Water sorption-based thermochemical storage materials: A review from material candidates to manufacturing routes

Abstract

A comprehensive and updated review is provided in this article, with a focus on water sorption-based thermochemical storage (WSTCS) materials, covering materials and their manufacturing routes. The state of the art of 22 most relevant salt hydrates is classified into seven groups (bromides, sulphates, carbonates, chlorides, nitrates, hydroxides, and sulphides) and studied as candidates. This is followed by a discussion on TCS material manufacturing, covering both conventional (shaping, pelletizing, etc.) and more advanced routes (e.g., extrusion, 3D printing, encapsulation, etc.). Finally, concluding remarks are presented, including limitations and future potentials for TCS research.

Highlights

1 Introduction

Nowadays, renewable energy (wind, solar, tidal, geothermal, etc.) is a hot topic as it offers energy resources with the potential of eventually reaching a zero-carbon energy future. One of the biggest challenges to achieve this is the mismatch between most renewable energy supply and end-user demand (Aydin et al., 2015). Energy storage has a key role to play in this regard, as it can store surplus energy to bridge generation and demand. Thermal energy storage (TES) is one of the storage technologies that have attracted increasing attention in recent years (N’Tsoukpoe et al., 2009). TES can be divided into sensible, latent, and thermochemical categories. This study concerns thermochemical heat storage (TCS), which has a very high energy density and virtually zero loss during storage and is particularly suitable for large-scale, medium- to-long-term energy storage, although it can also be used for meeting short-term needs. The TCS, however, is currently still in its early stage of development with the bulk of the activities still embedded in academic research (Delta Energy & Environment Ltd, 2016).

Among the TCS technologies, sorption offers a high storage capacity in a low-to-medium temperature range. Researchers have extensively worked on developing the systems up to the prototype level; however, the full understanding and complexity of these reactions’ mechanisms, given the poor scalability of the systems and the scattered information among the published studies, suppose a barrier to the TCS technology deployment. Although some systems have been tested at the reactor level in the literature (Zondag et al., 2013; de Boer et al., 2014; Michel et al., 2014; Michel et al., 2016), the levels have not yet been interconnected from the conceptualization–laboratory scale to prototype level.

Most of the reviews published in the area reported the materials used, systems implemented, and development status at the time being. A brief state of the art of the most relevant (most-cited) reviews published is provided in the following lines. N’Tsoukpoe et al. (2009) published a landmark review in 2009 that set the pathways to thermochemical energy storage. The authors presented the materials used in long-term sorption solar energy storage and the ongoing projects at that time. They highlighted closed absorption systems as the future direction for TCS systems, concluding that this system could lead to satisfying results with proper system optimization tasks. Cabeza et al. (2017) published an article in 2017 mainly oriented to absorption systems, discussing the operation principle of the technology, and the materials used or in search are listed and compared. The authors concluded that sorption had advanced very much due to the immense amount of research carried out around heat pumping and solar refrigeration. The same year, Donkers et al. (2017) provided a review on salt hydrates aimed to screen and select potential candidates for seasonal storage. Therefore, the authors performed a literature review followed by a framework of boundary conditions to narrow down to different candidates, according to certain operational conditions. As a result, potassium carbonate (K₂CO₃) showed the best performance, although requiring a change in the design of seasonal heat storage systems, which according to the authors is needed to overcome energy density and price issues. In 2019, Jarimi et al. (2019) published a review on the recent progress of TCS systems. The authors provided a comprehensive overview of three directions, namely, sorption materials, sorption reactor design, and sorption process design. In 2020, Clark et al. (2020) published an article reviewing the state of the art of hydrates in building applications. The authors outlined that to further develop this technology and bring it closer to commercialization, a merging of research from both material and system design viewpoints is needed. They also highlighted the main research lines to tackle at the material, reactor, and modeling level.

The reviews published are of outstanding importance for the scientific community as they drew the pathways for TCS during the last decades. However, the authors identified a gap in the literature when it comes to linking the technology progress from basic material research to material manufacturing at commercial levels. There is a need of understanding, for which have been the most studied materials by replying to the following burning questions: why have they been studied? What are the challenges that have been addressed? Which are the unresolved matters? How to move forward in the following years? With this review article, we aim to provide a guideline for energy researchers in the energy field to lay out the concepts and key challenges to researching the implementation of storage candidates. More importantly, we aim to give an overview of the current status to understand the blueprint of sorption thermochemical storage that can eventually help draw the progress and roadmap of the technology in the following years. The eventual goal is to assess the current state of the art and bring up the importance of water sorption-based materials in the energy scenario and propose a catalog of material candidates and their implementation challenges. The state of the art of an extensive list of water sorption candidates is reviewed, and their specific challenges are identified. Moreover, we dive into the commercialization challenges of thermochemical materials that are directly linked to the water sorption material formulation approaches to address the storage media challenges, by a review of conventional (shaping, impregnation, etc.) and emerging manufacturing routes (extrusion, encapsulation, etc.) and the possible manufacturing pathways.

2 Scope and guidance

This study aims for both newcomers and experienced researchers working on thermochemical energy storage and more specifically for researchers interested in developing and processing water sorption storage media. The main body of the article is divided into three main blocks. The first one provides an assessment of water sorption chemical storage with analyses on publication rates and the importance of water sorption in the energy scenario, and the second block includes a comprehensive literature review on water sorption screening candidates from low to medium application, namely, sulphates, nitrates, carbonates, chlorides, and bromides. The third block presents the current conventional and emerging manufacturing routes applied to TCS pure materials and composites. Finally, an outlook and conclusions are presented to tie up the concepts presented in the three previous blocks.

3 Importance of water sorption chemical storage

Sorption storage technologies store heat by breaking the binding forces, such as van der Waals forces or covalent forces, between a sorbent and a sorbate (Clark et al., 2020). The heat required to break the binding forces can be higher than that associated with the evaporation heat of a pure sorbate (e.g., water). As a result, the energy density of sorption-based TCS materials can be significantly higher than that of PCMs. Sorption-based systems are classified into sub-groups (Table 1): (1) solid adsorption, (2) liquid absorption, (3) chemical reactions, and (4) composites (Yan and Zhang, 2022). Sorption chemical reactions are divided into coordination reaction of ammoniate with ammonia and hydration reaction of salt hydrate with water (Soda and Beyene, 2016). In contrast to liquid absorption and solid adsorption, the chemical reaction is mono-variant, and the equilibrium uptake is defined by only one independent property (pressure or temperature) and the working temperature is easily controlled and adjusted by changing the pressure (Zhang et al., 2016). The sorption chemical reaction hysteresis may exist (Soda and Beyene, 2016). Sorption reaction systems provide high ESD and cover different ranges of heat source temperatures depending on the salt type (Posern and Osburg, 2017). WTES takes place by the absorption and desorption of water in salt hydrates. Salt hydration is viewed as two steps: water adsorption and hydrate formation. Salt hydrates form new crystal structures by dissociating or absorbing water molecules, and this process realizes the storage (Yan and Zhang, 2022).

Water sorption thermal energy storage (WSTES) technology is a promising thermal energy storage method that provides the inherent advantages of thermochemical storage systems of high energy storage density and negligible heat loss during storage periods. In addition to allowing the storage of heat in a strategic temperature range (25–200°C), which tackles a need for thermal energy storage in the current energy scenario for space heating and domestic hot water. A total of 66% of the current energy demand for heating in the industry and building sector in Europe accounts for space heating and hot water (<150°C), which falls in the working temperature range of sorption technologies (see Figure 1). Moreover, 28% of energy demand is for process heating at a temperature lower than 200°C. Process heating is most relevant and most challenging to decarbonize. In this framework, WSTES provides higher storage capacity than commercial phase change materials or water tanks for an added feature that they can operate as heat transformers.

FIGURE 1

In addition, WSTES has a key role in the seasonal and interseasonal thermal energy storage (STES) fields, which can be used to cover a portion or meet the whole space and water heat (SWH) demands in residential and commercial buildings (Alkhalidi et al., 2021; Donkers et al., 2017). Solar thermal energy can be stored in summer to be used in winter for SWH demand. Solar thermal systems use heat collectors to capture solar energy to be used according to the demand load. Energy can be stored for short-term and extended periods (seasonal) according to the storage system’s size. Water-based sorption is the preferred option for seasonal and interseasonal storages among thermochemical storage as it provides a higher energy density in the temperature range.

4 Assessment of the current status and challenges

Thermochemical research is currently successful in the thermal energy storage sector, see Figure 2. Sorption storage accounts for 80% of the thermochemical storage research, given the advanced material research undergone and the wide interest in seasonal storage and the strategic operational temperature range. WTES has been investigated for the application of short-duration, long-duration, or seasonal thermal energy storage by harvesting solar energy or industrial waste heat (Yan and Zhang, 2022). Although WTES has been highly praised in recent years (see Figure 2), its practical application reflects many technical shortcomings. Currently, the research on salt hydrate-based sorption heat storage is driven by material property studies focused on thermogravimetric analysis and energy density evaluation in the single hydration/dehydration process while not targeting the reaction kinetics (ref), which is one of the main challenges for their implementation.

FIGURE 2

4.1 Implementation challenges

WSTES is currently blocked at low TRLs (3–4), given their outstanding implementation challenges that particularly involve the structure stability on how to control the structure at the material and device levels to achieve optimal heat and mass transfer and reaction conversion rate. Researchers have highlighted the storage media stability as the main scalability-limiting factor. Water sorption materials are very dependent on any change in the system (particle size, density, temperature gradient, and kinetics) that can eventually translate into undesired side effects such as agglomeration, volume expansion, deliquescence (formation of a saturated solution), and chemical and physical degradation, see listed challenges in Table 2. The performance of a thermochemical material is defined by the temperature and reaction rate (Sögütoglu et al., 2020). At the same time, the temperature at which heat can be retrieved is limited by the reaction equilibrium. Therefore, to optimize the performance, the energy must be stored as close as possible to the equilibrium conditions (with workable reaction rates). Hence, ensuring a good knowledge and understanding of reaction kinetics is a key. Moreover, during charging/discharging cycles, the material suffers from volume expansion and pore reduction that leads to agglomeration, volume expansion, deliquescence (formation of a saturated solution), and chemical and physical degradation. All these factors are crucial for the technology implementation.

Over the years, authors have come up with different approaches to modify and adapt to those changes, while ensuring homogeneous particle distribution by the prevention of agglomeration effects (Afflerbach et al., 2017). Among the most popular strategies, there is an introduction of salt into a porous (from nano to micro size) solid matrix, which contains the water-based sorption TCM ensuring a stable structure over cycles (Korhammer et al., 2016a; Courbon et al., 2017a; Casey et al., 2017). The composite materials mainly consist of the active storage material (e.g., MgSO4/CaCl2/LiBr) in ranges from 20 to 40 %wt. and the supporting matrix, which can either provide sorption heat (e.g., silica gel/zeolites) or can retain larger amounts of salt but do not participate in the sorption process (e.g., graphite and magnesium oxide) (Scapino et al., 2017a). These composites have been reported to improve thermal stability and heat and mass transfer, allowing them to withstand larger thermal cycles. However, the use of a matrix also brings in new challenges such as the compromise between the large pore size and the improved thermal stability at the expense of the decrease in energy density and the increase in the production cost (Scapino et al., 2017b). These modifications should ideally minimize the cost production process and the low complexity of scalability. In addition, the step processes should not affect the conversion rate of the thermochemical material. Hence, given the high level of uncertainty in this paradigm, this has become the main challenge for the scientific community. This will be discussed in more detail in the manufacturing section of this article.

More efforts should be put into materials R&D, as this is currently the stagnation point for this technology, materials stay mainly in the laboratory research scale and only a few of them have been upgraded to the prototype scale. This is especially triggered by the level of understanding, which is scattered among the TCM materials, and the numerous challenges for the practical application of TCS. The former, level of understanding, is mainly a consequence of the long list of possible candidates and the fact that the material’s research outcomes are periodical; researchers focus on certain materials for years as they seem potential candidates disregarding the others, and this imbalances, even more, the level of knowledge (see Table 2). The latter, the main challenges for practical application, is common for all TCMs such as agglomeration, stability, and cyclability. The main general needs to tackle them are thermal conductivity enhancement, minimizing the cost of storage and the additional unit cost, and maximizing the conversion efficiency. However, there are specific challenges for each TCM that should be individually looked at and every single TCM that can be applicable for a certain application (waste heat, domestic hot water, industrial processes, etc). Thus, diversifying research and efforts can drive future material development and reveal novel candidates that have been overlooked because of applying a general systematic screening method. The main research focus and the related innovation needs of the candidates considered in this review are listed in Table 2, and it is to be noted that those are particularly materially oriented. These are extracted from the comprehensive state of the art presented in Section 5 (Table 3).

4.2 Candidates

The available candidates for WSTES salt hydrates, low to medium, have been reported and listed by researchers in the last few years (Donkers et al., 2017; Clark et al., 2020; Glasser, 2014; Hawwash et al., 2017). The outstanding candidates are listed in Figure 3, according to their deployment level. The preferred candidates by researchers and the ones that have reached higher development levels are magnesium chloride (Kim et al., 2014; Whiting et al., 2014; Nedea et al., 2016; Soda and Beyene, 2016; Pathak et al., 2017; Posern and Osburg, 2017; Sutton et al., 2018a), strontium bromide (Courbon et al., 2017a), (Zhang et al., 2016; Zhao et al., 2016; Fopah-Lele and Tamba, 2017; Fopah-Lele and Gaston, 2017; Gilles et al., 2018), magnesium sulfate (Posern and Kaps, 2008; Whiting et al., 2013; Ferchaud et al., 2014; Calabrese et al., 2018), sodium sulfate (Scapino et al., 2017b;Sharma et al., 1990; De Jong et al., 2014; Roelands et al., 2015; Solé et al., 2016), and calcium chloride (Molenda et al., 2012; Bouché et al., 2016; Jabbari-Hichri et al., 2017; van der Pal and Critoph, 2017; Sutton et al., 2018b). In the past 3 years, potassium carbonate has appeared as a new promising candidate for building applications, although this material has not reached a deployed level yet (Linnow et al., 2014; Sögütoglu et al., 2018; Gaeini et al., 2019; Shkatulov et al., 2020a).

FIGURE 3

An assessment of sorption-based thermochemical materials reviewed in the reported literature is included in this section. This assessment is aimed to identify the strengths, main challenges, and projections of the potential candidates included. In order to achieve that goal, we have gathered the number of publications over the last 25 years, see Figure 4. Looking at the publication trends, calcium chloride and magnesium chloride are the most studied TCM followed by magnesium sulfate, sodium sulfide, and strontium bromide. Calcium chloride was first studied in 1986 by Chaudhari et al. (1986) but increasingly attracted attention from 2005 onward, whereas magnesium sulfate was first published in 1981 (Steinmetz et al., 1981) and has been consistently studied since the early 2000s. Magnesium chloride was first published in 1976 for thermochemical water splitting (Yu Sung and Ulrichson, 1976) but increasingly attracted attention in 2004s. As a general trend, TCS technology is booming and it has following years of research and development to come as some materials have recently been added, possibly to stay, to the list of researched TCM; for example, potassium carbonate, sodium sulfide, strontium chloride, lithium chloride, sodium sulfite, etc.

FIGURE 4

5 Sorption material candidates

In this section, an extensive literature review is presented including all the WSTES candidates from low to medium temperature in Figure 3. The materials are gathered by their chemical groups, namely, sulphates, carbonates, bromides, nitrates, chlorides, hydroxides, and sulfides. Given that commonly, the reaction steps, reaction temperature, and key physical properties (melting point, density, and crystalline structure) of the different dehydrated states of the salt are not always easy to find and scattered in the literature, the authors provide a list of subsequence reactions and all related to them, see Table 4.

5.1 Sulphates

5.1.1 Magnesium sulfate

Magnesium sulfate is one of the TCMs that accounts for higher energy density (Abedin and Rosen, 2011), and it has been widely studied for seasonal storage (van Essen et al., 2009a; van Essen et al., 2009b; Hongois et al., 2011). Even though the melting point lies in the first dehydration step, this material meets other high-priority requirements such as hydration/dehydration temperature, energy density, deliquescence, and volume variation. Magnesium sulfate allows storing theoretical energy of up to 2.8 GJ m⁻³ (Okhrimenko et al., 2017). However, the use of magnesium sulfate powder is difficult in a storage reactor because the particles rapidly form agglomerates during dehydration/hydration cycles, thus liming gas transfer and causing reversibility issues and low temperature lift, resulting in poor system performance (Okhrimenko et al., 2017). The thermochemical storage pair works following reaction 1, the heat is released/stored by the hydration and dehydration of magnesium sulfate. The charging/discharging process of magnesium sulfate takes place at a temperature range of 30°C–275°C (van Essen et al., 2009b) and involves three stages.

Regarding its implementation challenges, the authors put their efforts into understanding the underlying mechanisms. Donkers et al. (2015) analyzed magnesium sulfate by NRM under dynamic conditions in a dry nitrogen atmosphere. It was observed that liquid water is formed in the pore of the crystal during dehydration (at 48°C) due to the local increase of the water vapor pressure. van Essen et al. (2009b) studied the influence of different particle sizes since that can also lead to the partial formation of liquid water inside the crystal, especially when working with large particle size distribution (200–500 μm). Previous experimental studies performed at the Energy research Centre of the Netherlands (ECN) showed that this material presents a storage energy density of 1 GJ m⁻³ when the material is used in a TC storage system with a 50% porosity-packed bed reactor (Ferchaud et al., 2012a). However, slow reaction kinetics under seasonal storage conditions (Ferchaud et al., 2014) might be related to the amorphization of the material (Ferchaud et al., 2012a). This phenomenon occurs when the system works under 13 mbar p (H₂O) (corresponding to the average value in northern Europe) (Ferchaud et al., 2014). Ferchaud et al. (2012a) studied the effect of vapor pressure in the dehydration reaction to find the optimal conditions that should be set in a TC storage system. The two consecutive reactions taking place during the dehydration of MgSO₄·7H₂O show an increase in the reaction rate with increasing p (H₂O) until 50 mbar. The reaction rate decreases again for p (H₂O) above 50 mbar. This phenomenon is known as the Topley–Smith effect, which is found in many salt hydrates (L’vov, 2007). This effect is often explained by assuming that the increasing water vapor pressure promotes the formation of additional structural defects (additional channels, cracks, and pores), which increases the water vapor removal out of the material and thereby facilitate the formation of a new lower hydrated phase. The authors concluded that the dehydration process is directly influenced by the water vapor pressure applied in a seasonal heat storage system. The kinetics of the reaction increases when the water pressure is below 50 mbar and decreases when pressure is above 50 mbar. When working at lower pressures (less than 50 mbar), the water pressure seems to promote the formation of structural defects, which facilitate the water removal of the material. However, above 50 mbar the water vapor saturation of the material surface reduces the removal of the water vapor in the material. Posern et al. (2015) explained the deliquescence phenomena that take place above the hydration reaction or the deliquescence humidity (DRH) at a certain temperature. Scapino et al. (2017a) also reviewed its potential use in open systems. As demonstrated by van Essen et al. (2009b), the authors also experienced that magnesium sulfate is unable to uptake water above 50°C unless very high relative humidity is applied . Magnesium sulfate has also been widely studied in composite salt in the matrix with numerous matrices to solve the agglomeration and stability issues (Whiting et al., 2014;Casey et al., 2014; Hongois et al., 2014; Posern et al., 2015; Xu et al., 2017; Sutton et al., 2018a; Xu et al., 2018; Wang et al., 2019a; Calabrese et al., 2019; Mahon et al., 2019; Miao et al., 2021).

5.1.2 Calcium sulfate

Calcium sulfate is a case of a less studied TCM material that presents significant potential for thermal energy storage. Calcium sulfate occurs in nature in three different forms, namely, CaSO₄∙2H₂O (gypsum), CaSO₄∙1.5H₂O (hemihydrate), and CaSO₄∙0.5H₂O (anhydrite) (Freyer and Voigt, 2003). The hydration reaction of calcium sulfate (2 H₂O to ½ H₂O) has been studied for gas–solid chemical pump applications (Xu et al., 2017). However, this material is mostly used in industrial processes (limestone–gypsum flue gas desulfurization (FGD), production of phosphoric acid or phosphate fertilizers) and has not been boarded to thermochemical storage. Some industrial processes are accompanied by the crystallization of calcium sulfate phases such as the wet limestone–gypsum flue gas desulfurization (FGD), the hydrometallurgical production of zinc and copper, and the recovery of natural gas and oil. Given its application as a binder and building material, attention has also been paid to the hydration–dehydration processes of calcium sulfate for chemical heat pumps. CaSO₄ hydration CHPs offer certain advantages, such as high heat density, using only safe and economical materials, no deliquescence, very small hysteresis, negligible expansion in hydration, long-term storage of the absorbent and products, and low heat loss (Shiren et al., 2020). As the main disadvantages, previous studies have reported the problem of inactivation of CaSO₄ due to hydration under high water vapor pressure (Lee et al., 2014; Richter et al., 2018); however, CaSO4 has the potential to improve durability by material development as shown by Shiren et al. (2020). Strydom et al. (1995) investigated the thermal dehydration of synthetic calcium sulfate at different heating rates. They concluded that dehydration takes place in three steps, which are highly influenced by the number of impurities present in the salt. The first dehydration stage is driven by nucleation, and the second and third stages are diffusion controlled.

The second dehydration (loss of 1.5 mol of water) overlaps with the first dehydration, and the reaction from dihydrate to hemihydrate is observed to be very slow under 95°C. While the last stage is uncertain to determine where it starts and ends, at 450°C the authors still observe some traces of CaSO₄·0.15 H₂O together with anhydrous. Hudson-Lamb et al. (1996) studied the dehydration of natural gypsum and pure calcium sulfate dihydrate. Although the authors stated that the dehydration takes place in two different steps, from dihydrate to hemihydrate and from hemihydrate to anhydrous, they also found traces of CaSO₄·0.15H₂O at 240 °C. Unlike Strydom et al. (1995), at 450 °C only anhydrite was present in the calcium sulfate. Badens et al. (1998) also studied the dehydration of gypsum powder, reporting the presence of two anhydride species γ-CaSO₄ and β-CaSO₄. The first one is called soluble anhydride because of its spontaneous hydration into hemihydrate under normal atmospheric conditions, and the beta form is called the insoluble anhydride. The transformation from alpha to beta takes place at approximately 127–227°C. The thermal path highly varies on the vapor pressure, and the authors found that at 500 Pa or below there is only one dehydration product from gypsum to γ-CaSO₄. At 900 Pa, the hemihydrate is an intermediate product between gypsum and γ-CaSO₄. Ogura et al. (2007) proposed a reversible chemical heat pump based on calcium sulfate and water reaction. They particularly studied the reaction from the hemihydrate to anhydrous state in the open and closed systems. The main outcomes were related to the reaction rate in the open system, which was not different from the semi-open system data and quite close to that in the closed system. Lee et al. (2014) studied the same reaction also for a CHP system, storing heat at around 100°C. The authors calculated the hydration and dehydration conversions after 450 cycles; they observed a reaction degradation given the irreversible transformation of III- CaSO₄ to II- CaSO₄, which are different forms of the crystal structure. From these results, they concluded that this phenomenon is dependent on proper pressure control (423 K at 30 kPa, 413 K at 151 kPa, and 403 K at 453 kPa) for ensuring the durability of the material. Richter et al. (2018), in their material screening study, also included calcium sulfate, which showed appropriate temperature levels for the reversible hydration reaction and a small reaction hysteresis. Recently, Shiren et al. (2020) studied calcium sulfate for waste heat recovery at a temperature of 423 K or less.

5.1.3 Copper sulfate

Copper sulfate (CuSO₄) is a salt hydrate material that has the advantage of having a high DRH at ambient temperature (25°C), and the dehydration of the first four water molecules can be driven by a low-temperature heat source of 60–85°C. Such attributes have attracted the researcher’s attention as a new working pair for thermally driven adsorption desalination–cooling systems (ADCSs). Ali et al. (2018) explored the use of copper sulfate in ADCS by studying the effect of changing relative pressure in the sorption reaction. The authors concluded that the copper sulfate system could be driven by solar energy or other low temperature renewable energy heat source (55–85°C), allowing the activation energy of 25.053 kJ/mol. The salt dehydration studies were mostly in the ‘60s and ‘80s (Borchardt and Daniels, 1957; Reisman and Karlak, 1958; Wendlandt, 1962; Mu and Perlmutter, 1981; Sørensen, 1981), and some comparative studies with other thermochemical materials have been published in the last years (Glasser, 2014), (Savchenko et al., 2006), (Al-Abbasi et al., 2017). Wendlandt (1962) studied the dehydration of copper sulfate, concluding that the dehydration from hepta to the trihydrate occurs in two steps and an intermediate phase CuSO₄∙4 H₂O at high pressures. Borchardt and Daniels (1957) studied the X-ray diffraction and dehydration stages. They found that no peaks appear between 275 and 725°C, and above 725 °C the two-stage decomposition from CuSO₄ to CuO with CuSO₄ and CuO as an intermediate was observed. However, they did not find any traces of the tetrahydrate, concluding that Borchardt and Daniels must have had a sample consisting of a mixture (CuSO₄∙3 H₂O and adhering solution). A number of up-to-date studies have studied hybrid composites, Savchenko et al. (2006) prepared a silica gel/copper sulfate “salt in a porous host matrix” composite, concluding that copper sulfate allowed the lowest salt content among the salts studied (17.3 wt% of salt content), although the ratio between the linked metal and the free metal was the highest, with an increasing salt content, and the free metal fraction increased as well. Al-Abbasi et al. (2017) overviewed the performance characteristics of copper sulfate, revealing that the salt shows a high thermal efficiency, slow reaction kinetics, and the melting during dehydration significantly affects the lifetime over consecutive cycles. The copper sulfate can be also used to produce hydrogen by water splitting using the decomposition reaction after full dehydration of the salt, as reported by Gonzales et al. (2009). Although the dehydration reaction is considered in the cycles, this is not the driving force for the hydrogen production (Bhosale et al., 2016).

5.1.4 Aluminum sulfate

Aluminum sulfate hydrated salt presents a reasonable dehydration temperature and high enthalpy of hydration/dehydration, which made it to be considered one of the most promising TCS materials. Aluminum sulfate hydrates, Al₂(SO₄)₃·nH₂O, range from 0 to 18 n and have been reported by several authors (Bassett and Goodwin, 1949; KŐmives et al., 1984; Mehrabadi and Farid, 2018; Guide for Authors, 2022). Further decomposition, as can be seen in reactions from 1 to 4,

andoccurs between 770 and 890°C in oxygen (Johnson and Gallagher, 1971).

The dehydration process of Al₂(SO₄)_3·18H₂O involves several phase transformations from high to low hydration phases until fully dehydrated Al₂(SO₄)₃. Jabbari-Hichri et al. (2016) expressed the disagreement on which is the more stable hydrated form, reviewing several authors’ studies where 14 or 17 (KŐmives et al., 1984) was claimed the most stable. Sixteen hydrated forms have also been found but are unstable. In addition, hydrates containing 12, 9 (Guide for Authors, 2022), and 6 mol of water have been studied by the decomposition of higher hydrated forms. The clear advantage of aluminum sulfate compared with other hydrate salts is the higher enthalpy of phase transition, which may significantly increase the heat storage capacity (Meisingset and Grønvold, 1986; Kruk et al., 1997). Çılgı and Cetişli (2009) studied the thermal decomposition of Al₂SO_4· 18H₂O by the TG method and four decomposition stages were found. Three of them were dehydration processes, Eqs 3–5, all endothermic reactions, where 2, 10, and 6 mol of water was lost, respectively. The last stage is sulfate decomposition.

Zondag et al. (2011a) considered the aluminum sulfate hydrated form as a candidate for working temperatures between 50 and 150 °C. The results of the study selected calcium and magnesium chlorides; chlorides showed a higher temperature lift of 11–19, whereas aluminium sulfate showed a lift of 1–2 °C (van Essen et al., 2009a). However, Al₂SO_4· 18H₂O, like others, present a melting temperature above the discharging temperature leading to systems without melting in the reactor during the discharging process, which complicates the design of the system. Therefore, it has also been studied in composites with supporting porous matrices such as mesoporous amorphous silica. The presence of the hydrated salt dispersed on the pores facilitated the hydration of both salt and supported adsorbing higher amounts of water, hence a consequently higher heat capacity (KŐmives et al., 1984). Mehrabadi and Farid (2018) also studied Al₂SO_4· 18H₂O along with other TCM in different composite/host matrices. Overall, Al₂SO_4· 18H₂O exhibits a very small temperature lift (DT = 10°C) during hydration of Al₂SO_4· 5H₂O at 25°C under 32 mbar; only 1–2°C of lift at 50°C under 13 mbar (N’Tsoukpoe et al., 2014a).

5.1.5. Other sulphates

Other sulfate hydrated salts have been studied and considered potential candidates for TCS. However, several undesired findings have been relegated to potential candidates. Li₂SO₄ and Na₂SO_4. have not shown significant reversibility under 100 °C when subjected to a heating rate of 10 K min (N’Tsoukpoe et al., 2014a). In addition, Li₂SO₄ presents acute toxicity and, like all lithium materials, limited lithium resources (N’Tsoukpoe et al., 2014a).

Yan et al. (2019) studied the melting and dehydrating processes of different hydrated salts. They determined that a high heating rate during the dehydrating process leads to the formation of molten hydrate, showing a lower dehydrating rate and higher activation energy when it is further dehydrated. In the case of Na₂SO₄, a dehydrating–dissolving process is suggested, and a mutual effect of melt and dehydration influences the equilibrium state. Moreover, Na₂SO₄ ·10H₂O high content and weakly linked water molecules can be considered a concentrated salt aqueous solution, presenting incongruent melting, which leads to the formation of lower hydrate salt, which is irreversible and leads to the loss of storage capacity. Zinc sulfate hexahydrate has also been studied recently by Rehman et al. (2021). The authors concluded that six water molecules were released with associated energy of 1747 J⋅g⁻¹, from 25°C to 150°C. Moreover, the water sorption study ensured that at 75% RH the material absorbs maximum water under ambient air temperature.

5.2 Carbonates

5.2.1 Potassium carbonate

Potassium carbonate was proposed by Donkers et al. (2017) for open and closed systems for domestic applications, although they also stated the inconvenience of its low energy density. Due to this, we decided to include this study, aiming to provide more experimental data and evidence of the physical and chemical properties of potassium carbonate. In Donkers’s work, they applied four criteria to select a TCM according to energy density on a material level above 1.3 GJ m⁻³, hydration temperature above 50°C, dehydration temperature below 120 °C, and a melting point above the dehydration temperature. Among 25 candidates, the reaction of K₂CO₃ (0–1.5) was the one that accounted for the lowest volume change while is not strongly corrosive, no higher hydrates are known, and it has no known unsafe side reactions. The interest in potassium carbonate lies in its easy availability, reasonable capacity for water uptake, and energy density at low temperatures (below 100 °C), while better stability than other salts that show higher energy density attributes. Since this work was published, some other authors have studied potassium carbonate for building applications. Gaeini et al. (2019) studied the reaction kinetics and mechanisms of de/re-hydration of K₂CO₃. According to their outputs, the dehydration takes place on a single step from sesquihydrate to anhydrous with an enthalpy of reaction of 60.88 kJ/mol and a bulk material energy density of 0.75 GJ m⁻³. The performance of the material improves upon cycling since its kinetics become faster after each cycle (Gaeini et al., 2019). Despite the advantages, the application of K₂CO₃ is challenged by the slow hydration rate, swelling/shrinking, and particle agglomeration (Shkatulov et al., 2020a).

5.3 Bromides

5.3.1 Strontium bromide hexahydrate

Strontium bromide has been detected as one of the most promising salts along with magnesium sulfate (N’Tsoukpoe et al., 2014a). SrBr₂·6H₂O has been already well-investigated at the laboratory and prototype level for various applications either for solar cooling and heat storage in a closed process (Lahmidi et al., 2006), or seasonal storage of solar energy in an open process (Michel et al., 2014;Marias et al., 2014). For short storage applications, it can be considered as storage and thermal comfort material (also as phase change material). The high enthalpy ( = 67,400 J) and density of hexahydrated strontium bromide (2.386 g/cm³) allow the salt to reach a high energy storage density (Courbon et al., 2017a). In addition, its high melting point enables a charging/discharging melting-free process. Its theoretical energy storage is very high with 628 kW h∙m³, indeed, it was found that the energy in an open system was 400 kW h∙m³ (Michel et al., 2014), and 531 kW h∙m³ (Lahmidi et al., 2006) for a closed system. Hence, it is one of the salts that allow high energy storage potential below 105°C (N’Tsoukpoe et al., 2014a). Application-wise, a temperature ranging from 80 to 90°C is sufficient to ensure the dehydration from hexahydrate to monohydrate without incongruent dissolution of water vapor in the solid phase, given the melting point (Fopah-Lele and Tamba, 2017). The dehydration of strontium bromide has been reported to take place in two stages; from hexahydrate to monohydrate and from monohydrate to anhydrous. Richter et al. (2018) studied strontium bromide in the framework of salt hydrates for a thermochemical heat transformer, concluding that SrBr₂ could withstand up to 10 charging/discharging cycles, obtaining appropriate temperature levels for the reversible hydration reaction and a small reaction hysteresis. In addition, SrBr₂ is a non-toxic material and forms a monohydrate with appropriate thermodynamics to thermally upgrade heat in an open-process mode. Strontium bromide forms hexahydrate that melts at 88.6°C, and it is dehydrated at a low heating rate below the melting point and stored at 110°C. To prevent a further reaction to the hexahydrate and deliquescence, the authors suggested a minimum temperature for hydration to 150°C, therefore, only working on the reversible reaction from monohydrate to anhydrous. Michel et al. (2012) presented an approximate plot of the limit of the saturated solution of the hexahydrate, calculated by interpolation between the melting point (88.62°C) and its eutectic (28°C). SrBr₂ is one of the salts included in this study that has been studied up to the prototype level (Zhao et al., 2016;Lahmidi et al., 2006;Guillaume, 2007; Mauran et al., 2008; Fopah Lele et al., 2015a), which denotes a higher potential for application and a deeper understanding of the salt reaction for both pure and composite formulations. According to the literature, under atmospheric pressure, the dehydration of hexahydrate to monohydrate is monotonous, desorbing 26% of the available water content (30.4%). The hydration occurs at 20 mbar and 25°C, even though a water vapor pressure can be applied above 40 mbar at 60°C. Above that temperature, no hydration is observed at saturation temperature (Fopah-Lele and Tamba, 2017). Given its latent heat, strontium bromide can also be used as a PCM. When used as a phase change material, it could present incongruent melting due to the insufficient release of water (uncompleted melting or charging) to dissolve the remaining salt crystals (Fopah-Lele and Tamba, 2017a). As the present salt just draws attention to heating and cooling storage application, its actual price is seen as a major inconvenient, 24–17 €/kg for hydrate and 320–210 €/kg for anhydrous. N’Tsoukpoe et al. (2014a) showed that when an external heat source is used for the evaporation and that heat is considered free and not considered in the evaluation of either the heat storage density or the thermal efficiency, the SrBr₂·6H₂O remained the best among over 125 screened materials from a thermodynamic point of view.

5.3.2 Lithium bromide monohydrate

Given that the DRH values for LiBr (7% at 30°C) are quite low, meaning that it is more likely for them to form liquid salt solutions directly in most cases, this salt is often applied in liquid/gas absorption cycles (Yu et al., 2015). If the desorption temperature is high enough to generate the solution to its crystallized state or even anhydrous state, the sorption process can be seen as a three-phase process, during which the solid crystal could be completely turned into a liquid solution (Yu et al., 2014). Thus, most of the studies are targeted at those used as an adsorption system for solar heating (N’Tsoukpoe et al., 2013; N’Tsoukpoe et al., 2014b; Myagmarjav et al., 2014; Manole and Lage, 1995). However, lithium bromide can be also considered for ‘open’ TES interseasonal systems, the reaction by water sorption without bringing the salt to the solution. Lefebvre et al. (2013) studied the hydration characteristics of lithium bromide concluding that two different transitions were observed corresponding to the transitions LiBr.2H₂O/LiBr.H₂O and LiBr.H₂O/anhydrous LiBr. LiBr crystallizes in the dihydrated form at the temperature of the storage tank surrounding (5–10°C) with a water content of 29 wt% of water (N’Tsoukpoe et al., 2014b; Lefebvre et al., 2013). Gordeeva et al. (1998a) studied in the ‘90s the confinement of the system “lithium bromide–silica gel” that was prepared by filling pores of silica gel. The authors also estimated the sorption capacity to be 1,600–2,100 kJ/kg. Myagmarjav et al. (2014) developed a novel composite by mixing pure magnesium hydroxide with lithium bromide, the addition of LiBr was found to decrease the estimated activation energy in the dehydration. Casey et al. (2014) studied the insertion of lithium chloride, among other hygroscopic salts, in different matrices (silica gel, zeolite, activated carbon, and vermiculite). The main outputs are that lithium bromide with vermiculite appeared to have significantly higher TES potential compared to the other candidates. Gordeeva et al. (2002) studied the water sorption equilibrium of LiBr confined to pores of a mesoporous synthetic carbon Sibunit and macroporous expanded graphite. They estimated the energy storage to be 1.1 and 2.8 kJ/g in the temperature range from 30 to 140°C. The authors also studied the water uptake over temperature and pressures, concluding that the hydrate is rather stable and possesses no transformation over 25–35 °C. At high temperatures, this hydrate undergoes decomposition toward an anhydrous LiBr (over 100°C). They also remarked on the formation of solid crystalline hydrates in the porous matrix.

5.4 Nitrates

5.4.1 Magnesium nitrate hexahydrate

Magnesium nitrate has been used as a phase change material (PCM), given its relatively high latent heat of fusion (Meisingset and Grønvold, 1986; Kruk et al., 1997; Jabbari-Hichri et al., 2016). However, it also allows for storing heat in the thermochemical reaction (Donkers et al., 2017). These dual materials are called reactive phase change materials. Although it shows a decent energy density, volume change, and a melting point above the hydration/dehydration process, it is usually not considered a strong candidate as it displays a tendency for significant supercooling and decomposes to magnesium oxide under strong heating conditions (Kenisarin and Mahkamov, 2016;Paulik et al., 1988). MgNO₃ hexahydrate turns into a dehydrated form if heated above 95°C, the equilibrium diagram of magnesium nitrate and water was reported by Kenisarin and Mahkamov (2016). The dehydration occurs in three steps; from the hexahydrate to the dihydrate showing that the slow dehydration is stable at lower hydrate states before decomposing at 400 °C. Two decomposition steps were observed by Paulik et al. (1988) and Drake et al. (2018), one from 300 to 400 °C with 56% mass loss of the initial hexahydrate and 450–500 °C with 17%. Drake et al. (2018) combined two energy storage methods aiming to harness the advantages of both (latent heat and thermochemical) by studying the eutectic composition of magnesium nitrate and water. For that purpose, they studied the melting and dehydration of magnesium nitrate. They found that from the dihydrate to the complete anhydride (at 270°C), no endothermic peaks can be observed, which they attributed to an obscured peak due to an experiment drift. It has been concluded that the use of quasi-isothermal and quasi-isobaric conditions is needed for an adequate resolution. Since the study was latent heat targeted, they also studied the melting, which they found to be incongruent (a common effect in salt hydrate PCMs). When the magnesium nitrate melts, the water is evenly dispersed, but on freezing, the original crystalline structure may not be perfectly reformed, and these different states may have an energy disparity (supercooling) (Drake et al., 2018). Likewise, higher dehydration temperatures are associated with increased supercooling and incongruent melting. The formation of a lower hydrate from incongruent melting is irreversible. The two stable hydrates of dehydrating are formed by incongruent melting of hexahydrate (Soda and Beyene, 2016).

5.4.2 Zinc nitrate hexahydrate

Similarly, magnesium nitrate, zinc nitrate, and calcium nitrate were proposed for latent heat storage (Xie et al., 2017) but have not yet been explored for thermochemical storage. The solid–liquid equilibrium was studied by Voigt and Zeng, (2002), and they were also screened by Donkers et al. (2017) for domestic applications. Both have an energy density of around 1.5–1.7 GJ m⁻³ and a dehydration–hydration temperature range that meets the system requirements. The thermal decomposition of zinc nitrate has been recently studied by Małecka et al. (2015), and they found that the dehydration starts at 30 °C, which is accompanied by the melting at 35°C. According to them, dehydration occurs in three consecutive steps that finish at 120 °C. The decomposition of nitrate groups starts at 75 °C and ends at 265 °C, forming ZnO as the final product. The literature shows that it is difficult to get high-purity zinc nitrate from zinc nitrate hexahydrate and compute the stoichiometric content of water (Kumar et al., 2018). Kumar et al. (2018) added nucleation additives to zinc nitrate hexahydrate, and, therefore, they also studied its thermal decomposition.

N’Tsoukpoe et al. (2014a) considered zinc nitrate hexahydrate in their systematic multi-step screening, and they remarked that the partial dehydration of this salt is preceded by the melting. The salt melts during the tests but remains reversible. However, the authors remarked that the reaction needs to be checked deeply because of the lack of coherence in the literature. Meanwhile, Kozak et al. (2003) suggest that the salt does not dehydrate but decomposes in other compounds involving Zn(OH)NO₃⋅H₂O and HNO₃ by following the reactions given below.

Zn(NO₃)2·6H₂O (solid phase) → pseudo-melting → HNO₃–H₂O (solution 1:6)→ evaporation (H₂O, HNO₃), hydrolysis, crystallization→Zn(OH)NO₃·H₂O (solid phase)→polycondensation, evaporation (H₂O, HNO₃)→Zn₃(OH)₄(NO₃)₂ → →dehydroxylation, thermal decomposition→ZnO (solid phase).

5.4.3 Calcium nitrate tetrahydrate

Calcium nitrate tetrahydrate is one of the most commonly used inorganic salts given its high latent heat and low melting point (Vranes et al., 2010). However, it has not been thoroughly studied for thermochemical storage purposes. Liu and Zhu (2015) used a double-layer coating technology to add an epoxy resin and cement layer to calcium nitrate tetrahydrate, which significantly enhanced the heat resistance of the crystals and reduced the deliquescence problems at room temperature. Guo et al. (2019) proposed a solid–liquid calcium-based composite by adding calcium nitrate tetrahydrate to calcium nitrate hexahydrate as a modifier for latent heat applications purposes. As for magnesium nitrate, Paulik et al. (1983) examined the dehydration of calcium nitrate under quasi-isothermal and quasi-isobaric conditions. They reported, as shown by other authors before, that calcium nitrate dehydrates in four steps. The salt melts at 42.7°C forming a saturated solution that with increasing temperature becomes unsaturated. Until the boiling point, the salt does not lose water, above 135 °C, the solution starts losing water. Therefore, the authors concluded that the dehydration process was not based on consecutive reactions but on the course and overlapping of physical processes such as evaporation, boiling, drying, and solid crust formation.

5.4.4 Lithium nitrate trihydrate

Lithium nitrate trihydrate has also been studied as a potential candidate for thermochemical energy storage (Sutton et al., 2018a; Sutton et al., 2018b). One of the main problems and reasons for not being extensively studied, as with all lithium materials, is their use limitation due to lithium resources (N’Tsoukpoe et al., 2014a).

Sutton et al. (2018b) impregnated LiNO₃ into porous vermiculite to form a salt matrix. They concluded that the single step of energy liberation of hydration for LiNO₃ allows a mechanism for the rapid hydration of LiNO₃ (liberating the energy) on the surface of the SIM and continues to liberate energy at a far slower rate as the salt within the core of the vermiculite particle is hydrated. The same authors in another publication (Sutton et al., 2018a) studied the experimental performance of mixed ‘salt in matrix’ materials. Layering and blending techniques established that the performance could be increased by up to 24% by the correct choice of mixing technique. Layering CaCl₂ on LiNO₃ provided the most efficient thermal release strategy and yielded a thermal storage density of 0.2 GJ/m³. N’Tsoukpoe et al. (2014a) considered lithium nitrate in their systematic screening concluding that the price might be a limitation due to ongoing pressure on lithium resources. Moreover, the authors could not identify a LiNO₃ hydrate with a melting temperature above 60°C. Casey et al. (2017) studied LiNO₃ with silica gel, vermiculite, activated carbon, and zeolite 13X, concluding that an energy density of 364 kJ/kg can be reached, with a promising working range of 30°C < T < 140°C, good isotherms, and longtime interval to reach the equilibrium moisture content across all relative humidity.

5.5 Chlorides

LiCl, MgCl_2, and CaCl₂ are of strong hygroscopicity, which could adsorb water vapor and form a solution in a closed system. This phenomenon is called deliquescence (see Section 4.2), which can bring salt segregation, corrosion in sorption reactors, and deterioration of heat and mass transfer of sorbents (Zhao et al., 2016). Calcium chloride and magnesium chloride are the other two TCMs targeted with significant potential for thermochemical heat storage (van Essen et al., 2009a). They exhibit high energy density at the desired operational temperatures. However, magnesium chloride suffers from thermal decomposition and HCl formation and deliquescence below 40 °C.

5.5.1 Calcium chloride

N’Tsoukpoe et al. (2014c) reviewed the use of calcium hydrate in applied thermal engineering, concluding that calcium chloride is a promising material for drying and heat storage applications. This material is already used for air dehumidification in buildings. Calcium chloride also has the potential to be used as a phase change material, given its melting point of around 30°C. Its use for thermochemical storage is promising although it presents challenges such as low melting point and a high tendency for agglomeration. Tang et al. (2017) studied the working pair zeolite/calcium chloride, by impregnating zeolite 13X with different concentrations of CaCl₂ aqueous solutions. They found out that the samples containing 5 wt% and 10 wt% have a better performance than the pure zeolite, the water uptakes equilibrium of formulation with 15 wt% and 20 wt% were lower than the pure zeolite. Richter et al. (2016) studied the CaCl₂·4H₂O dehydration for industrial waste heat upgrade by using an open system in the range of 100–200°C at 150 C, 130 C, and 100 °C at around 40 kPa, concluding that the system enables the use of waste heat at 100 °C with a thermal upgrade of 65 K. D’Ans et al. (2018) and Courbon et al. (2017b) proposed a working pair based on silica and calcium chloride, with a salt content of 43 wt% and 40–43 wt%, respectively. Meanwhile D’Ans et al. (2018) opted to encapsulate the salt in mesoporous silica, and Courbon et al. (2017b) proposed to successively impregnate the salt solution into silica gel, producing high content and stability composites. Both studies lead to a cycle loading lift of 0.4 g/g. Courbon et al. (2017b) found out that the mechanism of water sorption goes beyond hydration from anhydrous to 4 H₂O, the CaCl₂∙4 H₂O dissolves to form a CaCl₂ solution, leading to water trapped in the silica gel pores. Gough et al. (2016) studied the deliquescence of calcium chloride for exploring its potential on Mars. The authors used a Raman microscope to determine the occurrence of deliquescence of the salt for the three phases of calcium chloride (hexahydrate, dihydrate, and anhydrous). They observed that often CaCl₂∙2 H₂O deliquesced directly into liquid brine without forming CaCl₂∙6 H₂O phase, jumping the hydration step before deliquescence. Deliquescence was observed to lower with increasing temperature for the hexahydrate, while the dihydrate showed weaker temperature dependence. Savchenko et al. (2006) used the impregnation method to prepare silica gel/calcium chloride composites, concluding that the sorption equilibrium of the composites can be managed by varying conditions of the composite preparation. Jabbari-Hichri et al. (2017) prepared calcium chloride composites with silica gel, alumina, and bentonite (15 wt% of TCM), among all the matrices, silica gel showed the best performance given the low pore blocking and the high heat and water storage values. Jiang et al. (2017) explored the development of bi- and tri-salt graphite composite sorbents by combining magnesium chloride, calcium chloride, and ammonium chloride. The multilevel sorption alleviated the sorption hysteresis and led to a higher energy density. Karunadasa et al. (2018) investigated the correlation between the stability of hydrated/anhydrous products and microstructural changes of calcium chloride, finding that there are two different phases of anhydrous calcium after dehydration (large and small unit cell). The stability of the phases is dominated by the lattice strain, larger crystallite size, and small unit cell volume. Kim et al. (2013) studied a composite comprising a mixture of expanded graphite, magnesium oxide, and calcium chloride for a chemical heat pump application. From the study, the authors proposed a multiplied factor consisting of dehydration rate and mixing weight ratio, which suggested that 0.8 was the optimized value when the mixing molar ratio between CaCl₂ and Mg(OH)₂ was 0.10. Korhammer et al. (2016b) studied the reaction of the mixture between calcium chloride and magnesium chloride with alcohol, which revealed that the sorption behavior can be controlled by the variation of ethanol pressure and, as confirmed by other authors, that the mixtures of salts from the same family led to a superior thermal behavior with higher heat storage capacities. Gaeini et al. (2018) published a comprehensive study on the stability of calcium chloride-based materials and different manufacturing approaches: impregnated expanded graphite composite, impregnated vermiculite composite, and encapsulated calcium chloride powder. Of all the methods used, encapsulation is the one that shows the best cycling stability, although it allows for the lowest energy density, whereas the impregnated samples lead to an agglomerated structure similar to the dehydrated pure salt, which reduces the conversion rate over cycles. Molenda et al. (2013) studied the reversible reaction of calcium chloride at high H₂O partial pressures, which demonstrated that the reaction of calcium chloride is applicable for thermochemical energy storage up to 180 °C. They found intermediate hydration, a metastable step with 0.3 mol H₂O/mol CaCl₂, although they did not observe the formation of the monohydrate as Gaeini et al. (2018). Interestingly, they noticed the dehydration occurs in two processes depending on the pressure; at high pressures, the 0.3 H₂O intermediate step appears to be determinant, while at low pressures the monohydrate appears to be the intermediate step. This revealed a correlation between the formation temperatures, the intermediate species, and the partial vapor pressure. Richter et al. (2016) studied the endothermic dehydration of calcium chloride dihydrate at the laboratory scale exhibiting complete reversibility, thermal and cycling stability, and reasonable kinetics at temperatures up to 200 °C. They reported a thermal upgrade of 35 °C, discharging at 165°C and charging at 130 °C with a full conversion from the anhydrous to the dihydrate. By using the intermediate step (CaCl₂∙0.3 H₂O) reported by Molenda et al. (2013), the discharging could be conducted at 180 °C, which increases the thermal upgrade to 50 °C but reduces the storage density. Therefore, to increase the thermal upgrade hydration needs to take place at higher temperatures or dehydration at lower ones.

5.5.2 Magnesium chloride

Several studies revealed the promising potential of magnesium chloride for seasonal heat storage in terms of energy density and charging/discharging temperature. Apart from the common issues of instability and decomposition over cycles of other TCMs, this material is particularly disadvantaged by the fact that at ambient temperature it tends to overhydrate into a solution, which causes inhomogeneous dehydration. In addition, above 150 °C (some authors reported even lower temperatures) the chlorides decompose into magnesium hydroxy chloride involving a release of hydrochloric acid. Such a reaction causes obvious damage and reduces the quantity of active sorption material over time. The hydrolysis reaction occurs at the same time as the last dehydration reaction stage (from dihydrate to monohydrate) (Kirsh et al., 1987). Ferchaud et al. (2012a) studied the XRD patterns of hydration/dehydration intending to determine the temperature range over which magnesium chloride remains in a stable hydrated state during its dehydration/hydration reactions (13 mbar). The main outputs of this study are that the minimum hydration temperature should be set at 40 °C to avoid the overhydration of the material; the dehydrate/hydrates to tetra at 100 °C, and hexahydrate at 60 °C; the material remains in a solid state until 30 °C; no characteristic peaks of magnesium hydroxy chloride were observed at 150 °C after 3 h. Gutierrez et al. (2018) recently studied the reversible reaction of a ternary equilibrium solution KCl-MgCl₂-H₂O. The authors brought more evidence that at temperatures close to 180 °C, small amounts of HCl were observed from the decomposition of magnesium chloride. Also, decomposition and/or melting were confirmed, which had a strong effect on the cycling stability of the active material. According to Zondag et al. (2011b), the formation of HCl was found to occur at temperatures above 150 °C, before 180 °C. They also found out that the material closer to the evaporator was overhydrated, transforming to a gel-like structure after being remained long-term hydrated. Mamani et al. (2018) studied the viability of using bischofite (a material containing 97.4% of MgCl₂ ∙ 6 H₂O), bringing more shreds of evidence of the hydration/dehydration behavior of the active thermochemical material, following the previously published work. Huang et al. (2010) described the thermal decomposition of magnesium chloride in six steps (under air), from hexa to tetra, from tetra to di, from di to 0.3 H₂O/MgOHCl, from 0.3 H₂O/MgOHCl to MgOHCl, and from MgOHCl to MgO. Urs Rammelberg et al. (2012) studied the dehydration from hexahydrate to dihydrate in terms of water vapor pressure and conversion independence on the heating rate. The dehydration is completed after 2 h below 140 °C (heating rate 1 °C/min), and they found that the irreversible decomposition side reaction was running side-by-side with the dehydration MgCl₂∙H₂O at lower temperatures than other authors. According to Rammelberg et al., dehydration occurred in three overlapping stages, which are not highly dependent on the heating rate. Ferchaud et al. (2012b) studied magnesium chloride at a prototype level, they concluded that the thermochemical material could be dehydrated at temperatures below 130 °C, and the hydration was sufficient to provide tap water heating at 60 °C.

5.5.3 Lithium chloride

LiCl is a hygroscopic salt with high moisture capacity, easy renewability, and high chemical stability. It is the most widely used desiccant in commercial rotary dehumidifiers. LiCl/H₂O shows significant potential for sorption thermal energy storage with its large water sorption capacity. However, the fact that is a highly hygroscopic salt is likely to turn into a liquid solution when in contact with water vapor. In the temperature range of 19.1–93.5°C, there is anhydrous LiCl and monohydrate system, i.e., solid LiCl, a saturated solution containing monohydrate or dilute solution (Rau et al., 1991). Above 93.5°C, the solution contains anhydrous salt. Therefore, lithium chloride monohydrate dehydration takes place in one step from mono to anhydrous starting at 72°C until dehydration is finalized at 94.1°C. Below 19 °C, the solid phase consists of dehydrated LiCl. The one-step reaction allows for storing 2.08 GJ/m³ in an open system and 1.36 GJ/m³ in a closed system (Donkers et al., 2017).

5.5.4 Strontium chloride

The high energy storage density of SrCl₂⋅6H₂O (2.6 GJ/m³) further ensures its potential as a promising thermochemical energy storage material, especially at low temperatures as 80% of the energy storage capacity can be driven off at 90°C. The kinetics of this thermochemical material was first studied by Iyimen-Schwarz and Lechner (1983), the authors remarked that strontium chloride dehydrates in three steps in the temperature range from 50°C to 135°C; from hexa to dihydrate (), from dihydrate to monohydrate (), and from monohydrate to anhydrous (). In addition, the incongruent melting of the salt was also studied by Pistorius (1962). Although SrCl2·6H2O is a promising salt, it has not been widely studied in the literature and is mainly considered in multi-screening studies. N’Tsoukpoe et al. (2014a), in their systematic multi-step screening considered strontium chloride (along with SrBr₂⋅6H₂O) as the only two candidates that could provide a net energy storage density above 200 kW h/m³. Moreover, its DRH (71%) is high enough and is compatible with the usual ambient relative humidity on a typical cold day in many places (Mehrabadi and Farid, 2018). The main issue of this TCM material is that the salt grains tend to partially melt over the dehydration step at temperatures close to their melting point (61.3 °C), which as seen for the TCM materials gradually diminishes the porosity and permeability of a bed of salt and prevent vapor penetration through the bed, which is vital to recovering the stored heat (Mehrabadi and Farid, 2018).

5.5.5. Barium chloride

Barium chloride was widely studied in the early ’90s, and di- and monohydrate are the most commonly reported hydrates. The BaCl₂-H₂O system is one of the few hydrates that has been demonstrated to undergo fully reversible dehydration–hydration reactions (Lumpkin and Perlmutter, 1995). Dihydrate to monohydrate and monohydrate to anhydrous salt reactions are commonly observed to proceed in a stepwise manner; however, under moderate vacuum conditions the reactions overlap, and dihydrate reacts directly to form the anhydrous salt. Fenstad and Fray (2006) studied the H₂O-BaCl₂ equilibrium of the monohydrate and dihydrate proposing an updated diagram. No recent studies have been found for the system BaCl₂-H₂O; however, many studies have looked at the working pair BaCl₂-NH₃ (Jiang et al., 2013; Jenkins and Glasser, 2018; Martínez-Tejeda et al., 2018; Ma et al., 2019).

5.6 Hydroxides

Hydroxides, such as Fe(OH)₂, have been considered as potential TCM for solar seasonal storage. One of the studies carried out by Visscher and Veldhuis (2005) looked for materials to work under temperatures between 60 °C and 250 °C, an energy storage density higher than 1 GJ/m³ and abundant and easy to mine, simulated the use of Fe(OH)₂ according to the following reaction.

The energy storage density of the material, 2.2 GJ/m³ occurred at 150°C. It was observed that when the water vapor was released during the dissociation reaction into the atmosphere, a higher cycle storage density was reached. Also, when latent heat recovery was used, the cycle storage density increased by more than 60%, and the collector area needed was reduced by more than 40%. However, other practical applications were not considered such as corrosiveness or health and safety aspects. Hydroxides, like NaOH, and other corrosive alkalis cause chemical burns, permanent injury or scarring on human or animal tissue, and can cause blindness if contacts with the eye (Hui et al., 2011). Moreover, hydroxides are corrosive in contact with common system materials components such as stainless steel 304, which increases with mass fraction and temperature. Weber et al. (Fumey et al., 2014) reported a NaOH storage for long-term heat storage with high specific heat density designing a system with nearly three times the storage capacity of water ones.

5.7 Sulphides

Sodium sulphide is a widely studied TCM that has reached the prototype level and has been chosen as a candidate in several thermal storage projects like TEPIDIUS, SWEAT, and MERITS. One of the reasons behind that is its large energy density (2.66 GJ/m³ from penta to hemihydrate (Roelands et al., 2015)). Although it attracted significant interest in the past, Na₂S.nH₂O has some remarkable drawbacks, it is highly corrosive and reactive with the risk of outgassing H₂S (Donkers et al., 2017). In addition, physical instability involves phase transitions of sodium sulphide hydrate salts, as shown in the phase diagram of sodium sulphide (De Boer et al., 2002); (1) at a temperature of 49°C, a phase transition of the nonahydrate salt to the pentahydrate salt occurs while simultaneously a solution of sodium sulphide forms, (2) at a temperature of 83°C the pentahydrate structure partially dissolves while it forms a solution of sodium sulphide (Roelands et al., 2015). However, when researchers started to work with Na₂S, they were driven by the promising energy density but there was a lack of prototype studies results that later revealed its chemical, physical, and mechanical limitations.

Indeed, all the projects suffered initially from corrosion and faced variable results by overcoming such issues. Until now, there are still concerns raised regarding how the coatings perform in years, and how the last 1.5 water molecules, whereby the loading power is limited given the melting temperature and dehydration temperature are equal at the current vapor pressure. Moreover, the environmental issues due to the toxicity of the salt hydrate and its by-products such as hydrogen sulphide should not be overlooked. This salt has been studied thoroughly to the prototype level, and the main constraints have already been brought up.

6. Sorption storage material manufacturing routes

The challenges at the material level significantly influence the manufacturing strategies, which at the same time limit the technology readiness level of WSTES. In this section, we provide an overview of the manufacturing routes drawn by researchers in the past decade, and we also try to provide a pathway for future research. Currently, the TCM manufacturing routes are divided between conventional manufacturing procedures (shaping and insertion in a binder) and emerging manufacturing procedures (nano-alternatives, encapsulation, and extrusion), and all these procedures are described in this section. The manufacturing paths that have been considered by researchers and the emerging manufacturing routes are summarized in Figure 5.

FIGURE 5

Little has been carried out on large-scale manufacturing of thermochemical materials, and most work is still at a laboratory scale (from grams to kilos). Until now, only a few approaches for particle stabilization of thermochemical storage materials are reported in the literature ¹¹¹. The fact is that there is still no agreed approach in the manufacturing process at the laboratory scale, not to mention large scale manufacturing. The publications registered in the last 10 years for conventional and non-conventional manufacturing methods are presented in Figure 6. The binder matrix has been the most widely studied, while encapsulation and nanomaterial composite are attracting researchers’ attention since 2016 while increasing their publication rate yearly.

FIGURE 6

6.1 Conventional manufacturing routes

Conventional manufacturing routes can be classified into shaping or insertion in a binder matrix. Although these routes are not yet at a commercial scale, they are the most commonly used when TCMs are formulated in laboratories.

6.1.1 Shaping

Shaping or pelletizing pure thermochemical powder is the most convenient way to maximize energy density, as suggested by researchers. Despite shaping might appear as the cheapest and less complex solution, it does not tackle the TCM problems at once as after one cycle of hydration/dehydration a significant change in molar density and molar volume takes place, which leads to a breakage of the pellets or granules decomposing quickly upon repeated cycles (Afflerbach et al., 2017; Fujii et al., 1994).

6.1.2 Insertion in a binder matrix

FIGURE 7

The choice of the matrix can lead to different ways of preparing CSPMs, regarding the type of matrix used, the pore size, and microstructure, among others. A review of the state of the art is listed in Figure 8, the thermochemical material used, matrix, additive, and the preparation method used are thereof gathered as a schematic picture. Dry impregnation has been the method preferably chosen by researchers, given that the filtration step is omitted and the method is easier to follow (Gordeeva and Aristov, 2012). Nevertheless, when manufacturing large amounts of salt, the main defect is that special procedures and equipment are required to ensure a uniform distribution of the salt into the solution. Thus, even though at the laboratory scale, dry impregnation might stand as a feasible solution, many challenges must be faced before bringing it to the next level.

FIGURE 8

Carbon-based matrices are normally the ones used for solely matrix purposes, see Table 5. Among them, activated carbon has been used as a matrix, given it can increase the surface area, enhance the capillary force of the materials, and ensure it becomes a good carrier to conduct and hold water (Wang et al., 2016). Another alternative is expanded graphite (EG), which has been used as a binding material to enhance the heat transfer properties of the thermochemical reactants (Zamengo et al., 2014; Fopah Lele et al., 2015b;Cammarata et al., 2018). Similarly to activated carbon, EG is characterized by a high thermal conductivity, chemical stability, and large surface area and does not react with adsorbents reactants (Mauran et al., 1993), (Fujioka et al., 2008). In addition, EG has a worm-like particle shape that can host the TCM particles, which is required to remain unaltered after the binding method. Thus, some authors have experienced some issues when preparing the EG/TCM composites such as breakage or exfoliation (Zamengo et al., 2013). Non-carbon-based matrices are silica gel, zeolite, and vermiculite see Table 5. Mesoporous disordered silicas (silica gels) are widely studied as hydrophilic compounds due to their high affinity to water vapor (Ristić and Henninger, 2014). Silica gel composite shows a carryover problem at high humidity and temperatures, which depends on the ratio between the concentrations of salt/pores (Wang et al., 2016). Zeolite is a class of highly porous crystalline aluminosilicate materials that are most used in granular, cylindrical, or beaded forms. Zeolite provides a high specific surface area and accessible reaction sites to ensure good mass transfer, given the high absorbance capacity the salt can be absorbed into the pores (Shere et al., 2018). Vermiculite allows a single layer impregnation of the salt while absorbing large content of TCM than any other matrix used in the literature. As stated by Aydin et al. (2015), vermiculite stands as an excellent host matrix due to its low regeneration temperature (50–80 °C) and low cost.

6.2 Emerging manufacturing routes

In the previous manufacturing routes, we considered the option of artificially increasing the size of particles (by pelletizing) or re-structuring the material to prevent further agglomeration problems (impregnation). However, due to volume changes these structures can break, which leads to smaller particle size and (again) tend to agglomerate. In this framework, novel and emerging manufacturing routes to TCM manufacturing have proliferated in the last 5 years. In this section, we divide them into nano-alternatives, encapsulation, and extrusion.

6.2.2 Nano-alternatives

Nanocomposites are a new class of chemical heat storage materials for moderate-temperature storage owing to their comparatively lower decomposition temperatures (Cot-Gores et al., 2012). Such nanocomposites are generally co-precipitated metallic hydroxides, following a similar manufacturing method as the one reported in the “insertion in a binder” section. Researchers have used the co-precipitated method of two metallic hydroxides (Ryu et al., 2007; Kato et al., 2009; Ishitobi et al., 2013; Zheng et al., 2015), the advantage of this method is that a homogeneous nanocomposite can be obtained by the chemical reaction in the solution directly (Zheng et al., 2015). By using such a method, the decomposition temperature can be substantially decreased (300 °C lower) while increasing cyclability and achieving an optimal storage capacity.

Another nano-alternative is the one used in bulk industries; several flow agents are known to improvethe powder flowability by increasing the surface roughness (nanocoating) or minimize the contact area between particles to improve flow behavior, nanoparticles addition (flowing agents), especially for better handling properties. When applying this method to thermochemical applications, the main challenge is that the additives must maintain their benefits after several hydration/dehydration cycles. Silicon dioxide (SiO₂) nanoparticles (Pontiga et al., 2013) is a good candidate to be used as a doping agent, they either improve fluidization behavior or as a host matrix by using its agglomerates. Some researchers have already studied this strategy for thermochemical materials. Pontiga et al. (2013) (Roßkopf et al., 2014) used nanosilica to contain calcium hydroxide reaction, intending to improve fluidization, and the authors used the dry mixing technique to prepare the composite. Indeed, the use of nanosilica additive was useful to increase the flowability and enhance mass transfer. Interestingly, a commercialized nano-alternative is the nanocoating of the TCM with hydrophobic nanoparticles that improves material compatibility and provides a long-term service (Bolin and Glebov, 2016). The nanocoating (CaO, lithium bromide, lithium chloride, calcium chloride, calcium bromide, and sodium sulfate with silica nanoparticles) is prepared in a blender with the nanoparticles added to the salt solution and mixed at different intervals. The company (SaltX^©) owns the patent and manufactures and delivers the product at a commercial scale. In another study, Roßkopf et al. (2015) used a dry mixing process to coat calcium hydroxide with silicon dioxide nanoparticle agglomerates. They found some difficulties in the manufacturing process: the mixing intensity strongly influences the conversion of the reaction; higher intensity leads to higher conversion.

6.2.3 Encapsulation

The newest and one of the most promising methods to shape TCMs is encapsulation, showing the most efficient way to reduce low temperature lift and the interaction between the TCMs sorption process and their matrices. Encapsulation not only works as a TCM enclosure but also compensates energy losses as the salt hydrate particles are enveloped with a second inherently stable material to prevent coalescence and agglomeration. However, there is a key point to consider when working with encapsulated TCMs, the shell needs to ensure water penetration into the capsule during the heat absorption and release during desorption and a permeable capsule after thermal cycles is required. Apart from the challenge of having a semipermeable capsule to allow the steam to diffuse inside the shell, the shell must enable mechanical stability over several cycles. The capsule should be able to bear forces resulting from the volume expansion of the core material during charging–discharging. The material must be inert and not react or interact with the reversible reaction in any way, which can be challenging under hydrothermal conditions and temperatures up to 550 °C. Some researchers have published interesting studies on this matter as reviewed in the following. The Büchi spray dryer technique was used by Cuypers et al. (2013) to encapsulate CaCl₂·6H₂O. The authors conclude that the encapsulated TCM can store energy at lower temperatures, and the material shows a large thermal response to dehydration upon temperature increase. Gaeini et al. (2018) studied CaCl₂·6H₂O microencapsulation and their results show high multicycles stability and fastest kinetics in comparison with impregnation of graphite and vermiculite composite, although energy density was lower. A ceramic capsule was used by Afflerbach et al. (2017) to encapsulate CaCl₂·6H₂O following a four-step encapsulation method. The ceramic encapsulation not only increased the mechanical stability of the capsules but also the heterogeneities in the ceramic shell. A MgSO₄/polymer composite was synthesized by Kallenberger et al. (2016), which claimed some problems during the dehydration process with the monohydrate form. van Ravensteijn et al. (2021) performed a systematic evaluation of various commercially available polymers as stabilizing shell materials. The authors explored different preparation methods from polymer solutions, preparation of freestanding polymer films, cross-linked films, and coating of zeolite and potassium carbonate with polymer by fluidized bed coating. The authors identified ethylhydroxypropyl cellulose as a promising candidate combining ductility with sufficient water vapor permeabilities. The authors also concluded that the coating thickness has a distinct influence on permeability. Regarding the granular material study, coating of zeolite and potassium carbonate showed that the polymer layer remained intact and significantly improved the mechanical resilience, although the price must be assessed in this case. Shkatulov et al. (2020b) investigated the composites of three salt hydrates (CaCl₂, SrBr₂, and LiCl) encapsulated in mesoporous silica shells. The authors considered the silica shell capsules as promising for hot tap water and space heating, with emphasis on the LiCl encapsulation that confirmed significant stability after 50 hydration/dehydration cycles.

In summary, the encapsulation technique has a high potential to manufacture TCMs but involves additional steps that increase complexity and manufacturing costs. Also, the materials needed to encapsulate the TCM are expensive and not highly available, which will hinder the production of large quantities of encapsulated storage media.

6.3 Extrusion

Extrusion is one of the most common methods to manufacture polymers on a large scale and it has recently started to be used to shape TCMs through zeolites honeycombs. The extrusion of materials needs the addition of additives or binders to thermal process them. Many types of binders are available in the market depending on their origin (natural or synthetic), their nature (organic or inorganic), colloidal particle type or molecular type, and water-soluble or non-polar liquid soluble. The most common binders are the following (Bingre et al., 2018): kaolin, attapulgite, boehmite, aluminas, and/or silica or a combination of them. The paste extrusion technique was used by Li et al. (2001) to extrude 5A zeolite monolith using different bentonite percentages as a binder to improve the internal particle friction. The suggested amount recommended was 10–35% wt., less than the recommended amount will cause extrusion failure due to particle friction, and more than the recommended amount will cause difficulties in handling. To the best of the author’s knowledge, the scalability of this method has not been proven. As extrusion is widely used in other conventional manufacturing industries, adapting the manufacturing process with these materials should not be highly difficult due to the know-how and expertise in this process.

6.4 Summary of manufacturing techniques

Since thermochemical materials are still under material development, researchers have invested most of their efforts in searching for a proper way of stabilizing the salt particles to enable an efficient sorption process. This has limited the manufacturing routes studied in the field to a laboratory scale level, following a bottom-up approach. It is noted that most of the manufacturing routes that present better scalability (extrusion and shaping) are the ones that have been studied the least, whereas others such as impregnation and encapsulation are proven to increase the stability of the material but decrease in storage capacity and non-industrialized complex routes. To wrap up the manufacturing sections, we have elaborated a summary table that includes all the relevant parameters for the manufacturing routes included in the previous sections, see Table 6.

7. Outlook and concluding remarks

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