Advanced experimental investigation of double hydrated salts and their composite for improved cycling stability and metal compatibility for long-term heat storage technologies

doi:10.1016/j.renene.2020.08.085

Renewable Energy

Volume 162, December 2020, Pages 447-457

https://doi.org/10.1016/j.renene.2020.08.085 Get rights and content

Highlights

•
NH₄T-M present a potential TCM for low to mid-temperature TCHS applications.
•
An enhancement of 68.7% was recorded for NH₄T-Zn@EG with high cycling stability.
•
The compatibility of Cu/CTCM provided a corrosion resistance improvement of 39.6%.

Abstract

Thermochemical heat storage (TCHS) systems offer promising solutions to the global energy problem by storing energy produced by renewable sources in a very veritable manner. In this context, the structural and thermal energy storage performance of Ammonium Tutton salts NH₄T-M (M = Zn, Mg, Cu, Ni, Fe) were investigated as thermochemical heat storage materials (TCM). The thermal measurements of these materials revealed suitable operating conditions with a good storage density for low to mid-temperature applications. The compatibility test provided severe corrosion behavior of Cu/NH₄T-Cu and Cu/NH₄T-Fe due to the strong pitting corrosion damages confirmed by SEM microscopy and the corrosion products were revealed by XRD and Raman spectroscopy. The obtained results showed that NH₄T-Zn was qualified as the prospective candidate for TCHS with the highest storage density of 1214.6 kJ/kg and less corrosive behavior. Meanwhile, the cycling stability showed a significant storage density decrease of 24.2% after 20 cycles. A novel composite TCM was developed based on NH₄T-Zn impregnated into expanded graphite (EG). NH₄T-Zn@EG provided a high storage density of 1080.6 kJ/kg inducing an enhancement of 68.7% compared to the unimpregnated salt with high cycling stability. Also, the corrosion resistance of Cu/NH₄T-Zn@EG involved an improvement of 39.6% opening the route towards new researches concerning the development of the NH₄T-Zn@EG system.

Graphical abstract

Introduction

The interesting benefits of thermal energy storage technologies in general and thermochemical heat storage (TCHS), in particular, have been performed by numerous studies [1]. TCHS systems based on potential TCM present a green route towards a decarbonized energy future by increasing solar energy uses as a sustainable renewable resource for the recovery of energy demand-supply mismatch [2]. The various benefits of TCHS have been widely investigated such as the high volumic energy storage density minimizing space requirements; long-term storage; low heat losses reducing the insulation investment, large flexibility of the operating temperature range; and good reversibility. In contrast, TCHS presents multiple challenges such as materials performance, storage density stability, and system costs [3]. These issues have impeded the industrial application of this technology. Recently, extensive researches have been carried out on innovative TCHS systems based on the sorption heat storage concept and several pilot-scale-up have been developed and tested [4,5]. Different systems were evaluated using zeolites [4,6], hydrated salts such as MgCl₂·6H₂O [7,8], and SrBr₂·6H₂O [9] and interesting results have been reported. Hydrated salts have been considered as the most promising materials and a large number of studies based on theoretical approaches and experimental measurements have been conducted [[10], [11], [12], [13]]. Moreover, other classes of hydrated salts have been evaluated focused on double hydrated salts as potential TCM for low to medium temperature (<300 °C) [14]. Ait Ousaleh et al. [15] reported an excellent storage efficiency of 77.4% for Na₂Zn(SO₄)₂·4H₂O, a high storage density of 4.7 GJ m⁻³, and good cycling stability. Besides, double hydrated salts could present the reversibility and thermal stability issues [15]. Consequently, an investigation of reversibility and cycling stability remains necessary to prove the capability of these materials to be used as TCM.

The Tutton salts are an isomorphous series of crystals which crystallizes in a monoclinic mode with the general formula of M₂^IM^II(SO₄)₂·6H₂O enclosing two octa-hydrated complexes [M^II(H₂O)₆]²⁺, where M^II is a divalent cation (Co, Zn, Fe or an ion of group 3d) and M^I is a monovalent cation (K, Rb, Cs, and NH₄) [16]. Tutton salts have interesting physicochemical properties and considerable interest was given to the development of these materials as potential materials with high heat absorbance of the solar collectors [17,18]. The dehydration behavior of (NH₄)₂M(SO₄)₂·6H₂O (M = Ni, Zn, Co, Cu, Fe, Mn) occurs at a temperature below 250 °C which presents a worthy range for low to medium TCHS applications [[19], [20], [21]]. Moreover, Tutton salts have optimal melting temperatures, high dehydration enthalpies, and good thermal stability which could be suitable for storing energy absorbed by solar collectors [[22], [23], [24], [25], [26]]. These parameters increase the tendency of these hydrates to be applied as TCM.

Nonetheless, the practical application of the hydrated salts as TCM still needs research efforts, to overcome different issues arising in the application of a storage system. The most drawback is the low thermal conductivity [27], which impacts the heat transfer, the agglomeration phenomena limiting the vapor permeation, and involving degradation after the cycling process [28]. In order to prevent these undesirable issues, growing attention has been devoted to composite thermochemical storage materials (CTCM) based on hydrated salts confined into porous matrices such as zeolite [7,29], expanded vermiculite [30], MOFs [31], Silica gel [[32], [33], [34]], graphite [35], activated carbon foam [36], and expanded graphite [37]. The use of inert or active matrices can play an important role in the stability of the impregnated salts as well as on the storage performance which could be understood considering several factors such as the salt content, the operating conditions, and temperature regeneration issues in the case of sorbent such as zeolites, silica gel, graphite [36,38]. The CTCM results present a viable tool to control the operating conditions and enhance global storage properties.

TCM compatibility with the structural materials including copper heat exchanger metal (HX) presents another drawback that strongly impacts the long-term service of the TCHS unit. In this context, the corrosion behavior of several TCM based on hydrated salts have been widely studied in recent decades [39,40]. Solé et al. [41] indicated that copper is recommended with caution and the use of coating is often required to protect the metal from the TCM corrosive behavior. The hydrated salt compositions such as sulfates [42], chlorides [43], nitrates [44] have been intensively investigated. Besides, the pH of salt solution induced by the accidental TCM dissolution due to the overhydration phenomena has a strong impact on the passive layer stability [45]. Meanwhile, the confinement of salt within a porous matrix could have beneficial impacts not only for storing energy but also on the durability of structural materials. The used matrix as a physical barrier reduces the direct salt-metal contact decreasing the corrosion rate and preventing the severe metal surface damage [46]. Indeed, there are few literatures concerning the compatibility of CTCM with structural material, especially for hydrated salts composites.

The main objective of this paper is the development of new TCM for TCHS applications focus on ammonium Tutton’s salts (NH₄)₂M(SO₄)₂·6H₂O with (M = Zn, Mg, Cu, Ni, Mn, Fe). The investigation of dehydration behavior, structural reversibility, cycling performance, and corrosion behavior are presented. Furthermore, the development of novel CTCM based on the most prospective Tutton salt candidate confined into EG fillers is reported. The heat storage performances and its compatibility with copper metal are investigated.

Section snippets

Synthesis

(NH₄)₂M(SO₄)₂·6H₂O crystals were grown using the slow evaporation method of saturated aqueous solutions with an equimolar ratio containing (NH₄)₂SO₄ and MSO₄.nH₂O (M = Zn, Mg, Cu, Ni, and Fe) respectively. The solutions were completely diluted using a magnetic stirrer for 2 h at room temperature, then placed in an oven at 35 °C. After two weeks, the obtained crystals were recovered from the solutions, washed with acetone, and dried in air. The crystals were labeled as NH₄T-M with (M = Zn, Mg,

Structural investigation of NH₄T-M (M = Zn, Mg, Cu, Ni, Fe)

The SEM images of synthesized salts are illustrated in Fig. 2, the crystals of double hydrated salts showed prismatic morphology with defined forms and a micrometer scale. The X-ray powder diffraction patterns of the prepared materials NH₄T-M (M = Zn, Mg, Cu, Ni, Mn, Fe) (Fig. 3) are in good agreement with those reported in database files (NH₄T-Zn:01-072-1108; NH₄T-Mg: 01-089-2656; NH₄T-Cu: 01-071-2166; NH₄T-Ni:01-082-0531; NH₄T-Fe:01-071-2154). The XRD results confirm the high purity of the

Conclusion

In the present work, the thermochemical energy storage performance of ammonium Tutton hydrated salts NH₄T-M (M = Zn, Mg, Cu, Ni, Fe) was reported. The thermal analysis demonstrated the high storage density of these materials with suitable operating conditions for low to mid-temperature applications. Good reversibility for the overall double salts was proved while a significant storage density decrease of 34.8%, 33.1%, and 35.2% was recorded for NH₄T-Cu, NH₄T-Mg, and NH₄T-Ni respectively after

Authorship contributions

Category 1

Conception and design of study : Hanane Ait Ousaleh, Sair Said, El Bouari Abdeslam, Faik Abdessamad.

Acquisition of data : Zaki abdelali, Hanane Ait Ousaleh, Sair Said, Abboud Younes.

Analysis and/or interpretation of data: Sair Said, Hanane Ait Ousaleh, Abboud Younes.

Category 2

Drafting the manuscript: Sair Said, Hanane Ait Ousaleh.

Revising the manuscript critically for important intellectual content: Abbous Younes, Faik Abdessamad, El Bouari Abdeslam, Hanane Ait Ousaleh.

Category 3

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.

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Advanced experimental investigation of double hydrated salts and their composite for improved cycling stability and metal compatibility for long-term heat storage technologies

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Synthesis

Structural investigation of NH4T-M (M = Zn, Mg, Cu, Ni, Fe)

Conclusion

Authorship contributions

Declaration of competing interest

Renew. Sustain. Energy Rev.

Energy Procedia

Energy

Renew. Sustain. Energy Rev.

Sol. Energy Mater. Sol. Cell.

Energy

Appl. Energy

Sol. Energy

Appl. Energy

Appl. Energy

Sol. Energy Mater. Sol. Cell.

J. Cryst. Growth

J. Chem. Therm.

J. Mol. Struct.

Mater. Sci. Eng., B

J. Chem. Thermodyn.

Appl. Therm. Eng.

J. Energy Stor.

Sol. Energy Mater. Sol. Cell.

Energy

Sol. Energy

Sol. Energy Mater. Sol. Cell.

Energy Convers. Manag.

Appl. Energy

Appl. Therm. Eng.

Energy Procedia

Renew. Energy

Corrosion Sci.

Structural investigation of NH₄T-M (M = Zn, Mg, Cu, Ni, Fe)