*2.4.2. MOFs-ethanol pair*

the water loading capacity of four nitro or amino-functionalized MIL-101Cr materials (fully and partially functionalized) was assessed for heat transformation applications. The fully

, and partially aminated MIL-101Cr-pNH<sup>2</sup>

loadings (about 1.0 gw/gads) and proving the weak host-guest interactions and hence a lower regeneration temperature is required. Elsayed et al. [25] further improved the thermal conductivity and the water vapor capacity of MIL-101(Cr) to be used in adsorption heat pump application through using hydrophilic graphene oxide. Two methods have been used to develop MIL-101(Cr)/GrO composites. It was shown that introducing low amounts of GrO (2%) to the neat MIL-101(Cr) enhanced the water adsorption characteristics at high relative pressure but enhanced the heat transfer properties by 20–30% while using more than 2% of GrO reduced the water adsorption uptake but significantly enhanced the thermal conductivity by more than 2.5 times. Yan et al. [28] managed to improve the performance of the material through developing another composite (MIL-101@GO) of MIL-101(Cr) and graphite oxide (GO) with high-water vapor capacity for adsorption heat pumps (AHPs). It showed that MIL-101@GO possessed a super-high adsorption capacity for water vapor up to 1.58 gw/gads. This superior water vapor adsorption/desorption performance make MIL-101@GO a promising candidate as the water vapor adsorbent for adsorption heat pumps (AHPs) process. Another factor that was studied was the effect molding on the water adsorption properties of MIL-101(Cr) after pressing the prepared powder into a desired shape which was investigated by Rui et al. [29] . It showed that the forming pressure has a large influence on pore structure of shaped MIL-101, as the forming pressure increases from 3 to 5 MPa, the equilibrium adsorption capacity of water is up to 0.95 gw/gads at the forming pressure of 3 MPa. Other types of MOFs such as Al fumarate was investigated by Jeremias et al. [30] in the form of coating on a metal substrate via the thermal gradient approach. It was concluded that Al fumarate is a promising adsorbent for heat pumping applications as it can be regenerated at low temperature as low as 60°C with a water loading difference higher than 0.5 gw/gads. Fadhel et al. [31–33], generated cooling effect from using aluminum fumarate and MIL-101(Cr) in different multi-bed water adsorption systems. The performance was compared to other adsorbent materials such as AQSOA-Z02 and conventional silica gel. The isostructural CPO-27(Ni) was compared to aluminum fumarate by Elsayed et al. [34]. It was highlighted that the CPO-27(Ni) outperformed the aluminum fumarate at low evaporation temperatures, while the aluminum fumarate was more suitable for applications requiring high evaporation temperature. It was also mentioned that CPO-27(Ni) is suitable for systems operated with high desorption temperature while on the contrary aluminum fumarate can be regenerated

The performance of a number of MOFs such as HKUST-1 and MIL-100(Fe) was investigated and compared to silica gel RD-2060 by Rezk et al. [35]. They showed that HKUST-1 performed better than silica gel RD-2060 with an increase of water uptake of 93.2%, which could lead to a considerable increase in refrigerant flow rate, cooling capacity and/or reducing the size of the adsorption system. However, MIL-100(Fe) MOF showed reduced water uptake comparable to silica gel RD-2060 for water chilling applications with evaporation at 5°C. These results highlight the potential of using MOF materials to improve the efficiency of water adsorption cooling systems. Other MOFs such as MIL-53(Cr), MIL-53(Fe), Birm-1,

, showed the best water

aminated MIL-101Cr-NH<sup>2</sup>

82 Sustainable Air Conditioning Systems

at low desorption temperatures.

Saha et al. [44] presented experimental and theoretical investigations of adsorption characteristics of ethanol onto metal-organic framework namely MIL-101(Cr). The experiments have been conducted within relative pressures between 0.1 and 0.9 and adsorption temperatures ranging from 30 to 70°C, which are suitable for adsorption cooling applications. Adsorption isotherm data exhibit that 1 g of MIL-101(Cr) can adsorb as high as 1.1 g of ethanol at adsorption temperature of 30°C. The experimental results showed that the studied pair would be a promising candidate for developing high performance cooling device. Rezk et al. [45] experimentally investigated the ethanol adsorption characteristics of six MOF materials namely CPO-27(Ni), MIL-101(Cr), HKUST-1, MIL-100(Fe), MIL-53(Cr) and MIL-100(Cr) compared to that of silica gel as a conventional adsorbent material that is widely used in commercial adsorption systems. The results revealed that MIL-101(Cr) have shown superior performance with uptake value of 1.2 gw/gads. Also, MIL-101(Cr) proved to be stable through 20 successive cycles at 25°C. The results from theoretical modeling of a two-bed adsorption system with heat and mass recovery have shown that using MIL-101(Cr)/ethanol pair has remarkable potential in low temperature cooling applications.

#### *2.4.3. MOFs-methanol pair*

Jeremias et al. [46] showed that the use of alcohols (methanol) as working fluids turned be a good prospect for the application of otherwise promising, but hydrothermally unstable or not sufficiently hydrophilic materials like HKUST-1 or MIL-101(Cr), respectively, or for low temperature applications, where the vapor pressure of H2 O is not sufficient for acceptable kinetics and that heat and mass transfer could be optimized by various shaping procedures.

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

Adsorption Refrigeration Technologies http://dx.doi.org/10.5772/intechopen.73167 85

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

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

and discussed in Ref.s [53, 54] as for illustrated in **Figure 8** [55].

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

to inaccurate predictions for the overall performance.

**Figure 8.** Adsorbers' manufacturing procedure [55].

overall system performance [57–60].
