Preparation and thermal properties of novel eutectic salt/nano-SiO2/ expanded graphite composite for thermal energy storage
Introduction
Solar energy is considered as one of the most promising renewable energy sources due to its abundant availability [1,2]. However, intrinsic fluctuations and daily variability of solar energy require efficient thermal energy storage (TES) technologies to mitigate the mismatch between solar energy availability and demand, thus ensuring reliable and continuous operation of solar energy systems. Therefore, TES has gained considerable attention due to its potential in improving the utilization of renewable sources and better use of thermal energy [[3], [4], [5]]. A crucial challenge in TES is the development of adequate TES materials with advanced performance such as high thermal conductivity, high energy density and low cost.
Molten salts have been widely used as media for TES because of their relatively large heat capacity, wide operating temperature range, low working pressure, high chemical stability and low cost [[6], [7], [8]]. However, molten salts have disadvantages of low thermal conductivity, moderate specific heat, and need of container (e.g. macro-encapsulation or tank) to confine the liquid phase, which greatly limit their application to two-tank or single tank thermal storage systems [9,10].
The supporting matrices of inorganic material can be used to develop stable shape composites, bring improvements in the thermal conductivity and avoid leakage. Supporting matrices include foam metals, EG, graphite foam, diatomite, and alumina-silicate material. Compared with other material, porous EG has excellent thermochemical properties, such as high porosity and high thermal conductivity [[11], [12], [13]]. Using the porous EG to confine molten salts has the potential to solve the problems of leakage and using a container, and also can improve the thermal conductivity of the composite [[14], [15], [16]].
Different kinds of molten salts, such as KNO3-NaNO3 [17], NaCl-CaCl2 [15], can be combined with EG to form stable heat storage composite to enhance the thermal conductivity by different preparation methods. Zoubir Acem [18,19] adopted a cold-compression method to prepare KNO3-NaNO3/EG composite and revealed that the thermal conductivity of the composite was 20 times higher than that of eutectic nitrate by adding 15–20 wt% EG. However, the eutectic nitrate would occur leakage if there was not enough void space in the composite when melting. Liu [20] prepared MgCl2-KCl/EG composite through the compression-sintering method and discovered that the addition of EG enhanced the thermal conductivity by 11 times, and the composite had better homogeneity with smaller volume expansion. The thermal cycling test showed that the composite had good shape and structural stability. Ren [21] prepared the Ca(NO3)2-NaNO3/EG composite by impregnation and sintering two-step method. It was found that the thermal conductivity of the composite with 7 wt% EG increased 7.3 times compared with eutectic nitrate. The melting point and morphology of the composite did not change significantly after 500 thermal cycles. Tian [15] investigated the thermal properties of NaCl-CaCl2/EG composite fabricated by an impregnating method. The results showed that the thermal conductivity of composite with 0.5-20 wt% EG was 123.2-701.1% higher than that of pure eutectic chloride. The above results showed that the thermal conductivity of molten salts could be improved sharply when combining with EG by different methods.
The specific heat of molten salts is essential for thermal energy storage. Researches have tried to add different nanoparticles into the molten salts to improve its specific heat [[22], [23], [24], [25], [26], [27]]. Malik [24] dispersed different mass fraction Al2O3 nanoparticles into NaNO3-KNO3 and found that the specific heat increased by 30.6% when the mass fraction of Al2O3 was 0.78 wt%. Chieruzzi [25] observed that the specific heat of NaNO3-KNO3 increased by 1-22% in liquid state with the addition of 0.5, 1 and 1.5 wt% SiO2, Al2O3, and TiO2 nanoparticles, respectively. Others also confirmed that adding the small concentration of SiO2 nanoparticles could improve the specific heat of molten salts [26,27] due to the surface effect of nanoparticles.
In short, the addition of EG can enhance the thermal conductivity of molten salts, and inorganic nanoparticles can improve its specific heat in a liquid state. However, the nanoparticles in molten salts are easily agglomerated after placement for a certain time, and the molten salts in the liquid state require containment (e.g. tanks) or encapsulation to avoid leakage. If the molten salts with nanoparticles are combined with EG together, the molten salt nanofluid can enter into the interior of EG micropore and be absorbed by it, which can not only avoid the agglomeration of nanoparticles in molten salts to improve its specific heat, but also improve its thermal conductivity. However, until now, there are few researches on the influence of nanoparticles and EG on the physical properties of molten salts [28]. Also, molten salts in a liquid state are usually used for sensible heat storage, and its latent heat cannot be used simultaneously. While combining the molten salts with nanoparticles and EG to form a shape stable thermal storage material, its latent heat and sensible heat can be used in the meanwhile.
In this paper, NaNO3-KNO3/nano-SiO2 was used as heat storage material and EG as structural supporting material to form the NaNO3-KNO3/nano-SiO2/EG shape stable PCM by the mechanical dispersion method. On this foundation, its microstructure and chemical compatibility were characterized. The thermophysical properties were fully characterized, including latent heat, specific heat, and thermal conductivity. Especially, the specific heat and thermal conductivity of the pure binary nitrate and nitrate/nano-SiO2/EG composite were compared, and the effects of EG mass fraction (5–20 wt%) and SiO2 content (0.1-3 wt%) on heat storage property were investigated extensively. Moreover, the stability of the NaNO3-KNO3/nano-SiO2/EG composite was verified through heating-cooling thermal cycles and high-temperature experiment.
Section snippets
Materials
Potassium nitrate and sodium nitrate (AR, ＞ 99% in purity) were purchased from Beijing Chemical Plant. The basic physical parameters were shown in Table 1. The silica nanoparticles (average diameter 20 nm, specific surface area 143 m2/g, spherical, ＞ 99% in purity) supplied by Beijing Deco Island Gold Technology Co., Ltd., China. EG powder was purchased from Qingdao Furuite Graphite Co, Ltd. of China, which expandable volume was 300 mL/g, and granularity was mesh 80. EG was prepared from the
Microstructure characterization of samples
The preparation of composite materials (EG = 5, 10, 15, 20 wt% and SiO2 = 1 wt%) by the mechanical dispersion method. Fig. 3 shows the samples’ pictures with different EG content to observe the samples shape stability. It can be seen that the surface smoothness decreases and small pits appear in the surface of the sample with 5 wt% EG content. This is due to the thermal expansion of nitrate during the sintering process, and EG cannot absorb it completely, so the leakage of liquid nitrate
Conclusions
In this study, a novel shape-stable NaNO3-KNO3/nano-SiO2/EG composite was prepared by the mechanical dispersion method. The NaNO3-KNO3/nano-SiO2 was chosen as thermal energy storage material, and EG with different mass fractions (5, 10, 15, and 20 wt%) was chosen as supporting matrix material. Microstructure, chemical compatibility, specific heat, melting point, latent heat, thermal conductivity, and thermal cycle stability were studied. The main findings are the following:
- (1)
During the composite
CRediT authorship contribution statement
Qiang Yu: Methodology, Formal analysis, Investigation, Writing - original draft, Validation. Yuanwei Lu: Conceptualization, Resources, Writing - review & editing, Funding acquisition. Cancan Zhang: Formal analysis. Xiaopan Zhang: Formal analysis, Investigation. Yuting Wu: Resources, Writing - review & editing. Adriano Sciacovelli: Resources, Writing - review & editing, Funding acquisition.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work is supported by the Qinghai Science and Technology Project (No. 2017-GX-A3) and the National Natural Science Foundation of China (No. 51906003). The authors would like also to acknowledge the Royal Society (No. IEC\NSFC\170264) and International Cooperation and Exchange Project from NSFC (No. 51811530308) for the financial support.
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2023, Journal of Energy StorageCitation Excerpt :The latent heat of the base salt is 342.9 J·g−1, it's higher than that of the NaCl-KCl-MgCl2 (240.0 J·g−1), NaCl-CaCl2-MgCl2 (201.5 J·g−1) and the NaF-NaCl-Na2CO3 (326.9 J·g−1) in literature, and the latent heat of EG-PCMs are in the range from 148.4 to 236.0 J·g−1, while the latent heat of EG-GNPs-PCMs are in the range from 207.6 to 340.7 J·g−1, indicates that the latent heat of CPCMs nearly decreases linearly with the increasing amount of EG. Because nanomaterials don't undergo phase change, which will cause the decrement on the latent heat of CPCMs [40], and the results indicate that the carbon-based nanomaterials cannot boost the specific heat capacity and latent heat of molten salt effectively like SiO2 or TiO2 [39] nanomaterials. Thermal diffusivity of the base salt and CPCMs are obtained in this work, as shown in Fig. 11.