Elsevier

Applied Thermal Engineering

Volume 89, 5 October 2015, Pages 204-208
Applied Thermal Engineering

Research paper
Aluminum and silicon based phase change materials for high capacity thermal energy storage

https://doi.org/10.1016/j.applthermaleng.2015.05.037Get rights and content

Highlights

  • Six kinds of materials were investigated for thermal energy storage (550–1200 °C).

  • Partial melting of Al–Si materials show progressively changing temperatures.

  • Studied materials can be used in three different working temperature ranges.

  • Materials are potentially good candidates for thermal energy storage applications.

Abstract

Six compositions of aluminum (Al) and silicon (Si) based materials: 87.8Al-12.2Si, 80Al–20Si, 70Al–30Si, 60Al–40Si, 45Al–40Si–15Fe, and 17Al–53Si–30Ni (atomic ratio), were investigated for potentially high thermal energy storage (TES) application from medium to high temperatures (550–1200 °C) through solid–liquid phase change. Thermal properties such as melting point, latent heat, specific heat, thermal diffusivity and thermal conductivity were investigated by differential scanning calorimetry and laser flash apparatus. The results reveal that the thermal storage capacity of the Al–Si materials increases with increasing Si concentration. The melting point and latent heat of 45Al–40Si–15Fe and 17Al–53Si–30Ni are ∼869 °C and ∼562 J g−1, and ∼1079 °C and ∼960 J g−1, respectively. The measured thermal conductivity of Al–Si binary materials depend on Si concentration and is higher than 80 W m−1 K−1 from room temperature to 500 °C, which is almost two orders of magnitude higher than those of salts that are commonly used phase change material for thermal energy storage.

Introduction

Replacement of fossil fuels by renewable energy sources especially solar energy is a clear solution for the future of energy. With the decreased cost of photovoltaic (PV) and concentrated solar power (CSP) for electricity generation, the challenge of energy storage becomes more important due to the unavailability of sunlight at night time. Energy storage not only reduces the mismatch between the supply and the demand, but also improves the performance and reliability of energy systems, hence plays a crucial role in the future energy needs [1], [2]. For PV, batteries can be used for energy storage, however it is very expensive. In recent years, great attention has been drawn to CSP [3], in which the storage of thermal energy is much cheaper than electrical energy storage.

For thermal energy storage, either sensible heat or latent heat of the storage materials is of great interest. Sensible heat normally requires a large volume of heat storage material due to its relatively low thermal capacity plus another drawback that the heat exchange is not at a constant temperature. Phase change materials (PCMs) usually provide a large amount of heat due to phase change between different states during charging and discharging at a constant phase changing temperature [4], [5], [6], [7], [8], [9], [10], [11]. Among all the heat storage materials, up to now, salts are often used for this purpose due to the low cost and abundance in nature. However, the rather low thermal conductivity of salt can drastically decrease the efficiency of heat storage and extraction [9], [11]. Therefore, much effort has been devoted to improve the thermal conductivity of salt PCMs by utilizing a fin tube configuration, adding metal materials, etc. [12], [13], [14], [15], which, however, lead to significant weight and cost increasing. Besides, corrosion is also an important issue when inorganic salts are used as PCMs, particularly under the condition of high working temperatures.

Compared with other PCMs used in latent heat thermal energy storage method, metallic PCMs display high potential in high energy density TES systems because of their good thermal, physical and chemical properties [16]. Several studies have been reported on the thermophysical properties of different metals and alloys as PCMs [17], [18], [19], [20], [21]. It was reported that the large latent heat on a mass or volume basis were obtained in binary and ternary alloys with abundant elements Al, Cu, Mg, Si, and Zn [22], but not all of the metallic materials are suitable as PCMs for use in TES systems because of some drawbacks of physical and chemical properties such as high vapor pressure, short-term chemical stability, and fire hazard.

Considering the good performance of Al and Si based compositions: suitable phase change temperatures, high solid–liquid latent heat densities, and good thermal reliabilities, eutectic Al–Si alloy was mostly investigated as PCM in medium temperature TES systems over the past few decades [16], [23]. However, the syntheses and properties of none eutectic Al and Si compositions were seldom reported. For applications requiring temperature higher than 580 °C such as concentrated solar systems [24], eutectic Al–Si alloy is not suitable. Hence, it is very urgent to develop new Al and Si based PCMs with higher temperature suitable for solar TES systems by changing the ratio of Al/Si or adding new elements.

In this paper, six kinds of Al and Si based materials, which can be used in three different working temperature ranges due to their melting points, were prepared by arc-melting as medium to high temperature PCMs. The important physical properties of the materials including the melting temperature, the latent heat of the solid–liquid phase transition, and the temperature-dependent thermal conductivity of the compositions were investigated in details.

Section snippets

Preparation of materials

Six compositions of 87.8Al-12.2Si, 80Al–20Si, 70Al–30Si, 60Al–40Si, 45Al–40Si–15Fe, and 17Al–53Si–30Ni were prepared by arc-melting. Al (shot, 99.999%, Alfa Aesar), Si (bulk, 99.9999%, Alfa Aesar), Fe (particle, 99.98%, Alfa Aesar), and Ni (chunk, 99.95%, Alfa Aesar) were weighed according to the designed ratio. In order to ensure a homogeneous composition, each sample was melted for five times by arc-melting. The whole arc-melting process was conducted under the protection of argon gas. Before

Thermal properties of Al and Si based compositions

Fig. 1 shows the thermal behavior with melting temperature and latent heat of six compositions measured by DSC. From the curves, the melting point and latent heat can be obtained. Obviously, the PCMs absorb thermal energy during the melting process, all the compositions display sharp peaks and solid–liquid latent heats are higher than 450 J g−1, which is much higher than the typical value of about 300 J g−1 of the commonly used salts such as 56NaCl–44MgCl2 (mol.%) at 430 °C [25]. As shown in

Discussion

Six materials with different Al and Si compositions were studied as high temperature PCMs. The amount of heat stored in these materials depends on the composition of materials. The storage heat of Al–Si compositions increases with Si concentration because of the very high latent heat of Si. According to theoretical predictions [26], the latent heat of a material largely depends on the molar fraction of the elements, and the latent heat of pure Al and pure Si are about 397 and 1788 J g−1,

Conclusions

In this study, four binary Al–Si materials with different Al and Si ratios and two ternary materials with Fe or Ni added were studied as medium to high temperature PCMs for thermal energy storage applications. The results indicated that these Al and Si based materials are potentially good candidates for TES applications. The studied four binary materials: 87.8Al-12.2Si, 80Al–20Si, 70Al–30Si, 60Al–40Si, have heat storage capacity of ∼499, ∼553, ∼644, and ∼721 J g−1 with the same final melting

Acknowledgements

This work is supported by ARPA-E's HEATS program (Award Number: DE–AR000018, managed by Dr. Ravi Prasher and Dr. James Klausner) and the Open Research Subject of Key Laboratory of Special Materials and Preparation Technology of Sichuan Province (Award Number: szjj2015-087).

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    These authors contributed equally to this work.

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