*2.1.1. Silica gel-water systems*

**2. Working adsorbent/refrigerant pairs**

76 Sustainable Air Conditioning Systems

cooling capacity as it controls the duration of the adsorption cycle.

composite adsorbent/adsorbate pair.

**Figure 4.** Isotherms for type RD silica gel-water pair [5].

The overall performance as well as the design and operating parameters for an ARS are greatly affected by the employed working adsorbent/refrigerant pairs. In general, good adsorbents should have wider range of adsorption capacity with temperature variation, higher heat and mass transfer properties, along with thermal stability and low susceptibility to contamination. In addition, distinctive properties of a refrigerant should be examined, and that include heat of vaporization, thermal conductivity, boiling point and working pressures, reactivity and stability, toxicity, environmental impact and freezing point. The adsorption capacity of an adsorbent-refrigerant pair is commonly determined from plots known as adsorption isotherms as shown in **Figure 4** [5]. These isotherms give the amount of adsorbed mass taken up by the adsorbent, after reaching the thermodynamic equilibrium, as a function of pressure at constant temperatures. Accordingly, adsorbent-adsorbate pairs and their developments can be compared based on their isotherms. However, when the adsorbent domain undergoes transient operating conditions, a kinetic model is required to define the mass transfer kinetics and gives the instantaneous amount of adsorbate through a relation with the equilibrium uptake that is given by the isotherms. Mass transfer kinetics is a catch-all term related to intraparticle mass transfer resistance. The increase in the adsorption capacity increases capability of an ARS to have a large cooling capacity, where it sets up the total amount of refrigerant that can be adsorbed in a cycle. However, faster mass transfer kinetics is required to insure higher

The most commonly used adsorbent/refrigerant pairs are silica gel/water, zeolite/water, activated carbon/methanol, activated carbon/ammonia, calcium chloride/ammonia and composite adsorbent/ammonia. In general, according to the nature of the forces involved in the adsorption process, they are classified into three categories such as physical, chemical and Silica gel is an amorphous silicon dioxide, SiO<sup>2</sup> , made synthetically from sodium silicate, and has a granular, vitreous and highly porous form. The high-density silica is the common type of silica gel used in adsorption systems such as Fuji Davison types 'A' and 'RD' silica gel, which have pore diameter in the range of 2.0–3.5 nm, the pore volume is 0.3–0.4 cm3 /g and the specific surface area is 400–700 m<sup>2</sup> /g [6]. The other types of silica gel with relatively high pore sizes can be used as a host material in composite adsorbents. The thermodynamics characteristics of silica gel-water working pair were investigated experimentally by several researchers as in [5, 7], and the empirically determined parameters for the isotherm equations had been calculated from the experimental data. The performance of the two-bed silica gel-water was evaluated experimentally and analytically by several researchers [8–10].

In general, the main advantage of silica gel over other adsorbents is that the regeneration temperature is typically 85°C which makes such system to be suitable for solar energy use and low temperature waste heat sources. Moreover, it could be as low as 50°C when multi-stage configuration system is applied [11]. In such case, for non-regenerative cycle, the dynamic losses due to the heat capacities of the adsorber components will be reduced which lead to higher COPs since the adsorbent itself and the container vessel do not need to be heated to high temperatures. However, desorption temperature must not be too high. If it is higher than 120°C, silica gel will be destroyed. The adsorption heat is relatively higher than activated carbon pair between 2500–2800 kJ/kg. Also, silica gel porosity level is lower than activated carbon (100–1000 m2 /g). The maximum adsorption capacity at equilibrium could be between 0.35 and 0.4 kg/kg silica gel, while the net change in the instantaneous amount of adsorbate may not exceed 0.1 kg water/kg silica gel under typical operating conditions which is low. Another drawback is the limitation of evaporating temperature due to the freezing point of water and the uptake also is effected badly under a very low vacuum, that make silica gelwater refrigeration system be better to be applied in the air conditioning applications with large chilled water flow rates.

#### *2.1.2. Zeolite-water systems*

Zeolites are microporous, alumina silicate crystals composed of alkali or alkali soil. The zeolite-water working pair has a wide range of desorption temperature (70–250°C). Due to its stable performance at high temperatures, the adsorber can be directly heated by the exhaust gases from engines. Therefore, the zeolite-water system is simpler than that one driven by the hot water. However, the adsorption heat of zeolite-water is higher than that of silica gelwater, between about 3300 and 4200 kJ/kg [12], which will lead to low COPs, in addition to the drawbacks associated with using the water as a refrigerant. Several studies had been presented experimentally and theoretically to investigate and improve the performance of zeolite-water adsorption system particularly for vehicle air conditioning.

adsorption capacity, and they involve calcium chloride (CaCl<sup>2</sup>

transfer as well as the heat transfer by using composite adsorbents.

), barium chloride (BaCl<sup>2</sup>

Generally, chemical adsorbents have very large uptakes with specific adsorptions approaching 1 kg/kgads in some cases, and desorption temperatures varying from 40 to 80°C which are very promising. However, chemical adsorption systems stability is lower than that for physical adsorption systems due to agglomeration and swelling phenomena, which are common in chemical adsorbent beds. This instability reduces heat and mass transfer which limits the cooling capacities of chemical adsorbents. Consequently, heat-driven chillers utilizing these adsorbents have been less common than those using physical adsorbents. To overcome this problem, the porous heat transfer matrixes were put forward for the improvement of mass

Composite adsorbents, also called "Salt in Porous Matrix (CSPM)" represent the promising solution of aforementioned drawbacks associated with pure physical and chemical adsorbents. Thus, many of these composites, which are typically made of porous media and chemical adsorbents, have been developed synthetically to be applied in adsorption refrigeration systems as in Refs. [16–18]. In such composites, porous media work on improving the heat and mass transfer properties of the chemical adsorbents along with limiting the swelling characteristics of the chemical adsorbents, while the chemical adsorbents increase the refrigerant uptake of the adsorbent pair. The common examples of these composites are combinations of metal chlorides and AC, ACF, expanded graphite, silica gel or zeolite. For example, silica gel and chlorides/water which are known as selective water sorbents (SWSs) which are tested and studied by Aristov et al. [6]. Composite adsorbents of silica gel and chloride are usually produced using the impregnation method. The silica gel is immersed in a chloride salt solution and is then dried to remove the water. There are also four types of porous media were used with chlorides to produce composite adsorbents/ammonia: activated carbon, activated

Metal-organic frameworks (MOFs) are highly crystalline porous material that are widely regarded as promising materials for various applications such as catalysis [19], gas separation [20] and gas storage [21]. The high crystallinity of MOFs can be highlighted through the description of MOF-5 structure which was once described as "The zinc carboxylate cluster with the six carboxylate carbons forming a regular octahedron but with tetrahedral symmetry was elegantly beautiful especially when linked in such regular arrays like terracotta warriors" [22]. Usually, the approach of assembling new frameworks out of molecular building blocks or secondary building blocks held together by strong bonding has been considerably used in designing new materials even though it is a challenge to control the assembly of the basic building blocks in the solid state and thus predicting of the resulting structure. Based

), among others. For example, in CaCl<sup>2</sup>

magnesium chloride (MgCl<sup>2</sup>

chloride can adsorb 8 moles ammonia [15].

**2.3. Composite adsorbents-adsorbate pair**

carbon fiber, expanded graphite or vermiculite.

**2.4. Novel adsorbent materials: metal-organic frameworks (MOFs)**

cobalt chloride (CoCl<sup>2</sup>

), strontium chloride (SrCl<sup>2</sup>

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

/ammonia pair, 1 mole calcium

), manganese chloride (MnCl<sup>2</sup>

),

79

) and
