**2. Thermal Energy Storage Systems**

To provide a better understanding of TES challenges, in this section, briefly, the underpinning theory, as well their systematic characteristics, are explained. In general, TES systems have been used widely in the industrial sector (with some large-scale residential applications). These systems are divided into three main categories based on their storage methods: Sensible heat, Latent heat, and thermochemical heat, as presented in Figure 1. As can be seen in all of these systems, a high-capacity storage substance is implemented to receive, save, hold, or carry the thermal energy. The performance process of these substances can be characterized in three main steps, charging, storage, and discharging.

In Figure 2, these three processes are presented for the different TES categories. As presented in Figure 2a, Sensible heat storage systems use the simple method of storing heat by discharging excess heat from the material without phase change. In Latent heat storage (Figure 2b), the stored heat is the result of a phase change; thus, they can attain a higher thermal density compared to the Sensible method. In Thermochemical systems (Figure 2c), unlike the two other methods, the stored heat is obtained from a reversible thermochemical reaction.

**Figure 1.** Classification of different thermal storage methods.

**Figure 2.** Methods of thermal energy storage (**a**) Sensible heat, (**b**) latent heat, (**c**) thermochemical heat (Sorption heat) [1].

Thermochemical systems are particularly efficient in storing heat for long-term storage applications because heat loss from the system is low.

In addition, Thermochemical storage systems, due to their high-density storage characteristics, provide a spatially efficient and compact storage system compared to the other heat storage. A volumetric comparison between the three main categories is presented in Figure 3; as can be seen, for instance, replacement of latent system by Thermochemical would save up to 10 times the required space, which makes these types of systems advantageous for residential sector application [2].

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**Figure 3.** Volume needed to full cover the annual storage need of an energy efficient passive house (6480 MJ) [3].

Given that zeolite 13X stocks heat by adsorption method that belongs to the thermochemical category, in this review, the focus will be on adsorption heat storage.

Adsorption is the movement of atoms or molecules from a bulk phase (which might be solid, liquid, or gas) to a solid or liquid surface. Heat is released as a result of attractive interactions between the surface (adsorbent) and the molecules being adsorbed (adsorbate) [2], as presented in Figure 4.

**Figure 4.** Charging and discharging thermochemical heat by sorption reaction [4].

The attractive potential for this thermochemical reaction can be explained with Lennard-Jones potential (Figure 5) equation as a function of distance:

$$V\_{\rm LJ}(r) = 4\varepsilon \left[ \left( \frac{\sigma}{r} \right)^{12} - \left( \frac{\sigma}{r} \right)^{6} \right] \tag{1}$$

where *σ* is the distance at which the potential is zero, and *ε* is the depth of the potential well. According to Equation (1), higher *ε* means more heat would be released after adsorption. The extent of this potential varies from 8 to 800 kJ mol−1. The amount of adsorption depends on the inherent properties of the material in reaction, such as specific surface area, and the affinity of the couple adsorbent/adsorbate. The adsorption process also is subject to operating conditions, such as temperature, pressure, and concentrate of the adsorbate. For instance, high specific surface area, high pressure, and low temperature increase adsorption reaction rate.

**Figure 5.** Lennard-Jones potential.

#### **3. Adsorption Heat Materials**

Generally, all solids are adsorbents, but only those with a high specific surface area are interesting for sorption applications. In this section, a few adsorbent families are briefly introduced.

In industrial applications, activated carbons are the most often used adsorbents. It is mostly employed in the purification of gases. Non-polar activated carbons are commonly utilized for water treatment. As a result of their poor affinity for water, they are not employed in heat storage applications. Silica gel is a synthetically porous type of sodium silicone dioxide. It may be represented as SiO2.nH2O in its chemical compound. Treating synthesis (PH and presence of cations) control carefully permits pores to be controlled. Its large specific surface area (from 600 to 800 m2.g−1) and hydrophilic characteristics make it an excellent desiccant, and it has a very high adsorption capacity at low pressures and temperatures. When silica gel is saturated with water, it loses its capacity to create heat for long periods of time, which is one of its limitations as a heat storage medium.

Zeolites are porous crystalline minerals composed of silicon (Si), aluminum (Al), and oxygen (O) atoms. In tetrahedral configurations, each Si or Al atom is linked to four oxygen atoms (SiO4 and AlO4). Each oxygen atom is shared with another tetrahedron that connects Si or Al atoms, as presented in Figure 6. This configuration results in atomic angles remaining the same and, thus, a uniform distribution in pore sizes. On the other hand, since each oxygen atom is shared between two tetrahedral Si or Al atoms, the stoichiometric composition of each tetrahedral unit is SiO2 or AlO2 with minimum Si/Al ratio to be 1.0 without any upper limit. Given the fact that rich Aluminium sieves has a high affinity for water (since each Al atom introduces a negative charge in the material and polar molecules, such as water, are sensitive to these charges), it demonstrates a high level of hydrophilic behavior. Conversely, since rich silicon sieves have a hydrophobic behavior, heat induced transition between hydrophilic and hydrophobic behavior (usually occurs at a Si/Al ratio between 8 and 10 [5], plus uniform pore sizes, introduces zeolites as an exceptional adsorbent fit for TES systems.

**Figure 6.** Zeolite framework structures: (**a**) A, (**b**) X and Y, (**c**) Rho. The H2 molecule is shown to scale for comparison [6].

There are various type of zeolite and their composite used for heat storage [7]. Among the zeolite family, zeolite 13X (Figure 6b) has the largest permeability for water molecules due to its inner hole size, which plays a key role in heat storage efficiency.

In addition, while employing zeolitic composite as heat storage may be more efficient, the use of added materials and the synthesis process raises health concerns that zeolite 13X, in turn, does not.

To make use of this exceptional adsorptive capacity, zeolites must be placed inside reactor beds to undergo the aforesaid three performance processes (charging, storing, and discharging). The efficiency of performance processes is closely related to controlling the reactor bed operational condition, such as temperature, flow rate, and moisture level.

There exist varieties of reactor systems that can serve the purpose, among them open adsorption systems with the fixed bed being the most used type. A schematic of these systems is presented in Figure 7. To charge zeolite in this system, a stream of hot air with minimum 120 ◦C must be injected into the reactor, where zeolite is placed. The injected hot stream into the bed, while charging zeolites, also extracts and transports its trapped moisture.

**Figure 7.** Open adsorption process.

Regarding the storage step, thanks to the unique zeolite structure, the stored heat will remain untouched until moisture re-enters the reactor, which makes this system needless for thermal isolation (a specific advantage compared to other systems). In these systems, heat sorption capacity is defined as:

$$
\mathbb{Q}\text{ sorbtion} = -V \mid \Delta H \mid \Delta q \tag{2}
$$

where *V* is the bed volume, |Δ*H*| is the specific sorption heat, and Δ*q* presents the sorbate uptake capacity between charging and discharging phase [8]. During the discharge (adsorption) process, ambient airflow with precisely controlled humidity enters the reactor. Given that zeolite is hydrophilic at ambient temperature, it absorbs the humidity and releases the stored energy in the form of heat. The outlet air temperature and the discharging duration depend not only on the inherent property of zeolite but also on the effectiveness of reactor system design and its operational conditions. This makes reactors a key player of the heat storage systems, after selecting appropriate materials.

Given that the proper access to zeolite's unique storage capacity and its efficient delivery bonds to reactor parameters, the design and optimization of reactors would then become one of the main challenges in using heat storage systems in the residential sector. Figure 8 presents the reactor most important variables in three main categories: Reactor Structure, Auxiliary Equipment, and Design Parameters, each introducing their challenges.

**Figure 8.** Influence impact on reactor optimization [9].

The main goal in TES systems is to optimize mass/heat transfer and decrease heat loss. To attain this, several configurations have been examined so far, such as staced, tabular, honeycomb, plate, modular, etc., in all these configurations parameters, such as reactor diameter, length, and connectors, have significant effects on the efficiency of heat storage and have been investigated by many researchers. Anderson et al. [10] found that bed length and heat loss have direct relationship, so, by reducing the length, heat loss can be

reduced. Lahmidi et al. [11] used a stacted reactor. In this design, to improve mass transfer a nozzle device is added to the system, which increases the interaction area between solid and water vapor conducive. To further improve mass and heat capacity Stitou et al. [12] developed a pilot plan using high thermal conductivity of ENG. Layer thickness is another key parameter for reactor design, directly affecting the hydration time. Van Essen et al. [13] observed that decreasing the thickness speeds up the hydration process. Oktariani et al. developed system for generation steam by using a zeolite 13X-water system [14]. They found that the flow direction of feeding water from the top of the reactor using nozzle configuration could conform a better result than feeding water from the reactor bottom. Considering the aforementioned facts, it seems simpler reactors, such as a high efficiency staced reactor, could be the better option for the residential sector; however, thanks to the advancement of fabrication technology, modification to reactor design is yet to present future enhancements.

Evaluation of an efficient design can be tested using TES systematic performance parameters. In general, performance parameters can be expressed by three terms: thermal power density with unit (kW·m−3), thermal storage density with unit (kWh·m−3), and the Coefficient of Performance (COP) with the first two being proportional, and can be obtained from various design and condition parameters and the third to be defined as (Figure 9):

$$\text{COP} = \frac{Q\_H}{W} \tag{3}$$

where *W* and *QH* are the power required to run the discharging process and the amount of extracted heat, respectively. As can be seen, the provided COP in the article, although present in the effectiveness of the discharge process, leaves the charging process untouched. To cover the whole performance cycle, it might be better to introduce TES cycle thermal efficiency *η* as:

$$\eta = \frac{Q\_H}{\mathcal{W} + Q\_{\rm in}} \tag{4}$$

where *Qin* presents the provided heat to the system. It is now vivid that the thermal efficiency of the TES systems indeed depends on a variety of factors, on top of which the effectiveness of the charging process is present. The multivariable nature of TES ongoing performance process opens several challenges for further optimization and TES systems new applications (e.g., residential applications), to be investigated in the next section.

**Figure 9.** Coefficient of performance [15].
