51.6 | 204.5 | — | — | |
CA | 32.14 | 156.04 | 32.53 | 154.24 | Karaipekli et al. [ |
MA | 53.86 | 192.58 | 53.74 | 190.11 | |
CA | 31.17 | 169.4 | 31.69 | 170.3 | This study |
MA | 52.68 | 188.6 | 51.63 | 193.1 | |
CA-MA eutectic mixture | 18.21 | 148.5 | 17.40 | 134.0 | This study |
DSC curves of pure CA and pure MA.
In a binary system, if the two solid components are completely immiscible and can form a eutectic system, then the phase system will become a eutectic binary system [
Thus, the binary eutectic mixture fatty acids have a lower phase change point than any of the fatty acids. Zhang et al. [
For fatty acids,
The relevant parameters of CA and MA in Table
The theoretical phase diagram of the CA-MA eutectic mixtures can be drawn, and the corresponding theoretical ratios and phase change temperatures can be determined by equation (
However, experiments have proven that some errors occur if equation (
The effects of the composition mass ratios on the melting temperatures of the CA-MA binary eutectic mixtures are shown in Figure
Effects of composition mass ratios (wt%) on the melting temperatures of CA-MA binary eutectic mixtures.
The DSC curve of the prepared CA-MA PCM is shown in Figure
DSC curve of the CA-MA binary eutectic mixture.
The phase change temperatures and latent heat of the CA-MA mixtures in some references [
Thermal properties of CA-MA mixtures in this study and other references.
PCM | Phase change temperature (°C) | Phase change latent heat (J/g) | References |
---|---|---|---|
CA/MA mixture (75.0/25.0 wt%) | 22.17 | 153.19 | Sarı et al. [ |
CA/MA mixture (76.0/24.0 wt%) | 23.64 | 147.70 | Fu et al. [ |
CA/MA mixture (78.0/22.0 wt%) | 19.65 | 149.02 | Gao and Qian [ |
CA/MA mixture (73.0/27.0 wt%) | 21.70 | 168.37 | Karaipekli et al. [ |
CA/MA mixture (72.0/28.0 wt%) | 18.21 | 148.50 | This study |
FTIR spectroscopy was conducted to ascertain the chemical structure of the CA-MA PCM. The FTIR spectra of the single fatty acid CA and MA and the CA-MA mixture are shown in Figure
FTIR spectra of CA, MA, and CA-MA.
The infrared spectrum curves of CA and MA in Figure
The infrared spectrum curve of the CA-MA in Figure
Thermal stability refers to the resistance of PCMs to high temperatures. Thus, whether a significant mass loss occurs in the temperature range in which the PCM was used can be determined. Generally, PCMs, especially organic PCMs, often undergo significant mass loss because of oxidation, decomposition, and volatilization reactions when subjected to high-temperature tests. Therefore, the temperature range must be controlled when using those materials. The thermal stability of fatty acid PCMs is commonly analyzed via TGA.
As shown by the thermal gravimetric (TG) and DTG curves of the CA-MA PCM in Figure
TG and DTG curves of CA-MA PCM.
The thermal cycle reliability of PCMs refers to whether the thermal energy storage performance decays after repeated storage/discharge processes. The reliability is an important parameter to measure the service life of PCMs. The thermal cycle reliability of PCMs is often tested by accelerated thermal cycling to study the changes of two important thermodynamic parameters, phase change temperature, and latent heat, before and after thermal cycling. The DSC curves of the CA-MA PCMs after 500, 1000, and 2000 thermal cycles are shown in Figure
DSC curves of CA-MA PCM after 500, 1000, and 2000 thermal cycles.
The changes in melting and freezing temperatures of the CA-MA PCM with the thermal cycling number are shown in Figure
Variations of the phase change temperatures of the CA-MA PCM with the thermal cycling number.
As the thermal cycling number increases, the variations of the melting and freezing latent heat values of the CA-MA PCM are shown in Figure
Changes in the latent heat of CA-MA PCM with the thermal cycling number.
As shown in Figures
FTIR spectra of the uncycled and cycled CA-MA PCMs.
The CA-MA binary eutectic mixture PCM was prepared. The thermal properties, thermal stability, and long-term cycling reliability of the binary eutectic mixtures were studied.
The DSC results show that the CA-MA PCM is highly suitable for LHTES and as backfill materials around the buried pipe for GSHP systems because of the phase change temperatures ( The TGA results show that the CA-MA PCM has excellent thermal stability below 100°C. Thermal cycling tests show that the CA-MA PCM has good long-term cycling thermal reliability because of the small variations of phase change temperatures and latent heat with thermal cycling number The FTIR results indicate that the molecular structure of the CA-MA binary eutectic mixture has not changed; the phase change heat storage performance and chemical properties of fatty acid are maintained. In addition, the CA-MA PCM does not undergo any chemical degradation after thermal cycling, and its thermal property changes only because the single fatty acids supplied contain a certain amount of impurities
The CA-MA PCM has promising application prospects in LHTES and as backfill materials around the buried pipe of GSHP systems due to its good performance.
The data availability of the manuscript can be found at
The authors declare that there are no conflicts of interest.
This study is supported by the National Natural Science Foundation of China (Project No. 51776226), Natural Science Foundation of Hunan Province (Grant No. 2018JJ2366), Scientific Research Fund of Hunan Provincial Education Department (Grant No. 18K097), and Key R&D Project of Hunan Province (Grant No. 2018GK2074).