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1 Introduction

Sorption TES belongs to the wider class of thermochemical energy storage. The definition ‘sorption’ was first reported by McBain [1] in 1909 to describe the interaction occurring between a gaseous sorbate and a liquid or solid sorbent. The former interaction was identified as absorption while the latter as adsorption. Absorption was defined as ‘a modified solid solution in which practically only the outer layers become saturated, owing to the difficulty of diffusion in solids’, while adsorption was defined as ‘condensation on the outside of the surface of the solid’. In most of the TESs based on sorption technology, the sorbate is represented by water vapour. For this reason, a classification among the materials for sorption TES is mainly referred to the sorbent employed, as represented in Fig. 1. The classification is performed taking into account the main reaction occurring between the sorbate and the sorbent materials. The absorption into liquids relates to absorption of water vapour inside salt solutions, or ammonia inside liquid water. The solid adsorption is related to the surface interaction between the sorbate (i.e. water vapour) with the solid surface, through weak bonds, like Van der Waals and hydrogen bonds. Nevertheless, in some cases different phenomena can coexist. In particular, the sorption composites present an intermediate behaviour between physical adsorption onto the solid (i.e. the host matrix) and chemical hydration reaction (i.e. in the embedded salt).

Fig. 1
figure 1

Categorisation of sorbent materials for sorption TES

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Generally, there are two system configurations for sorption TES: closed and open cycle. Figure 2 reports the working phases of a closed sorption TES. During charging phase, the reactor, in which the sorbent material is saturated of sorbate (e.g. water), is regenerated exploiting heat coming from the heat source, Qdes. The desorbed vapour is then condensed in the condenser, and the heat of condensation, Qcond, is either dissipated in the ambient or delivered to the load, if the temperature level is sufficient to satisfy it. Once the charging process is completed (i.e. the sorbent material is almost dry), the connection between the condenser and the reactor is closed. Under this condition, the system can keep the stored energy for indefinite time, since the thermal energy is stored as sorption potential between sorbate and sorbent material. In order to get back the stored thermal energy, the connection between liquid sorbate reservoir, which in this phase acts as evaporator and sorbent material in the reactor is again opened. During this discharging phase, the sorbate is evaporated by means of the heat from the ambient, Qevap, then the vapour fluxes to the sorbent material, since the sorption process is exothermic, heat is released to the load, Qsorp. Clearly, this process is defined closed since the sorbate is continuously condensed/evaporated in a closed system without any mass exchange with the ambient.

Fig. 2
figure 2

Closed sorption TES cycle: charging and discharging phase

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Differently, the open sorption TES system, represented in Fig. 3, continuously exchanges mass (sorbate) with the ambient. Actually, the two charging/discharging phases are similar to the ones already described for the closed cycle. Main difference is that, in this case, heat is provided and extracted by fluxing air through the sorbent material contained in the reactor. Particularly, during charging/regeneration phase, a hot and dry air flux enters the storage, causing the desorption of sorbate (i.e. water), and exiting at lower temperature and higher humidity content. During discharging/sorption phase, the humid and cooled air flux is provided to the dry sorbent material, which triggers the sorption and the consequent release of stored heat, which reflects on the exiting hot and dry air flux.

Fig. 3
figure 3

Open sorption TES cycle: charging and discharging phase

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In the literature, several reviews focus on the analysis of the peculiarities of each class of sorbent materials for TES applications, some suggested readings are [2,3,4,5,6]. The following sections focus on the most recent advancements achieved in the sorption TES technology during last years.

2 Sorption Materials for TES Applications

2.1 Liquid Absorption

Liquid absorption technology was mainly investigated for absorption heat pumps and chillers applications [7]. In such a context, LiBr-water and ammonia-water are the working pairs commonly used for these applications, thanks to their good thermodynamic properties as well as their high cycling stability [7]. This technology has been proposed also for long-term thermal energy storage, investigating different possible salt solutions as sorbent. In the literature, the ones that showed the most attractive features are the aqueous solutions based on LiBr [8] and NaOH [9].

An interesting comparison among possible candidates for liquid absorption thermal energy storage was presented by [10]. They investigated seven working pairs, (aqueous solutions of CaCl2, Glycerine, KOH, LiBr, LiCl, NaOH and water/ammonia working pairs) analysing also the effect of allowing partial crystallisation of the salt inside the solution. In the paper, different features were analysed, taking into account optimal charging temperatures, cost of the materials and corrosion resistance. In Table 1 the achievable TES densities, minimum charging temperatures required, the storage system efficiency and the absolute pressure of the system are summarised. The data are reported for both solutions working without any crystallisation and solutions achieving a crystallisation up to a solid salt mass four times higher than the salt solution mass.

Table 1 Main features of investigated working pairs for liquid absorption TES
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As can be highlighted in Table 1, when no crystallisation is allowed, the most attractive solution is the one employing LiCl as salt, presenting also low charging temperature and good efficiency. Similar result is obtained when the crystallisation happens. Clearly, neither glycerine/water nor water/ammonia working pairs can allow any crystallisation under the operating conditions typical of the TES. Generally, crystallisation improves the storage density, lowering at the same time the efficiency of the system. More discussions on the proper selection of liquid absorption working pairs can be found in [11, 12].

2.2 Solid Adsorption

In the solid adsorption TES, the adsorbate molecules interact with the solid on the external surface of the adsorbent by means of physical bonds. The adsorbent materials usually employed in solid adsorption TES are zeolites [13], silico-aluminophosphates [14] and silica gels [15]. Recently, also activities on Metalorganic Frameworks (MOFs) are under development for this purpose [16].

Zeolites are crystalline aluminosilicate adsorbents with microporous structure able to host molecules of different nature. Several different structures exist, but, for sorption TES applications with water as sorbate, the most commonly employed are 4A, 54, 13X and Y [17]. These materials are quite well established and investigated in the literature. Nevertheless, still some activities are currently performed. For instance, in [18], a comparison between commercial 13X with Na+ as cation and exchanged samples with Mg2+ and Ca2+ was performed. It demonstrated that the effect of the cation allows obtaining higher storage density when the Mg2+ cations replace the Na+ in the zeolite structure. Another analysis performed on the exchanged cations in 13X samples was recently reported in [19]. In this case, different cations were employed, demonstrating that the increasing trend of the storage density depends on the exchanged cations (i.e. Zn > Sr > Cd > Na). This confirms the possibility of tuning it by properly varying the exchanged cations. In general, zeolites are characterised by high charging temperatures (i.e. above 150 °C), which make them suitable for industrial TES applications but less attractive for low temperature solar TES.

Aluminophosphates (AlPOs) and other derived structures like silico-aluminophosphates (SAPOs) are characterised by a crystalline and porous structure, similar to the classical zeolites. For this reason they are often referred as zeotypes [20, 21]. These materials are usually characterised by an adsorption isotherm V according to IUPAC classification [22], showing a partially hydrophobic behaviour at low relative pressure and a partially hydrophilic behaviour at high relative pressure. This isotherm is sometimes defined as ‘S-shaped’ since a high water vapour exchange can be achieved in a narrow relative pressure range, making these materials suitable for sorption TES applications [21, 23]. Recently, Brancato and Frazzica [21] comparatively analysed the achievable performance of three among the most attractive zeotypes for adsorption heat transformation, namely, AlPO-18, SAPO-34 and FAPO-34. In particular, for TES application, they demonstrated that the storage density achievable by these materials, when heat source at 90 °C is available, can reach up to 0.14 kWh/kg, to provide space heating, which represents a promising performance indicator for sorption TES application. Nevertheless, the available micro pores volume typical of these materials, which cannot allow further increasing the sorption capacity and thus the related TES density, limits this value. Some other papers dealing with the analysis of these materials are the following [14, 24].

Silica gels are porous materials with an amorphous structure, which have been historically considered for sorption TES applications, thanks to their ability as water sorbents. These materials can be synthesised under different conditions, achieving porous structure from the micro- to meso-pores [20]. They attracted a lot of attention, especially thanks to their low cost and wide commercial availability. Nevertheless, the interest towards this class of materials has been recently reduced, since outcomes of some research projects, like Modestore [25], demonstrated that they have low water vapour exchange, which limits the achievable TES density.

2.3 Composite Materials

The development of composite sorbent materials was proposed to overcome some of the main issues of the pure salt applied for sorption TES (i.e. cycling stability, low thermal conductivity and vapour transfer resistance) [26, 27]. Indeed, in these materials the salt is embedded inside a porous matrix, which disperses the salt, allowing lowering mass transfer resistance and limiting agglomeration problems. The research activity on the development and testing of this class of materials is gaining a lot of attention, thanks to high achievable TES density as well as their flexibility in terms of composition that makes them adaptable for different operating conditions.

Several salts coupled to different porous matrixes (e.g. zeolites, silica gels, carbonaceous materials) were investigated and reported in the literature. A deep literature review is reported in [6].

In Table 2, an overview of the composite sorbents developed and tested so far is reported. As can be seen, also from the storage density point of view, these materials show an intermediate behaviour between pure salts and physical adsorbents.

Table 2 Some of the most interesting composite sorbents reported in the literature for sorption TES applications
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Table 3 reports a comparison among the average thermo-physical parameters of different sorbent materials for TES applications. As can be seen, the composites present the most attractive performance in terms of sorption capacity, which means achievable TES density. While, among the pure adsorbents, MOFs present attractive features. Nevertheless, both categories presents some open issues to be investigated, such as the cycling stability and the cost, especially for MOFs. This confirms the needs for further research and development activities.

Table 3 Thermo-physical parameters of different sorbent materials for TES applications
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3 Components and Systems for Sorption TES Application

Material development represents only the first stage in the design of a TES system, since the realisation of a complete system generally involves several technical challenges. Research on sorption storages, useful for application in the residential sector, still requires significant efforts for increasing the technological level. Nonetheless, during the last years, significant efforts were devoted to the study of components and prototypes, demonstrating the practical feasibility of the technology. To present a complete overview of the most recent systems reported in the literature, Table 4 summarises the relevant features and findings regarding the prototypes, while Table 5 presents the testing conditions and main experimental outcomes of a vast amount of prototypes of thermochemical storages available in the literature.

Table 4 Sorption storage systems recently presented
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Table 5 Boundary conditions and main results of the prototypes of sorption storages presented in literature
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Among absorption systems, a wide experimental activity was performed on NaOH/water working pair for the development of a pilot demonstrator [42]. However, the results of the experimental campaign, despite proving the successful operation of the system, evidenced that a proper design of the components is needed to avoid low thermal power output.

In the field of solid adsorption, several prototypes were developed for space heating application, employing zeolite 13X as adsorbent within open cycles. The main peculiarity of the prototypes presented in [43,44,45] is their modularity, which allows parallel or series operation, to extend the time during which the useful effect is produced or increase the thermal power output. Instead, only a few prototypes were developed using zeolite (i.e. FAM Z02) for closed adsorption applications in space heating and cooling, which have further stressed the need for an optimised design of the storage unit itself and the auxiliary components, such as the evaporator.

On the contrary, during the last years, the intense activity on composite materials lead to the development of an increasing number of prototypes. Apart from the tailoring of the composite for the specific application, the main peculiarity of several of these prototypes is the use under different conditions than the standard charge/discharge cycle presented in Fig. 2. For instance, the prototypes developed in [46, 47] are used as ‘sorption thermal battery’ for cold and heat energy storage: based on user request (i.e. cold or heat) connection to the evaporator or to the sorption bed of the thermal battery can be selected, even to achieve contemporary heat and freezing useful effect. The prototype reported in [48] is a resorption thermal battery that can be operated in different modes according to the external ambient temperature.

A common outcome of many experimentations is the need to address design issues, either to increase the achievable storage capacity or, in the case of the prototypes containing salts, to avoid salt leakage and increase the thermal conductivity of the system. It is however possible to foresee that, thanks also to the increasing number of projects involving sorption storages, a technological growth will be achieved within the next years.