The potential of the use of salt hydrates MgCl 2 •6H 2 O (bischofite) with typical impurities of the Salar de Atacama as a thermal energy storage material was evaluated with special attention to its corrosion behavior. Bischofite behavior is compared with that of commercial salt MgCl 2 •6H 2 O. The corrosion tests were conducted with metal sheets (copper, aluminum and stainless steel) partially immersed in molten salt hydrates at a temperature of 120°C during 1500 hrs. The results showed minimum corrosion on all the immersed surfaces of all the metals. However, very sever corrosion was observed at the salt/air interface due to a known phenomenon of oxygen enhanced corrosion usually found even with water at ambient temperature. The corrosion products were determined with scanning electron microscopy (SEM) and X-ray diffraction (XRD) technique. For salts hydrates bischofite and MgCl 2 •6H 2 O, the results show the formation of cuprite (Cu 2 O) and hematite (Fe 2 O 3) on copper and stainless steel samples, respectively. For all cases studied in the present work, several chloride compounds were identified as corrosion products.
In this study, an inorganic mixture of 40 wt.% MgCl 2 •6H 2 O and 60 wt.% Mg(NO 3) 2 •6H 2 O was impregnated into two types of the expanded graphite (EG): EG flakes and EG matrix. Different methods of preparation usually used in the organic composite preparation, such as direct blending, vacuum impregnation, ultrasonic impregnation and immersion were applied in this work. Considering the samples with EG matrix, it can be concluded that the optimal result of the encapsulation and ΔH ratio can be reached with the experimental time of 1, 3 and 4 hours using the vacuum, ultrasound and immersion methods, respectively. Moreover, the percentage of encapsulation increases with the experimental time and with the EG percentage for the EG matrix and for the EG flakes, respectively. Concerning the supercooling phenomena, the results show that the presence of EG reduces the difference between fusion and crystallization temperature for up to 65%, acting as а nucleating agent. The resulting composite samples (CPCM) were proved to have good latent heat and a significant reduction of the supercooling effect, which eliminates the need to use nucleating agents, that are essential for pure PCM of 40 wt.% MgCl 2 •6H 2 O and 60 wt.% Mg(NO 3) 2 •6H 2 O. Moreover, using EG suppresses significantly the melting time which indicates the heat transfer enhance of the mixture. This circumstance could allow to empower the utilization of the mentioned mixture in different fields of thermal energy storage at low temperature.
Summary
In this study, an inorganic mixture based on bischofite (industrial by‐product) was developed and characterized for its application as a phase change material for low‐temperature thermal energy storage. The most appropriate composition was established as 40 wt% bischofite and 60 wt% Mg(NO3)2 · 6H2O. Thermophysical properties were defined and compared with those of the mixture with synthetic MgCl2 · 6H2O. The heat of fusion and melting temperature were measured as 62.0°C and 132.5 kJ kg−1 for the mixture with MgCl2 · 6H2O and 58.2°C and 116.9 kJ kg−1 for the mixture with bischofite. The specific heat capacity values, cycling, and thermal stability for both mixtures were also determined. For the mixture with MgCl2 · 6H2O, the densities of the solid and liquid states were 1517 kg m−3 (ambient temperature) and 1515 kg m−3 (at 60‐70°C), respectively. For the mixture with bischofite, the densities of the solid and liquid states were 1525 kg m−3 (ambient temperature) and 1535 kg m−3 (at 60‐70°C), respectively. Both mixtures show supercooling of about 23.4 and 34.1°C for the mixture with bischofite and MgCl2 · 6H2O, respectively. In addition, it was shown that supercooling may be reduced by increasing the quantity of material tested. Thereby, it was established that an inorganic mixture based on bischofite is a promising PCM for low‐temperature thermal energy storage applications.
Summary
The battery thermal management system based on phase change materials (PCMs) has proven to be a safe, inexpensive, and high‐performance technology, which is currently gaining popularity over the other battery thermal management systems. PCMs are able to absorb heat generated by batteries, prolong battery life, and improve their performance. Organic compounds, such as paraffin, are widely used as phase change materials for the battery thermal management systems; however, there are no published data on the application of inorganic PCMs. Therefore, the main objective of this work is developing of two composite inorganic PCMs based on magnesium chloride hexahydrate and characterizing their properties for passive thermal control of lithium‐ion battery packs. Moreover, to have a valid baseline and compare the behavior of two inorganic PCMs with currently commercialized and well investigated organic PCM, paraffin wax was used as reference material for both mixtures. All three PCMs were impregnated into the expanded graphite matrix to enhance their thermal conductivity. The material characterization studies, including thermal properties investigation, density and viscosity measurements, soaking and compression testing, evaluation of thermal expansion, thermal conductivity, and micro X‐ray fluorescence analysis, were conducted for all PCMs. The results indicate that both inorganic mixtures are appropriate for thermal management of Li‐ion battery packs. Future work with the developed and characterized composite inorganic PCMs will include electrical cycling studies and nail penetration tests to reveal their effectiveness for passive thermal management of Li‐ion battery packs.
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