Elsevier

Renewable Energy

Volume 145, January 2020, Pages 2629-2636
Renewable Energy

Functional phase change composites with highly efficient electrical to thermal energy conversion

https://doi.org/10.1016/j.renene.2019.08.007Get rights and content

Highlights

  • A novel FSPCM with highly efficient electrical to thermal energy conversion is reported.

  • The composite increased thermal conductivities by 252%.

  • Its resistivity is 2 orders of magnitude lower than that of pure PEG.

  • The composite achieves rapid efficiently electrothermal conversion up to 70.2%.

Abstract

Electrothermal functional phase change materials (PCMs) have poor mechanical properties and low thermal conductivities (k). To address this problem, a new electrothermal functional composite PCM, denoted PEG2000-CaCl2/CNTs, was synthesized in one step by ligand substitution using a polyethylene glycol with a molecular weight of 2000 (PEG2000) as the PCM, carbon nanotubes (CNTs) as a k- and electrical conductivity (σ)-enhancing framework material and Cl−1 as the ligand. The PEG2000-CaCl2/CNTs composite PCM was characterized in situ by differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy, thermogravimetric analysis and scanning electron microscopy. The experimental results demonstrated the following. The coordinate covalent bonds in the PEG2000-CaCl2/CNTs composite PCM enhanced the mechanical performance of the material. The compressive strength of the composite PCM sample with 20 wt% CNTs exhibited excellent compressive strength at 80 °C. Adding CNTs to the PEG2000 PCM at a ratio of 20 wt% increased k by 252% and reduced the electrical resistivity from 9500 to 90 Ω∙m. The energy stored in the composite PCM could be triggered and released under relatively low voltages (1.5–2.0 V). This result could significantly reduce energy consumption. Additionally, after 100 melt and crystallization cycles, the DSC curve of the composite PCM changed by less than 3.0%. After 50 electrical-to-thermal energy conversion cycles, the heat storage/release curves of the composite PCM changed by less than 5.0%. The synthesized functional composite PCM exhibits excellent σ, k and thermostability and exceptional mechanical properties, and it opens up new applications for PCMs.

Introduction

As the pace of the human industrialization process increases, global energy consumption is rising sharply. Global energy demands and fossil fuel-based energy consumption patterns continue to exacerbate problems such as energy shortages, unreasonable energy structures, environmental pollution and climate change [[1], [2], [3], [4]]. To address these problems, people are rapidly exploring renewable energy sources, such as solar, wind, nuclear and recycled thermal energies, as new energy sources to replace conventional energy sources (e.g., fossil fuels) [[5], [6], [7], [8]]. Renewable energy sources have garnered large amounts of attention thanks to their environmentally friendly acquisition process, renewability and abundance [9]. Particularly in recent years, high-efficiency thermal energy storage has attracted extensive attention due to its capacity to adjust the imbalance between supply and demand and its crucial role in sustainable energy [10,11]. Of the energy storage technologies, phase-change energy storage technology involving the use of phase-change materials (PCMs) has become an effective way to store thermal energy thanks to the advantages of PCMs, such as high energy storage density and a nearly isothermal energy storage process [7,12]. Moreover, PCMs have other advantages. For example, these materials undergo relatively small volumetric changes during the phase-change process, are nontoxic, and have long life cycles [13,14].

An inexpensive, controllable heat source is the key to using a PCM to achieve thermophysical storage. The heat sources currently in use, including solar energy [15], geothermal energy [16] and industrial waste heat [17], are limited by external factors and thus cannot be used extensively. As a clean and easily controllable and convertible energy source, electrical energy has become a high-efficiency heat source for thermal storage using PCMs. On the other hand, phase-change energy storage technology can also address the problem of the differences between the peaks and valleys of electrical power [18,19]. Using PCMs to realize electrical-to-thermal energy conversion can improve the efficiency of energy use, thereby meeting the goal of using energy at high efficiency. To manage the conversion of electrical to thermal energy, developing high-energy storage density electrothermal PCMs has become a focus of research.

The electrical and thermal conductivities (σ and k) of a PCM are important factors affecting its capacity to convert electrical energy to thermal energy. However, typical PCMs have an electrical resistivity (ρ) on the order of magnitude of 7–12 and a k of 0.1–0.5 W/m∙K. The extremely low electronic σ and k limit the application of PCMs in practice [[20], [21], [22], [23]]. In the past few years, researchers have added various base materials (e.g., carbon fibers, metal fillers, graphite, metal nanowires, metal foams, expanded graphite and sponges [[24], [25], [26], [27]]) into PCMs as their frameworks to enhance their k and σ. Due to their exceptional properties (excellent k, high chemical inertness, low density, high surface area, large pore volume and excellent σ), carbon-based support materials have been extensively investigated [28]. As a typical one-dimensional structural carbon material, carbon nanotubes (CNTs) have been extensively used in studies to improve the k and σ of PCMs. For example, Wang et al. introduced CNTs fillers into a paraffin-based PCM and found that the CNTs fillers formed heat transfer paths in the organic matrix, resulting in a 40% increase in the k of the PCM [29]. Li et al. [30] prepared a composite PCM with CNTs and stearic acid and found that the phase-change latent heat (ΔH) of the PCM reached 111.8 J/g and the energy storage efficiency increased significantly as a result of the addition of CNTs. Wang et al. [31] prepared a new CNTs/PCM composite by introducing CNTs into polyethylene glycol (PEG)-10,000 (PEG10000)-co-N,N′-dihydroxyethyl aniline and found that the energy storage efficiency of the composite was significantly higher than that of the pure PEG10,000. Liu et al. [32] used a CNTs as a porous skeleton to encapsulate n-eicosane (C20) and made an electrically conductive nanocomposite with modulated performance. Chen et al. [33] reported a carbon nanotube sponge encapsulated paraffin wax composite that can store thermal energy by applying a small voltage or by light absorption with high electro-to-heat or photo-to-thermal storage efficiencies (40%–60%).

The morphology of a PCM during the electrical-to-thermal energy conversion process is another important parameter that affects its application. When a PCM melts and becomes a liquid, the electronic current collectors are prone to short-/open-circuiting. Therefore, electrically and thermally conductive composite PCMs should also possess high compressive strength in a high-temperature (T) environment (i.e., after melting). However, as a framework, CNTs are unable to prevent the leakage of PCMs during the solid-to-liquid phase-change process. A PEG/CNTs composite has an unstable form (the PCM collapses after T exceeds its melting point) [34]. While capillary action is conducive to limiting a liquid PCM within the pores, extensive volume expansion during the phase-change process causes the PCM to disperse from the pores, and consequently, the PCM forms a network of seepage through the matrix [35]. Therefore, preparing form-stable PCMs (FSPCMs) is the most effective means to address the aforementioned problem.

As PCMs, PEGs [36] have advantages such as high phase change enthalpy, adjustable phase-change T (TC), excellent chemical stability, nontoxicity, noncorrosiveness and low saturated vapor pressure, and thereby, PEG-based PCMs have become ideal T-controlled materials that have attracted particular interest. In our previous study [37], we successfully prepared a PEG-CaCl2 FSPCM using the coordination chemistry approach and ligand substitution technique. On this basis, to improve the k and σ of the PEG-CaCl2 FSPCM, in this study, we used electrically conductive CNTs as an electrically and thermally conductive matrix and the PEG-CaCl2 coordination complex as the PCM. We prepared a new composite PCM for thermal energy storage, denoted PEG2000-CaCl2(1:2)/CNTs, in one step by adding electrically and thermally conductive CNTs to a mixture of ethanol, CaCl2 and a PEG with a molecular weight of 2000 (PEG2000). The PEG2000-CaCl2/CNTs composite PCM has excellent properties such as a stable form and high compressive strength, σ and k.

Section snippets

Materials

All the experimental chemicals were of analytical grade and were used without further purification. The polyethylene glycol (PEG2000, purity: 98%, m. p.: 60–63 °C) was acquired from the Aladdin Company. The calcium chloride (CaCl2) was acquired from the Chengdu Kelong Chemical Reagent Factory. The multiwall carbon nanotubes (CNTs, purity: >95%, OD: 20–30 nm, ID: 5–10 nm, length: 10–30 μm, bulk density: 0.28 g/cm3, true density: 2.1 g/cm3) were supplied by Chengdu Organic Chemicals Co. Ltd.,

Macroscopic and microscopic characteristics of the composite PCM

To examine the form characteristics of the composite, the PEG2000-CaCl2/CNTs-20 wt% composite PCM sample was characterized by a leak test. Fig. 1 shows the results. The PEG2000, PEG2000-CaCl2 and PEG2000-CaCl2/CNTs-20 wt% samples were pressed into flake structures, which were then placed in 25 °C and 80 °C environments, respectively, for 2 h. At 25 °C, all the samples maintained a solid state, and no leakage occurred. When the samples were placed in the 80 °C (higher than the melting point of

Conclusions

In this study, a new electrothermal functional composite PCM, PEG2000-CaCl2/CNTs, was synthesized in one step using the ligand substitution method with PEG2000 as the PCM, CNTs as thermal and electrical conduction-enhancing framework material and CaCl2 as the ligand and was subsequently characterized in situ. The prepared PEG2000-CaCl2/CNTs composite PCM exhibited excellent stability and compressive strength, mainly because the PEG-CaCl2 framework was synthesized based on the coordination

Acknowledgements

The work is supported by Natural Science Foundation of China (NO: 51678488), Sichuan Province Science and Technology Plan Project (No. 2018GZ0161) and Sichuan Province Science and Technology Plan Project (No. 2017JY0252).

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