**3.1. Adsorbent developments**

Intensifying the heat transfer of an adsorbent depends mainly on increasing its thermal conductivity where the conduction is the major way to transfer the heat through the adsorbent. Consolidating the adsorbent or using additives with good thermal conductivity into the adsorbent are the common approaches used to enhance the heat transfer in the adsorbent [50, 51]. However, such approaches always decrease the permeability of the adsorbent leading to a decrease in inter-particle mass transfer. The overall performance of the bed will be affected by this contradiction between *heat transfer* and *mass transfer* in the adsorbent. Thus, it should be considered that the increase in thermal conductivity 20-fold, for example, does not mean a similar great enhancement in the overall performance due to the reduction in mass transfer. On microscopic level, the distribution of micro layers inside the samples of an adsorbent affects both the thermal conductivity and permeability, and then investigation can be applied for enhancing both of them as made in Ref. [52]. Therefore, sample preparation and filling techniques can be optimized to enhance both heat and mass transfer. Testing adsorbents in various forms and sizes is also an effective way to investigate the best HMT performance of the adsorber. Developments of the composite adsorbents are another active area used to enhance refrigerant uptake of the pure adsorbents and their stability. More recently, there are two trends: coating the adsorbent over the heat transfer metal surfaces of adsorbers, aiming at the elimination both of thermal contact resistances and large inter-particle voids, or using the new metal-organic frameworks (MOFs) materials which provide attractive adsorption characteristics compared to common adsorbents.

#### *3.1.1. Adsorbent coatings*

From the standpoint that loose grains and consolidated adsorbent beds have poor heat transfer and mass transfer properties, respectively, the concept of coated adsorber has been developed to introduce adsorbers with efficient heat and mass transfer. Applying direct synthesis, or using a binder for deposition a layer of adsorbent over walls of the metal heat exchangers are two common technologies of adsorbent coatings. Different approaches have been reported and discussed in Ref.s [53, 54] as for illustrated in **Figure 8** [55].

### **3.2. Optimization of the adsorber design and cycle modes**

not sufficiently hydrophilic materials like HKUST-1 or MIL-101(Cr), respectively, or for low

kinetics and that heat and mass transfer could be optimized by various shaping procedures.

Enhancing the heat and mass transfer (HMT) of the adsorber is the most crucial part in developing ARSs. For a given cooling capacity, higher specific cooling capacity (SCC) means smaller amount of adsorbent to be used, and that can be a direct result of improving the heat and mass transfer performance of the adsorber. Besides, a lighter weight and smaller volume are existed in such case. As the adsorption system consumes less heat during regeneration modes, the COP is increased. Two methods are commonly used to increase the HMT: one is the development of adsorbents and the second is the optimization of the adsorber designs

Intensifying the heat transfer of an adsorbent depends mainly on increasing its thermal conductivity where the conduction is the major way to transfer the heat through the adsorbent. Consolidating the adsorbent or using additives with good thermal conductivity into the adsorbent are the common approaches used to enhance the heat transfer in the adsorbent [50, 51]. However, such approaches always decrease the permeability of the adsorbent leading to a decrease in inter-particle mass transfer. The overall performance of the bed will be affected by this contradiction between *heat transfer* and *mass transfer* in the adsorbent. Thus, it should be considered that the increase in thermal conductivity 20-fold, for example, does not mean a similar great enhancement in the overall performance due to the reduction in mass transfer. On microscopic level, the distribution of micro layers inside the samples of an adsorbent affects both the thermal conductivity and permeability, and then investigation can be applied for enhancing both of them as made in Ref. [52]. Therefore, sample preparation and filling techniques can be optimized to enhance both heat and mass transfer. Testing adsorbents in various forms and sizes is also an effective way to investigate the best HMT performance of the adsorber. Developments of the composite adsorbents are another active area used to enhance refrigerant uptake of the pure adsorbents and their stability. More recently, there are two trends: coating the adsorbent over the heat transfer metal surfaces of adsorbers, aiming at the elimination both of thermal contact resistances and large inter-particle voids, or using the new metal-organic frameworks (MOFs) materials which provide attractive adsorption

From the standpoint that loose grains and consolidated adsorbent beds have poor heat transfer and mass transfer properties, respectively, the concept of coated adsorber has been developed

O is not sufficient for acceptable

temperature applications, where the vapor pressure of H2

**3. Heat and mass transfer enhancements**

characteristics compared to common adsorbents.

*3.1.1. Adsorbent coatings*

and cycle modes.

**3.1. Adsorbent developments**

84 Sustainable Air Conditioning Systems

Optimized parameters and sophisticated designs of adsorber configurations can help in enhancing the inter-particle mass transfer in the adsorbent domain, along with facilitating the heat transfer between the adsorbent and heat transfer fluid HTF. Extended metal surfaces 'fins' are commonly used to intensify the heat transfer and overcome the low thermal conductivity of the adsorbent materials. However, the COP of an adsorption system is strongly affected by the metal-adsorbent mass ratio [56]. Therefore, the net effects of the fins parameters such as fin spacing, height and thickness should be investigated carefully to optimize the overall system performance [57–60].

In the same context, operational control parameters, such as adsorber modes' durations and fluid flow rates, influence considerably the ARSs' performance and need to be also optimized. Basically, in view of the fact that the diffusion of mass within the adsorbent particles is better with higher temperatures, therefore, the desorption process is carried out faster than the adsorption process. That explains why the differences between equilibrium and instantaneous amount of adsorbate (Weq–*w*) in desorption and adsorption modes are not identical during the cyclic steady state. A larger difference is required during an adsorption mode to adsorb the same total amount desorbed during a desorption mode for making cyclic steady state. Increasing the cooling water velocity and/or adsorption duration over heating water velocity and/or desorption mode are the common ways to reach steady state cycle. And that increases need to be optimized for maximizing the adsorption system performance, as by adsorption/ desorption times reallocation, [61]. Operating under low pressures is another challenge as in the case of water and methanol as refrigerants. In this case, the poor mass transfer in adsorbers can lessen greatly the difference in the refrigerant uptakes during the cycle. That requires more developed designs for such adsorbers to improve their performances. It is important to mention that studying the net effect of any operating parameter on the adsorption kinetic during only one mode (adsorption or desorption) based on given initial conditions may lead to inaccurate predictions for the overall performance.
