Elsevier

Applied Energy

Volume 148, 15 June 2015, Pages 87-92
Applied Energy

Thermal conductivities and characteristics of ternary eutectic chloride/expanded graphite thermal energy storage composites

https://doi.org/10.1016/j.apenergy.2015.03.020Get rights and content

Highlights

  • A new ternary chloride/expanded graphite was designed for solar energy storage.

  • The composition of eutectic salts were predicted from calculated phase diagrams.

  • Thermal properties of composites were measured including heat capacity, specific heat.

  • The effect of expanded graphite on thermal conductivities was investigated.

Abstract

Ternary eutectic chloride (NaCl–CaCl2–MgCl2)/expanded graphite (EG) composites were prepared for thermal energy storage applications at a solar thermal power plant. Heat capacity and latent heat thermal energy storage (LHTES) characteristics of the composites including the melting temperature and latent heat capacity were investigated using a differential scanning calorimetry (DSC) technique, and the effects of EG additives in the composite on thermal conductivities were evaluated using a hot disk analyzer. The ternary eutectic chloride/EG composites with expanded volumes ranging from 150 to 250 g/L and the EG mass fractions ranging from 0 to 5 wt% were prepared by absorbing liquid chlorides into the EG at high temperature. Experimental results indicated that the specific heat capacity of solid composites decreased with EG mass fraction and temperature. In the liquid state, the effect of the EG loading on the specific heat was not uniform. Specific heat capacity reached maximum at 1 wt% of EG loading and the specific heat capacities of all samples rose with temperature. The melting temperatures of the composites were the same as the pure ternary eutectic chloride, but the phase change latent heat decreased with the mass percentages of EG in the composites. The thermal conductivities of the composites were 1.35–1.78 times higher than that of the pure ternary eutectic chloride.

Introduction

Thermal energy storage is a key process which has been widely applied in various energy transportation and utilization system. Three major technologies currently being considered for heat storage include: sensible heat-, latent heat- and thermo-chemical-heat storage. Among the three technologies, latent heat thermal energy storage (LHTES) is most promising for heat storage due to the consistent temperature and large thermal capacity [1], [2], [3].

Molten salts exhibit desirable thermal characteristics such as the wide operating temperature range, large latent heat capacity, small supercooling, low vapor pressure and good thermal stability, and thus being considered as potential phase change materials (PCMs) for LHTES applications [4], [5], [6], [7]. Molten salts can be used high-temperature heat storage at concentrating solar power (CSP) plants potentially. Solar Salt (weight ratio of NaNO3/KNO3 to be 60/40) and HITEC Salt (weight ratio of NaNO3/KNO3/NaNO2 to be 7/53/40) are two commercial eutectic salts as thermal energy storage media used in several CSP plants [8], [9]. Still, it is urged to develop more eutectic salts for various temperature applications. Reddy developed eutectic salts, LiNO3–NaNO3–KNO3 and LiNO3–NaNO3–KNO3–NaNO2, which had a lower melting point suitable for use in parabolic trough solar power generation [4], [10]. Fernández studied the effects of the two additives, LiNO3 and Ca(NO3)2, on the thermal properties of the Solar Salt for high-temperature energy storage [11]. Due to the higher melting point, chlorides are enable to generate higher temperature steam for a better Rankine cycle performance. Oztekin reported that a eutectic chloride salt (weight ratio of NaCl/MgCl2 to be 57/43, melting point = 444 °C) encapsulated in stainless steel were used to store solar thermal energy in CSP plants [12], [13]. In addition, that there are huge amount of waste chloride salts in Chinese salt lake, thus the material cost will be dramatically decreased.

However, the low thermal conductivity of salts induces the low heat transfer rate during melting and solidification processes, which is the major drawback that limited the application of molten salts as PCMs at a large scale. Progresses have been made to enhance heat transfer by various methods, such as dispersing high thermal conductivity materials, adding finned configurations, and encapsulating PCMs [14], [15], [16], [17], [18]. Sarı and Zhang studied the effects of EG addition on thermal conductivities and melting time, melting temperatures, as well as latent heat capacities of paraffin/expanded graphite composites to obtain form-stable composite as PCMs [19], [20]. For form-stable composite formed by polyethylene glycol/expanded graphite, Wang showed that the maximum weight percentage of polyethylene glycol was as high as 90 wt% without any leakage during the melting period, with the latent heat of 161.2 J/g and the melting point of 61.46 °C [21]. Wang reported a novel sebacic acid/expanded graphite composite PCM for medium-temperature solar heat storage, which had the phase change temperature of 128 °C and the maximum latent heat of 187 J/g [22]. Based on the previous work, it has been realized that porous expanded graphite (EG) is a promising candidate as heat transfer promoter, due to its excellent features of high thermal conductivity, good stability, lightweight and low cost. However, most work focused on organics/EG composite PCMs for low-temperature energy storage, and there is a lack of attention on the performance of eutectic salts/EG composites for high-temperature usage.

In our group’s previous work, a new ternary eutectic chloride (NaCl–CaCl2–MgCl2) was developed as PCMs because of its suitable application in high-temperature and abundant reserves, which was prepared by a statically mixing method. Based on this eutectic chloride, in this paper, ternary eutectic chloride/EG composites were prepared with various mass fractions of EG from 0.5 to 5 wt%. The thermo-physical properties and thermal conductivities of pure ternary eutectic chloride and composites were experimentally investigated and further discussed.

Section snippets

Expanded graphite preparation

Natural graphite flakes (purity of 99%, 50–60 mesh, produced in Ha Da Company of Qingdao, China) was acid treated by concentrated sulfuric acid (H2SO4, 95–98%). Potassium permanganate (KMnO4) was added as an oxidative agent. First, H2SO4 and natural graphite flakes were poured into a beaker and mixed sufficiently. Then, KMnO4 was added into the mixture, and stirred 3 min followed by adding distilled water into a beaker and stirred 30 min at 70 °C. After that, the obtained slurry was poured into

Morphology of ternary eutectic chloride/EG composite

Fig. 2 shows SEM images of natural graphite flakes, EG and ternary eutectic chloride/EG composites. In Fig. 2a, natural graphite flake was stratiform, and the interaction force between layer and layer was van der Waals force. After oxidative treatment of intercalation and microwave radiation treatment, the space was enlarged along horizontal and vertical direction, and then EG was formed with worm-like microstructure as showed in Fig. 2b. The increased pores and space made it easy to absorb and

Conclusions

A new ternary eutectic chloride/EG composite was prepared and studied as high temperature PCMs for CSP plants in the work. The ternary eutectic chloride was fully filled by the porous EG at the EG loading higher than 5 wt%, and no leakage from EG was allowed in the solid–liquid phase change process. The specific heat capacity of composites decreased with EG loading and temperature in the solid state. The effect of the EG loading on the specific heat was not uniform in liquid state. Specific heat

Acknowledgements

The authors are gratefully acknowledge the financial support from State Key Program of National Natural Science of China (51436009), National Nature Science Foundation of China (51106185), National Natural Science Foundation of China (51376067), National Basic Research Program of China (2010CB227103), Nature Science Foundation of Guangdong Province (S2012040007694), and Fundamental Research Funds for the Central Universities (12lgpy23).

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