**4. Technical Challenge**

According to the facts mentioned in the previous section, one can easily conclude that to introduce adsorption systems in the residential sector a compact system must be designed due to space restrictions. However, given that providing the required amount of household power is directly proportional to the amount of zeolite, TES sizing would become a significant limiting factor in the development of an adsorption system for residential application. As a result, many researchers are attempting to improve TES system efficiency to make them capable of storing more energy in less space (highly efficient system). This transition introduces several challenges in executing TES performance processes (charge and discharge), such as efficient charging resources, precise humidity control, reactor design, manufacturing, etc. In this section, the main obstacles in achieving such a compact and efficient system will be discussed.

#### *4.1. Required Temperature Supply*

To begin the charging phase in adsorption systems, the inlet air temperature must reach the hydration reaction temperature. Depending on the materials employed, different storage systems require different input temperatures; for zeolite 13X, this temperature must be above 120 ◦C [16]. As a way to increase efficiency, many researchers investigated the effect of increasing input temperature on system function. Johannes et al., in 2015, used open-source heat storage, including two containers of zeolite 13X. They examined two different temperatures levels for the charging phase and compared their effects on the system charging phase. They discovered increasing temperature from 120 ◦C to 180 ◦C can enhance storage density by 40%, while decreasing charging time by 7% (Figure 10).

**Figure 10.** The effect inlet temperature on the heat storage density (**a**) and the charging time (**b**) [17].

Required high temperature flow can be supplied from various resources. Table 1 illustrates some of the prototypes that were built, as well as the temperatures and heat sources that were used. As can be seen in the highlighted column, electric heaters have been employed to deliver the required heat in almost all instances. However, electric heaters have high consumption due to their low COP as a heating system. With a typical heater (consuming between 2 to 4 kWh), achieving higher temperature input flow seems a less than ideal solution to provide Qin, unless a cost-effective power source can be provided. This necessity makes the intake power supply of compact TES systems a significant challenge to be addressed for residential applications.


**Table 1.** Used temperature for prototypes.

#### *4.2. Relative Humidity Control*

The level of humidity entering to reaction area in the collector with open systems has the most influence on power density and storage density [22]. Relative humidity has direct effect on the sorbate uptake (as reflected in Equation (2)), which makes it a key parameter for storage density. Most prototypes employ an electrical humidifier, water tank, and sensor to supply and control the humidity of the system's incoming air [23]. Figure 11 shows how lowering the relative humidity in the charging process from 0.5 to 0.1 percent enhances both heat storage density (a) and charging time (b). It also demonstrates that storage density and power density are very sensitive to humidity changes. A change in humidity of 0.4 percent caused a significant power change, as can be seen in Figure 11. These characteristics must, thus, be considered to build an appropriate power system.

**Figure 11.** The influence of inlet relative humidity on the heat storage density and power density (**a**) as well as the charging time (**b**) [18].

For household applications, the inlet relative humidity becomes more important as unstable ambient air is used for the charging and discharging process [24]. Due to the strong dependency of stored and released heat on relative humidity, the use of TES system in the residential sector would introduce a significant challenge that necessitates implementing precise humidity monitoring for closed loop control, especially when variable setpoints are desired to adjust variable system's needs.

#### *4.3. Reduce Reactor Size*

Reducing the size of the reactor itself can improve their residential development. Smaller reactors occupy less space and are simpler to integrate with an energy source. However, as mentioned earlier, due to the direct relationship between zeolite volume and system power, constructing a smaller reactor faces numerous configuration and manufacturing limitations. Several investigations have been performed to identify and improve effective size criteria, such as bed length, cross section area, and tank volume by Kuznik et al. [22]

and Michel, Mazet, and Neveu [24]. Gondre et al. [17] found that there exists a linear between the outlet power versus cross section area, heat storage capacity versus storage tank volume, and charging time and autonomy versus bed length, as shown in Figure 12.

**Figure 12.** (**a**) The influence of cross section area on outlet power; (**b**) The impact of storage tank volume on hear storage capacity; (**c**) The influence of bed length against chrging time; (**d**) The impact of bed length on discharging time [17].

Their experiment presented the following significant outcomes, as illustrated in the Figure 12:


Reducing the size of the reactor, as predicted, reduces the power of the system. Therefore, ways to optimize the reactor's dimensions should be investigated if residential applications are in the list.

#### *4.4. System Output Power*

Another point of interest in TES system optimization is the output power during its discharge process. Many researchers have been able to enhance TES system output power by selectively adjusting the discharge process parameters. Several prototypes in laboratories were designed for this purpose, presented in Figure 13. Among them, Jahannes et al. developed a high power open sorption system (STAID), which contains two reactors of 80 kg of zeolite13X [18]. The discharge parameter of this system was adjusted to generate heat for the residential sector during peak hours. Input ambient air flow temperature was considered to be 20 ◦C, and outlet temperature was 57 ◦C. It was observed that their system delivered 6 h of continuous heating during the discharge phase, equal to a maximum 2.25 kw of thermal power output.

**Figure 13.** Comparison of mass power obtained from different physical adsorption systems, where STAID is the present work [18].

Zettl et al., of the Austria Solar Innovation Center (ASIC), used different techniques to increase the discharge power. They designed a prototype with a rotating bed that was capable to generate maximum outlet power 1.5 kW only using 50 kg zeolite 4A. The purpose of using rotating bed was to avoid the formation of dead zone to increase the outlet power. The input temperature was 25 ◦C, and maximum outlet temperature was 60 ◦C [19].

ADEnergy research Center of the Netherlands developed an open sorption concept using two beds with 150 kg zeolite 13X with the compact bed. This system was designed to supply warm air for the residential sector. The air is humidified with 12 mbar water vapor pressure, and air flow rate is 80 m3/h. This system generates maximum 0.4 kW, and output temperature is 70 ◦C [25].

MonoSorp prototype was designed as heat storage system with opened bed for space heating. The input temperature is around 20 ◦C, and the maximum outlet temperature is approximately 42 ◦C. They used zeolite as extruded honeycomb structures to avoid pressure loss. This system was able to deliver maximum thermal power of 1.5 kW [13].

Figure 13 collectively present these TES systems specific power output concerning their involved substance mass and volume The research process, as shown in Figure 13, is aimed at improving extractable power based on material mass (W/kg material) and improving heat storage density (kwh/m3), which, in addition to designing smaller systems, enables these systems to be used for a longer period of time, allowing them to deliver the required heat not for several hours, but for many days.

Although the current attempt has already proven the feasibility of utilizing such systems for residential use, it seems that further improvement of system output power is required market justifications. This would become a significant challenge as power optimization would directly point toward zeolite's physical limitations.

#### *4.5. Efficiency*

As mentioned before, zeolites have the unique capacity to store heat, and, due to their high structural endurance, they can tolerate numerous thermal cycles. These characteristics make zeolites known for their high thermal efficiency; however, utilizing them in TES systems has yet to reveal anything near to their optimum capacity. In this regard, in the literature, one can find two different efficiencies reported by scientists and engineers which indicates zeolite material efficiency and its system efficiency. Figure 14 shows these efficiencies for TES system containing zeolite in an open bed reactor. In this prototype, developed by Kuznik, the reported material efficiency was about 70%. However, one can clearly see that, when it is placed in the storage systems, this efficiency drops to around 36%.

**Figure 14.** (**a**) The influence of charging time on the temperature profile (**b**) The impact of discharging time on the temperature profile; (**c**) Heat losses in the TES system [22].

As can be seen in Figure 14c, there exist several losses associated with all the three TES main processes which drop the engineering efficiency significantly. Nearly half of the injected heat is directly discharged through the outlet in the charging process.

The type of reactor and the inherent feature of zeolite can be responsible for the large amount of energy wasted during the charging phase. Zeolites, despite their high energy storage capacity, have poor thermal conductivity. Thermal conductivity is very important for increasing the internal temperature of zeolites to a level where the zeolite's internal moisture can release as a gas. Figure 14a shows the consequences of low thermal conductivity, as the reactor outlet temperature begins to increase after two hours of the charging process begins.

During the storing period, there exist charge conversion losses, cool down losses, and, finally, discharge conversion losses for the discharge process. There is no energy loss during the discharging phase, and the outlet temperature remains constant for six hours Figure 14b. All these losses, on the other hand, provide a window of opportunity to be addressed by engineers which, of course, brings many new challenges to the table.

#### **5. Conclusions**

Different aspects of utilizing zeolite 13X as a heat storage medium have been briefly discussed, and the main challenges have been summarized. It was found that zeolite 13X has high potential to be used in the residential sector; however, there exist several challenges that should be addressed prior for this to happen. This review demonstrated and discussed the variety of challenges from different perspectives, such as compact system design, charging supply, humidity management, system output power, and system efficiency. It was concluded that, to bring this technology to the residential sector, compact efficient designs are required, including techniques that lead to increasing the contact area while reducing the size of the system, a contradiction to be solved. In addition, it was concluded that, to make system energy intake justified, more affordable energy resources should be identified and implemented. Overall, the current review reveals that, although the implication of the zeolites TES system as a heating system seems feasible, they are, by far, set to become a serious competitor for current heating appliances in the residential market. This outcome, although it may negate the business side of the technology, offers several nitce opportunities for potential experts of the field.

**Author Contributions:** Conceptualization, A.B. and A.Z.; methodology, A.B.; investigation, A.B.; resources, A.B.; writing—original draft preparation, A.B.; writing—review and editing, A.B. and A.Z.; visualization, A.B.; supervision, A.Z.; project administration, A.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** The paper has not been supported by any funding resources.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

#### **References**

